We revisit the Hi size-mass (D HI -M HI ) relation of galaxies with a sample of more than 500 nearby galaxies covering over five orders of magnitude in Hi mass and more than ten B-band magnitudes. The relation is remarkably tight with a scatter σ ∼0.06 dex, or 14%. The scatter does not change as a function of galaxy luminosity, Hi richness or morphological type. The relation is linked to the fact that dwarf and spiral galaxies have a homogenous radial profile of Hi surface density in the outer regions when the radius is normalised by D HI . The early-type disk galaxies typically have shallower Hi radial profiles, indicating a different gas accretion history. We argue that the process of atomic-to-molecular gas conversion or star formation cannot explain the tightness of the D HI -M HI relation. This simple relation puts strong constraints on simulation models for galaxy formation.

The Faint Irregular Galaxies GMRT Survey (FIGGS) is a Giant Metrewave Radio Telescope (GMRT) based H I imaging survey of a systematically selected sample of extremely faint nearby dwarf irregular galaxies. The primary goal of FIGGS is to provide a comprehensive and statistically robust characterization of the neutral interstellar medium properties of faint, gas-rich dwarf galaxies. The FIGGS galaxies represent the extremely low mass end of the dwarf irregular galaxies population, with a median M B ∼ −13.0 and median H I mass of ∼3 × 10 7 M , extending the baseline in mass and luminosity space for a comparative study of galaxy properties. The H I data are supplemented with observations at other wavelengths. In addition, distances accurate to ∼10 per cent are available for most of the sample galaxies. This paper gives an introduction to FIGGS, describes the GMRT observations and presents the first results from the H I observations. From the FIGGS data, we confirm the trend of increasing H I to optical diameter ratio with decreasing optical luminosity; the median ratio of D H I /D Ho for the FIGGS sample is 2.4. Further, on comparing our data with aperture synthesis surveys of bright spirals, we find at best marginal evidence for a decrease in average surface density with decreasing H I mass. To a good approximation, the discs of gas-rich galaxies, ranging over three orders of magnitude in H I mass, can be described as being drawn from a family with fixed H I average surface density.

Abstract. We present deep GMRT 21-cm absorption spectra of 10 damped Lyman-α systems (DLAs), of which 8 are at redshifts z > ∼ 1.3. HI absorption was detected in only one DLA, the z = 0.5318 absorber toward PKS 1629+12. This absorber has been identified with a luminous spiral galaxy; the spin temperature limit (T s ≤ 310 K) derived from our observations continues the trend of DLAs associated with bright spirals having low spin temperatures. In seven of the remaining 9 systems, the observations place strong lower limits on the spin temperature of the HI gas. We combine this sample with data taken from the literature to study the properties of all known DLAs with 21-cm absorption studies. The sample of DLAs which have been searched for 21-cm absorption now consists of 31 systems, with T s estimates available in 24 cases; of these, 16 are at z < 2 and 8 at z > 2, with 11 (all at z < 1) having optical identifications. For the latter 11 DLAs, we find that all of the low T s DLAs have been identified with large, luminous galaxies, while all the DLAs with high spin temperature (T s > ∼ 1000 K) have been identified either with LSBs or dwarfs. Further, we find no correlation between impact parameter and spin temperature; it is thus unlikely that the high measured T s values for DLAs arise from lines of sight passing through the outskirts of large disk galaxies. Instead, the spin temperature of DLAs appears to correlate with the host galaxy type. The trend (noted earlier by Chengalur & Kanekar 2000) that low z DLAs exhibit both high and low T s values while high redshift (z > ∼ 3) DLAs only show high spin temperatures is present in this expanded data set. Based on this difference in spin temperatures, the Gehan test rules out the hypothesis that DLAs at z > 2 and DLAs at z < 2 are drawn from the same parent population at ∼99% confidence level. Finally, we use the new GMRT spectra along with 2 spectra from the literature to estimate upper limits on the fraction of cold HI, f CNM , in DLAs at z > ∼ 3. For local spirals, f CNM ∼ 0.5; in contrast, we find that f CNM < 0.3 in all 7 high z absorbers, and f CNM < 0.1 in 5 of the 7 cases.

We use observations from the Giant Metrewave Radio Telescope (GMRT) to measure the atomic hydrogen gas content of star‐forming galaxies at z= 0.24 (i.e. a look‐back time of ∼3 Gyr). The sample of galaxies studied were selected from Hα‐emitting field galaxies detected in a narrow‐band imaging survey with the Subaru Telescope. The Anglo‐Australian Telescope was used to obtain precise optical redshifts for these galaxies. We then co‐added the H i 21‐cm emission signal for all the galaxies within the GMRT spectral line data cube. From the co‐added signal of 121 galaxies, we measure an average atomic hydrogen gas mass of (2.26 ± 0.90) × 109  M⊙. We translate this H i signal into a cosmic density of neutral gas at z= 0.24 of Ωgas= (0.91 ± 0.42) × 10−3. This is the current highest redshift at which Ωgas has been constrained from 21‐cm emission and our value is consistent with that estimated from damped Lyα systems around this redshift. We also find that the correlations between the Hα luminosity and the radio continuum luminosity and between the star formation rate (SFR) and the H i gas content in star‐forming galaxies at z= 0.24 are consistent with the correlations found at z= 0. These two results suggest that the star formation mechanisms in field galaxies ∼3 Gyr ago were not substantially different from the present, even though the SFR is three times higher.

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