ALBERT

Description: The Arecibo Ultra-Deep Survey (AUDS) combines the unique sensitivity of the telescope with the wide field of the Arecibo L -band Feed Array (ALFA) to directly detect 21 cm ${\rm H\,{\small i}}$ emission from galaxies at distances beyond the local Universe bounded by the lower frequency limit of ALFA ( z  = 0.16). AUDS has collected 700 h of integration time in two fields with a combined area of 1.35 deg 2 . In this paper, we present data from 60 per cent of the total survey, corresponding to a sensitivity level of 80 μJy. We discuss the data reduction, the search for galaxies, parametrization, optical identification and completeness. We detect 102 galaxies in the mass range of $\log (M_{{\rm H\,{\small {I}}}}/M)-2\log h=5.6{\rm -}10.3$ . We compute the ${\rm H\,{\small i}}$ mass function (HIMF) at the highest redshifts so far measured. A fit of a Schechter function results in α = – 1.37 ± 0.03, * = (7.72 ± 1.4) x 10 –3 h 3 Mpc –3 and $\log \,(M_{\rm H\,{\small i}}^{*}/{\rm M}_{{\odot }})=({9.75\pm 0.041})+2\log h$ . Using the measured HIMF, we find a cosmic ${\rm H\,{\small i}}$ density of $\Omega _{\rm H\,{\small i}}=({2.33\pm 0.07})\times 10^{-4}\, h^{-1}$ for the sample ( z  = 0.065). We discuss further uncertainties arising from cosmic variance. Because of its depth, AUDS is the first survey that can determine parameters for the HIMF in independent redshift bins from a single homogeneous data set. The results indicate little evolution of the comoving mass function and $\Omega _{\rm H\,{\small i}}$ within this redshift range. We calculate a weighted average for $\Omega _{\rm H\,{\small i}}$ in the range 0 〈  z  〈 0.2, combining the results from AUDS as well as results from other 21 cm surveys and stacking, finding a best combined estimate of $\Omega _{\rm H\,{\small i}}=( 2.63\pm 0.10)\times 10^{-4}\, h^{-1}$ .

Description: We present a statistical method for measuring the average H  i spin temperature in distant galaxies using the expected detection yields from future wide-field 21 cm absorption surveys. As a demonstrative case study, we consider an all-southern-sky simulated survey of 2-h per pointing with the Australian Square Kilometre Array Pathfinder for intervening H  i absorbers at intermediate cosmological redshifts between z  = 0.4 and 1. For example, if such a survey yielded 1000 absorbers, we would infer a harmonic-mean spin temperature of $\overline{T}_\mathrm{spin} \sim 100$  K for the population of damped Lyman α absorbers (DLAs) at these redshifts, indicating that more than 50 per cent of the neutral gas in these systems is in a cold neutral medium (CNM). Conversely, a lower yield of only 100 detections would imply $\overline{T}_\mathrm{spin} \sim 1000$  K and a CNM fraction less than 10 per cent. We propose that this method can be used to provide independent verification of the spin temperature evolution reported in recent 21 cm surveys of known DLAs at high redshift and for measuring the spin temperature at intermediate redshifts below z 1.7, where the Lyman α line is inaccessible using ground-based observatories. Increasingly more sensitive and larger surveys with the Square Kilometre Array should provide stronger statistical constraints on the average spin temperature. However, these will ultimately be limited by the accuracy to which we can determine the H  i column density frequency distribution, the covering factor and the redshift distribution of the background radio source population.

Description: We present global VLBI observations of the 21-cm transition of atomic hydrogen seen in absorption against the radio source J0855+5751. The foreground absorber (SDSS J085519.05+575140.7) is a dwarf galaxy at z  = 0.026. As the background source is heavily resolved by VLBI, the data allow us to map the properties of the foreground H i gas with a spatial resolution of 2 pc. The absorbing gas corresponds to a single coherent structure with an extent 〉35 pc, but we also detect significant and coherent variations, including a change in the H i optical depth by a factor of 5 across a distance of 6 pc. The large size of the structure provides support for the Heiles & Troland model of the interstellar medium, as well as its applicability to external galaxies. The large variations in H i optical depth also suggest that caution should be applied when interpreting T S measurements from radio-detected DLAs. In addition, the distorted appearance of the background radio source is indicative of a strong jet–cloud interaction in its host galaxy. We have measured its redshift ( z  = 0.541 86) using optical spectroscopy on the William Herschel Telescope and this confirms that J0855+5751 is an FR II radio source with a physical extent of 〈1 kpc and supports the previous identification of this source as a compact symmetric object. These sources often show absorption associated with the host galaxy and we suggest that both H i and OH should be searched for in J0855+5751.

Description: We examine the H i -based star formation efficiency ( ${\rm SFE}_{{\rm H\,\small {I}}}$ ), the ratio of star formation rate to the atomic hydrogen (H i ) mass, in the context of a constant stability star-forming disc model. Our observations of H i -selected galaxies show ${\rm SFE}_{{\rm H\,\small {I}}}$ to be fairly constant (log ${\rm SFE}_{{\rm H\,\small {I}}}=-9.65$  yr –1 with a dispersion of 0.3 dex) across ~5 orders of magnitude in stellar masses. We present a model to account for this result, whose main principle is that the gas within galaxies forms a uniform stability disc and that stars form within the molecular gas in this disc. We test two versions of the model differing in the prescription that determines the molecular gas fraction, based on either the hydrostatic pressure or the stellar surface density of the disc. For high-mass galaxies such as the Milky Way, we find that either prescription predicts ${\rm SFE}_{{\rm H\,\small {I}}}$ similar to the observations. However, the hydrostatic pressure prescription is a more accurate ${\rm SFE}_{{\rm H\,\small {I}}}$ predictor for low-mass galaxies. Our model is the first model that links the uniform ${\rm SFE}_{{\rm H\,\small {I}}}$ observed in galaxies at low redshifts to star-forming discs with constant marginal stability. While the rotational amplitude V max is the primary driver of disc structure in our model, we find that the specific angular momentum of the galaxy may play a role in explaining a weak correlation between ${\rm SFE}_{{\rm H\,\small {I}}}$ and effective surface brightness of the disc.

Description: We study the origin of the wide distribution of angles between the angular momenta of the stellar and gas components, α G, S , in early-type galaxies (ETGs). We use the GALFORM model of galaxy formation, set in the cold dark matter framework, and coupled it with a Monte Carlo simulation to follow the angular momenta flips driven by matter accretion on to haloes and galaxies. We consider a gas disc to be misaligned with respect to the stellar body if α G,S  〉 30 deg. By assuming that the only sources of misalignments in galaxies are galaxy mergers, we place a lower limit of 2–5 per cent on the fraction of ETGs with misaligned gas/stellar components. These low fractions are inconsistent with the observed value of 42 ± 6 per cent in ATLAS 3D . In the more general case, in which smooth gas accretion in addition to galaxy mergers can drive misalignments, our calculation predicts that 46 per cent of ETGs have α G, S  〉 30 deg. In this calculation, we find correlations between α G, S and stellar mass, cold gas fraction and star formation rate, such that ETGs with high masses, low cold gas fractions and low star formation rates are more likely to display aligned cold gas and stellar components. We confirm these trends observationally for the first time using ATLAS 3D data. We argue that the high fraction of misaligned gas discs observed in ETGs is mostly due to smooth gas accretion (e.g. cooling from the hot halo of galaxies) which takes place after most of the stellar mass of the galaxy is in place and comes misaligned with respect to the stellar component. Galaxies that have accreted most of their cold gas content prior to the time where most of the stellar mass was in place show aligned components.

Description: We report the detection and mapping of atomic hydrogen in H  i 21 cm emission from ESO 184-G82, the host galaxy of the gamma-ray burst 980425. This is the first instance where H  i in emission has been detected from a galaxy hosting a gamma-ray burst (GRB). ESO 184-G82 is an isolated galaxy and contains a Wolf–Rayet region close to the location of the GRB and the associated supernova, SN 1998bw. This is one of the most luminous H  ii regions identified in the local Universe, with a very high inferred density of star formation. The H  i 21 cm observations reveal a high H  i mass for the galaxy, twice as large as the stellar mass. The spatial and velocity distribution of the H  i 21 cm emission reveals a disturbed rotating gas disc, which suggests that the galaxy has undergone a recent minor merger that disrupted its rotation. We find that the Wolf–Rayet region and the GRB are both located in the highest H  i column density region of the galaxy. We speculate that the merger event has resulted in shock compression of the gas, triggering extreme star formation activity, and resulting in the formation of both the Wolf–Rayet region and the GRB. The high H  i column density environment of the GRB is consistent with the high H  i column densities seen in absorption in the host galaxies of high-redshift GRBs.

Description: We measure the neutral atomic hydrogen (H i ) gas content of field galaxies at intermediate redshifts of z  ~ 0.1 and ~0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope. In order to make high signal-to-noise ratio detections, an H i signal stacking technique is applied: H i emission spectra from multiple galaxies, optically selected by the second Canadian Network for Observational Cosmology redshift survey project, are co-added to measure the average H i mass of galaxies in the two redshift bins. We calculate the cosmic H i gas densities ( H i ) at the two redshift regimes and compare those with measurements at other redshifts to investigate the global evolution of the H i gas density over cosmic time. From a total of 59 galaxies at z  ~ 0.1 we find H i  = (0.33 ± 0.05) 10 –3 , and at z  ~ 0.2 we find H i  = (0.34 ± 0.09) 10 –3 , based on 96 galaxies. These measurements help bridge the gap between high- z damped Lyman α observations and blind 21-cm surveys at z  = 0. We find that our measurements of H i at z  ~ 0.1 and ~0.2 are consistent with the H i gas density at z  ~ 0 and that all measurements of H i from 21-cm emission observations at z 0.2 are in agreement with no evolution of the H i gas content in galaxies during the last 2.4 Gyr.

Description: We study the contribution of galaxies with different properties to the global densities of star formation rate (SFR), atomic (H i ) and molecular hydrogen (H 2 ) as a function of redshift. We use the galform model of galaxy formation, which is set in the cold dark matter (CDM) framework. This model includes a self-consistent calculation of the SFR, which depends on the H 2 content of galaxies. The predicted SFR density and how much of this is contributed by galaxies with different stellar masses and infrared luminosities are in agreement with observations. The model predicts a modest evolution of the H i density at z  〈 3, which is also in agreement with the observations. The H i density is predicted to be always dominated by galaxies with SFR 〈 1 M  yr –1 . This contrasts with the H 2 density, which is predicted to be dominated by galaxies with SFR 〉10 M  yr –1 at z  〉 1. Current high-redshift galaxy surveys are limited to detect carbon monoxide in galaxies with SFR 30 M  yr –1 , which in our model make up, at most, 20 per cent of the H 2 in the universe. In terms of stellar mass, the predicted H 2 density is dominated by massive galaxies, M stellar  〉 10 10 M , while the H i density is dominated by low-mass galaxies, M stellar  〈 10 9 M . In the context of upcoming neutral gas surveys, we suggest that the faint nature of the galaxies dominating the H i content of the Universe will hamper the identification of optical counterparts, while for H 2 , we expect follow-up observations of molecular emission lines of already existing galaxy catalogues to be able to uncover the H 2 density of the Universe.

Description: We develop a model for the distribution of the interstellar medium (ISM) and star formation in galaxies based on recent studies that indicate that galactic discs stabilize to a constant stability parameter, which we combine with prescriptions of how the phases of the ISM are determined and for the star formation law (SFL). The model predicts the gas surface mass density and star formation intensity of a galaxy given its rotation curve, stellar surface mass density and the gas velocity dispersion. This model is tested on radial profiles of neutral and molecular ISM surface mass density and star formation intensity of 12 galaxies selected from the H i Nearby Galaxy Survey sample. Our tests focus on intermediate radii (0.3 to 1 times the optical radius) because there are insufficient data to test the outer discs and the fits are less accurate in detail in the centre. Nevertheless, the model produces reasonable agreement with the ISM mass and star formation rate integrated over the central region in all but one case. To optimize the model, we evaluate four recipes for the stability parameter, three recipes for apportioning the ISM into molecular and neutral components, and eight versions of the SFL. We find no clear-cut best prescription for the two-fluid (gas and stars) stability parameter Q 2f and therefore for simplicity, we use the Wang and Silk approximation ( Q WS ). We found that an empirical scaling between the molecular-to-neutral ISM ratio ( R mol ) and the stellar surface mass density proposed by Leroy et al. works marginally better than the other two prescriptions for this ratio in predicting the ISM profiles, and noticeably better in predicting the star formation intensity from the ISM profiles produced by our model with the SFLs we tested. Thus, in the context of our modelled ISM profiles, the linear molecular SFL and the two-component SFL work better than the other prescriptions we tested. We incorporate these relations into our ‘constant Q disc’ model.

Description: We study the atomic (H i ) and molecular hydrogen (H 2 ) contents of early-type galaxies (ETGs) and their gas sources using the galform model of galaxy formation. This model uses a self-consistent calculation of the star formation rate, which depends on the H 2 content of galaxies. We first present a new analysis of H i Parkes All-Sky Survey and ATLAS 3D surveys, with special emphasis on ETGs. The model predicts H i and H 2 contents of ETGs in agreement with the observations from these surveys only if partial ram pressure stripping of the hot gas is included, showing that observations of neutral gas in ‘quenched’ galaxies place stringent constraints on the treatment of the hot gas in satellites. We find that 90 per cent of ETGs at z = 0 have neutral gas contents supplied by radiative cooling from their hot haloes, 8 per cent were supplied by gas accretion from minor mergers that took place in the last 1 Gyr, while 2 per cent were supplied by mass-loss from old stars. The model predicts neutral gas fractions strongly decreasing with increasing bulge fraction. This is due to the impeded disc regeneration in ETGs, resulting from both active galactic nuclei feedback and environmental quenching by partial ram pressure stripping of the hot gas.