HI Column Density Research Articles

Abundances of Deuterium, Nitrogen, and Oxygen toward HZ 43A: Results from the FUSE Mission

We present an analysis of interstellar absorption along the line of sight to the nearby white dwarf star HZ43A. The distance to this star is 68+/-13 pc, and the line of sight extends toward the north Galactic pole. Column densities of OI, NI, and NII were derived from spectra obtained by the Far Ultraviolet Spectroscopic Explorer (FUSE), the column density of DI was derived from a combination of our FUSE spectra and an archival HST GHRS spectrum, and the column density of HI was derived from a combination of the GHRS spectrum and values derived from EUVE data obtained from the literature. We find the following abundance ratios (with 2-sigma uncertainties): DI/HI = (1.66 +/- 0.28) x 10^-5, OI/HI = (3.63 +/- 0.84) x 10^-4, and NI/HI = (3.80 +/- 0.74) x 10^-5. The NII column density was slightly greater than that of NI, indicating that ionization corrections are important when deriving nitrogen abundances. Other interstellar species detected along the line of sight were CII, CIII, OVI, SiII, ArI, MgII, and FeII; an upper limit was determined for NIII. No elements other than HI were detected in the stellar photosphere.

Abundant molecular gas in tidal dwarf galaxies: On-going galaxy formation

[Abridged...] We investigate the process of galaxy formation as can be observed in the only currently forming galaxies -- the so-called Tidal Dwarf Galaxies, hereafter TDGs -- through observations of the molecular gas detected via its CO emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (two of them have already been published in Braine et al. 2000), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10^8 M_sun. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H2.... Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin....

Simulations of Damped Lyα and Lyman Limit Absorbers in Different Cosmologies: Implications for Structure Formation at High Redshift

We use hydrodynamic cosmological simulations to study damped Lyman-alpha (DLA) and Lyman limit (LL) absorption at redshifts z=2-4 in five variants of the cold dark matter scenario. Our standard simulations resolve the formation of dense concentrations of neutral gas in halos with circular velocity v_c roughly 140 km/s for Omega_m=1 and 90 km/s for Omega_m=0.4, at z=2; an additional LCDM simulation resolves halos down to v_c approximately 50 km/s at z=3. We find a clear relation between HI column density and projected distance to the center of the nearest galaxy, with DLA absorption usually confined to galactocentric radii less than 10-15 kpc and LL absorption arising out to projected separations of 30 kpc or more. Detailed examination provides evidence of non-equilibrium effects on absorption cross-section. If we consider only absorption in the halos resolved by our standard simulations, then all five models fall short of reproducing the observed abundance of DLA and LL systems at these redshifts. If we extrapolate to lower halo masses, we find all four models are consistent with the observed abundance of DLA systems if the the extrapolated behavior extends to circular velocities roughly 50-80 km/s, and they may produce too much absorption if the relation continues to 40 km/s. Our results suggest that LL absorption is closely akin to DLA absorption, arising in less massive halos or at larger galactocentric radii but not caused by processes acting on a radically different mass scale.

Gas-rich galaxies and the H i mass function

We have developed an automated cross-correlation technique to detect 21cm emission in sample spectra obtained from the HI Parkes All Sky Survey. The initial sample selection was the nearest spectra to 2435 low surface brightness galaxies in the catalogue of Morshidi-Esslinger et al. (1999). The galaxies were originally selected to have properties similar to Fornax cluster dE galaxies. As dE galaxies are generally gas poor it is not surprising that there were only 26 secure detections. All of the detected galaxies have very high values of $(M_{H}/L_{B})_{\odot}$. Thus the HI selection of faint optical sources leads to the detection of predominately gas rich galaxies. The gas rich galaxies tend to reside on the outskirts of the large scale structure delineated by optically selected galaxies, but they do appear to be associated with it. These objects appear to have similar relative dark matter content to optically selected galaxies. The HI column densities are lower than the 'critical density' necessary for sustainable star formation and they appear, relatively, rather isolated from companion galaxies. These two factors may explain their high relative gas content. We have considered the HI mass function by looking at the distribution of velocities of HI detections in random spectra on the sky. The inferred HI mass function is steep though confirmation of this results awaits a detailed study of the noise characteristics of the HI survey.

Detection of a multi-phase ISM at ${\vec z=}$ 0.2212

We present sensitive Giant Metrewave Radio Telescope (GMRT) and high-resolution Arecibo HI 21-cm observations of the damped Lyman-$\alpha$ absorber (DLA) at $z=0.2212$ towards OI 363. The spectra are in excellent agreement and yield a spin temperature $T_s = 890 \pm 160$ K, far higher than $T_s$ values measured in local spirals, but similar to values obtained in the majority of DLAs ($T_s \ga 1000$ K). The high velocity resolution of the Arecibo spectra enables us to obtain estimates of physical conditions in the absorbing clouds by fitting multiple Gaussians to the absorption profile. The spectra are well fit by a three-component model with two narrow and one wide components, with temperatures ${T_{k_1}} = 308 \pm 24$ K, ${T_{k_2}} = 180 \pm 30$ K and ${T_{k_3}} = 7600 \pm 1250$ K, respectively. The last of these is in excellent agreement with the expected temperatures for the WNM ($5000 - 8000$ K). The mere fact that components are seen with lower temperatures than the estimated $T_s$ implies that the absorber must have a multi-phase medium. We use the measured 21-cm optical depth and the above estimates of the kinetic temperature to obtain the HI column density in the various components. The total column density in the narrow components is found to be $\NHI (CNM) \le 1.9 \pm 0.25 \times 10^{20}$ cm, while that in the wide component is $\NHI (WNM) \ge 1.26 \pm 0.49 \times 10^{21}$ cm. Thus, the WNM contains at least 75% of the total HI in the $z = 0.2212$ DLA, unlike our Galaxy, in which the CNM and WNM have equitable contributions; this accounts for the difference in the spin temperatures of the $z = 0.2212$ system and local spirals, suggesting that the DLA is probably a dwarf or LSB type galaxy (abridged).

Damped [CLC]Lyα[/CLC] Absorption from a Nearby Low Surface Brightness Galaxy

Ground-based & HST images of the nearby galaxy SBS 1543+593 (z=0.009) show it to be a Low Surface Brightness (LSB) galaxy with a central surface brightness of mu_B(0)=23.2 mag/arcsec-2 and scale length 0.9 h-1 kpc, values typical for the local LSB galaxy population. The galaxy lies directly in front of the QSO HS 1543+5921 (z=0.807); an HST STIS spectrum of the quasar reveals a damped Lyman-alpha (DLA) line at the redshift of the interloper with an HI column density of log N(HI) = 20.35, as well as several low-ionization metal lines with strengths similar to those found in the Milky Way interstellar medium. Our data show that LSB galaxies are certainly able to produce the DLA lines seen at higher redshift, and fuels the speculation that LSB galaxies are a major contributor to that population of absorbers.

New Observations of Extra-Disk Molecular Gas in Interacting Galaxy Systems, Including a Two-Component System in Stephan’s Quintet

We present new CO (1 - 0) observations of eleven extragalactic tails and bridges in nine interacting galaxy systems, almost doubling the number of such features with sensitive CO measurements. Eight of these eleven features were undetected in CO to very low CO/HI limits, with the most extreme case being the NGC 7714/5 bridge. This bridge contains luminous H II regions and has a very high HI column density (1.6 X 10^21 cm^-2 in the 55 CO beam), yet was undetected in CO to rms T(R)* = 2.4 mK. The HI column density is higher than standard H2 and CO self-shielding limits for solar-metallicity gas, suggesting that the gas in this bridge is metal-poor and has an enhanced N(H2)/I(CO) ratio compared to the Galactic value. Only one of the eleven features in our sample was unambiguously detected in CO, a luminous HI-rich star formation region near an optical tail in the compact group Stephan's Quintet. We detect CO at two widely separated velocities in this feature, at ~6000 km/s and ~6700 km/s. Both of these components have HI and H-alpha counterparts. These velocities correspond to those of galaxies in the group, suggesting that this gas is material that has been removed from two galaxies in the group. The CO/HI/H-alpha ratios for both components are similar to global values for spiral galaxies.

No Diffuse H[TINF]2[/TINF] in the Metal-deficient Galaxy I Z[CLC]w[/CLC] 18

The metal deficient starburst galaxy IZw18 has been observed with FUSE in a search for H2 molecules. The spectrum obtained with an aperture covering the full galaxy shows no absorption lines of diffuse H2 at the radial velocity of the galaxy. The upper limit for the diffuse H2 column density is found to be very low: N(H2) <~ 10^15 cm-2 (10 sigma), unlike our Galaxy where H2 is generally present for even low HI column densities. Although the HI column density is here as high as N(HI) ~ 2.10^21 cm-2, we observe 2N(H2)/N(HI)<< 10^-6. We cannot exclude the possibility that some H2 could be in very dense, small and discrete clumps which cannot be detected with the present observation. However, the remarkable absence of diffuse H2 in this metal-poor galaxy can be explained by the low abundance of dust grains (needed to form this molecule from H-atoms), the high ultraviolet flux and the low density of the HI cloud surrounding the star-forming regions. Thus having eliminated diffuse H2 as a significant contributor to the total mass, it appears that the gas of the galaxy is dominated by HI, and that the high dynamical mass is not composed of cold and diffuse baryonic dark matter.

Chemical enrichment at high redshifts

We have tried to understand the recent observations related to metallicity in Ly-α forest clouds in the framework of the two component model. The model consists of mini-halos having circular velocities smaller than ∼ 55 km s−1, with no star formation and galactic halos with higher circular velocities ≤ 250 km s−1, having clouds, star formation and consequent metal enrichment. We find that even if the mini-halos were chemically enriched by an earlier generation of stars, to have [C/H] ≃ −2.5, the number of C IV lines with column density >1012 cm−2, contributed by the mini-halos, at the redshift of 3, would be only about 10% of the total number of lines, for a chemical enrichment rate of (1 + z)−3 in the galaxies. Recently reported absence of heavy element lines associated with most of the Ly-α lines with HI column density between 1013.5 cm−2 and 1014 cm−2 by Lu et al [13], if correct, gives an upper limit on [C/H]= −3.7, not only in the mini-halos, but also in the outer parts of galactic halos. However, the mean value of 7×10−3 for the column density ratio of C IV and HI, determined by Cowie and Songaila (1998) for low Ly-α optical depths, implies an abundance of [C/H]= −2.5 in mini-halos as well as most of the region in galactic halos. The redshift and column density distribution of C IV has been shown to be in reasonable agreement with the observations.

Detection of Magnetic Fields toward M17 through the HiZeeman Effect

We have carried out VLA Zeeman observations of HI absorption lines toward the HII region in the M17 giant molecular cloud complex. The HI absorption lines toward M17 show between 5 and 8 distinct velocity components which vary spatially in a complex manner across the source. We explore possible physical connections between these components and the M17 region based on calculations of HI column densities, line of sight magnetic field strengths, as well as comparisons with a wide array of previous optical, infrared, and radio observations. In particular, an HI component at the same velocity as the southwestern molecular cloud (M17 SW) ~20 km/s seems to originate from the edge-on interface between the HII region and M17 SW, in un-shocked PDR gas. We have detected a steep enhancement in the 20 km/s HI column density and line of sight magnetic field strengths (Blos) toward this boundary. A lower limit for the peak 20 km/s HI column density is N_{HI}/T_s > 5.6 x 10^{19} cm^{-2}/K while the peak Blos is ~ -450 muG. In addition, blended components at velocities of 11-17 km/s appear to originate from shocked gas in the PDR between the HII region and an extension of M17 SW, which partially obscures the southern bar of the HII region. The peak N_{HI}/T_s and Blos for this component are > 4.4 x 10^{19} cm^{-2}/K and +550 muG, respectively. Comparison of the peak magnetic fields detected toward M17 with virial equilibrium calculations suggest that ~1/3 of M17 SW's total support comes from its static magnetic field and the other 2/3 from its turbulent kinetic energy which includes support from Alfven waves.

HI Column Density Distribution Function at z = 0: Connection to Damped Lyα Statistics

AbstractWe present a measurement of the HI column density distribution function f(Nhi) at the present epoch for column densities &gt; 1020 cm−2. These high column densities compare to those measured in damped Lyα lines seen in absorption against background quasars. Although observationally rare, it appears that the bulk of the neutral gas in the Universe is associated with these damped Lyα systems. In order to obtain a good anchor point at z = 0 we determine f(Nhi) in the local Universe by using 21 cm synthesis observations of a complete sample of spiral galaxies. We show that f(Nhi) for damped Lyα systems has changed significantly from high z to the present and that change is greatest for the highest column densities. The measurements indicate that low surface brightness galaxies make a minor contribution to the cross section for HI, especially for NHI &gt; 1021 cm−2.

HI in Karachentsev Objects Properties of new nearby Dwarf Galaxies

AbstractThis is a report on HI observations of newly discovered nearby dwarf galaxies, most of which of low surface brightness, from the first section of the Karachentsev catalog. Observations were performed using the 100-m radiotelescope at Effelsberg, the Nançay radiotelescope, and the compact array of the Australia Telescope. We observed 220 galaxies with a detection rate of about 60%. 35 of the observed galaxies are located within the Local Volume (i.e., within 10 Mpc). The smallest detected galaxies have diameters around 0.3 kpc and HI-masses of a few times 106 solar masses. We confirm a correlation between HI column density and surface brightness of gas-rich disk galaxies.

Star Formation Thresholds in Low Surface Brightness Dwarf Galaxies

AbstractThe results of a comparative study of star formation thresholds in gas-rich, low surface brightness, dwarf galaxies are presented. Approximately half the galaxies in the study were “high MH/LB” galaxies, which appear to have inefficient star formation properties. The comparison sample comprised of otherwise “normal” dwarf galaxies, with moderate current star formation rates. In all systems, sites of active star formation were associated with local peaks in the HI column density. For both types of galaxies, the azimuthally averaged gas column density is low. Similar to other LSB systems, the global gas densities are well below the critical threshold for star formation throughout the system. Thus, star formation is inhibited globally, but occurs locally in these gas-rich dwarf galaxies.

A “Minihalo” Model for the Lyman Limit Absorption Systems at High Redshift

We propose that a large fraction of the QSO Lyman limit absorption systems (LLS) observed at high redshift (z > 3) originate from gas trapped in small objects, such as minihalos, that form prior to reionization. In the absence of a strong UV flux, the gas is predominantly neutral and may form clouds with HI column density NHI > 10^18 cm^-2. Due to their high densities and high HI column densities, these clouds are not destroyed by the onset of the UV background at a later time. Thus, if not disrupted by other processes, such as mergers into larger systems or `blow away' by supernovae, they will produce LLS. We show that the observed number density of LLS at high redshifts can well be reproduced by the survived `minihalos' in hierarchical clustering models such as the standard cold dark matter model. The number density of LLS in such a population increases with z even beyond the redshifts accessible to current observations and dies off quickly at z < 2. This population is distinct from other populations because the absorbing systems have small velocity widths and a close to primordial chemical composition. The existence of such a population requires that the reionization of the universe occurs late, at z < 20.

3.21. Atomic ISM in the nuclear starburst regions of M82 &amp; NGC 253

M82 (NGC 3034) is a nearby (D = 3.3 Mpc) “prototypical” starburst galaxy which emits most of its luminosity in the infrared (LIR = 3×1010L⊙, see Rieke et al. 1980). M82 is also a strong radio source 3C 231, with numerous compact knots which are thought to be young SNR's (Kronberg et al., 1981; Muxlow et al., 1994). Its strong 1.4 GHz radio continuum is extended over the entire 500 pc nuclear starburst region, and the HI absorption is easily mapped at 2″ (30 pc) resolution using the VLA. The resulting velocity integrated optical depth (τΔV) map can be converted to HI column density map if HI spin temperature (Tsp) is known.

Probing Lyα Absorbers in Cosmological Simulations with Double Lines of Sight

We perform a double line of sight (DLOS) analysis of the Lyman-alpha forest structures that form and evolve in cosmological N-body/hydrodynamic simulations. Pairs of simulated spectra, extracted from lines of sight separated by distances from D=12.5kpc up to 800kpc, and a ``control sample'' of unrelated lines of sight, are analyzed at redshifts 3, 2, and 1. Coincident line samples are defined for HI column density thresholds of Nco = 10^{12.5}, 10^{13}, and 10^{14} per square cm. We find that: 1) Under the assumption of a single structure size, a Bayesian analysis yields sizes that are larger for smaller Nco, and at fixed Nco the size decreases with decreasing redshift. However, these derived sizes are found to increase with increasing D indicating that the assumption of a single structure size is invalid. 2) The column densities of coincident pairs are highly correlated for small D, with increasing scatter as D is increased, consistent with structures that have a centrally peaked N(HI) that decreases gradually with radius. 3) The velocity difference distribution for coincident lines is very narrow for small D, and widens as D is increased to meet the expectation for chance coincidences in unrelated lines of sight. This behavior is indicative of organized motion within the structures. 4) For small D, the distribution of anticoincident line column densities, Nac, falls steeply as Nac increases from the cutoff value, but has a significant tail at large values which is inconsistent with a population of spherical absorbers with sharp edges, and consistent with a flattened geometry. The conclusions reached on the basis of the DLOS analysis are validated by an examination of the three-dimensional structures and velocity flows in the simulation data.

Gravitational lensing by damped Ly-α absorbers

Assuming that (i) damped Ly-α absorbers (DLAs) arise in present-day-like spiral galaxies which are immersed in isothermal dark matter halos, (ii) that these galaxies obey the Tully-Fisher σ σ ∗ = ( L L ∗ ) 1 α TF and the Holmberg R L R ∗ = ( L L ∗ ) α H relations, and (iii) that they follow the Schechter luminosity distribution, we describe how their observed number density ( dN dz ), distribution of column density ( f( N)) as well as inferred cosmological density of HI ( ω HI derived from DLA surveys are affected by gravitational lensing (GL). The ‘by-pass’ effect causes the lines-of-sight (LOSs) towards background QSOs to avoid the central parts of galaxies and reduces their effective cross-section for absorption; the ‘amplification bias’ leads observers to select QSOs whose LOSs preferentially cross galaxies close to their Einstein radius. As a consequence, the determination of the quantities dN dz , f( N) and ω HI from DLA surveys does not only depend on the redshift z and luminosity L of galaxies responsible for the absorbers but also on the column density profile of HI within the galaxies and on the redshift z q and magnitude b q of the background QSOs. For most of the existing surveys using b q ≲ 19 QSOs, the amplification bias dominates the combined effect resulting in a slight overestimate of dN dz , f( N) and ω HI. We mainly find that observational strategies presently used to produce high- z DLA surveys result in avoiding the signature of significant GL effects: following our model, we determine that an overestimate of ω HI by more than 10% is unlikely for the z ⋍ 1.7 existing surveys, but may reach ⋍ 35% for the low redshift ones. However, we show that, in the absence of extinction by dust and micro-lensing effects, surveys ideally designed to enhance GL effects, i.e. to search for DLAs at z ∼ 0.5 in front of very bright ( b q ⋍ 16 ), high- z ( z q > 1) QSOs, may lead 1) to overestimate by up to ⋍90% the number of DLAs per unit redshift; 2) to bias the survey towards high HI column density systems so that it could contain up to 4 times as many such systems, thus 3) to overestimate by up to ⋍170% the cosmological density of gas associated with those DLAs. Identification of the galaxies responsible for the DLAs may be severely biased towards luminous galaxies if 2 α TF − α H > 0 ; this latter effect is greatly increased for log N HI > 21 DLAs.

H I Column Density Oscillation in Interstellar Space

The author proposes the possibility to observe the HI column density oscillation in small scales (∼ AU-scale). If such variance of the column density were to be observed, he would suggest that the observational results show the existence of some type of waves in two-phase gas mixture. Such wave is known to be a void wave in the context of multi-phase fluid dynamics. If the variation of HI column density is representation of the void wave, we can also expect the HII density variations have anti-correlation to the HI density fluctuations, in the context of classical two phase model of interstellar medium.

Local HI: Constraints on the Evolution of the HI Content of the Universe

AbstractAnalyses of QSO absorption lines show that the HI content has evolved over the redshift range z = 5 to z = 0. The 21-cm line measurements of the z = 0 HI content avoid several biases inherent in the absorption-line technique, such as the influence of evolving dust content in the absorbers, and will produce a reliable measure to anchor theories of galaxy evolution. Examples of important questions to be addressed by local HI surveys are: (1) Is there a significant population of gas-rich galaxies or intergalactic clouds that is missing from the census of optically selected galaxies? (2) Is there an adequate reservoir of neutral gas to substantially prolong star formation at its present rate? (3) Are there massive objects of such low HI column density that they can have escaped detection in the ‘unbiased’ HI surveys that have been conducted so far?

APM z ≳ 4 survey: distribution and evolution of high column density H i absorbers

Eleven candidate damped Lya absorption systems were identified in 27 spectra of the quasars from the APM z>4 survey covering the redshift range 2.8<z(abs)<4.4 (8 with z(abs)>3.5). High resolution echelle spectra (0.8A FWHM) have been obtained for three quasars, including 2 of the highest redshift objects in the survey. Two damped systems have confirmed HI column densities of N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold. We have discovered the highest redshift damped Lya absorber known at z=4.383 in QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. We combine this high redshift sample with other quasar samples covering the redshift range 0.008 < z < 4.7 to study the redshift evolution and the column density distribution function for absorbers with log N(HI)>=17.2. In the HI column density distribution f(N)=kN^-beta we find evidence for breaks in the power law, flattening for 17.2< log N(HI)<21 and steepening for log N(HI)>21.2. The column density distribution function for the data with log N(HI)>=20.3 is better fit with the form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the number density per unit redshift is evident in the higher column density systems with an apparent decline in N(z) for z>3.5.