# Posts Tagged hi absorption

## Today's Postings

### Probing Intergalactic Neutral Hydrogen by High Precision Analysis of the Red Damping Wing of Gamma-Ray Burst 130606A Afterglow Spectrum at z = 5.913

The unprecedentedly bright optical afterglow of GRB 130606A at a redshift close to the reionization era (z = 5.913) provides a new opportunity to probe the ionization status of intergalactic medium (IGM). Here we present a high-precision analysis of the red Ly alpha damping wing of the afterglow spectrum taken by Subaru/FOCAS during 10.4-13.2 hr after the burst. We find that the minimal model including only the baseline power-law and HI absorption in the host galaxy does not give a good fit, leaving residuals showing concave curvature in 8400-8900 A with an amplitude of about 0.6% of the flux. Such a curvature in the short wavelength range cannot be explained either by extinction at the host with standard extinction curves or by the known systematic uncertainties in the observed spectrum. The red damping wing by intervening HI gas outside the host can reduce the residual by about 3 sigma statistical significance. We find that a damped Ly alpha system is not favored as the origin of this intervening HI absorption, from the observed Ly beta and metal absorption features. Therefore absorption by diffuse IGM remains as a plausible explanation. A fit by a simple uniform IGM model requires HI neutral fraction of f_{HI} ~ 0.1-0.5 depending on the distance to the GRB host, implying high f_{HI} IGM associated with the observed dark Gunn-Peterson (GP) troughs. This gives a new evidence that the reionization is not yet complete at z ~ 6. Further investigations using more GRB afterglows to various sightlines, in comparison with realistic theoretical simulations, are desirable.

### A hadronic scenario for HESS J1818-154

Aims: G15.4+0.1 is a faint supernova remnant (SNR) that has recently been associated with the gamma-ray source HESS J1818-154. We investigate a hadronic scenario for the production of the gamma-ray emission. Methods: Molecular 13CO (J=1-0) taken from the Galactic Ring Survey (GRS) and neutral hydrogen (HI) data from the Southern Galactic Plane Survey (SGPS) have been used in combination with new 1420 MHz radio continuum observations carried out with the Giant Metrewave Radio Telescope (GMRT). Results: From the new observations and analysis of archival data we provided for the first time a reliable estimate for the distance to the SNR G15.4+0.1 and discovered molecular clouds located at the same distance. On the basis of HI absorption features, we estimate the distance to G15.4+0.1 in 4.8+/-1.0 kpc. The 13CO observations clearly show a molecular cloud about 5 arcmin in size with two bright clumps, labeled A and B, clump A positionally associated with the location of HESS J1818-154 and clump B in coincidence with the brightest northern border of the radio SNR shell. The HI absorption and the 13CO emission study indicates a possible interaction between the molecular material and the remnant. We estimate the masses and densities of the molecular gas as (1.2+/-0.5)X10^3 M_sun and (1.5+/-0.4)X10^3 cm^-3 for clump A and (3.0+/-0.7)X10^3 M_sun and (1.1+/-0.3)X10^3 cm^-3 for clump B. Calculations show that the average density of the molecular clump A is sufficient to produce the detected gamma-ray flux, thus favoring a hadronic origin for the high-energy emission.

### On the connection between the intergalactic medium and galaxies: The HI-galaxy cross-correlation at z < 1

We present a new optical spectroscopic survey of 1777 ‘star-forming’ (‘SF’) and 366 ‘non-star-forming’ (‘non-SF’) galaxies at redshifts z < 1 (2143 in total), 22 AGN and 423 stars, observed by instruments such as DEIMOS, VIMOS and GMOS, in 3 fields containing 5 quasi-stellar objects (QSOs) with HST UV spectroscopy. We also present a new spectroscopic survey of 165 ‘strong’ (10^14 < NHI < 10^17 cm^-2), and 489 ‘weak’ (10^13 < NHI < 10^14 cm^-2) intervening HI absorption line systems at z < 1 (654 in total), observed in the spectra of 8 QSOs by COS and FOS on the HST. Combining these new data with previously published galaxy catalogs such as VVDS and GDDS, we have gathered a sample of 654 HI absorption systems and 17509 galaxies at transverse scales < 50 Mpc. We present observational results on the HI-galaxy and galaxy-galaxy correlations at transverse scales r < 10 Mpc, and the HI-HI auto-correlation at transverse scales r < 2 Mpc. The two-point correlation functions are measured both along and transverse to the line-of-sight. We constrain the HI-galaxy statistical connection, as a function of both HI column density and galaxy star-forming activity. Our results are consistent with the following conclusions: (1) the bulk of HI systems on Mpc scales have little velocity dispersion (<120 km/s) with respect to the bulk of galaxies; (2) the vast majority of strong HI systems and SF galaxies are distributed in the same locations, together with 75+-15% of non-SF galaxies, all of which typically reside in dark matter haloes of similar masses; (3) 25+-15% of non-SF galaxies reside in galaxy clusters and are not correlated with strong HI systems at scales < 2 Mpc; and (4) 50% of weak HI systems reside within galaxy voids (hence not correlated with galaxies), and are confined in dark matter haloes of masses smaller than those hosting… [abridged]

### Quasars Probing Quasars VI. Excess HI Absorption Within One Proper Mpc of z~2 Quasars

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar’s environment in absorption. We use a sample of 650 projected quasar pairs to study the HI Lya absorption transverse to luminous, z~2 quasars at proper separations of 30kpc < R < 1Mpc. In contrast to measurements along the line-of-sight, regions transverse to quasars exhibit enhanced HI Lya absorption and a larger variance than the ambient intergalactic medium, with increasing absorption and variance toward smaller scales. Analysis of composite spectra reveals excess absorption characterized by a Lya equivalent width profile W = 2.3A (R/100kpc)^-0.46. We also observe a high (~60%) covering factor of strong, optically thick HI absorbers (HI column log NHI > 17.3) at separations R<200kpc, which decreases to ~20% at R~1Mpc, but still represents a significant excess over the cosmic average. This excess of optically thick absorption can be described by a quasar-absorber cross-correlation function xi_QA(r) = (r/r_0)^gamma with a large correlation length r_0 = 12.5+2.7-1.4 Mpc/h (comoving) and gamma = 1.68+0.14-0.30. The HI absorption measured around quasars exceeds that of any previously studied population, consistent with quasars being hosted by massive dark matter halos Mhalo~10^12.5 Msun at z~2.5. The environments of these massive halos are highly biased towards producing optically thick gas, and may even dominate the cosmic abundance of Lyman limit systems and hence the intergalactic opacity to ionizing photons at z~2.5. The anisotropic absorption around quasars implies the transverse direction is much less likely to be illuminated by ionizing radiation than the line-of-sight, which we interpret in terms of the same obscuration effects frequently invoked in unified models of active galactic nuclei.

### HI Absorption Toward HII Regions at Small Galactic Longitudes

We make a comprehensive study of HI absorption toward HII regions located within Galactic longitudes less than 10 degrees. Structures in the extreme inner Galaxy are traced using the longitude-velocity space distribution of this absorption. We find significant HI absorption associated with the Near and Far 3kpc Arms, the Connecting Arm, Banias Clump 1 and the H I Tilted Disk. We also constrain the line of sight distances to HII regions, by using HI absorption spectra together with the HII region velocities measured by radio recombination lines.

### A Thorough Investigation of Distance and Age of the Pulsar Wind Nebula 3C58

A growing number of researchers present evidence that the pulsar wind nebula 3C58 is much older than predicted by its proposed connection to the historical supernova of A.D. 1181. There is also a great diversity of arguments. The strongest of these arguments rely heavily on the assumed distance of 3.2 kpc determined with HI absorption measurements. This publication aims at determining a more accurate distance for 3C58 and re-evaluating the arguments for a larger age. I have re-visited the distance determination of 3C58 based on new HI data from the Canadian Galactic Plane Survey and our recent improvements in the knowledge of the rotation curve of the outer Milky Way Galaxy. I have also used newly determined distances to objects in the neighbourhood, which are based on direct measurements by trigonometric parallax. I have derived a new more reliable distance estimate of 2 kpc for 3C58. This makes the connection between the pulsar wind nebula and the historical event from A.D. 1181 once again much more viable.

### Kinematic distance study of the planetary nebulae-supernova remnant-HII region complex at G35.6-0.5

Two possible planetary nebulae (PN G035.5-00.4 and IRAS 18551+0159), one newly re-identified supernova remnant (SNR G35.6-0.4), and one HII region (G35.6-0.5) form a line-of-sight-overlapped complex known as G35.6-0.5. We analyze 21 cm HI absorption spectra towards the complex to constrain their kinematic distances. PN G035.5-00.4 has a distance from 3.8+/-0.4 kpc to 5.4+/-0.7 kpc. IRAS 18551+0159 is at 4.3+/-0.5 kpc. We discuss the distance for SNR 35.6-0.4, for which the previous estimate was 10.5 kpc, and find plausible for it to be 3.6+/-0.4 kpc. The new distance of SNR G35.6-0.4 and the derived mass for the ~55 km/s CO molecular cloud can accommodate an association with HESS J1858+020. We also conclude that SNR G35.6-0.4 is unlikely associated with PSR J1857+0210 or PSR J1857+0212, which are projected into the SNR area.

### X-ray and Radio Observations of the gamma Cygni Supernova Remnant G78.2+2.1

We present an analysis of ROSAT and CHANDRA ACIS X-ray observations of the gamma Cygni supernova remnant (G78.2+2.1, DR4) and also analyze radio HI absorption spectra. The ROSAT All-Sky-Survey image shows extended X-ray emission from G78.2+2.1 and also an adjacent limb-brightened shell on its northern boundary. A new mosaic created from ROSAT pointed PSPC data shows details of the X-ray emission over the entire face of G78.2+2.1, including X-ray bright features along the southern, eastern and northern rim but a faint center and a faint western rim. We assemble all available Chandra archival data to create a new mosaic which covers a large part of the northern rim and central regions of G78.2+2.1. The HI absorption spectrum is used to obtain a distance limit on the SNR of 2-4 kpc. Chandra spectral parameters are used to constrain the physical properties of G78.2+2.1: we find an X-ray temperature of 0.6-1.2 keV (90% error), and that the SNR is well explained by a Sedov model with age of 8000-16000 yr and blast energy of $0.5-4 \times 10^{51}$ erg. The two brightest hard spectrum sources in G78.2+2.1 are found to be an extragalactic background source and a power-law spectrum compact source. The compact source has an X-ray column density consistent with that of G78.2+2.1 and has also been identified recently as a TeV emitter. Spectra from the northern rim of the SNR include an additional low column density component, which is the same as that from an X-ray spectrum of the northern shell. Thus the northern shell overlaps the northern part of G78.2+2.1. The low column density of the northern shell is consistent with the distance of a B3 star at distance of 980 pc. A stellar wind bubble origin for this northern shell is proposed.

### X-ray and Radio Observations of the gamma Cygni Supernova Remnant G78.2+2.1 [Replacement]

We analyse \textit{ROSAT} and \textit{Chandra} ACIS X-ray observations and HI absorption spectra of the $\gamma$ Cygni supernova remnant (G78.2+2.1, DR4). The \textit{ROSAT} All-Sky-Survey image shows G78.2+2.1 has an adjacent limb-brightened shell north of it. A new \textit{ROSAT} mosaic shows details of the X-ray emission over the entire face of G78.2+2.1. We also create \textit{Chandra} mosaics which cover much of the northern rim and central regions of G78.2+2.1. HI absorption spectra result in association of G78.2+2.1 with the $\gamma$ Cygni nebula, with distance 1.7 to 2.6 kpc. Chandra spectra for G78.2+2.1 give an X-ray temperature of 0.6-1.2 keV (90% error), and that a Sedov model has age of 6800-10000 yr. A compact power-law X-ray source in G78.2+2.1 is consistent with the same distance as G78.2+2.1. The northern X-ray shell is identified with a B3 star at distance of 980 pc and is proposed as a stellar wind bubble.

### Discovery of a small diameter young supernova remnant G354.4+0.0

We report discovery of a shell like structure G354.4+0.0 of size 1.6′ that shows morphology of a shell supernova remnant. Part of the structure show polarized emission in NRAO VLA sky survey (NVSS) map. Based on 330 MHz, 1.4 GHz Giant Metrewave Radio Telescope (GMRT) observations and existing observations at higher frequencies, we conclude the partial shell structure showing synchrotron emission is embedded in an extended HII region of size ~4′. The spectrum of the diffuse HII region turns over between 1.4 GHz and 330 MHz. HI absorption spectrum shows it to be located more than 5 kpc away from Sun. Based on morphology, non-thermal polarized emission and size, this object is one of the youngest supernova remnants discovered in the Galaxy with an estimated age of about 100-500 years.

### A high-resolution mm and cm study of the obscured LIRG NGC 4418 - A compact obscured nucleus fed by in-falling gas?

The aim of this study is to constrain the dynamics, structure and feeding of the compact nucleous of NGC4418, and to reveal the nature of the main hidden power source: starburst or AGN. We obtained high spatial resolution observations of NGC4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz with the SMA very extended configuration. We use the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate and the dust temperature. Emission lines are used to study the kinematics through position-velocity diagrams. Molecular emission is studied with population diagrams and by fitting an LTE synthetic spectrum. We detect bright 1mm line emission from CO, HC3N, HNC and C34S, and 1.4 GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s-1. We detect vibrationally excited HC3N and HNC, with Tvib 300K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160 and 300 K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 1e4 Msun pc-2 is extremely high, and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in Arp220, with an age of 3-10 Myr and a star formation rate >10 Msun yr-1. If an AGN is present, it must be extremely Compton-thick.