Posts Tagged ism

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Prospects of Turbulence Studies in High-Energy Density Laser-Generated Plasma: Numerical Investigations in Two Dimensions [Cross-Listing]

We investigate the possibility of generating and studying turbulence in plasma by means of high-energy density laser-driven experiments. Our focus is to create supersonic, self-magnetized turbulence with characteristics that resemble those found in the interstellar medium (ISM). We consider a target made of a spherical core surrounded by a shell made of denser material. The shell is irradiated by a sequence of laser pulses sending inward-propagating shocks that convert the inner core into plasma and create turbulence. In the context of the evolution of the ISM, the shocks play the role of supernova remnant shocks and the core represents the ionized interstellar medium. We consider the effects of both pre-existing and self-generating magnetic fields and study the evolution of the system by means of two-dimensional numerical simulations. We find that the evolution of the turbulent core is generally, subsonic with rms-Mach number $M_t\approx 0.2$. We observe an isotropic, turbulent velocity field with an inertial range power spectra of $P(k)\propto k^{-2.3}$. We account for the effects of self-magnetization and find that the resulting magnetic field has characteristic strength $\approx 3\times 10^{4}$ G. The corresponding plasma beta is $\approx 1\times 10^{4}$–$1\times 10^{5}$, indicating that the magnetic field does not play an important role in the dynamical evolution of the system. The natural extension of this work is to study the system evolution in three-dimensions, with various laser drive configurations, and targets with shells and cores of different masses. The latter modification may help to increase the turbulent intensity and possibly create transonic turbulence. One of the key challenges is to obtain transonic turbulent conditions in a quasi-steady state environment.

On the Origin of Highly Ionized X-ray Absorbers Detected in the Galactic X-ray Binaries

X-ray observations of the Galactic X-ray binaries (XRB) revealed numerous highly ionized metal absorption lines. However, it is unclear whether such lines are produced by the hot interstellar medium (ISM) or the circumstellar medium (CSM) intrinsic to the binaries. Here we present a Chandra X-ray absorption line study of 28 observations on 12 X-ray binaries, with a focus on the NeIX and FeXVII lines. We report the first detections of these lines in a significant amount of observations. We do not find significant dependence of the line equivalent width on the distance of the XRBs, but we do see weak dependence on the source X-ray luminosity. We also find two out of twelve selected targets show strong temporal variation of the NeIX absorbers. While the line ratio between the two ion species suggests a temperature consistent with the previous predictions of the ISM, comparing with two theoretical models of the ISM shows the observed column densities are significantly higher than predictions. On the other hand, photoionzation by the XRBs provides reasonably good fit to the data. Our findings suggest that a significant fraction of these X-ray absorbers may originate in the hot gas intrinsic to the X-ray binaries, and that the ISM makes small, if not negligible, contribution. We briefly discuss the implications to the study of the Milky Way hot gas content.

The Pressure of the Star Forming ISM in Cosmological Simulations

We examine the pressure of the star-forming interstellar medium (ISM) of Milky-Way sized disk galaxies using fully cosmological SPH+N-body, high resolution simulations. These simulations include explicit treatment of metal-line cooling in addition to dust and self-shielding, $\mathrm{H_{2}}$ based star formation. The 4 simulated halos have masses ranging from a few times $10^{10}$ to nearly $10^{12}$ solar masses. Using a kinematic decomposition of these galaxies into present-day bulge and disk components, we find that the typical pressure of the star-forming ISM in the present-day bulge is higher than that in the present-day disk by an order of magnitude. We also find that pressure of the star-forming ISM at high redshift is on average, higher than ISM pressures at low redshift. This explains the why the bulge forms at higher pressures: the disk assembles at lower redshift, when the ISM is lower pressure and the bulge forms at high redshift, when the ISM is at higher pressure. If ISM pressure and IMF variation are tied together as suggested in studies like \cite{Conroy2012}, these results could indicate a time-dependent IMF in Milky-Way like systems, as well as a different IMF in the bulge and the disk.

Magnetized HI Fibers and the Rolling Hough Transform

We present observations of a new group of structures in the diffuse Galactic ISM: slender, linear HI features we dub "fibers" that extend for many degrees at high Galactic latitude. To characterize and measure the extent and strength of these fibers, we present the Rolling Hough Transform (RHT), a new machine vision method for parameterizing the coherent linearity of structures in the image plane. With this powerful new tool we show the fibers are oriented along the interstellar magnetic field as probed by starlight polarization. We find that these low column (N(HI) ~ 5 x 10^18 cm^-2) fiber features are most likely a component of the local cavity wall, about 100 pc away. The HI data we use to demonstrate this alignment at high latitude are from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey and the Parkes Galactic All Sky Survey (GASS). We find better alignment in the higher resolution GALFA-HI data, where the fibers are more visually evident. This trend continues in our investigation of magnetically aligned linear features in the Riegel-Crutcher HI cold cloud, detected in the Southern Galactic Plane Survey (SGPS). We propose an application of the RHT for estimating the field strength in such a cloud, based on the Chandrasekhar-Fermi method. We conclude that data-driven, quantitative studies of ISM morphology can be very powerful predictors of underlying physical quantities.

3D simulations of the early stages of AGN jets: geometry, thermodynamics and backflow

We investigate the interplay between jets from Active Galactic Nuclei (AGNs) and the surrounding InterStellar Medium (ISM) through full 3D, high resolution, Adaptive Mesh Refinement simulations performed with the FLASH code. We follow the jet- ISM system for several Myr in its transition from an early, compact source to an extended one including a large cocoon. During the jet evolution, we identify three major evolutionary stages and we find that, contrary to the prediction of popular theoretical models, none of the simulations shows a self-similar behavior. We also follow the evolution of the energy budget, and find that the fraction of input power deposited into the ISM (the AGN coupling constant) is of order of a few percent during the first few Myr. This is in broad agreement with galaxy formation models employing AGN feedback. However, we find that in these early stages, this energy is deposited only in a small fraction (< 1%) of the total ISM volume. Finally we demonstrate the relevance of backflows arising within the extended cocoon generated by a relativistic AGN jet within the ISM of its host galaxy, previously proposed as a mechanism for self-regulating the gas accretion onto the central object. These backflows tend later to be destabilized by the 3D dynamics, rather than by hydrodynamic (Kelvin- Helmholtz) instabilities. Yet, in the first few hundred thousand years, backflows may create a central accretion region of significant extent, and convey there as much as a few millions of solar masses.

Size distribution of supernova remnants and the interstellar medium: the case of M33

The size distribution of supernova remnants (SNRs) can help to clarify the various aspects of their evolution and interaction with the interstellar medium (ISM). Since the observed samples of SNRs are a collection of objects with very different ages and origin that evolve in different conditions of the ISM, statistical Monte Carlo methods can be used to model their statistical distributions. Based on very general assumptions on the evolution, we have modeled samples of SNRs at various initial and environmental conditions, which were then compared with observed collections of SNRs. In the evolution of SNRs the pressure of the ISM is taken into account, which determines their maximum sizes and lifetimes. When comparing the modeled and observed distributions, it is very important to have homogeneous observational data free from selection effects. We found that a recently published collection of SNRs in M33 (Long et al. 2010, ApJS,187,495) satisfies this requirement if we select the X-ray SNRs with hardness ratios in a limited range of values. An excellent agreement between distributions of this subset of SNRs and the subset of modeled SNRs was reached for a volume filling-factor of the warm phase of the ISM (partly ionized gas with $n_{\rm H}\sim 0.2-0.5~ \rm {cm}^{-3}; T \sim 8000-10000~K$) in M33 of $\sim\ 90%$. The statistical distributions constructed in this way, which reproduce practically all the statistical properties of observed SNRs, allowed us to obtain one of the important parameters of M33: the birthrate is one SNR every ${140} – {150}$ yr, and the total number of SNRs with a shock Mach number $M_{s} \geq 2$ is larger than $\sim 1000$.

Two-phase model for Black Hole feeding and feedback

We study effects of AGN feedback outflows on multi-phase inter stellar medium (ISM) of the host galaxy. We argue that SMBH growth is dominated by accretion of dense cold clumps and filaments. AGN feedback outflows overtake the cold medium, compress it, and trigger a powerful starburst — a positive AGN feedback. This predicts a statistical correlation between AGN luminosity and star formation rate at high luminosities. Most of the outflow’s kinetic energy escapes from the bulge via low density voids. The cold phase is pushed outward only by the ram pressure (momentum) of the outflow. The combination of the negative and positive forms of AGN feedback leads to an $M-\sigma$ relation similar to the result of King (2003). Due to porosity of cold ISM in the bulge, SMBH influence on the low density medium of the host galaxy is significant even for SMBH well below the $M-\sigma$ mass. The role of SMBH feedback in our model evolves in space and time with the ISM structure. In the early gas rich phase, SMBH accelerates star formation in the bulge. During later gas poor (red-and-dead) phases, SMBH feedback is mostly negative everywhere due to scarcity of the cold ISM.

Dust Scattering In Turbulent Media: Correlation Between The Scattered Light and Dust Column Density

Radiative transfer models in a spherical, turbulent interstellar medium (ISM) in which the photon source is situated at the center are calculated to investigate the correlation between the scattered light and the dust column density. The medium is modeled using fractional Brownian motion structures that are appropriate for turbulent ISM. The correlation plot between the scattered light and optical depth shows substantial scatter and deviation from simple proportionality. It was also found that the overall density contrast is smoothed out in scattered light. In other words, there is an enhancement of the dust-scattered flux in low-density regions, while the scattered flux is suppressed in high-density regions. The correlation becomes less significant as the scattering becomes closer to be isotropic and the medium becomes more turbulent. Therefore, the scattered light observed in near-infrared wavelengths would show much weaker correlation than the observations in optical and ultraviolet wavelengths. We also find that the correlation plot between scattered lights at two different wavelengths shows a tighter correlation than that of the scattered light versus the optical depth.

Cosmic Ray Sampling of a Clumpy Interstellar Medium

How cosmic rays sample the multi-phase interstellar medium (ISM) in starburst galaxies has important implications for many science goals, including evaluating the cosmic ray calorimeter model for these systems, predicting their neutrino fluxes, and modeling their winds. Here, we use Monte Carlo simulations to study cosmic ray sampling of a simple, two-phase ISM under conditions similar to those of the prototypical starburst galaxy M82. The assumption that cosmic rays sample the mean density of the ISM in the starburst region is assessed over a multi-dimensional parameter space where we vary the number of molecular clouds, the galactic wind speed, the extent to which the magnetic field is tangled, and the cosmic ray injection mechanism. We evaluate the ratio of the emissivity from pion production in molecular clouds to the emissivity that would be observed if the cosmic rays sampled the mean density, and seek areas of parameter space where this ratio differs significantly from unity. The assumption that cosmic rays sample the mean density holds over much of parameter space; however, this assumption begins to break down for high cloud density, injection close to the clouds, and a very tangled magnetic field. We conclude by evaluating the extent to which our simulated starburst region behaves as a proton calorimeter and constructing the time-dependent spectrum of a burst of cosmic rays.

Narrow Band X-ray Photometry as a Tool for Studying Galaxy and Cluster Mass Distributions

We explore the utility of narrow band X-ray surface photometry as a tool for making fully Bayesian, hydrostatic mass measurements of clusters of galaxies, groups and early-type galaxies. We demonstrate that it is sufficient to measure the surface photometry with the Chandra X-ray observatory in only three (rest frame) bands (0.5–0.9 keV, 0.9–2.0 keV and 2.0–7.0 keV) in order to constrain the temperature, density and abundance of the hot interstellar medium (ISM). Adopting parametrized models for the mass distribution and radial entropy profile and assuming spherical symmetry, we show that the constraints on the mass and thermodynamic properties of the ISM that are obtained by fitting data from all three bands simultaneously are comparable to those obtained by fitting similar models to the temperature and density profiles derived from spatially resolved spectroscopy, as is typically done. We demonstrate that the constraints can be significantly tightened when exploiting a recently derived, empirical relationship between the gas fraction and the entropy profile at large scales, eliminating arbitrary extrapolations at large radii. This "Scaled Adiabatic Model" (ScAM) is well suited to modest signal-to-noise data, and we show that accurate, precise measurements of the global system properties are inferred when employing it to fit data from even very shallow, snapshot X-ray observations. The well-defined asymptotic behaviour of the model also makes it ideally suited for use in Sunyaev-Zeldovich studies of galaxy clusters.

Progenitor Type Identification for Supernova Remnant N103B in the Large Magellanic Cloud by Suzaku and Chandra Observations

This paper presents a detailed analysis of supernova remnant (SNR) N103B located in the Large Magellanic Cloud (LMC), based on Suzaku and Chandra observations. The spectrum of the entire SNR was reproduced using 3 ISM components with the kT of 0.32, 0.56, and 0.92keV and one ejecta component of 3.96keV, based on spectral analysis of the Suzaku/XIS data. The ejecta was overabundant in heavy elements, such as Mg, Si, S, Ca, Fe, and Ni. The unprecedentedly high quality of data obtained by XIS, allowed us to correctly distinguish between the emissions from the ISM and the ejecta for the first time. Combining XIS spectral analysis with Chandra/ACIS image analysis, we verified that the ejecta distributions for elements from Si to Fe-K were similar to one another, although Fe-K emission was located slightly inward compared with that of lighter elements such as Si, S, Ar, and Ca. The onion-like structure of the ejecta was maintained after the SN. In addition, the ISM emission represented by O and Fe-L was located inside the ejecta emission. We compared hydrogen-rich ejecta plasma, which is indicative of Type II SNRs, with plasma rich in heavy elements and poor in hydrogen, which is mainly observed in Type Ia. In the case of N103B, we could not determine whether the origin of the continuum emission in the 4.0-6.0keV band was from ejecta or high-temperature ISM only based on the spectral modeling of XIS data. High-energy continuum images in the 5.2-6.0keV band obtained by ACIS were extremely similar to those of ejecta, implying that the origin of the high-energy continuum might indeed be the ejecta. By combining spectral analysis with high-energy continuum images, we found some indications for H-dominated plasma, and as a result, that the progenitor of N103B might have been a Type II. The progenitor mass was estimated to be 13 Msun based on the abundance patterns of Mg, Fe, and Ni relative to Si.

IBEX, SWCX and a Consistent Model for the Local ISM

The Local Interstellar Medium (LISM) makes its presence felt in the heliosphere in a number of ways including inflowing neutral atoms and dust and shaping of the heliosphere via its ram pressure and magnetic field. Modelers of the heliosphere need to know the ISM density and magnetic field as boundary conditions while ISM modelers would like to use the data and models of the heliosphere to constrain the nature of the LISM. An important data set on the LISM is the diffuse soft X-ray background (SXRB), which is thought to originate in hot gas that surrounds the local interstellar cloud (LIC) in which the heliosphere resides. However, in the past decade or so it has become clear that there is a significant X-ray foreground due to emission within the heliosphere generated when solar wind ions charge exchange with inflowing neutrals. The existence of this SWCX emission complicates the interpretation of the SXRB. We discuss how data from IBEX and models for the Ribbon in particular provide the possibility of tying together heliosphere models with models for the LISM, providing a consistent picture for the pressure in the LISM, the ionization in the LIC and the size and shape of the heliosphere.

Thermals in stratified regions of the ISM

We present a model of a "thermal" (i.e., a hot bubble) rising within an exponentially stratified region of the ISM. This model includes terms representing the ram pressure braking and the entrainment of environmental gas into the thermal. We then calibrate the free parameters associated with these two terms through a comparison with 3D numerical simulations of a rising bubble. Finally, we apply our "thermal" model to the case of a hot bubble produced by a SN within the stratified ISM of the Galactic disk.

Chemistry of Dark Clouds: Databases, Networks, and Models

Chemical models have been developed over the years by astrophysicists to study the pro- cesses at play in the various environments of the interstellar medium (ISM) that define the chemical composition of the gas and the dust. These qualitative aspects of the model predictions have been improved from a chemical point of view thanks to many recent developments of the experimental technics and theoretical methods that aim at studying the individual reactions in conditions as close to the ISM conditions as possible and characterize the rate constants of their efficiency. These models have also been more and more associated with dynamical evolution of the ISM physical conditions (for star forming regions for instance) since the chemical composition is far from steady-state in such regions. In this paper, we try to assess the state of the art concerning the chemical modeling of dark clouds, the initial step for the formation of stars and disks.

Cosmic ray driven outflows

We present simulations of the magnetized interstellar medium (ISM) in models of massive star forming (40 Msun / yr) disk galaxies with high gas surface densities (~100 Msun / pc^2) similar to observed star forming high-redshift disks. We assume that type II supernovae deposit 10 per cent of their energy into the ISM as cosmic rays and neglect the additional deposition of thermal energy or momentum. With a typical Galactic diffusion coefficient for CRs (3e28 cm^2 / s) we demonstrate that this process alone can trigger the local formation of a strong low density galactic wind maintaining vertically open field lines. Driven by the additional pressure gradient of the relativistic fluid the wind speed can exceed 1000 km/s, much higher than the escape velocity of the galaxy. The global mass loading, i.e. the ratio of the gas mass leaving the galactic disk in a wind to the star formation rate becomes of order unity once the system has settled into an equilibrium. We conclude that relativistic particles accelerated in supernova remnants alone provide a natural and efficient mechanism to trigger winds similar to observed mass-loaded galactic winds in high-redshift galaxies. These winds also help explaining the low efficiencies for the conversion of gas into stars in galaxies as well as the early enrichment of the intergalactic medium with metals. This mechanism can be at least of similar importance than the traditionally considered momentum feedback from massive stars and thermal and kinetic feedback from supernova explosions.

Cosmic rays can drive strong outflows from gas-rich high-redshift disk galaxies [Replacement]

We present simulations of the magnetized interstellar medium (ISM) in models of massive star forming (40 Msun / yr) disk galaxies with high gas surface densities (~100 Msun / pc^2) similar to observed star forming high-redshift disks. We assume that type II supernovae deposit 10 per cent of their energy into the ISM as cosmic rays and neglect the additional deposition of thermal energy or momentum. With a typical Galactic diffusion coefficient for CRs (3e28 cm^2 / s) we demonstrate that this process alone can trigger the local formation of a strong low density galactic wind maintaining vertically open field lines. Driven by the additional pressure gradient of the relativistic fluid the wind speed can exceed 1000 km/s, much higher than the escape velocity of the galaxy. The global mass loading, i.e. the ratio of the gas mass leaving the galactic disk in a wind to the star formation rate becomes of order unity once the system has settled into an equilibrium. We conclude that relativistic particles accelerated in supernova remnants alone provide a natural and efficient mechanism to trigger winds similar to observed mass-loaded galactic winds in high-redshift galaxies. These winds also help explaining the low efficiencies for the conversion of gas into stars in galaxies as well as the early enrichment of the intergalactic medium with metals. This mechanism can be at least of similar importance than the traditionally considered momentum feedback from massive stars and thermal and kinetic feedback from supernova explosions.

Probing Oort Cloud and local ISM properties via dust produced in cometary collisions

The Oort Cloud remains one of the most poorly explored regions of the Solar System. We propose that its properties can be constrained by detecting and studying from space a population of dust grains produced in collisions of comets in the outer Solar System. We explore the dynamics of micron-size grains outside the heliosphere (beyond ~250 AU), which are affected predominantly by the magnetic field of the interstellar medium (ISM) flow past the Sun. We derive analytic models for the production and motion of small particles as a function of their birth location in the Cloud and calculate particle flux and velocity distribution in the inner Solar System. These models are verified by direct numerical simulations. We show that grains originating in the Oort Cloud have a unique distribution of arrival directions (mainly perpendicular to both the ISM wind velocity and the ISM magnetic field), which should easily distinguish them from both interplanetary and interstellar dust populations. We also demonstrate that the distribution of particle arrival velocities is uniquely related to the spatial distribution of the dust production inside the Cloud. The latter is, in turn, determined both by the mass distribution in the Cloud and the physical properties of comets. Cometary collisions within the Oort Cloud are expected to produce a flux of micron-size grains in the inner Solar System of up to several m^{-2} yr^{-1}. The next-generation dust detectors may be sensitive enough to detect and constrain this dust population, which will illuminate us about the Oort Cloud’s properties. We also show that the recently-detected mysterious population of large (micron-size) unbound particles, which seems to arrive with the ISM flow is unlikely to be of a cometary origin.

Using synthetic emission maps to constrain the structure of the Milky Way

We present the current standing of an investigation into the structure of the Milky Way. We use smoothed particle hydrodynamics (SPH) to simulate the ISM gas in the Milky Way under the effect of a number of different gravitational potentials representing the spiral arms and nuclear bars, both fixed and time-dependent. The gas is subject to ISM cooling and chemistry, enabling us to track the CO and HI density. We use a 3D grid-based radiative transfer code to simulate the emission from the SPH output, allowing for the construction of synthetic longitude-velocity maps as viewed from the Earth. By comparing these maps with the observed emission in CO and HI from the Milky Way (Dame et al. 2001, Kalberla et al. 2005), we can infer the arm/bar geometry that provides a best fit to our Galaxy. By doing so we aim to answer key questions concerning the morphology of the Milky Way such as the number of the spiral arms, the pattern speeds of the bar(s) and arms, the pitch angle of the arms and shape of the bar(s)

Using synthetic emission maps to constrain the structure of the Milky Way [Replacement]

We present the current standing of an investigation into the structure of the Milky Way. We use smoothed particle hydrodynamics (SPH) to simulate the ISM gas in the Milky Way under the effect of a number of different gravitational potentials representing the spiral arms and nuclear bars, both fixed and time-dependent. The gas is subject to ISM cooling and chemistry, enabling us to track the CO and HI density. We use a 3D grid-based radiative transfer code to simulate the emission from the SPH output, allowing for the construction of synthetic longitude-velocity maps as viewed from the Earth. By comparing these maps with the observed emission in CO and HI from the Milky Way (Dame et al. 2001, Kalberla et al. 2005), we can infer the arm/bar geometry that provides a best fit to our Galaxy. By doing so we aim to answer key questions concerning the morphology of the Milky Way such as the number of the spiral arms, the pattern speeds of the bar(s) and arms, the pitch angle of the arms and shape of the bar(s)

The Smooth MgII gas distribution through the interstellar/extra-planar/halo interface

We report the first measurements of MgII absorption systems associated with spectroscopically confirmed z~0.1 star-forming galaxies at projected distances of D<6kpc. We demonstrate the data are consistent with the well known anti-correlation between rest-frame MgII equivalent width, Wr(2796), and impact parameter, D, represented by a single log-linear relation derived by Nielsen et al. (MAGIICAT) that converges to ~2A at D=0kpc. Incorporating MAGIICAT, we find that the halo gas covering fraction is unity below D~25kpc. We also report that our D<6kpc absorbers are consistent with the Wr(2796) distributions of the Milky Way interstellar medium (ISM) and ISM+halo. In addition, quasar sight-lines of intermediate redshift galaxies with 6<D<25kpc have an equivalent width distribution similar to that of the Milky Way halo, implying that beyond ~6kpc, quasar sight-lines are likely probing halo gas and not the ISM. As inferred by the Milky Way and our new data, the gas profiles of galaxies can be fit by a single log-linear Wr(2796)-D relation out to large scales across a variety of gas-phase conditions and is maintained through the halo/extra-planar/ISM interfaces, which is remarkable considering their kinematic complexity. These low redshift, small impact parameter absorption systems are the first steps to bridge the gap between quasar absorption-line studies and HI observations of the CGM.

The nature of the ISM in galaxies during the star-formation activity peak of the Universe [Replacement]

We combine a semi-analytic model of galaxy formation, which tracks atomic and molecular phases of cold gas, with a three-dimensional radiative-transfer and line tracing code to study the sub-mm emission from several atomic and molecular species (CO, HCN, C, C+, [OI]) in galaxies. We aim to understand if the physics that drives the formation of stars at the epoch of peak star formation in the Universe is similar to or different from that in local galaxies. We find that normal star-forming galaxies at high redshift have much higher CO-excitation peaks than their local counterparts, higher HCN/CO ratios and that CO cooling predominantly takes place through molecules with higher excitation levels. We find an increase in the ratio between [OI] and [CII] in typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to counterparts at z = 0. All our model results suggest that typical star-forming galaxies at high redshift consist of much denser and warmer star-forming clouds than their local counterparts and form their stars under significantly different ISM conditions. Galaxies belonging to the tail of the SF activity peak of the Universe (z = 1.2) are already less dense and cooler than counterparts during the actual peak of SF activity (z = 2.0). We use our results to discuss how future ALMA surveys can best confront our predictions and constrain models of galaxy formation.

The nature of the ISM in galaxies during the star-formation activity peak of the Universe

We combine a semi-analytic model of galaxy formation, which tracks atomic and molecular phases of cold gas, with a three-dimensional radiative-transfer and line tracing code to study the sub-mm emission from several atomic and molecular species (CO, HCN, C, C+, [OI]) in galaxies. We aim to understand if the physics that drives the formation of stars at the epoch of peak star formation in the Universe is similar to or different from that in local galaxies. We find that normal star-forming galaxies at high redshift have much higher CO-excitation peaks than their local counterparts, higher HCN/CO ratios and that CO cooling predominantly takes place through molecules with higher excitation levels. We find an increase in the ratio between [OI] and [CII] in typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to counterparts at z = 0. All our model results suggest that typical star-forming galaxies at high redshift consist of much denser and warmer star-forming clouds than their local counterparts and form their stars under significantly different ISM conditions. Galaxies belonging to the tail of the SF activity peak of the Universe (z = 1.2) are already less dense and cooler than counterparts during the actual peak of SF activity (z = 2.0). We use our results to discuss how future ALMA surveys can best confront our predictions and constrain models of galaxy formation.

Variability in Spectropolarimetric properties of Sy 1.5 galaxy Mrk 6

Here we present an analysis of spectro-polarimetric observations of type 1.5 AGN Mrk 6. The galaxy was observed with 6m telescope SAO RAN in 12 epochs, from 2010 to 2013. Additionally, the inter-stellar mater (ISM) polarization has been observed and its contribution to the AGN spectra has been taken into account. To explore the nature of the polarization mechanism and the structure of the AGN central part we investigated the variability in the polarization properties of Mrk 6 in the continuum and in the broad H$\alpha$ line. We measured Stokes parameters and determined the percent of polarization and polarization angle in 12 spectra with and without correction for the ISM polarization. We estimated the time lag between the unpolarized and polarized continuum flux variation of about $\sim 2$ days, that indicates a compact scattering region which contributes to the variability in the polarized continuum light. The polarization in the line profile shows three components in the BLR, one that is coming from the disc, and two which indicate outflows in different parts of the BLR. We found that the ISM polarization has a very significant influence on the measured AGN polarization parameters. After correcting the observations for the ISM polarization we were able to detect the Keplerian motion in the BLR (using changes in polarization angle across the H$\alpha$ line profile). We give a new method for the black hole mass estimation using spectro-polarimetric observation in the line profile. Assuming that the scattering region is located in the inner part of the torus ($\sim 220$ light days), the black hole mass in Mrk 6 is $M_{BH}\sim 1.16\cdot 10^8M_\odot$, that is in a good agreement with estimates obtained from reverberation mapping.

Extreme Galaxies During Reionization: Testing ISM and Disk Models [Replacement]

We test the ability of equilibrium galactic disk and one-zone interstellar medium models to describe the physical and emission properties of quasar hosts, submillimeter galaxies, and Lyman-alpha emitters at z>~6. The size, line widths, star formation rates, black hole accretion rates, gas masses and temperatures, and the relationships between these properties are all well-described by our model, and we provide approximate fitting formulae for comparison with future observations. However, comparing our carbon line predictions to observations reveals differences between the ISM at low and high redshifts. Our underestimate of the [CII] line emission indicates either higher star formation efficiencies in high-redshift molecular clouds or less depletion of metals into dust at fixed metallicity. Further, our over-prediction of the CO(6-5)/CO(1-0) ratio suggests that molecular clouds in real high-redshift galaxies have a lower turbulent Mach number and more subthermal CO(6-5) emission than expected owing either to sizes smaller than the local Jeans mass or to a pressure support mechanism other than turbulence.

Circum-stellar medium around rotating massive stars at solar metallicity [Replacement]

Aims. Observations show nebulae around some massive stars but not around others. If observed, their chemical composition is far from homogeneous. Our goal is to put these observational features into the context of the evolution of massive stars and their circumstellar medium (CSM) and, more generally, to quantify the role of massive stars for the chemical and dynamical evolution of the ISM. Methods. Using the A-MAZE code, we perform 2d-axisymmetric hydrodynamical simulations of the evolution of the CSM, shaped by stellar winds, for a whole grid of massive stellar models from 15 to 120 Msun and following the stellar evolution from the zero-age main-sequence to the time of supernova explosion. In addition to the usual quantities, we also follow five chemical species: H, He, C, N, and O. Results. We show how various quantities evolve as a function of time: size of the bubble, position of the wind termination shock, chemical composition of the bubble, etc. The chemical composition of the bubble changes considerably compared to the initial composition, particularly during the red-supergiant (RSG) and Wolf-Rayet (WR) phases. In some extreme cases, the inner region of the bubble can be completely depleted in hydrogen and nitrogen, and is mainly composed of carbon, helium and oxygen. We argue why the bubble typically expands at a lower rate than predicted by self-similarity theory. In particular, the size of the bubble is very sensitive to the density of the ISM, decreasing by a factor of around 2.5 for each additional dex in ISM density. The bubble size also decreases with the metallicity of the central star, as low-metallicity stars have weaker winds. Our models qualitatively fit the observations of WR ejecta nebulae.

The Interstellar Medium White Paper [Replacement]

The interstellar medium is the engine room for galactic evolution. While much is known about the conditions within the ISM, many important areas regarding the formation and evolution of the various phases of the ISM leading to star formation, and its role in important astrophysical processes, remain to be explained. This paper discusses several of the fundamental science problems, placing them in context with current activities and capabilities, as well as the future capabilities that are needed to progress them in the decade ahead. Australia has a vibrant research community working on the interstellar medium. This discussion gives particular emphasis to Australian involvement in furthering their work, as part of the wider international endeavour. The particular science programs that are outlined in this White Paper include the formation of molecular clouds, the ISM of the Galactic nucleus, the origin of gamma-rays and cosmic rays, high mass star and cluster formation, the dense molecular medium, galaxy evolution and the diffuse atomic medium, supernova remnants, the role of magnetism and turbulence in the Galactic ecology and complex organic molecules in space.

Dust-to-metal ratios in damped Lyman-alpha absorbers: Fresh clues to the origins of dust and optical extinction towards gamma-ray bursts [Replacement]

Motivated by the anomalous dust-to-metal ratios derived in the literature for gamma-ray burst (GRB) damped Lyman-alpha absorbers (DLAs), we measure these ratios using the dust-depletion pattern observed in UV/optical afterglow spectra associated with the ISM at the GRB host-galaxy redshifts. Our sample consists of 20 GRB absorbers and a comparison sample of 72 QSO-DLAs with redshift 1.2 < z < 4.0 and down to Z = 0.002 Z_Sol metallicities. The dust-to-metal ratio in QSO- and GRB-DLAs increases both with metallicity and metal column density, spanning ~10–110% of the Galactic value and pointing to a non universal dust-to-metal ratio. The low values of dust-to-metal ratio suggest that low-metallicity systems have lower dust fractions than typical spiral galaxies and perhaps that the dust in these systems is produced inefficiently, i.e. by grain growth in the low-metallicity regime with negligible contribution from supernovae (SNe) and asymptotic giant branch (AGB) stars. On the other hand, some GRB- and QSO-DLAs show high dust-to-metal ratio values out to z ~ 4, requiring rapid dust production, such as in SN ejecta, but also in AGB winds and via grain growth for the highest metallicity systems. GRB-DLAs overall follow the dust-to-metal-ratio properties of QSO-DLAs, GRBs probing up to larger column and volume densities. For comparison, the dust-to-metal ratio that we derive for the SMC and LMC are ~82–100% and ~98% of the Galactic value, respectively. The literature dust-to-metal ratio of the low-metallicity galaxy I Zw 18 (< 37%) is consistent with the distribution that we find. The dust extinction Av increases steeply with the column density of iron in dust, N(Fe)dust, calculated from relative metal abundances, confirming that dust extinction is mostly occurring in the host galaxy ISM. Most GRB-DLAs display log N(Fe)dust > 14.7, above which several QSO-DLAs reveal H2 (abridged).

Direct Simulation Monte Carlo for astrophysical flows: II. Ram pressure dynamics [Replacement]

We use the Direct Simulation Monte Carlo (DSMC) method combined with an n-body code to study the dynamics of the interaction between a gas-rich spiral galaxy and intracluster or intragroup medium, often known as the ram pressure scenario. The advantage of this gas kinetic approach over traditional hydrodynamics is explicit treatment of the interface between the hot and cold, dense and rarefied media typical of astrophysical flows and the explicit conservation of energy and momentum and the interface. This approach yields some new physical insight. Owing to the shock and backward wave that forms at the point ICM–ISM contact, ICM gas is compressed, heated and slowed. The shock morphology is Mach-disk-like. In the outer galaxy, the hot turbulent post-shock gas flows around the galaxy disk, while heating and ablating the initially cool disk gas. The outer gas and angular momentum are lost to the flow. In the inner galaxy, the hot gas pressurizes the neutral ISM gas causing a strong two-phase instability. As a result, the momentum of the wind is no longer impulsively communicated to the cold gas as assumed in the Gunn-Gott (1972) formula, but oozes through the porous disk, transferring its linear momentum to the disk en masse. The escaping gas mixture has a net positive angular momentum and forms a slowly rotating sheath. The shear flow caused by the post-shock ICM flowing through the porous multiphase ISM creates a strong Kelvin-Helmholtz instability in the disk that results in Cartwheel-like ring and spoke morphology.

The metals-to-dust ratio to very low metallicities using GRB and QSO absorbers; extremely rapid dust formation [Replacement]

Among the key parameters defining the ISM of galaxies is the fraction of the metals that are locked up in dust: the metals-to-dust ratio. This ratio bears not only on the ISM and its evolution, but particularly on the origin of cosmic dust. We combine extinction and abundance data from GRB afterglows, from QSO absorbers, as well as from galaxy-lensed QSOs, to determine the metals-to-dust ratios for lines-of-sight through a wide diversity of galaxies from blue, dwarf starbursts to massive ellipticals, across a vast range in redshift z=0.1-6.3, and nearly three orders of magnitude in column density and metal abundance. We thus determine the metals-to-dust ratio in a unique way, providing direct determinations of in situ gas and dust columns without recourse to assumptions with large uncertainties. We find that the metals-to-dust ratios in these systems are surprisingly close to the value for the local group (10^{21.3} cm-2 A_V mag-1), with a mean value of 10^{21.2} cm-2 A_V mag-1 and a standard deviation of 0.3 dex. There is no evidence of deviation from this mean ratio as a function of metallicity, even down to our lowest metallicity of 0.01 Z/Z_sun. The lack of any obvious dependence of the metals-to-dust ratio on either column density, galaxy type or age, redshift, or metallicity indicates a close correspondence between the formation of the metals and the formation of dust. Any delay between the formation of metals and dust must be shorter than the typical metal-enrichment times of these galaxies. Formation of the bulk of the dust in low mass stars is therefore ruled out by these data at any cosmic epoch. Furthermore, dust destruction must not dominate over formation/growth in virtually any galaxy environment. The correlation between metals and dust is a natural consequence of the formation of the bulk of dust in SNe [Abridged].

The Cosmic Evolution of the IMF Under the Jeans Conjecture with Implications for Bottom-Heavy Ellipticals [Replacement]

We examine the cosmic evolution of a stellar initial mass function (IMF) in galaxies that varies with the Jeans mass in the interstellar medium, paying particular attention to the K-band stellar mass to light ratio (M/L_K) of present-epoch massive galaxies. We calculate the typical Jeans mass using high-resolution hydrodynamic simulations coupled with a fully radiative model for the ISM, which yields a parameterisation of the IMF characteristic mass as a function of galaxy star formation rate (SFR). We then calculate the star formation histories of galaxies utilising an equilibrium galaxy growth model coupled with constraints on the star formation histories set by abundance matching models. We find that at early times, energetic coupling between dust and gas drive warm conditions in the ISM, yielding bottom-light/top- heavy IMFs associated with large ISM Jeans masses for massive star-forming galaxies. Owing to the remnants of massive stars that formed during the top-heavy phases at early times, the resultant M/L_K(sigma) in massive galaxies at the present epoch is increased relative to the non- varying IMF case. At late times, lower cosmic ray fluxes allow for cooler ISM temperatures in massive galaxies, and hence newly formed clusters will exhibit bottom-heavy IMFs, further increasing M/L_K(sigma). Our central result is hence that a given massive galaxy may go through both top-heavy and bottom-heavy IMF phases during its lifetime, though the bulk of the stars form during a top-heavy phase. Qualitatively, the variations in M/L_K(sigma) with galaxy mass are in agreement with observations, however, our model may not be able to account for bottom-heavy mass functions as indicated by stellar absorption features.

Hierarchically Clustered Star Formation in the Magellanic Clouds

We present a cluster analysis of the bright main-sequence and faint pre–main-sequence stellar populations of a field ~ 90 x 90 pc centered on the HII region NGC 346/N66 in the Small Magellanic Cloud, from imaging with HST/ACS. We extend our earlier analysis on the stellar cluster population in the region to characterize the structuring behavior of young stars in the region as a whole with the use of stellar density maps interpreted through techniques designed for the study of the ISM structuring. In particular, we demonstrate with Cartwrigth & Whitworth’s Q parameter, dendrograms, and the Delta-variance wavelet transform technique that the young stellar populations in the region NGC 346/N66 are hierarchically clustered, in agreement with other regions in the Magellanic Clouds observed with HST. The origin of this hierarchy is currently under investigation.

The Shapes of the HI Velocity Profiles of the THINGS Galaxies

We analyze the shapes of the HI velocity profiles of The HI Nearby Galaxy Survey (THINGS) to study the phase structure of the neutral interstellar medium (ISM) and its relation to global galaxy properties. We use a method analogous to the stacking method sometimes used in high redshift HI observations to construct high signal-to-noise (S/N) profiles. We call these high S/N profiles super profiles. We analyze and discuss possible systematics that may change the observed shapes of the super profiles. After quantifying these effects and selecting a sub-sample of unaffected galaxies, we find that the super profiles are best described by a narrow and a broad Gaussian component, which are evidence of the presence of the Cold Neutral Medium (CNM) and the Warm Neutral Medium (WNM). The velocity dispersion of the narrow component range from ~3.4 to ~8.6 km/s with an average of 6.5+/-1.5 km/s, whereas that of the broad component range from ~10.1 to ~24.3 km/s with an average of 16.8+/-4.3 km/s. We find that the super profile parameters correlate with star formation indicators such as metallicity, FUV-NUV colors and H_alpha luminosities. The flux ratio between the narrow and broad components tends to be highest for high metallicity, high star formation rate (SFR) galaxies. We show that the narrow component identified in the super profiles is associated with the presence of star formation, and possibly with molecular hydrogen.

Discovery of a halo around the Helix Nebula NGC 7293 in the WISE all-sky survey [Replacement]

We report the discovery of an extended halo (40′ in diameter) around the planetary nebula NGC7293 (the Helix Nebula) observed in 12micron band from the Wide-field Infrared Survey Explorer all-sky survey. The mid-infrared halo has an axisymmetric structure with a sharp boundary to the northeast and a more diffuse boundary to the southwest, suggesting an interaction between the stellar wind and the interstellar medium (ISM). The symmetry axis of the halo is well aligned with that of a northeast arc, suggesting that the two structures are physically associated. We have attempted to fit the observed geometry with a model of a moving steady-state stellar wind interacting with the ISM. Possible combinations of the ISM density and the stellar velocity are derived from these fittings. The discrepancies between the model and the observations suggest that the stellar mass loss has a more complicated history, including possible time and angle dependences.

Discovery of a halo around the Helix Nebula NGC 7293 in the WISE all-sky survey

We report the discovery of an extended halo (40′ in diameter) around the planetary nebula NGC7293 (the Helix Nebula) observed in 12micron band from the Wide-field Infrared Survey Explorer all-sky survey. The mid-infrared halo has an axisymmetric structure with a sharp boundary to the northeast and a more diffuse boundary to the southwest, suggesting an interaction between the stellar wind and the interstellar medium (ISM). The symmetry axis of the halo is well aligned with that of a northeast arc, suggesting that the two structures are physically associated. We have attempted to fit the observed geometry with a model of a moving steady-state stellar wind interacting with the ISM. Possible combinations of the ISM density and the stellar velocity are derived from these fittings. The discrepancies between the model and the observations suggest that the stellar mass loss has a more complicated history, including possible time and angle dependences.

Discovery of a halo around the Helix Nebula NGC 7293 in the WISE all-sky survey [Replacement]

We report the discovery of an extended halo (40′ in diameter) around the planetary nebula NGC7293 (the Helix Nebula) observed in 12micron band from the Wide-field Infrared Survey Explorer all-sky survey. The mid-infrared halo has an axisymmetric structure with a sharp boundary to the northeast and a more diffuse boundary to the southwest, suggesting an interaction between the stellar wind and the interstellar medium (ISM). The symmetry axis of the halo is well aligned with that of a northeast arc, suggesting that the two structures are physically associated. We have attempted to fit the observed geometry with a model of a moving steady-state stellar wind interacting with the ISM. Possible combinations of the ISM density and the stellar velocity are derived from these fittings. The discrepancies between the model and the observations suggest that the stellar mass loss has a more complicated history, including possible time and angle dependences.

Multiwavelength view of ISM in the merger remnant Fornax A galaxy [Replacement]

We present multiwavelength imagery along with the X-ray emission maps of merger remnant galaxy NGC 1316 to study the content of dust and its association with other phases of ISM. Color-index maps as well as extinction maps derived for this galaxy reveals an intricate and complex morphology of dust i.e., in the inner part it exist in the form of a prominent lane while at about 6–7\,kpc it apparently forms an arc-like pattern extended along the North-East direction. Apart from this, several other clumps and knots are also evident in these maps. Dust emission mapped using \textit{Spitzer} detection at 8 $\mu$m indicate even complex structure its morphology. Extinction curve derived over optical through near-IR bands exhibit identical extinction properties of dust and can be assessed from the parallely running extinction curve with that for the Galaxy. Quantum of dust estimated from optical extinction measurement is found to be 2.13$\times\, 10^5$\Msun\,, while that from IRAS flux densities is 2.11$\times\, 10^6$\Msun\, and from integrated flux densities at 24$\mu$m, 70$\mu$m and 160 $\mu$m from MIPS is 3.2$\times\, 10^7$\Msun\,, significantly larger than the estimates from the optical extinction. High resolution \textit{Chandra} observations of this merger remnant system have provided with an unprecedented look at the complex nature of distribution of X-ray emission that closely matches with that of the ionized gas and to some extent with the dust distribution also. X-ray color-color plot of the 80 resolved X-ray sources within optical D$_{25}$ extent of the galaxy enabled us to separate them in different classes.

Multiwavelength view of ISM in the merger remnant Fornax A galaxy

We present multiwavelength imagery along with the X-ray emission maps of merger remnant galaxy NGC 1316 to study the content of dust and its association with other phases of ISM. Color-index maps as well as extinction maps derived for this galaxy reveals an intricate and complex morphology of dust i.e., in the inner part it exist in the form of a prominent lane while at about 6–7\,kpc it apparently forms an arc-like pattern extended along the North-East direction. Apart from this, several other clumps and knots are also evident in these maps. Dust emission mapped using \textit{Spitzer} detection at 8 $\mu$m indicate even complex structure its morphology. Extinction curve derived over optical through near-IR bands exhibit identical extinction properties of dust and can be assessed from the parallely running extinction curve with that for the Galaxy. Quantum of dust estimated from optical extinction measurement is found to be 2.13$\times\, 10^5$\Msun\,, while that from IRAS flux densities is 2.11$\times\, 10^6$\Msun\, and from integrated flux densities at 24$\mu$m, 70$\mu$m and 160 $\mu$m from MIPS is 3.2$\times\, 10^7$\Msun\,, significantly larger than the estimates from the optical extinction. High resolution \textit{Chandra} observations of this merger remnant system have provided with an unprecedented look at the complex nature of distribution of X-ray emission that closely matches with that of the ionized gas and to some extent with the dust distribution also. X-ray color-color plot of the 80 resolved X-ray sources within optical D$_{25}$ extent of the galaxy enabled us to separate them in different classes.

The metal content of hot DA white dwarf spectra

A study of high ion metal absorption features present in the spectra of hot DA white dwarfs is presented. An analysis of three DAs is performed, where previous studies came to conflicting conclusions as to the stars’ nitrogen configurations. The nitrogen abundances were found to be in keeping with DAs of higher Teff, with a homogeneous distribution. A search for circumstellar gas discs was performed on eight stars, where circumstellar pollution may explain the differences between predicted and observed metal abundances. No positive detections were made. Already the subject of previous studies, the circumstellar absorption features seen at many hot DAs were again analysed, using a more advanced technique than those implemented in previous studies. This allowed, for the first time, column density measurements for all non-photospheric absorbing material. The derived column density measurements are consistent with those predicted to exist in white dwarf Stromgren Spheres, and the velocities of the absorbing material are not far from the velocities of either the observed ISM or predicted LISM clouds along the stars’ sight lines. However, given the distances to some of the stars, it is unlikely that the ionised material resides in the LISM in all cases; it may however be loosely related to it. The observations here could not conclusively rule out the ionisation of circumstellar material about the stars, though no evidence for such material has yet been found. The velocity of the circumstellar material at WD2218+706 is inconsistent with the expansion velocity of the PN at the star, implying that the circumstellar material does not reside in the PN, though it may have originated there. Once though to be related to these circumstellar features, mass loss at the DAs has been ruled out, since the high log g of these stars prohibits the loss of significant mass in a stellar wind.

Electron attachment rates for pah anions in interstellar and circumstellar media: dependence on chemical properties

Context. The attachment of free electrons to polycondensed aromatic ring molecules (PAHs) is studied for a variety of such molecules with different numbers of condensed rings and over a broad range of electron temperatures, using a multichannel quantum approach. The calculations of the relevant cross sections are used in turn to obtain the corresponding attachment rates for each of the systems under study, and these rates are parametrized as a function of temperature using a commonly employed expression for two-body processes. Aims. The scope of this work is to establish from first principles the influence of their chemical properties on the efficiency of electron-attachment process for PAHs. Methods. Quantum multichannel scattering methods are employed to generate the relevant cross sections and hence the attachment rates. Results. The rates obtained for the present molecules are found to markedly vary within the test ensemble of the present work and to be smaller than the values currently used for the full class of PAHs when models of their evolutions in ISM environments are employed. The effects of such differences on the evolutions of chemical networks that include both PAH and PAH- species are analysed in some details and related to previous calculations.

Evolved star water maser cloud size determined by star size

Cool, evolved stars undergo copious mass loss but the details of how the matter is returned to the ISM are still under debate. We investigated the structure and evolution of the wind at 5 to 50 stellar radii from Asymptotic Giant Branch and Red Supergiant stars. 22-GHz water masers around seven evolved stars were imaged using MERLIN, at sub-AU resolution. Each source was observed at between 2 and 7 epochs (several stellar periods). We compared our results with long-term Pushchino single dish monitoring. The 22-GHz emission is located in ~spherical, thick, unevenly filled shells. The outflow velocity doubles between the inner and outer shell limits. Water maser clumps could be matched at successive epochs separated by <2 years for AGB stars, or at least 5 years for RSG. This is much shorter than the decades taken for the wind to cross the maser shell, and comparison with spectral monitoring shows that some features fade and reappear. In 5 sources, most of the matched features brighten or dim in concert from one epoch to the next. One cloud in W Hya was caught in the act of passing in front of a background cloud leading to 50-fold, transient amplification. The masing clouds are 1-2 orders of magnitude denser than the wind average and contain a substantial fraction of the mass loss in this region, with a filling factor <1%. The RSG clouds are ~10x bigger than those round the AGB stars. Proper motions are dominated by expansion, with no systematic rotation. The maser clouds survive for decades (the shell crossing time) but the masers are not always beamed in our direction. Radiative effects cause changes in flux density throughout the maser shells on short timescales. Cloud size is proportional to parent star size; clouds have a similar radius to the star in the 22-GHz maser shell. Stellar properties such as convection cells must determine the clumping scale.

Low frequency radio-FIR correlation in normal galaxies at ~1 kpc scales

We study the radio–FIR correlation between the nonthermal (synchrotron) radio continuum emission at \lambda 90 cm (333 MHz) and the far infrared emission due to cool (~20 K) dust at \lambda 70\mu m in spatially resolved normal galaxies at scales of ~1 kpc. The slope of the radio–FIR correlation significantly differs between the arm and interarm regions. However, this change is not evident at a lower wavelength of \lambda 20 cm (1.4 GHz). We find the slope of the correlation in the arm to be 0.8 \pm 0.12 and we use this to determine the coupling between equipartition magnetic field (B_{eq}) and gas density (\rho_{gas}) as B_{eq} \propto \rho_{gas}^{0.51 \pm 0.12}. This is close to what is predicted by MHD simulations of turbulent ISM, provided the same region produces both the radio and far infrared emission. We argue that at 1 kpc scales this condition is satisfied for radio emission at 1.4 GHz and may not be satisfied at 333 MHz. Change of slope observed in the interarm region could be caused by propagation of low energy (~1.5 GeV) and long lived (~ 10^8 yr) cosmic ray electrons at 333 MHz.

Ram pressure stripping in elliptical galaxies: I. the impact of the interstellar medium turbulence

Elliptical galaxies contain X-ray emitting gas that is subject to continuous ram pressure stripping over timescales comparable to cluster ages. The gas in these galaxies is not in perfect hydrostatic equilibrium. Supernova feedback, stellar winds, or active galactic nuclei (AGN) feedback can significantly perturb the interstellar medium (ISM). Using hydrodynamical simulations, we investigate the effect of subsonic turbulence in the hot ISM on the ram pressure stripping process in early-type galaxies. We find that galaxies with the stronger turbulent ISM produce longer, wider, and more smoothly distributed tails of the stripped ISM than those characterised by weaker ISM turbulence. Our main conclusion is that even very weak internal turbulence, at the level of <15% of the average ISM sound speed in a galaxy, can accelerate the gas removal from the galaxy via ram pressure stripping and remove a significant fraction (~50%) of the preexisting ISM in ~6 Gyr. The magnitude of this effect increases sharply with the strength of the turbulence. As most of the gas stripping takes place near the boundary between the ISM and the intraclustermedium (ICM), the boost in the ISM stripping rate is due to the "random walk" of the ISM from the central regions of the galactic potential well to larger distances, where the ram pressure is able to permanently remove the gas from the galaxy. The ICM can be temporarily trapped inside the galactic potential well due to the mixing of the turbulent ISM with the ICM. The galaxies with more turbulent ISM, yet still characterised by very weak turbulence, can hold larger amounts of the ICM. [Abridged]

Ram pressure stripping in elliptical galaxies: I. the impact of the interstellar medium turbulence [Replacement]

Elliptical galaxies contain X-ray emitting gas that is subject to continuous ram pressure stripping over timescales comparable to cluster ages. The gas in these galaxies is not in perfect hydrostatic equilibrium. Supernova feedback, stellar winds, or active galactic nuclei (AGN) feedback can significantly perturb the interstellar medium (ISM). Using hydrodynamical simulations, we investigate the effect of subsonic turbulence in the hot ISM on the ram pressure stripping process in early-type galaxies. We find that galaxies with more turbulent ISM produce longer, wider, and more smoothly distributed tails of the stripped ISM than those characterised by weaker ISM turbulence. Our main conclusion is that even very weak internal turbulence, at the level of <15% of the average ISM sound speed, can significantly accelerate the gas removal from galaxies via ram pressure stripping. The magnitude of this effect increases sharply with the strength of turbulence. As most of the gas stripping takes place near the boundary between the ISM and the intraclustermedium (ICM), the boost in the ISM stripping rate is due to the "random walk" of the ISM from the central regions of the galactic potential well to larger distances, where the ram pressure is able to permanently remove the gas from galaxies. The ICM can be temporarily trapped inside the galactic potential well due to the mixing of the turbulent ISM with the ICM. The galaxies with more turbulent ISM, yet still characterised by very weak turbulence, can hold larger amounts of the ICM. [Abridged]

The supernova-regulated ISM. II. The mean magnetic field

The origin and structure of the magnetic fields in the interstellar medium of spiral galaxies is investigated with 3D, non-ideal, compressible MHD simulations, including stratification in the galactic gravity field, differential rotation and radiative cooling. A rectangular domain, 1x1x2 kpc^{3} in size, spans both sides of the galactic mid-plane. Supernova explosions drive transonic turbulence. A seed magnetic field grows exponentially to reach a statistically steady state within 1.6 Gyr. Following Germano (1992) we use volume averaging with a Gaussian kernel to separate magnetic field into a mean field and fluctuations. Such averaging does not satisfy all Reynolds rules, yet allows a formulation of mean-field theory. The mean field thus obtained varies in both space and time. Growth rates differ for the mean-field and fluctuating field and there is clear scale separation between the two elements, whose integral scales are about 0.7 kpc and 0.3 kpc, respectively.

Suppressing hot gas accretion to supermassive black holes by stellar winds

We argue that one of the basic assumptions of the Bondi accretion process, that the accreting object has zero pressure, might not hold in many galaxies because of the pressure exerted by stellar winds of star orbiting the central super massive black hole (SMBH). Hence, the Bondi accretion cannot be used in these cases, such as in the galaxy NGC 3115. The winds of these high-velocity stars are shocked to temperatures above the virial temperature of the galaxy, leading to the formation of a hot bubble of size ~0.1-10 pc near the center. This hot bubble can substantially reduce the mass accretion rate by the SMBH. If the density of the hot bubble is lower than that of the interstellar medium (ISM), a density-inversion layer is formed. Adding to other problems of the Bondi process, our results render the Bondi accretion irrelevant for AGN feedback in cooling flow in galaxies and small groups of galaxies and during galaxy formation.

The linewidth-size relationship in the dense ISM of the Central Molecular Zone

The linewidth (sigma) – size (R) relationship has been extensively measured and analysed, in both the local ISM and in nearby normal galaxies. Generally, a power-law describes the relationship well with an index ranging from 0.2-0.6, now referred to as one of "Larson’s Relationships." The nature of turbulence and star formation is considered to be intimately related to these relationships, so evaluating the sigma-R correlations in various environments is important for developing a comprehensive understanding of the ISM. We measure the sigma-R relationship in the Central Molecular Zone (CMZ) of the Galactic Centre using spectral line observations of the high density tracers N2H+, HCN, H13CN, and HCO+. We use dendrograms, which map the hierarchical nature of the position-position-velocity (PPV) data, to compute sigma and R of contiguous structures. The dispersions range from ~2-30 km/s in structures spanning sizes 2-40 pc, respectively. By performing Bayesian inference, we show that a power-law with exponent 0.3-1.1 can reasonably describe the sigma-R trend. We demonstrate that the derived sigma-R relationship is independent of the locations in the PPV dataset where sigma and R are measured. The uniformity in the sigma-R relationship suggests turbulence in the CMZ is driven on the large scales beyond >30 pc. We compare the CMZ sigma-R relationship to that measured in the Galactic molecular cloud Perseus. The exponents between the two systems are similar, suggestive of a connection between the turbulent properties within a cloud to its ambient medium. Yet, the velocity dispersion in the CMZ is systematically higher, resulting in a coefficient that is nearly five times larger. The systematic enhancement of turbulent velocities may be due to the combined effects of increased star formation activity, larger densities, and higher pressures relative to the local ISM.

The role of star formation for the galactic dynamo

Magnetic field amplification by a fast dynamo is seen in local box simulations of SN-driven ISM turbulence, where the self-consistent emergence of large-scale fields agrees very well with its mean-field description. We accordingly derive scaling laws of the turbulent transport coef- ficients in dependence of the SN rate, density and rotation. These provide the input for global simulations of regular magnetic fields in galaxies within a mean-field MHD framework. Using a Kennicutt-Schmidt relation between the star formation (SF) rate and midplane density, we can reduce the number of free parameters in our global models. We consequently present dynamo models for different rotation curves and radial density distributions.

Giant Molecular clouds: what are they made from, and how do they get there?

We analyse the results of four simulations of isolated galaxies: two with a rigid spiral potential of fixed pattern speed, but with different degrees of star-formation induced feedback, one with an axisymmetric galactic potential and one with a `live’ self-gravitating stellar component. Since we use a Lagrangian method we are able to select gas that lies within giant molecular clouds (GMCs) at a particular timeframe, and to then study the properties of this gas at earlier and later times. We find that gas which forms GMCs is not typical of the interstellar medium at least 50 Myr before the clouds form and reaches mean densities within an order of magnitude of mean cloud densities by around 10 Myr before. The gas in GMCs takes at least 50 Myr to return to typical ISM gas after dispersal by stellar feedback, and in some cases the gas is never fully recycled. We also present a study of the two-dimensional, vertically-averaged velocity fields within the ISM. We show that the velocity fields corresponding to the shortest timescales (that is, those timescales closest to the immediate formation and dissipation of the clouds) can be readily understood in terms of the various cloud formation and dissipation mechanisms. Properties of the flow patterns can be used to distinguish the processes which drive converging flows (e.g.\ spiral shocks, supernovae) and thus molecular cloud formation, and we note that such properties may be detectable with future observations of nearby galaxies.

Hierarchical Structure of Magnetohydrodynamic Turbulence In Position-Position-Velocity Space

Magnetohydrodynamic turbulence is able to create hierarchical structures in the interstellar medium that are correlated on a wide range of scales via the energy cascade. We use hierarchical tree diagrams known as dendrograms to characterize structures in synthetic Position-Position-Velocity (PPV) emission cubes of optically thin isothermal magnetohydrodynamic turbulence. We show that the structures and degree of hierarchy observed in PPV space are related to the physics of the gas, i.e. self-gravity and the global sonic and Alfvenic Mach number. Simulations with higher Alfvenic Mach number, self-gravity and supersonic flows display enhanced hierarchical structure. We observed a strong sonic and Alfvenic dependency when we apply the the statistical moments (i.e. mean, variance, skewness, kurtosis) to the dendrogram distribution. Larger magnetic field and sonic Mach number correspond to larger values of the moments. Application of the dendrogram to 3D density cubes, also known as Position-Position-Position cubes (PPP), reveals that the dominant emission contours in PPP and PPV are related for supersonic gas but not for subsonic. We also explore the effects of smoothing, thermal broadening and velocity resolution on the dendrograms in order to make our study more applicable to observational data. These results all point to hierarchical tree diagrams as being a promising additional tool for studying ISM turbulence and star forming regions in the direction of obtaining information on the degree of self-gravity, the Mach numbers and the complicated relationship between PPV and PPP.

Molecular and atomic line surveys of galaxies II: unbiased estimates of their star formation mode

We make use of our ‘minimal’ cold interstellar medium (ISM) emission line model that predicts the molecular and atomic line emission per unit dense, star-forming gas mass (Geach & Papadopoulos 2012; Paper I) to examine the utility of key line ratios in surveys of the so-called star formation ‘mode’ as traced by xi_SF = M_dense(H_2)/M_total(H_2). We argue that xi_SF and its proxies provide very sensitive, extinction-free discriminators of rapid starburst/merger-driven versus secular quiescent/disk-like stellar mass assembly, with the most promising diagnostic to be applied in the near-future being CO(4-3)/[CI](1-0). These lines are accessible across nearly the full range 0<z<2 (thus covering the bulk of galaxy evolution) with the Atacama Large Millimeter Array. In addition to their diagnostic power, another advantage of this combination is the similar observed frequencies (Delta nu_0 ~ 30 GHz) of the lines, resulting in nearly spatially-matched beams for a fixed aperture, thus mitigating the effects of resolution/morphology bias in the interpretation of galaxy-averaged line ratios. Finally we discuss the capability of deep blind redshift surveys with the high frequency component of the Square Kilometer Arrray (SKA) in discovering H_2-rich galaxies with very low xi_SF values. These could be the progenitors of starburst galaxies seen prior to the onset of star formation; such galaxies could be a class of extreme outliers from local (gas surface density)-(star formation rate) scaling laws, which would exclude them from current star formatation or stellar mass selected samples. Our conservative model suggests that SKA could detect such systems residing at z~3 at a rate of 20-200 per hour.