HI Column Density Research Articles

HIDEEP - an extragalactic blind survey for very low column-density neutral hydrogen

We have earned out an extremely long integration time (9000 s beam - 1 ) 21-cm blind survey of 60 deg 2 in Centaurus using the Parkes multibeam system. We find that the noise continues to fall as √t o b s throughout, enabling us to reach an HI column-density limit of 4.2 x 10 1 8 cm - 2 for galaxies with a velocity width of 200 km s - 1 in the central 32 deg 2 region, making this the deepest survey to date in terms of column density sensitivity. The H i data are complemented by very deep optical observations from digital stacking of multi-exposure UK Schmidt Telescope R-band films, which reach an isophotal level of 26.5 R mag arcsec - 2 (≃27.5 B mag arcsec - 2 ). 173 Hi sources have been found, 96 of which have been uniquely identified with optical counterparts in the overlap area. There is not a single source without an optical counterpart. Although we have not measured the column densities directly, we have inferred them from the optical sizes of their counterparts. All appear to have a column density of N H 1 = 10 2 0 . 6 5 ′ 0 . 3 8 . This is at least an order of magnitude above our sensitivity limit, with a scatter only marginally larger than the errors on N H 1 . This needs explaining. If confirmed it means that H 1 surveys will only find low surface brightness (LSB) galaxies with high M H 1 /L B . Gas-rich LSB galaxies with lower HI mass to light ratios do not exist. The paucity of low column-density galaxies also implies that no significant population will be missed by the all-sky H 1 surveys being carried out at Parkes and Jodrell Bank.

HI absorption in a gravitational lens atz~ 0.7645

We have used the Westerbork Synthesis Radio Telescope to detect HI 21 cm absorption at in the gravitational lens system towards PMN J0134-0931. The 21 cm profile has two broad components, with peak optical depths of and , at heliocentric redshifts and , respectively. The redshift of the stronger component matches that of CaII H and K absorption detected earlier. The absorption has a total velocity width of ~500 km s-1 (between nulls) and an equivalent width of km s-1. This would imply a total HI column density of cm-2, for a spin temperature of 200 K and a covering factor of unity. The high estimated HI column density is consistent with the presence of large amounts of dust at the lens redshift; the intervening dust could be responsible for the extremely red colour of the background quasar.

A homogeneous sample of sub-damped Lyman alpha systems - I. Construction of the sample and chemical abundance measurements

In this first paper of a series, we report on the use of quasar spectra obtained with the high-resolution Ultraviolet-Visual Echelle Spectrograph (UVES) and available through the European Southern Observatory (ESO) Very Large Telescope (VLT) archive to build the first homogeneous and unbiased sample of sub-damped Lyman a (sub-DLA) systems, absorbers with Hi column densities >10 1 9 cm - 2 but lower than the classical definition of damped Lyman a systems (2 x 10 2 0 cm - 2 ). A systematic investigation of the properties of these systems and a comparison with those of the DLAs (Paper II of this series) is expected to provide new clues on the association of high column density absorbers with galaxies and on the overall evolution of the neutral hydrogen gas mass and metal content in the Universe. In the spectra of 22 quasars which were found suitable for a sub-DLA search, we identified 12 sub-DLAs and one borderline case between the DLA and sub-DLA systems in the redshift interval z = 1.8-4.3. We measured the column densities of H I and of up to 16 additional ions of low, intermediate and high ionization: O I, C II, C IV, Si II, Si iv, N I, S II, Mg I, Mg II, Al II, Al III, Fe II, Fe III, Ni II, Zn II and Cr II. We further investigated the significance of the ionization corrections in the determination of the chemical abundances from the low-ionization ions in the sub-DLA HI column density range. Using the predictions of different ion ratios as a function of the ionization parameter computed using the CLOUDY software package, we have estimated that with the exception of one case, the ionization corrections to the abundances of nine systems for which we were able to constrain the ionization parameter are lower than 0.2 dex for all of the elements except Al II and Zn n down to H I column densities of log N(HI)=19.3 cm - 2 . Finally, we present the first sub-DLA chemical abundance data base, which contains the abundance measurements of 11 different elements (O, C, Si, N, S, Mg, Al, Fe, Ni, Zn and Cr). We took advantage of the lower H I column densities in sub-DLAs to measure, in particular, the O and C abundances using lines that are normally saturated in DLAs.

The large-scale anomalous microwave emission revisited byWMAP

We present a new study of the high latitude galactic contributions to the millimeter sky, based on an analysis of the WMAP data combined with several templates of dust emission (DIRBE/COBE and FIRAS/COBE) and gas tracers (HI and Halpha). To study the IR to millimeter properties of the diffuse sky at high galactic latitude, we concentrate on the emission correlated with the HI gas. We compute the emission spectrum of the dust/free-free/synchrotron components associated with HI gas from low to large column densities. A significant residual WMAP emission over the free-free, synchrotron and the dust contribution is found from 3.2 to 9.1 mm. We show that this residual WMAP emission (normalised to 10$^{20}$ atoms/cm$^2$) (1) exhibits a constant spectrum from 3.2 to 9.1 mm and (2) significantly decreases in amplitude when N$_{HI}$ increases, contrary to the HI-normalised far-infrared emission which stays rather constant. It is thus very likely that the residual WMAP emission is not associated with the Large Grain dust component. The decrease in amplitude with increasing opacity ressembles in fact to the decrease of the transiently heated dust grain emission observed in dense interstellar clouds. This is supported by an observed decrease of the HI-normalised 60 microns emission with HI column densities. Although this result should be interpreted with care due to zodiacal residual contamination at 60 microns, it suggests that the WMAP excess emission is associated with the small transiently heated dust particles. On the possible models of this so-called ``anomalous microwave emission'' linked to the small dust particles are the spinning dust and the excess millimeter emission of the small grains, due to the cold temperatures they can reach between two successive impacts with photons.

The presence and distribution of H I absorbing gas in sub-galactic sized radio sources

We consider the incidence of HI absorption in intrinsically small sub-galactic sized extragalactic sources selected from sources classified as Gigahertz Peaked Spectrum (GPS) and Compact Steep Spectrum (CSS) sources. We find that the smaller sources ( 0.5 kpc). Both a spherical and an axi-symmetric gas distribution, with a radial power law density profile, can be used to explain this anti-correlation between projected linear size and HI column density. Since most detections occur in objects classified as galaxies, we argue that if the unified schemes apply to GPS/CSSs a disk distribution for the HI is more likely. The most favoured explanation for the compact sizes of the GPS/CSSs is that they are young sources evolving in a power law density medium. For the GPSs with measured expansion velocities, our derived densities are within an order of magnitude of those estimated from ram-pressure confinement of the lobes assuming equipartition. Our results therefore support the youth model.

Interstellar Deuterium, Nitrogen, and Oxygen Abundances toward GD 246, WD 2331−475, HZ 21, and Lanning 23: Results from theFUSEMission

The interstellar abundances of D i ,N i, and O i in the local ISM are studied using high-resolution spectra of four hot white dwarfs. The spectra of GD 246, WD 2331� 475, HZ 21, and Lan 23 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) in the wavelength range 905–1187 A ˚ . The line of sight to GD 246 probes the Local Interstellar Cloud and at least one other H i cloud inside the Local Bubble, which contains most of the gas seen along this line of sight. The column densities of Hi ,C ii*, Sii, and Siii are measured using archival Hubble Space Telescope STIS echelle-mode observations. The H i column density is deter... . .. . .. . .. . .. ... ...

A window to the Galactic X‐ray halo: The ISM towards the Lockman hole

We present a combined X-ray/HI-analysis of the ISM towards the Lockman hole. This sky region is considered as the "window to the distant universe" because of its absolute lowest HI column density on the whole sky. The Lockman hole appears to be not as transparent as the HI data suggest. We propose that about half of the ISM towards the Lockman hole is in form of ionized hydrogen rather than HI.

Hi Absorption in GPS/CSS Sources

AbstractCombining our own observations with data from the literature, we consider the incidence of HI absorption in gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) sources. Here we present our preliminary results, where we find that the smaller GPS sources (<1 kpc) on average have larger HI column densities than the larger CSS sources (>1 kpc). Both a spherical and an axi-symmetric gas distribution, with a radial power law density profile, can be used to explain this anti-correlation between projected linear size and HI column density. Since most detections occur in galaxy classified objects, we argue that if the unified schemes apply to the GPS/CSS sources, a disk distribution for the HI is more likely.

The warm-hot intergalactic medium at $\mathsf{\vec {z}\sim2.2}$: Metal enrichment and ionization source

Results are presented for our search for warm-hot gas towards the quasar Q 0329-385. We identify ten OVI systems of which two are within 5000 km/s of z_em and a third one should be of intrinsic origin. The seven remaining systems have HI column densities 10^13.7<N(HI)<10^15.6 cm^-2. At least ~1/3 of the individual OVI sub-systems have temperatures T<1x10^5 K and cannot originate in collisionally ionized gas. Photoionization by a hard UV background field reproduces well the ionic ratios for metallicities in the range 10^-2.5 - 10^-0.5 solar, with possibly sub-solar N/C relative abundance. For [O/C]=0, the sizes inferred for the OVI clouds are in some cases larger than the maximum extent implied by the Hubble flow. This constraint is fulfilled assuming a moderate overabundance of oxygen relative to carbon. For a soft UV ionizing spectrum, an overabundance of O/C is required, [O/C]~0.0-1.3. For a hard(soft) UV spectrum and [O/C]=0(1), the OVI regions have overdensities in 10-40.

The Bright Gamma‐Ray Burst of 2000 February 10: A Case Study of an Optically Dark Gamma‐Ray Burst

The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow. It is therefore one of the events recently classified as dark GRBs or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear. Chandra observations allowed us to localize this GRB within ~1" and a radio transient was detected with the VLA. We identify the likely (P=0.01) host galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the optical flux of this object is consistent with the above HI column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a Giant Molecular Cloud. We therefore conclude that either the gas is local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3) clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy. Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be substantially different from that observed in GRBs with optical afterglows.

Modelling Neutral Hydrogen Discs of Spiral Galaxies

We present an analytical model of a neutral hydrogen disc in a spiralgalaxy. The gas disc of the spiral galaxy is assumed to have an exponentialsurface density profile and to be ionized by the cosmic ultravioletbackground. To compare with observations, we consider the disc positionangle and inclination angle for a line of sight going through the galaxydisc. The HI column densities depend on the strength of ionizing fieldand disc position and inclination. The model was applied to NGC 3198 andthe results were compared with observational data. The HI disc profileat large disc radii can be tested by further HI observations usingradio telescopes with a larger aperture than the present facilities. This HIdisc model can be used to predict quasar absorption line systems bygalaxy discs if quasar lines of sight go through the discs.

Searching for an Intrinsic Stellar Population in Compact High-Velocity Clouds

We are investigating the hypothesis that Compact High-Velocity Clouds (CHVCs) are the left-over building blocks of Local Group galaxies. To this end, we are searching for their embedded stellar populations using FORS at the VLT. The search is done with single-star photometry in V and I bands, which is sensitive to both, young and old, stellar populations. Five CHVCs of our sample have been observed so far down to I=24. We pointed the VLT towards the highest HI column density regions, as determined in Effelsberg radio data. In an alternate approach, we searched 2MASS public data towards those 5 CHVCs down to K=16. While the VLT data probe the central regions out to distance moduli of about 27, the 2MASS data are sensitive to a population of red giant stars to distance moduli of about 20. The 2MASS data, on the other hand, cover a much wider field of view than the VLT data (radius of 1 degree versus FORS field of 6.8 arcmin). We did not find a stellar population intrinsic to the CHVCs in either data. In this paper, we illustrate our search methods.

Probing the absorbing haloes around two high-redshift radio galaxies with VLT-UVES

We present VLT-UVES echelle spectroscopy of the HI and CIV absorption in the spatially-extended Lyman-alpha emission around two high-redshift radio galaxies 0200+015 (z=2.23) and 0943-242 (z=2.92). The absorbers in 0943-242 exhibit little additional structure compared with previous low-resolution spectroscopy and the main absorber is still consistent with HI column density of ~10^19 cm^-2. This is consistent with a picture in which the absorbing gas has low density and low metallicity and is distributed in a smooth absorbing shell located beyond the emission-line gas. However, the main absorbers in 0200+015 are very different. The previous single absorber fit of HI column density \~10^19 cm^-2, now splits into two ~4 x 10^14 cm^-2 absorbers which extend more than 15kpc to obscure additional Ly-alpha emission coincident with a radio lobe in these high-resolution observations. Although consistent with the shell-like distribution for the absorption systems, 0200+015 requires a much higher metal enrichment than 0943-242. The metallicity, inferred from the CIV absorption, is considerably lower in 0943-242 than in 0200+015. We explain these differences with an evolutionary scenario based on the size of the radio source. In both sources the HI absorption gas originates from either a gas-rich merger or pristine cluster gas which cools and collapses towards the centre of the dark matter halo. The higher metallicity in the larger radio source (0200+015) may be a result of a starburst driven superwind (concurrent with the triggering of the radio emission) which has engulfed the outer halo in this older source.

The Soft X‐Ray Emission in a Large Sample of Galaxy Clusters with theROSATPosition Sensitive Proportional Counter

The study of soft X-ray emission of 38 X-ray selected galaxy clusters observed by ROSAT PSPC indicates that the soft excess phenomenon may be a common occurrence in galaxy clusters. Excess soft X-ray radiation, above the contribution from the hot intra-cluster medium, is evident in a large fraction of sources, and is clearly detected with large statistical significance in the deepest observations. The investigation relies on new, high resolution 21 cm HI observations. The sample selection also features analysis of infra-red images, to further ensure reliability of results with respect to the characteristics of Galactic absorption. The possibility of background or calibration effects as cause of the excess emission is likewise investigated; a detailed analysis of the distribution of the excess emission with respect to detector position and Galactic HI column density shows that the excess emission is a genuine celestial phenomenon. We find evidence for a preferential distribution of the soft excess emission at distances larger than ~150-200 kpc from the centers of clusters; this behavior may be naturally explained in the context of a non-thermal Inverse-Compton scenario. Alternatively, we propose that the phenomenon may be caused by thermal emission of very large-scale `warm' filaments seen in recent hydrodynamic simulations. This new interpretation relieves the very demanding requirements of either the traditional intra-cluster `warm' gas and the non-thermal scenarios. We also investigate the possibility of the soft excess originating from unresolved, X-ray faint cluster galaxies.

Multiwavelength observations of the M15 intermediate-velocity cloud

We present Westerbork Synthesis Radio Telescope HI images, Lovell Telescope multibeam HI wide-field mapping, Wisconsin H-alpha Mapper facility images, William Herschel Telescope longslit echelle CaII observations, and IRAS ISSA 60 and 100 micron coadded images towards the intermediate velocity cloud located in the general direction of the M15 globular cluster. When combined with previously-published Arecibo data, the HI gas in the IVC is found to be clumpy, with peak HI column density of 1.5x10^(20) cm^(-2), inferred volume density (assuming spherical symmetry) of 24 cm^(-3)/(D kpc), and maximum brightness temperature at a resolution of 81x14 arcsec of 14 K. The HI gas in the cloud is warm, with a minimum FWHM value of 5 km/s, corresponding to a kinetic temperature, in the absence of turbulence, of 540 K. There are indications in the HI data of 2-component velocity structure in the IVC, indicative of cloudlets. This velocity structure is also tentatively seen in the CaK spectra, although the SNR is low. The main IVC condensation is detected by WHAM in H-alpha with intensities uncorrected for Galactic absorption of upto 1.3 Rayleigh, indicating that the cloud is partially ionised. The FWHM of the ionised component, at a resolution of 1 degree, exceeds 30 km/s. The spatial and velocity coincidence of the H-alpha and HI peaks in emission is qualitatively good. Finally, the 100 and 60 micron IRAS images show spatial coincidence over a 0.7 degree field, with low and intermediate-velocity gas, respectively, indicating that the IVC may contain dust.

Atomic Hydrogen Gas in Dark Matter Minihalos and the Compact High‐Velocity Clouds

We calculate the coupled hydrostatic and ionization structures of pressure-supported gas clouds that are confined by gravitationally dominant dark-matter (DM) mini-halos and by an external bounding pressure provided by a hot medium. We focus on clouds that are photoionized and heated by the present-day background metagalactic field and determine the conditions for the formation of warm (WNM), and multi-phased (CNM/WNM) neutral atomic hydrogen (HI) cores in the DM-dominated clouds. We consider LCDM dark-matter halos, and we compute models for a wide range of halo masses, total cloud gas masses, and external bounding pressures. We present models for the pressure-supported HI structures observed in the Local Group dwarf galaxies Leo A and Sag DIG. We then construct minihalo models for the multi-phased (and low-metallicity) compact high-velocity HI clouds (CHVCs). If the CHVCs are drawn from the same family of halos that successfully reproduce the dwarf galaxy observations, then the CHVCs must be "circumgalactic objects" with characteristic distances of 150 kpc. For such systems we find that multi-phased behavior occurs for peak WNM HI column densities between 2e19 and 1e20 cm^-2, consistent with observations. If the large population of CHVCs represent "missing low-mass satellites" of the Galaxy, then these clouds must be pressure-confined to keep the gas neutral. For an implied CHVC minihalo scale velocity of v_s=12 km s^-1, the confining pressure must exceed ~50 cm^-3 K. A hot (2e6 K) Galactic corona could provide the required pressure at 150 kpc.

Star formation in the warped outer pseudoring of the spiral galaxy NGC 3642

NGC 3642 was classified as a spiral galaxy with three rings and no bar. We have performed an HI and optical study of this nearly face-on galaxy. We find that the nuclear ring might in fact be part of an inner one-armed spiral, that could be driving nuclear accretion and feeding the central activity in the inner kpc. The inner ring is faint, and the outer ring is a rather ill-defined pseudoring. Furthermore, the size ratio of the rings is such that they cannot be due to a single pattern speed linking them together. The outer pseudoring is peculiar, since it lies in the faint outer parts of the disk, where star formation is still going on at 1.4 times the optical radius. Higher HI column densities are associated with these regions and the atomic gas layer is warped. These perturbations affect only the outer disk, since the kinematics within the main body conforms well to an ordinary differentially rotating disk. We propose here that both nuclear activity and star formation in the warped outer parts might be linked to the fact that NGC 3642 is located in a rich environment, where its close neighbors show clear signs of merging. Our suggestion is that NGC 3642 has captured recently a low-mass, gas-rich dwarf, and star formation was triggered in this infalling external gas that produced also a pronounced warp in the gaseous disk.

Hydrogen Column Density Evaluations toward Capella: Consequences on the Interstellar Deuterium Abundance

The deuterium abundance evaluation in the direction of Capella has for a long time been used as a reference for the local interstellar medium (ISM) within our Galaxy. We show here that broad and weak HI components could be present on the Capella line of sight, leading to a large new additional systematic uncertainty on the N(HI) evaluation. The D/H ratio toward Capella is found to be equal to 1.67 (+/-0.3)x10^-5 with almost identical chi^2 for all the fits (this range includes only the systematic error; the 2 sigma statistical one is almost negligible in comparison). It is concluded that D/H evaluations over HI column densities below 10^19 cm^-2 (even perhaps below 10^20 cm^-2 if demonstrated by additional observations) may present larger uncertainties than previously anticipated. It is mentionned that the D/O ratio might be a better tracer for DI variations in the ISM as recently measured by the Far Ultraviolet Spectroscopic Explorer (FUSE).

HI 21 cm imaging of a nearby damped Lyman-α system

We present Giant Metrewave Radio Telescope (GMRT) HI 21 cm emission images of the z = 0.009 damped Lyman-α (DLA) absorber towards the QSO HS 1543+5921. The DLA has been earlier identified as a low surface brightness galaxy SBS 1543+593 (Reimers & Hagen 1998; Bowen et al. 2001a) with small (∼400 pc) impact parameter to the QSO line of sight. The extremely low redshift of the absorber allows us to make, for the first time, spatially resolved images of the 21 cm emission; besides the HI mass, this also enables us to determine the velocity field of the galaxy and, hence, to estimate its dynamical mass. We obtain a total HI mass of ∼1.4 × 109 M⊙, considerably smaller than the value of M∗HI determined from blind 21 cm emission surveys. This continues the trend of low HI mass in all low redshift DLAs for which HI emission observations have been attempted. We also find that the QSO lies behind a region of low local HI column density in the foreground galaxy. This is interesting in view of suggestions that DLA samples are biased against high HI column density systems. Finally, the dynamical mass of the galaxy is found to be Mdyn ∼ 5 × 109 M⊙.

A new 21-cm absorber identified with an $L \sim L^\star$ galaxy

We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted 21-cm absorption from the $z=0.437$ metal line absorption system towards PKS 1243-072. HI absorption is clearly detected; the absorption profile has a velocity spread of $\sim 20$ km/s. Detection of 21-cm absorption indicates that the absorber has an HI column density large enough to be classified as a damped Lyman-$\alpha$ system. Follow up ground based optical imaging and spectroscopy allow us to identify the absorber with an $L \sim L^\star$ galaxy at an impact parameter of $\sim 9.8$ kpc from the line of sight to the QSO. The absorbing galaxy is unusual in that it has bright emission lines. On the basis of the optical spectrum we are unable to uniquely classify the galaxy since its emission line ratios lie in the transition region between starburst and Seyfert II type spectra.