Total HI Research Articles

Accurate measurement of the H i column density from H i 21 cm absorption-emission spectroscopy

We present a detailed study of an estimator of the HI column density, based on a combination of HI 21cm absorption and HI 21cm emission spectroscopy. This "isothermal" estimate is given by $N_{\rm HI,ISO} = 1.823 \times 10^{18} \int \left[ \tau_{\rm tot} \times {\rm T_B} \right] / \left[ 1 - e^{-\tau_{\rm tot}} \right] {\rm dV}$, where $\tau_{\rm tot}$ is the total HI 21cm optical depth along the sightline and ${\rm T_B}$ is the measured brightness temperature. We have used a Monte Carlo simulation to quantify the accuracy of the isothermal estimate by comparing the derived $N_{\rm HI,ISO}$ with the true HI column density $N_{\rm HI}$. The simulation was carried out for a wide range of sightlines, including gas in different temperature phases and random locations along the path. We find that the results are statistically insensitive to the assumed gas temperature distribution and the positions of different phases along the line of sight. The median value of the ratio of the true H{\sc i} column density to the isothermal estimate, $N_{\rm HI}/{N_{\rm HI, ISO}}$, is within a factor of 2 of unity while the 68.2% confidence intervals are within a factor of $\approx 3$ of unity, out to high HI column densities, $\le 5 \times 10^{23}$\,cm$^{-2}$ per 1 km s$^{-1}$ channel, and high total optical depths, $\le 1000$. The isothermal estimator thus provides a significantly better measure of the HI column density than other methods, within a factor of a few of the true value even at the highest columns, and should allow us to directly probe the existence of high HI column density gas in the Milky Way.

THE CORRELATION BETWEEN DISPERSION MEASURE AND X-RAY COLUMN DENSITY FROM RADIO PULSARS

Pulsars are remarkable objects that emit across the entire electromagnetic spectrum, providing a powerful probe of the interstellar medium. In this study, we investigate the relation between dispersion measure (DM) and X-ray absorption column density NH using 68 radio pulsars detected at X-ray energies with the Chandra X-ray Observatory or XMM-Newton. We find a best-fit empirical linear relation of NH (10^20 cm^-2)= 0.30+0.13-0.09 DM (pc cm^-3), which corresponds to an average ionization of 10+4-3%, confirming the ratio of one free electron per ten neutral hydrogen atoms commonly assumed in the literature. We also compare different NH estimates and note that some NH values obtained from X-ray observations are higher than the total Galactic HI column density along the same line of sight, while the optical extinction generally gives the best NH predictions.

The evolution of H i and C iv quasar absorption line systems at 1.9 <z< 3.2

We have investigated the distribution and evolution of ~3100 intergalactic HI absorbers with HI column densities log N(HI) = [12.75, 17.0] at 1.9 < z < 3.2, using 18 high resolution, high S/N quasar spectra obtained from the ESO VLT/UVES archive. We used two sets of Voigt profile fitting analysis, one including all the available high-order Lyman lines to obtain reliable HI column densities of saturated lines, and another using only the Ly-alpha lines. There is no significant difference between the results from the two fits. Combining our results with literature data, the mean number density at 0 < z < 4 is not well described by a single power law and strongly suggests that its evolution slows down at z < 1.5 at the high and low column density ranges. We also divided our entire HI absorbers at 1.9 < z < 3.2 into two samples, the unenriched forest and the CIV-enriched forest, depending on whether HI lines are associated with CIV at log N(CIV) > 12.2 within a given velocity range. The entire HI column density distribution function (CDDF) can be described as the combination of these two well-characterised populations which overlap at log N(HI) ~ 15. At log N(HI) < 15, the unenriched forest dominates, showing a similar power-law distribution to the entire forest. The CIV-enriched forest dominates at log N(HI) > 15, with its distribution function proportional to N(HI)^(-1.45). However, it starts to flatten out at lower N(HI), since the enriched forest fraction decreases with decreasing N(HI). The deviation from the power law at log N(HI) = [14, 17] shown in the CDDF for the entire HI sample is a result of combining two different HI populations with a different CDDF shape. The total HI mass density relative to the critical density is Omega(HI) ~ 1.6 x 10^(-6) h^(-1), where the enriched forest accounts for ~40% of Omega(HI).

Simulation of hydrogen isotope retention in the gap of tungsten divertor tiles

The use of castellated divertor structures in fusion devices may increase the amount of hydrogen isotope retention because of the presence of gaps between the tiles. To address this issue, a two dimensional Hydrogen Isotope Inventory Processes code (HIIPC2D) is developed. To represent the effect of the gap geometry on the incident plasma flux reasonably, a two dimensional kinetic edge plasma code is integrated into HIIPC2D. The simulation results indicate that the existence of gaps of divertor tiles does increase the HI retention in the short term, but in the long term its contribution to the total HI retention will be small.

GEOCHEMICAL INVESTIGATION OF LACUSTRINE OIL SHALE IN THE LUNPOLA BASIN (TIBET): IMPLICATIONS FOR PALEOENVIRONMENT AND PALEOCLIMATE

The Dingqinghu Formation oil shale, located in the centre of the Lunpola basin, represents a potential large lacustrine oil shale resource in Tibet. A geochemical investigation of the oil shale was performed to reconstruct paleoenvironment and paleoclimate during deposition. The total organic carbon (TOC) contents (1.46-11.85%), S2 values (4.79-115.80 mg HC/mg rock) and HI (328-1040 mg HC/mg TOC) of oil shale samples are high, and indicate that the oil shale has a good oil source rock potential. The thermal maturity assessed from PI (0.01-0.09) and Tmax (429-440 °C) shows an immature to early mature stage of the organic matter. The oil shale exhibits characteristics of odd-over-even predominance, maximum n-alkanes peak at nC25 or nC23, a higher proportion of C29 sterane, low δ 13 Corg values (-29.9 to -26.7‰), a low Pr/Ph ratio (0.03-0.40), high values of the gammacerane index (up to 25.24), and presence of β-carotane, which is consistent with a reducing, stratified and hypersaline palaeo-lake with the main contribution of algae and bacteria to the organic matter (OM). The development history of palaeo-lakes from the Oligocene to the Early Miocene indicates that the climate of the Lunpola basin region during the deposition of oil shale was arid.

TRACING COLD H I GAS IN NEARBY, LOW-MASS GALAXIES

We analyze line-of-sight atomic hydrogen (HI) line profiles of 31 nearby, low-mass galaxies selected from the Very Large Array - ACS Nearby Galaxy Survey Treasury (VLA-ANGST) and The HI Nearby Galaxy Survey (THINGS) to trace regions containing cold (T $\lesssim$ 1400 K) HI from observations with a uniform linear scale of 200 pc/beam. Our galaxy sample spans four orders of magnitude in total HI mass and nine magnitudes in M_B. We fit single and multiple component functions to each spectrum to isolate the cold, neutral medium given by a low dispersion (<6 km/s) component of the spectrum. Most HI spectra are adequately fit by a single Gaussian with a dispersion of 8-12 km/s. Cold HI is found in 23 of 27 (~85%) galaxies after a reduction of the sample size due to quality control cuts. The cold HI contributes ~20% of the total line-of-sight flux when found with warm HI. Spectra best fit by a single Gaussian, but dominated by cold HI emission (i.e., have velocity dispersions <6 km/s) are found primarily beyond the optical radius of the host galaxy. The cold HI is typically found in localized regions and is generally not coincident with the very highest surface density peaks of the global HI distribution (which are usually areas of recent star formation). We find a lower limit for the mass fraction of cold-to-total HI gas of only a few percent in each galaxy.

COLD GAS ACCRETION BY HIGH-VELOCITY CLOUDS AND THEIR CONNECTION TO QSO ABSORPTION-LINE SYSTEMS

We combine HI 21cm observations of the Milky Way, M31, and the local galaxy population with QSO absorption-line measurements to geometrically model the three-dimensional distribution of infalling neutral gas clouds (HVCs) in the extended halos of low-redshift galaxies. We demonstrate that the observed distribution of HVCs around the Milky Way and M31 can be modeled by a radial exponential decline of the mean HI volume filling factor in their halos. Our model suggests a characteristic radial extent of HVCs of R_halo=50 kpc, a total HI mass in HVCs of ~10^8 M_sun, and a neutral-gas-accretion rate of ~0.7 M_sun/yr for M31/Milky-Way-type galaxies. Using a Holmberg-like luminosity scaling of the halo size of galaxies we estimate R_halo=110 kpc for the most massive galaxies. The total absorption-cross section of HVCs at z=0 most likely is dominated by galaxies with total HI masses between 10^8.5 and 10^10 M_sun. Our model indicates that the HI disks of galaxies and their surrounding HVC population can account for 30-100 percent of intervening QSO absorption-line systems with log N(HI)>17.5 at z=0. We estimate that the neutral-gas accretion rate density of galaxies at low redshift from infalling HVCs is dM_HI/dt/dV=0.022 M_sun/yr/Mpc^3, which is close to the measured star-formation rate density in the local Universe. HVCs thus may play an important role for the on-going formation and evolution of galaxies.

NGC 4656UV: A UV-SELECTED TIDAL DWARF GALAXY CANDIDATE

We report the discovery of a UV-bright tidal dwarf galaxy candidate in the NGC 4631/4656 galaxy group, which we designate NGC 4656UV. Using survey and archival data spanning from 1.4 GHz to the ultraviolet we investigate the gas kinematics and stellar properties of this system. The HI morphologies of NGC 4656UV and its parent galaxy NGC 4656 are extremely disturbed, with significant amounts of counterrotating and extraplanar gas. From UV-FIR photometry, computed using a new method to correct for surface gradients on faint objects, we find that NGC 4656UV has no significant dust opacity and a blue spectral energy distribution. We compute a star formation rate of 0.027 M_sun yr^-1 from the FUV flux and measure a total HI mass of 3.8x10^8 M_sun for the object. Evolutionary synthesis modeling indicates that NGC 4656UV is a low metallicity system whose only major burst of star formation occurred within the last ~260-290 Myr. The age of the stellar population is consistent with a rough timescale for a recent tidal interaction between NGC 4656 and NGC 4631, although we discuss the true nature of the object--whether it is tidal or pre-existing in origin--in the context of its metallicity being a factor of ten lower than its parent galaxy. We estimate that NGC 4656UV is either marginally bound or unbound. If bound, it contains relatively low amounts of dark matter. The abundance of archival data allows for a deeper investigation into this dynamic system than is currently possible for most TDG candidates.

COSMOLOGICAL EVOLUTION OF ATOMIC GAS AND IMPLICATIONS FOR 21 cm H I ABSORPTION

Galaxy disks are shown to contain a significant population of atomic clouds of 100pc linear size which are self-opaque in the 21cm transition. These objects have HI column densities as high as 10^23 and contribute to a global opacity correction factor of 1.34+/-0.05 that applies to the integrated 21cm emission to obtain a total HI mass estimate. Opacity-corrected images of the nearest external galaxies have been used to form a robust z=0 distribution function of HI, f(N_HI,X,z=0), the probability of encountering a specific HI column density per unit comoving distance. This is contrasted with previously published determinations of f(N_HI,X) at z=1 and 3. A systematic decline of moderate column density (18<log(N_HI)<21) HI is observed that corresponds to a decline in surface area of such gas by a factor of five since z=3. The number of equivalent DLA absorbers (log(N_HI)>20.3) has also declined systematically over this redshift interval by a similar amount, while the cosmological mass density in such systems has declined by only a factor of two to its current, opacity corrected value of Omega_HI^DLA(z=0) = 5.4 +/- 0.9x10^-4. We utilize the tight, but strongly non-linear dependence of 21cm absorption opacity on column density at z=0 to transform our HI images into ones of 21cm absorption opacity. These images are used to calculate distribution and pathlength functions of integrated 21cm opacity. The incidence of deep 21cm absorption systems is predicted to show very little evolution with redshift, while that of faint absorbers should decline by a factor of five between z=3 and the present. We explicitly consider the effects of HI absorption against background sources that are extended relative to the 100pc intervening absorber size scale. Future surveys of 21cm absorption will require very high angular resolution, of about 15mas, for their unambiguous interpretation. (Abridged.)

THEGALEXARECIBO SDSS SURVEY. V. THE RELATION BETWEEN THE H I CONTENT OF GALAXIES AND METAL ENRICHMENT AT THEIR OUTSKIRTS

We have obtained long-slit spectra of 174 star-forming galaxies with stellar masses greater than 10^10 M_\odot from the GALEX Arecibo SDSS (GASS) survey. These galaxies have both HI and H_2 mass measurements. The average metallicity profile is strikingly flat out to R_90, the radius enclosing 90% of the r-band light. Metallicity profiles which decline steadily with radius are found primarily for galaxies in our sample with low stellar mass (Log(M_*)<10.2), concentration, and/or mean stellar mass density. Beyond ~R_90, however, around 10 percent of the galaxies in our sample exhibit a sharp downturn in metallicity. Remarkably, we find that the magnitude of the outer metallicity drop is well correlated with the total HI content of the galaxy (measured as f_HI=M_HI/M_*). We examine the radial profiles of stellar population ages and star formation rate densities, and conclude that the galaxies with largest outer metallicity drops are actively growing their stellar disks, with mass doubling times across the whole disk only one third as long as a typical GASS galaxy. We also describe a correlation between local stellar mass density and metallicity, which is valid across all galaxies in our sample. We argue that much of the recent stellar mass growth at the edges of these galaxies can be linked to the accretion or radial transport of relatively pristine gas from beyond the galaxies' stellar disks.

NEW EVIDENCE FOR MASS LOSS FROM δ CEPHEI FROM H I 21 cm LINE OBSERVATIONS

Recently published Spitzer observations of the classical Cepheid archetype delta Cephei revealed an extended dusty nebula surrounding this star and its hot companion. The infrared emission resembles a bow shock aligned with the direction of space motion of the star, indicating that delta Cep is undergoing mass-loss through a stellar wind. Here we report HI 21-cm line observations with the VLA to search for neutral atomic hydrogen associated with this wind. Our VLA data reveal a spatially extended HI nebula (~13' or 1 pc across) surrounding the position of delta Cep. The nebula has a head-tail morphology, consistent with circumstellar ejecta shaped by the interaction between a stellar wind and the ISM. We directly measure a mass of circumstellar hydrogen M_HI\approx0.07M_odot, although the total HI mass may be larger. The HI data imply a stellar wind with an outflow velocity V_o=35.6\pm1.2 km/s and a mass-loss rate of M_dot=(1.0\pm0.8)x10**-6 M_dot/yr. We have computed theoretical evolutionary tracks that include mass loss across the instability strip and show that a mass-loss rate of this magnitude, sustained over the preceding Cepheid lifetime of delta Cep, could be sufficient to resolve a significant fraction of the discrepancy between the pulsation and evolutionary masses for this star. (abridged)

THE H I MASS DENSITY IN GALACTIC HALOS, WINDS, AND COLD ACCRETION AS TRACED BY Mg II ABSORPTION

It is well established that MgII absorption lines detected in background quasar spectra arise from gas structures associated with foreground galaxies. The degree to which galaxy evolution is driven by the gas cycling through halos is highly uncertain because their gas mass density is poorly constrained. Fitting the MgII equivalent width (W) distribution with a Schechter function and applying the N(HI)-W correlation of Menard & Chelouche, we computed Omega(HI)_MgII ~ Omega(HI)_halo =(1.41 +0.75 -0.44)x10^-4 for 0.4<z<1.4. We exclude DLAs from our calculations so that Omega(HI)_halo comprises accreting and/or outflowing halo gas not locked up in cold neutral clouds. We deduce the cosmic HI gas mass density fraction in galactic halos traced by MgII absorption is Omega(HI)_halo/Omega(HI)_DLA=15% and Omega(HI)_halo/Omega_b=0.3%. Citing several lines of evidence, we propose infall/accretion material is sampled by small W whereas outflow/winds are sampled by large W, and find Omega(HI)_infall is consistent with Omega(HI)_outflow for bifurcation at W=1.23^{+0.15}_{-0.28}\AA; cold accretion would then comprise no more than ~7% of of the total HI mass density. We discuss evidence that (1) the total HI mass cycling through halos remains fairly constant with cosmic time and that the accretion of HI gas sustains galaxy winds, and (2) evolution in the cosmic star formation rate depends primarily on the rate at which cool HI gas cycles through halos.

THE M81 GROUP DWARF IRREGULAR GALAXY DDO 165. I. HIGH-VELOCITY NEUTRAL GAS IN A POST-STARBURST SYSTEM

We present new multi-configuration VLA HI spectral line observations of the M81 group dIrr post-starburst galaxy DDO 165. The HI morphology is complex, with multiple column density peaks surrounding a large region of very low HI surface density that is offset from the center of the stellar distribution. The bulk of the neutral gas is associated with the southern section of the galaxy; a secondary peak in the north contains ~15% of the total HI mass. These components appear to be kinematically distinct, suggesting that either tidal processes or large-scale blowout have recently shaped the ISM of DDO 165. Using spatially-resolved position-velocity maps, we find multiple localized high-velocity gas features. Cross-correlating with radius-velocity analyses, we identify eight shell/hole structures in the ISM with a range of sizes (~400-900 pc) and expansion velocities (~7-11 km/s). These structures are compared with narrow- and broad-band imaging from KPNO and HST. Using the latter data, recent works have shown that DDO 165's previous "burst" phase was extended temporally (>1 Gyr). We thus interpret the high-velocity gas features, HI holes, and kinematically distinct components of the galaxy in the context of the immediate effects of "feedback" from recent star formation. In addition to creating HI holes and shells, extended star formation events are capable of creating localized high velocity motion of the surrounding interstellar material. A companion paper connects the energetics from the HI and HST data.

Detecting neutral hydrogen in emission at redshift z ≃ 1

We use a large N-body simulation to examine the detectability of HI in emission at redshift z ~ 1, and the constraints imposed by current observations on the neutral hydrogen mass function of galaxies at this epoch. We consider three different models for populating dark matter halos with HI, designed to encompass uncertainties at this redshift. These models are consistent with recent observations of the detection of HI in emission at z ~ 0.8. Whilst detection of 21 cm emission from individual halos requires extremely long integrations with existing radio interferometers, such as the Giant Meter Radio Telescope (GMRT), we show that the stacked 21 cm signal from a large number of halos can be easily detected. However, the stacking procedure requires accurate redshifts of galaxies. We show that radio observations of the field of the DEEP2 spectroscopic galaxy redshift survey should allow detection of the HI mass function at the 5-12 sigma level in the mass range 10^(11.4) M_sun/h < M_halo < 10^(12.5)M_sun/h, with a moderate amount of observation time. Assuming a larger noise level that corresponds to an upper bound for the expected noise for the GMRT, the detection significance for the HI mass function is still at the 1.7-3 sigma level. We find that optically undetected satellite galaxies enhance the HI emission profile of the parent halo, leading to broader wings as well as a higher peak signal in the stacked profile of a large number of halos. We show that it is in principle possible to discern the contribution of undetected satellites to the total HI signal, even though cosmic variance limitation make this challenging for some of our models.

CAUGHT IN THE ACT: STRONG, ACTIVE RAM PRESSURE STRIPPING IN VIRGO CLUSTER SPIRAL NGC 4330

We present a multi-wavelength study of NGC 4330, a highly-inclined spiral galaxy in the Virgo Cluster which is a clear example of strong, ongoing ICM-ISM ram pressure stripping. The HI has been removed from well within the undisturbed old stellar disk, to 50% - 65% of R_25. Multi-wavelength data (WIYN BVR and H-alpha, VLA 21-cm HI and radio continuum, and GALEX NUV and FUV) reveal several one-sided extraplanar features likely caused by ram pressure at an intermediate disk-wind angle. At the leading edge of the interaction, the H-alpha and dust extinction curve sharply out of the disk in a remarkable and distinctive "upturn" feature that may be generally useful as a diagnostic indicator of active ram pressure. On the trailing side, the ISM is stretched out in a long tail which contains 10% of the galaxy's total HI emission, 6 - 9% of its NUV-FUV emission, but only 2% of the H-alpha. The centroid of the HI tail is downwind of the UV/H-alpha tail, suggesting that the ICM wind has shifted most of the ISM downwind over the course of the past 10 - 300 Myr. Along the major axis, the disk is highly asymmetric in the UV, but more symmetric in H-alpha and HI, also implying recent changes in the distributions of gas and star formation. The UV-optical colors indicate very different star formation histories for the leading and trailing sides of the galaxy. On the leading side, a strong gradient in the UV-optical colors of the gas-stripped disk suggests that it has taken 200-400 Myr to strip the gas from a radius of >8 to 5 kpc, but on the trailing side there is no age gradient. All our data suggest a scenario in which NGC 4330 is falling into cluster center for first time and has experienced a significant increase in ram pressure over the last 200-400 Myr.

ERRATUM: “DETECTION OF DIFFUSE NEUTRAL INTRAGROUP MEDIUM IN HICKSON COMPACT GROUPS” (2010, ApJ, 710, 385)

We present new Green Bank Telescope (GBT) 21 cm neutral hydrogen (HI) observations of a complete distance limited sample of 22 Hickson Compact Groups (HCGs) with at least four true members. We detected an average HI mass of $8 \times 10^{9} M_{\odot}$ (median= $6\times 10^{9} M_{\odot}$), which is significantly larger than previous single-dish measurements. Consequently, the HI-deficiencies for these HCGs have been reduced, although not completely eliminated. Spectral comparison of the GBT data with complementary Very Large Array (VLA) data shows significant HI excess in the GBT spectra. The observed excess is primarily due to the high surface brightness sensitivity of the GBT detecting diffuse, low column density HI in these groups. The excess gas forms a faint diffused neutral medium which is an intermediate stage in the evolution of high-surface brightness HI tidal debris in the intra-group medium (IGM) before it is fully ionized. The excess gas mass fraction (ratio of excess HI to total HI) for our complete sample varies from 5% to 81% with an average of 36% (median=30%). The excess gas mass fraction is highest in slightly HI deficient groups where the tidal debris has had enough time to evolve. We also find the excess gas content increases with the evolutionary phase of the group described in Verdes-Montenegro et al. 2001. Theoretical calculations indicate that an HI cloud of radius >200 pc would survive in an IGM of 2 million Kelvin for more than the typical dynamical lifetime of a group. However, smaller clouds get evaporated and assimilated into the hot IGM in a much shorter timescale.

Modelling the H i halo of the Milky Way

Aims: we studied the global distribution and kinematics of the extra-planar neutral gas in the Milky Way. Methods: we built 3D models for a series of Galactic HI layers, projected them for an inside view, and compared them with the Leiden-Argentina-Bonn 21-cm observations. Results: we show that the Milky Way disk is surrounded by an extended halo of neutral gas with a vertical scale-height of 1.6[+0.6/-0.4] kpc and an HI mass of 3.2[+1.0/-0.9]x10^8 solar masses, which is 5-10% of the total Galactic HI. This HI halo rotates more slowly than the disk with a vertical velocity gradient of -15[+/-4] km/s/kpc. We found evidence for a global infall motion, both vertical (20[+5/-7] km/s) and radial (30[+7/-5]km/s). Conclusions: the Milky Way HI halo shows properties similar to the halos of external galaxies and is compatible with being predominantly produced by supernova explosions in the disk. It is most likely composed of distinct gas complexes with masses of 10^4-10^5 solar masses of which the Intermediate Velocity Clouds are the local manifestations. The classical High Velocity Clouds appear to be a separate population.

Cold gas in massive early-type galaxies: the case of NGC 1167

We present a study of the morphology and kinematics of the neutral hydrogen in the gas-rich (M_HI=1.5x10^{10}Msun), massive early-type galaxy NGC 1167, which was observed with the Westerbork Synthesis Radio Telescope (WSRT). The HI is located in a 160kpc disk (~3xD_25) and has low surface density (<2Msun pc^{-2}). The disk shows regular rotation for r<65kpc but several signs of recent and ongoing interaction and merging with fairly massive companions are observed. No population of cold gas clouds is observed - in contrast to what is found in some spiral galaxies. This suggests that currently the main mechanism bringing in cold gas to the disk is the accretion of fairly massive satellite galaxies, rather than the accretion of a large number of small gas clumps. NGC 1167 is located in a (gas-) rich environment: we detect eight companions with a total HI mass of ~6x10^9Msun within a projected distance of 350kpc. Deep optical images show a disrupted satellite at the northern edge of the HI disk. The observed rotation curve shows a prominent bump of about 50km/s (in the plane of the disk) at r=1.3xR_25. This feature in the rotation curve occurs at the radius where the HI surface density drops significantly and may be due to large-scale streaming motions in the disk. We suspect that both the streaming motions and the HI density distribution are the result of the interaction/accretion with the disrupted satellite. Like in other galaxies with wiggles and bumps in the rotation curve, HI scaling describes the observed rotation curve best. We suggest that interactions create streaming motions and features in the HI density distribution and that this is the reason for the success of HI scaling in fitting such rotation curves.

The GALEX Arecibo SDSS Survey - II. The star formation efficiency of massive galaxies

We use measurements of the HI content, stellar mass and star formation rates in ~190 massive galaxies with stellar masses greater than 10^10 Msun, obtained from the Galex Arecibo SDSS Survey (GASS) described in Paper I (Catinella et al. 2010) to explore the global scaling relations associated with the bin-averaged ratio of the star formation rate over the HI mass, which we call the HI-based star formation efficiency (SFE). Unlike the mean specific star formation rate, which decreases with stellar mass and stellar mass surface density, the star formation efficiency remains relatively constant across the sample with a value close to SFE = 10^-9.5 yr^-1 (or an equivalent gas consumption timescale of ~3 Gyr). Specifically, we find little variation in SFE with stellar mass, stellar mass surface density, NUV-r color and concentration. We interpret these results as an indication that external processes or feedback mechanisms that control the gas supply are important for regulating star formation in massive galaxies. An investigation into the detailed distribution of SFEs reveals that approximately 5% of the sample shows high efficiencies with SFE > 10^-9 yr^-1, and we suggest that this is very likely due to a deficiency of cold gas rather than an excess star formation rate. Conversely, we also find a similar fraction of galaxies that appear to be gas-rich for their given specific star-formation rate, although these galaxies show both a higher than average gas fraction and lower than average specific star formation rate. Both of these populations are plausible candidates for "transition" galaxies, showing potential for a change (either decrease or increase) in their specific star formation rate in the near future. We also find that 36+/-5% of the total HI mass density and 47+/-5% of the total SFR density is found in galaxies with stellar mass greater than 10^10 Msun. [abridged]

THE DISCOVERY OF HOST GALAXY H I ABSORPTION IN CTA 21

We report the discovery of HI 21-cm absorption towards the well-studied GHz Peaked-Spectrum source CTA 21 (4C 16.09) using the Arecibo Telescope on 2009 September 20 and 21. Recently, the frequency band between 700 and 800 MHz was temporarily opened up to radio astronomy when US TV stations were mandated to switch from analog to digital transmissions, with new frequency allocations. The redshifted HI frequency for CTA 21 falls within this band. CTA 21 has a complex radio structure on a range of scales. The innermost prominent components are separated by ~12 mas while weak diffuse emission extends for up to ~300 mas. The HI absorption profile that we find has two main components, one narrow, the other wider and blue-shifted. The total HI column density is 7.9 x 10^20 cm^-2, assuming a covering factor of unity and a spin temperature of 100 K. This HI absorption confirms the recently determined optical redshift of this faint galaxy of z ~ 0.907. We discuss this new detection in the light of HI absorption studies towards compact radio sources, and also the possibility that CTA 21 may be exhibiting multiple cycles of nuclear activity. This new detection in CTA 21 is consistent with a strong trend for detection of HI absorption in radio galaxies with evidence of episodic nuclear/jet activity.