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We present the first estimate, based on direct HI 21 cm observations, of the HI mass function (HIMF) of star-forming galaxies at $z\approx1$, obtained by combining our measurement of the scaling relation between HI mass ($M_{HI}$) and B-band luminosity ($M_B$) of star-forming galaxies with literature estimates of the B-band luminosity function at $z\approx1$. We determined the $M_{HI}-M_B$ relation by using the GMRT-CATz1 survey of the DEEP2 fields to measure the average HI mass of blue galaxies at $z=0.74-1.45$ in three separate $M_B$ subsamples. This was done by separately stacking the HI 21 cm emission signals of the galaxies in each subsample to detect, at (3.5-4.4)$\sigma$ significance, the average HI 21 cm emission of each subsample. We find that the $M_{HI}-M_B$ relation at $z\approx1$ is consistent with that at $z\approx0$. We combine our estimate of the $M_{HI}-M_B$ relation at $z\approx1$ with the B-band luminosity function at $z\approx1$ to determine the HIMF at $z\approx1$. We find that the number density of galaxies with $M_{HI}>10^{10} M_\odot$ (higher than the knee of the local HIMF) at $z\approx1$ is a factor of $\approx4-5$ higher than that at $z\approx0$, for a wide range of assumed scatters in the $M_{HI}-M_B$ relation. We rule out the hypothesis that the number density of galaxies with $M_{HI}>10^{10} M_\odot$ remains unchanged between $z \approx 1$ and $z\approx0$ at $\gtrsim99.7$\% confidence. This is the first statistically significant evidence for evolution in the HIMF of galaxies from the epoch of cosmic noon., Comment: 9 pages, 5 figures; minor changes to match version in press at ApJ Letters

We present here estimates of the average rates of accretion of neutral gas onto main-sequence galaxies and the conversion of atomic gas to molecular gas in these galaxies at two key epochs in galaxy evolution: (i) $z\approx1.3-1.0$, towards the end of the epoch of peak star-formation activity in the Universe, and (ii) $z\approx1-0$, when the star-formation activity declines by an order of magnitude. We determine the net gas accretion rate $\rm{R_{Acc}}$ and the molecular gas formation rate $\rm{R_{Mol}}$ by combining the relations between the stellar mass and the atomic gas mass, the molecular gas mass, and the star-formation rate (SFR) at three epochs, $z=1.3$, $z=1.0$, and $z=0$, with the assumption that galaxies evolve continuously on the star-forming main-sequence. We find that, for all galaxies, $\rm{R_{Acc}}$ is far lower than the average SFR $\rm{R_{SFR}}$ at $z\approx1.3-1.0$; however, $\rm{R_{Mol}}$ is similar to $\rm{R_{SFR}}$ during this interval. Conversely, both $\rm{R_{Mol}}$ and $\rm{R_{Acc}}$ are significantly lower than $\rm{R_{SFR}}$ over the later interval, $z\approx1-0$. We find that massive main-sequence galaxies had already acquired most of their present-day baryonic mass by $z\approx1.3$. At $z\approx1.3-1.0$, the rapid conversion of the existing atomic gas to molecular gas was sufficient to maintain a high average SFR, despite the low net gas accretion rate. However, at later times, the combination of the lower net gas accretion rate and the lower molecular gas formation rate leads to a decline in the fuel available for star-formation, and results in the observed decrease in the SFR density of the Universe over the last 8 Gyr., Comment: 8 pages, 3 figures; Accepted for publication in ApJ Letters

Star-forming galaxies are believed to replenish their atomic gas reservoir, which is consumed in star-formation, through accretion of gas from their circumgalactic mediums (CGMs). However, there are few observational constraints today on the gas accretion rate in external galaxies. Here, we use our recent measurement of the scaling relation between the atomic hydrogen (HI) mass $M_{HI}$ and the stellar mass $M_*$ in star-forming galaxies at $z \approx 0.35$, with the relations between the star-formation rate (SFR) and $M_*$, and the molecular gas mass $M_{Mol}$ and $M_*$, and the assumption that star-forming galaxies evolve along the main sequence, to determine the evolution of the neutral gas reservoir and the average net gas accretion rate onto the disks of star-forming galaxies over the past 4 Gyr. For galaxies with $M_* \gtrsim 10^9 M_{\odot}$ today, we find that both $M_*$ and $M_{HI}$ in the disk have increased, while $M_{Mol}$ has decreased, since $z \sim 0.35$. The average gas accretion rate onto the disk over the past 4 Gyr is similar to the average SFR over this period, implying that main-sequence galaxies have maintained a stable HI reservoir, despite the consumption of gas in star-formation. We obtain an average net gas accretion rate (over the past 4 Gyr) of $\approx 6 M_{\odot} yr^{-1}$ for galaxies with the stellar mass of the Milky Way. At low redshifts, $z \lesssim 0.4$, the reason for the decline in the cosmic SFR density thus appears to be the inefficiency in the conversion of atomic gas to molecular gas, rather than insufficient gas accretion from the CGM., Comment: Accepted for publication in The Astrophysical Journal Letters

The atomic hydrogen (HI) properties of star-forming galaxies in the local Universe are known to correlate with other galaxy properties via the ``HI scaling relations''. The redshift evolution of these relations serves as an important constraint on models of galaxy evolution. However, until recently, there were no estimates of the HI scaling relations at cosmological distances. Using data from a deep Giant Metrewave Radio Telescope HI 21 cm survey of the Extended Groth Strip, and the technique of spectral line stacking, we determine the scaling relation between the HI mass and the stellar mass for star-forming galaxies at $z\approx0.35$. We use this measurement, along with the main-sequence relation in galaxies, to infer the dependence of the HI depletion timescale of these galaxies on their stellar mass. We find that massive star-forming galaxies at $z\approx0.35$, with stellar mass $\rm M_* \gtrsim10^{9.5}\:M_{\odot}$, are HI-poor compared to local star-forming galaxies of a similar stellar mass. However, their characteristic HI depletion time is lower by a factor of $\approx 5$ than that of their local analogues, indicating a higher star-formation efficiency at intermediate redshifts (similar to that at $z \approx 1$). While our results are based on a relatively small cosmic volume and could thus be affected by cosmic variance, the short characteristic HI depletion timescales ($\lesssim 3$ Gyr) of massive star-forming galaxies at $z \approx 0.35$ indicate that they must have acquired a significant amount of neutral gas through accretion from the circumgalactic medium over the past four Gyr, to avoid quenching of their star-formation activity., Comment: Accepted for publication in the Astrophysical Journal Letters

The neutral atomic hydrogen (HI) mass function (HIMF) describes the distribution of the HI content of galaxies at any epoch; its evolution provides an important probe of models of galaxy formation and evolution. Here, we report Giant Metrewave Radio Telescope HI 21cm spectroscopy of blue star-forming galaxies at $z\approx0.20-0.42$ in the Extended Groth Strip, which has allowed us to determine the scaling relation between the average HI mass ($\rm{M_{HI}}$) and the absolute B-band magnitude ($\rm{M_B}$) of such galaxies at $z \approx 0.35$, by stacking the HI 21cm emission signals of galaxy subsamples in different $\rm{M_B}$ ranges. We combine this $\rm{M_{HI}-M_B}$ scaling relation (with a scatter assumed to be equal to that in the local Universe) with the known B-band luminosity function of star-forming galaxies at these redshifts to determine the HIMF at $z\approx0.35$. We show that the use of the correct scatter in the $\rm{M_{HI}-M_B}$ scaling relation is critical for an accurate estimate of the HIMF. We find that the HIMF has evolved significantly from $z\approx0.35$ to $z\approx0$, i.e. over the last four Gyr, especially at the high-mass end. High-mass galaxies, with $\rm{M_{HI}\gtrsim10^{10}\ M_\odot}$, are a factor of $\approx3.4$ less prevalent at $z\approx0.35$ than at $z \approx 0$. Conversely, there are more low-mass galaxies, with $\rm{M_{HI} \approx10^9\ {M}_\odot}$, at $z\approx0.35$ than in the local Universe. While our results may be affected by cosmic variance, we find that massive star-forming galaxies have acquired a significant amount of HI through merger events or accretion from the circumgalactic medium over the past four Gyr., Comment: Accepted for publication in The Astrophysical Journal Letters

We present a comparison between the average atomic gas mass, $\langle M_{Atom}\rangle$ (including HI and He), the average molecular gas mass, $\langle M_{Mol}\rangle$, and the average stellar mass, $\langle M_*\rangle$, of a sample of star-forming galaxies at $z\approx0.75-1.45$, to probe the baryonic composition of galaxies in and during the epoch of peak star-formation activity in the universe. The $\langle M_{Atom}\rangle$ values of star-forming galaxies in two stellar-mass matched samples at $z=0.74-1.25$ and $z=1.25-1.45$, were derived by stacking their HI 21cm signals in the GMRT-CAT$z1$ survey. We find that the baryonic composition of star-forming galaxies at $z\gtrsim 1$ is dramatically different from that at $z\approx0$. For star-forming galaxies with $\langle M_*\rangle\approx10^{10} M_\odot$, the contribution of stars to the total baryonic mass, $M_{Baryon}$, is $\approx61\%$ at $z\approx0$, but only $\approx16\%$ at $z\approx1.3$, while molecular gas constitutes $\approx6\%$ of the baryonic mass at $z\approx0$, and $\approx14\%$ at $z\approx1.3$. Remarkably, we find that atomic gas makes up $\approx70\%$ of $M_{Baryon}$ in star-forming galaxies at $z\approx1.3$. We find that the ratio $\langle M_{Atom}\rangle/\langle M_*\rangle$ is higher both at $z\approx1.0$ and at $z\approx1.3$ than in the local Universe, with $\langle M_{Atom}\rangle/\langle M_*\rangle\approx1.4$ at $z\approx1.0$, and $\approx4.4$ at $z\approx1.3$, compared to its value of $\approx0.5$ today. Further, we find that the ratio $\langle M_{Atom}\rangle/\langle M_{Mol}\rangle$ in star-forming galaxies with $\langle M_*\rangle \approx10^{10} M_\odot$ is $\approx2.3$ at $z\approx1.0$ and $\approx5.0$ at $z\approx1.3$. Overall, we find that atomic gas is the dominant component of the baryonic mass of star-forming galaxies at $z\approx1.3$, during the epoch of peak star-formation activity in the universe., Comment: 9 Pages, 4 Figures; Accepted for publication in ApJ Letters. Updated with minor changes to match version in press

We describe the design, data analysis, and basic results of the Giant Metrewave Radio Telescope Cold-HI AT $z\approx1$ (GMRT-CAT$z$1) survey, a 510-hour upgraded GMRT HI 21 cm emission survey of galaxies at $z=0.74-1.45$ in the DEEP2 survey fields. The GMRT-CAT$z$1 survey is aimed at characterising HI in galaxies during and just after the epoch of peak star-formation activity in the Universe, a key epoch in galaxy evolution. We obtained high-quality HI 21 cm spectra for 11,419 blue star-forming galaxies at $z=0.74-1.45$, in seven pointings on the DEEP2 subfields. We detect the stacked HI 21 cm emission signal of the 11,419 star-forming galaxies, which have an average stellar mass of $M_* \approx 10^{10} M_\odot$, at $7.1\sigma$ statistical significance, obtaining an average HI mass of $\langle M_{HI}\rangle=(13.7\pm1.9)\times10^{9} M_\odot$. This is significantly higher than the average HI mass of $\langle M_{HI} \rangle=(3.96 \pm 0.17)\times10^{9} M_\odot$ in star-forming galaxies at $z \approx 0$ with an identical stellar-mass distribution. We stack the rest-frame 1.4 GHz continuum emission of our 11,419 galaxies to infer an average star-formation rate (SFR) of $8.07\pm0.82 M_\odot yr^{-1}$. Combining our average HI mass and average SFR estimates yields an HI depletion timescale of $1.70\pm0.29$ Gyr, for star-forming galaxies at $z\approx1$, $\approx3$ times lower than that of local galaxies. We thus find that, although main-sequence galaxies at $z\approx1$ have a high HI mass, their short HI depletion timescale is likely to cause quenching of their star-formation activity in the absence of rapid gas accretion from the circumgalactic medium., Comment: 27 pages, 15 figures; The Astrophysical Journal, 937, 103; updated to match published version

Measurements of the atomic hydrogen (HI) properties of high-redshift galaxies are critical to understanding the decline in the star-formation rate (SFR) density of the Universe after its peak $\approx8-11$ Gyr ago. Here, we use $\approx510$ hours of observations with the upgraded Giant Metrewave Radio Telescope to measure the dependence of the average HI mass of star-forming galaxies at $z=0.74-1.45$ on their average stellar mass and redshift, by stacking their HI 21 cm emission signals. We divide our sample of 11,419 main-sequence galaxies at $z=0.74-1.45$ into two stellar-mass ($M_*$) subsamples, with $M_*>10^{10} M_\odot$ and $M_*<10^{10} M_\odot$, and obtain clear detections, at $>4.6\sigma$ significance, of the stacked HI 21 cm emission in both subsamples. We find that galaxies with $M_*>10^{10} M_\odot$, which dominate the decline in the cosmic SFR density at $z\lesssim1$, have HI reservoirs that can sustain their SFRs for only a short period, $0.86\pm0.20$ Gyr, unless their HI is replenished via accretion. We also stack the HI 21 cm emission from galaxies in two redshift subsamples, at $z=0.74-1.25$ and $z=1.25-1.45$, again obtaining clear detections of the stacked HI 21 cm emission signals, at $>5.2\sigma$ significance in both subsamples. We find that the average HI mass of galaxies with $\langle M_* \rangle\approx10^{10} M_\odot$ declines steeply over a period of $\approx1$ billion years, from $(33.6\pm6.4) \times 10^9 M_\odot$ at $\langle z\rangle\approx1.3$ to $(10.6\pm1.9)\times10^9 M_\odot$ at $\langle z\rangle\approx1.0$, i.e. by a factor $\gtrsim3$. We thus find direct evidence that accretion of HI onto star-forming galaxies at $z\approx1$ is insufficient to replenish their HI reservoirs and sustain their SFRs, thus resulting in the decline in the cosmic SFR density 8 billion years ago., Comment: 11 Pages, 7 Figures; Astrophysical Journal Letters, 931 L34; a minor typographical error is fixed in this version

We study the relationship between the H{\sc i} specific angular momentum (j$_{\rm g}$) and the H{\sc i} mass (M$_{\rm g}$) for a sample of galaxies with well measured H{\sc i} rotation curves. We find that the relation is well described by an unbroken power law \jg $\propto$ \mg$^{\alpha}$ over the entire mass range (10$^{7}$-10$^{10.5}$ M$_{\odot}$), with $\alpha = 0.89 \pm 0.05$ (scatter 0.18 dex). This is in reasonable agreement with models which assume that evolutionary processes maintain H{\sc i} disks in a marginally stable state. The slope we observe is also significantly different from both the $j \propto M^{2/3}$ relation expected for dark matter haloes from tidal torquing models and the observed slope of the specific angular momentum-mass relation for the stellar component of disk galaxies. Our sample includes two H{\sc i}-bearing ultra diffuse galaxies, and we find that their angular momentum follows the same relation as other galaxies. The only discrepant galaxies in our sample are early-type galaxies with large rotating H{\sc i} disks which are found to have significantly higher angular momentum than expected from the power law relation. The H{\sc i} disks of all these early-type galaxies are misaligned or counter-rotating with respect to the stellar disks, consistent with the gas being recently accreted. We speculate that late stage wet mergers, as well as cold flows play a dominant role in determining the kinematics of the baryonic component of galaxies as suggested by recent numerical simulations., Comment: Accepted for publication in MNRAS.15 pages, 22 figures, 6 tables

We construct mass models of eight gas rich dwarf galaxies that lie in the Lynx-Cancer void. From NFW fits to the dark matter halo profile, we find that the concentration parameters of halos of void dwarf galaxies are similar to those of dwarf galaxies in normal density regions. We also measure the slope of the central dark matter density profiles, obtained by converting the rotation curves derived using 3-D (\fat) and 2-D (ROTCUR) tilted ring fitting routines, into mass densities. We find that the average slope ($\alpha = -1.39 \pm 0.19$), obtained from 3-D fitting is consistent with that expected from an NFW profile. On the other hand, the average slope measured using the 2-D approach is closer to what would be expected for an isothermal profile. This suggests that systematic effects in velocity field analysis have a significant effect on the slope of the central dark matter density profiles. Given the modest number of galaxies we use for our analysis, it is important to check these results using a larger sample., Comment: Accepted for publication in MNRAS, 24 pages, 58 figures, and 6 tables

We have used the upgraded Giant Metrewave Radio Telescope to carry out a deep (117 on-source hours) L-band observation of the Extended Groth Strip, to measure the average neutral hydrogen (HI) mass and median star formation rate (SFR) of star-forming galaxies, as well as the cosmic HI mass density, at $0.2 < z < 0.4$. This was done by stacking the HI 21cm emission and the rest-frame 1.4 GHz radio continuum from 445 blue star-forming galaxies with $\rm M_B \leq -17$ at $z_{\rm mean} \approx 0.34$. The stacked HI 21cm emission signal is detected at $\approx 7\sigma$ significance, implying an average HI mass of $\rm \langle M_{HI} \rangle = (4.93 \pm 0.70) \times 10^9 \: M_{\odot}$. We also stacked the rest-frame 1.4 GHz radio continuum emission of the same galaxies, to obtain a median SFR of $(0.54 \pm 0.06) \: {\rm M}_\odot$ yr$^{-1}$; this implies an average atomic gas depletion time scale of $\rm \langle \Delta t_{HI}\rangle \approx$ 9 Gyr, consistent with values in star-forming galaxies in the local Universe. This indicates that the star-formation efficiency does not change significantly over the redshift range $0 - 0.4$. We used the detection of the stacked HI 21cm emission signal to infer the normalized cosmic HI mass density $(\rm \rho_{HI}/\rho_{c,0})$ in star-forming galaxies at $z \approx 0.34$. Assuming the local relation between HI mass and absolute B-magnitude, we obtain $\rm \rho_{HI}/\rho_{c,0} = (4.81 \pm 0.75) \times 10^{-4}$, implying no significant evolution in $\rm \rho_{HI}/\rho_{c,0}$ from $z \approx 0.4$ to the present epoch., Comment: 7 pages, 4 figures

Carbon Radio Recombination Lines trace the interface between molecular and atomic gas. We present GMRT observations of Carbon Radio Recombination Lines (CRRL), C$\alpha$244-C$\alpha$250 towards Cassiopeia A. We use a novel technique of stacking the emission lines in the visibility domain to obtain, for the first time, sub-pc resolution optical depth maps of these CRRLs. The emission shows a wide range of spatial and velocity structures, some of which are unresolved within our synthesis beam of 0.29 pc and velocity channel of 0.55 km/s. These variations in the emission optical depth and line width are indicative of inhomogeneity and fragmentation in the intervening Perseus Arm gas. We compare the distribution of the CRRL emission with that of diffuse and dense molecular gas using existing CO and H$_2$CO studies. We find that the CO emission in the -47 km/s Perseus Arm component is primarily concentrated along an elongated structure detected in our CRRL maps, to the south of which lies the high-density molecular clumps traced by H$_2$CO. This spatial distribution of CRRL and molecular tracers is similar to what one would observe for a Photo Dissociation Region (PDR). In the other Perseus Arm component centered at -37 km/s, there is evidence for high column density (N$_{H_2}$ ~ $10^{22}$cm$^{-2}$) molecular clumps embedded in diffuse CO as well as CRRL emission towards the center of Cassiopeia A. We propose that the CRRL emissions coincident with molecular tracers originates from the line of sight integrated component of the C$^{+}$ envelope of the molecular gas., Comment: 11 pages, 7 figures. Accepted for publication in MNRAS

We present high resolution H{\sc i} 21cm Giant Meterwave Radio Telescope (GMRT) observations of the superthin galaxy FGC1540 with a spatial resolution of 10$''$ $\times$ 8$''$ and a spectral resolution of 1.73 kms$^{-1}$ and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial H{\sc i} surface density profile by fitting a 3-dimensional tilted ring model directly to the H{\sc i} data cubes by using the publicly-available software, Fully Automated Tirrific (FAT). We also present the rotation curve of FGC1540 derived from its optical spectroscopy study using the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We use the rotation curve, the H{\sc i} surface density profile together with Spitzer 3.6 $\mu$m and the SDSS $i$--band data to construct the mass models for FGC1540. We find that both the Pseudo-isothermal (PIS), as well as Navarro-Frenk-White (NFW) dark matter (DM) halos, fit the observed rotation curve equally well. The PIS model indicates a compact dark matter halo ($R_{\rm C}/R_{\rm D}$ < 2), with the best-fitting core radius ($R_{\rm C}$) approximately half the exponential stellar disc scale length ($R_{\rm D}$), which is in agreement with the mass models of superthin galaxies studied earlier in the literature. Since the vertical thickness of the galactic stellar disc is determined by a balance between the net gravitational field and the velocity dispersion in the vertical direction, the compact dark matter halo may be primarily responsible in regulating the superthin vertical structure of the stellar disc in FGC1540 as was found in case of the superthin galaxy UGC7321., Comment: Accepted for publication in MNRAS

Mass and specific angular momentum are two fundamental physical parameters of galaxies. We present measurements of the baryonic mass and specific angular momentum of 11 void dwarf galaxies derived from neutral hydrogen (H{\sc i}) synthesis data. Rotation curves were measured using 3D and 2D tilted ring fitting routines, and the derived curves generally overlap within the error bars, except in the central regions where, as expected, the 3D routines give steeper curves. The specific angular momentum of void dwarfs is found to be high compared to an extrapolation of the trends seen for higher mass bulge-less spirals, but comparable to that of other dwarf irregular galaxies that lie outside of voids. As such, our data show no evidence for a dependence of the specific angular momentum on the large scale environment. Combining our data with the data from the literature, we find a baryonic threshold of $\sim 10^{9.1}~M_{\odot}$ for this increase in specific angular momentum. Interestingly, this threshold is very similar to the mass threshold below which the galaxy discs start to become systematically thicker. This provides qualitative support to the suggestion that the thickening of the discs, as well as the increase in specific angular momentum, are both results of a common physical mechanism, such as feedback from star formation. Quantitatively, however, the amount of star formation observed in our dwarfs appears insufficient to produce the observed increase in specific angular momentum. It is hence likely that other processes, such as cold accretion of high angular momentum gas, also play a role in increasing the specific angular momentum., Comment: Accepted for publication in MNRAS

We report a deep Giant Metrewave Radio Telescope (GMRT) search for Galactic H{\sc i} 21cm absorption towards the quasar B0438$-$436, yielding the detection of wide, weak H{\sc i} 21cm absorption, with a velocity-integrated H{\sc i} 21cm optical depth of $0.0188 \pm 0.0036$~km~s$^{-1}$. Comparing this with the H{\sc i} column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of $3760 \pm 365$~K, one of the highest measured in the Galaxy. This is consistent with most of the H{\sc i} along the sightline arising in the stable warm neutral medium (WNM). The low peak H{\sc i} 21cm optical depth towards B0438$-$436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H{\sc i} 21cm absorption and emission spectra. This yields a gas kinetic temperature of $\rm T_k \leq (4910 \pm 1900)$~K, and a spin temperature of $\rm T_s = (1000 \pm 345)$~K for the gas that gives rise to the H{\sc i} 21cm absorption. Our data are consistent with the H{\sc i} 21cm absorption arising from either the stable WNM, with $\rm T_s \ll T_k$, $\rm T_k \approx 5000$~K, and little penetration of the background Lyman-$\alpha$ radiation field into the neutral hydrogen, or from the unstable neutral medium, with $\rm T_s \approx T_k \approx 1000\;K$., Comment: 5 pages, 3 figures; accepted for publication in MNRAS-Letters

We use observations made with the Giant Metrewave Radio Telescope (GMRT) to probe the neutral hydrogen (HI) gas content of field galaxies in the VIMOS VLT Deep Survey (VVDS) 14h field at $z \approx 0.32$. Because the HI emission from individual galaxies is too faint to detect at this redshift, we use an HI spectral stacking technique using the known optical positions and redshifts of the 165 galaxies in our sample to co-add their HI spectra and thus obtain the average HI mass of the galaxies. Stacked HI measurements of 165 galaxies show that 95 per cent of the neutral gas is found in blue, star-forming galaxies. Among these galaxies, those having lower stellar mass are more gas-rich than more massive ones. We apply a volume correction to our HI measurement to evaluate the HI gas density at $z \approx 0.32$ as $\Omega_{HI}=(0.50\pm0.18) \times 10^{-3}$ in units of the cosmic critical density. This value is in good agreement with previous results at z < 0.4, suggesting no evolution in the neutral hydrogen gas density over the last $\sim 4$ Gyr. However the $z \approx 0.32$ gas density is lower than that at $z \sim 5$ by at least a factor of two., Comment: 16 pages, 14 figures, accepted for publication in MNRAS

The extended Schmidt law (ESL) is a variant of the Schmidt law which relates the surface densities of gas and star formation, with the surface density of stellar mass added as an extra parameter. We empirically investigate for the first time whether low metallicity faint dwarf irregular galaxies (dIrrs) follow the ESL. Here we consider the `global' law where surface densities are averaged over the galactic discs. dIrrs are unique not only because they are at the lowest end of mass and star formation scales for galaxies, but also because they are metal-poor compared to the general population of galaxies. Our sample is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) which is the largest survey of atomic hydrogen in such galaxies. The gas surface densities are determined using their atomic hydrogen content. The star formation rates are calculated using GALEX far ultraviolet fluxes after correcting for dust extinction, whereas the stellar surface densities are calculated using Spitzer 3.6 $\mu$m fluxes. All surface densities are calculated over stellar discs defined by the 3.6 $\mu$m images. We find dIrrs indeed follow the extended Schmidt law. The mean deviation of the FIGGS galaxies from the relation is 0.01 dex, with a scatter around the relation of less than half that seen in the original relation. In comparison, we also show that the FIGGS galaxies are much more deviant when compared to the `canonical' Kennicutt-Schmidt relation. Our results help strengthen the universality of the extended Schmidt law, especially for galaxies with low metallicities. We suggest that models of star formation in which feedback from previous generations of stars set the pressure in the ISM, are promising candidates for explaining the ESL. We also confirm that ESL is an independent relation and not a form of a relation between star formation efficiency and metallicity., Comment: Accepted for publication in Astronomy & Astrophysics. Figure 2 on Page 5 shows the main result

We derive the specific baryonic angular momentum of five gas rich dwarf galaxies from HI kinematics complemented by stellar mass profiles. Since the gas mass of these galaxies is much larger than the stellar mass, the angular momentum can be determined with relatively little uncertainty arising from the uncertainties in the stellar mass to light ratio. We compare the relation between the specific baryonic angular momentum (j) and the total baryonic mass (M) for these galaxies with that found for spiral galaxies. Our combined sample explores the j-M plane over 3 orders of magnitude in baryon mass. We find that our sample dwarf have significantly higher specific angular momentum than expected from the relation found for spiral galaxies. The probability that these gas rich dwarf galaxies follow the same relation as spirals is found to be $<10^{-6}$. This implies a difference in the evolution of angular momentum in these galaxies compared to larger ones. We suggest that this difference could arise due to one or more of the following: a lower baryon fraction in dwarf galaxies, particularly that arising from preferential outflows low angular momentum gas as found in high resolution simulations that include baryonic feedback; "cold mode" anisotropic accretion from cosmic filaments. Our work reinforces the importance of the j-M plane in understanding the evolution of galaxies., Comment: 10 Pages, 7 figures. Accepted for publication in MNRAS

We present HI 21cm and optical observations of UGC 3672 which is located near the centre of the nearby Lynx-Cancer void. We find that UGC 3672 consists of an approximately linearly aligned triplet of gas rich dwarfs with large scale velocity continuity along the triplet axis. The faintest component of the triplet is extremely gas-rich (MHI/LB ~ 17) and also extremely metal deficient (12+log(O/H) ~ 7.0). The metallicity of this dwarf is close to the 'floor' observed in star forming galaxies. Low resolution HI images show that the galaxy triplet is located inside a common HI envelope, with fairly regular, disk like kinematics. At high angular resolution however, the gas is found to be confined to several filamentary tidal tails and bridges. The linear alignment of the galaxies, along with the velocity continuity that we observe, is consistent with the galaxies lying along a filament. We argue that the location of this highly unusual system in an extremely low density environment is not a coincidence, but is a consequence of structure formation proceeding more slowly and also probing smaller scales than in regions with average density. Our observations also indicate that wet mergers of galaxies flowing along filaments is a possible pathway for the formation of gas rich disks. The UGC 3672 system provides an interesting opportunity to study the kind of interactions typical between high redshift extremely gas rich unevolved small systems that lie at base of the hierarchical galaxy formation model., Comment: 12 pages, 8 figures, accepted for publication in MNRAS

We present the observations and first results from the FIGGS2 survey. FIGGS2 is an extension of the earlier Faint Irregular Galaxies GMRT survey (FIGGS) towards faint luminosity end. The sample consists of 20 galaxies of which 15 were detected in HI 21cm line using the Giant Meter-wave Radio Telescope (GMRT). The median blue band magnitude of our sample is ~ -11.6, which is more than one magnitude fainter than earlier FIGGS survey. From our GMRT observations we find that, for many of our sample galaxies, the HI disks are offset from their optical disks. The HI diameters of the FIGGS2 galaxies show a tight correlation with their HI mass. The slope of the correlation is 2.08 +/- 0.20 similar to what is found for FIGGS galaxies. We also find that for almost all galaxies, the HI disks are larger than the optical disks which is a common trend for dwarf or spiral galaxies. The mean value of the ratio of HI to optical diameter is ~ 1.54., Comment: 12 pages, 8 figures, accepted for publication in Astrophysical Bulletin

We present the results of HI spectral stacking analysis of Giant Metrewave Radio Telescope (GMRT) observations targeting the COSMOS field. The GMRT data cube contains 474 field galaxies with redshifts known from the zCOSMOS-bright 10k catalogue. Spectra for the galaxies are co-added and the stacked spectrum allows us to make a $\sim 3\sigma$ measurement of the average HI mass. Using this average HI mass along with the integral optical $B$-band luminosity of the galaxies and the luminosity density of the COSMOS field, a volume normalisation is applied to obtain the cosmic HI mass density ($\Omega_{\rm HI}$). We find a cosmic HI mass density of $\Omega_{\rm HI}$ = (0.42 $\pm$ 0.16) $\times$ 10$^{-3}$ at $z \sim 0.37$, which is the highest-redshift measurement of $\Omega_{\rm HI}$ ever made using HI spectral stacking. The value we obtained for $\Omega_{\rm HI}$ at $z \sim 0.37$ is consistent with that measured from large blind 21-cm surveys at $z = 0$ as well as measurements from other HI stacking experiments at lower redshifts. Our measurement in conjunction with earlier measurements indicates that there has been no significant evolution of HI gas abundance over the last 4 Gyr. A weighted mean of $\Omega_{\rm HI}$ from all 21-cm measurements at redshifts $z \lesssim 0.4$ gives $\Omega_{\rm HI}$ = (0.35 $\pm$ 0.01) $\times$ 10$^{-3}$. The $\Omega_{\rm HI}$ measured (from HI 21-cm emission measurements) at $z \lesssim 0.4$ is however approximately half that measured from Damped Lyman-$\alpha$ Absorption (DLA) systems at $z \gtrsim 2$. Deeper surveys with existing and upcoming instruments will be critical to understand the evolution of $\Omega_{\rm HI}$ in the redshift range intermediate between $z \sim 0.4$ and the range probed by DLA observations., Comment: 12 pages, 11 figures, accepted for publication in MNRAS

We revisit the HI size-mass (D$_{\rm HI}$-M$_{\rm HI}$) 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 $\sigma \sim$0.06 dex, or 14%. The scatter does not change as a function of galaxy luminosity, HI richness or morphological type. The relation is linked to the fact that dwarf and spiral galaxies have a homogenous radial profile of HI surface density in the outer regions when the radius is normalised by D$_{\rm HI}$. The early-type disk galaxies typically have shallower HI radial profiles, indicating a different gas accretion history. We argue that the process of atomic-to-molecular gas conversion or star formation cannot explain the tightness of the D$_{\rm HI}$-M$_{\rm HI}$ relation. This simple relation puts strong constraints on simulation models for galaxy formation., Comment: 9 pages, 6 figures, accepted for publication at MNRAS

We present the results of a study of the amount and distribution of cold atomic gas, as well its correlation with recent star formation in a sample of extremely faint dwarf irregular galaxies. Our sample is drawn from the Faint Irregular Galaxy GMRT Survey (FIGGS) and its extension, FIGGS2. We use two different methods to identify cold atomic gas. In the first method, line-of-sight HI spectra were decomposed into multiple Gaussian components and narrow Gaussian components were identified as cold HI. In the second method, the brightness temperature (T_B) is used as a tracer of cold HI. We find that the amount of cold gas identified using the T_B method is significantly larger than the amount of gas identified using Gaussian decomposition. We also find that a large fraction of the cold gas identified using the T_B method is spatially coincident with regions of recent star formation, although the converse is not true. That is only a small fraction of the regions with recent star formation are also covered by cold gas. For regions where the star formation and the cold gas overlap, we study the relationship between the star formation rate density and the cold \HI column density. We find that the star formation rate density has a power law dependence on the HI column density, but that the slope of this power law is significantly flatter than that of the canonical Kennicutt-Schmidt relation., Comment: Accepted for publication in MNRAS

Observations are presented of the isolated dwarf irregular galaxy And IV made with the Hubble Space Telescope Advanced Camera for Surveys and the Giant Metrewave Radio Telescope in the 21 cm HI line. We determine the galaxy distance of $7.17\pm0.31$ Mpc using the Tip of Red Giant Branch method. The galaxy has a total blue absolute magnitude of -12.81 mag, linear Holmberg diameter of 1.88 kpc and an HI-disk extending to 8.4 times the optical Holmberg radius. The HI mass-to-blue luminosity ratio for And IV amounts $12.9~M_{\odot}/L_{\odot}$. From the GMRT data we derive the rotation curve for the HI and fit it with different mass models. We find that the data are significantly better fit with an iso-thermal dark matter halo, than by an NFW halo. We also find that MOND rotation curve provides a very poor fit to the data. The fact that the iso-thermal dark matter halo provides the best fit to the data supports models in which star formation feedback results in the formation of a dark matter core in dwarf galaxies. The total mass-to-blue luminosity ratio of $162~M_{\odot}/L_{\odot}$ makes And IV among the darkest dIrr galaxies known. However, its baryonic-to-dark mass ratio ($M_{gas}+M^*)/M_T = 0.11$ is close to the average cosmic baryon fraction, 0.15., Comment: 9 pages, 10 figures, 2 tables, accepted in Astronomische Nachrichten

We present HI 21cm emission observations of the z ~ 0.00632 sub-damped Lyman-alpha absorber (sub-DLA) towards PG1216+069 made using the Arecibo Telescope and the Very Large Array (VLA). The Arecibo 21cm spectrum corresponds to an HI mass of ~ 3.2x10^7 solar masses, two orders of magnitude smaller than that of a typical spiral galaxy. This is surprising since in the local Universe the cross-section for absorption at high HI column densities is expected to be dominated by spirals. The 21cm emission detected in the VLA spectral cube has a low signal-to-noise ratio, and represents only half the total flux seen at Arecibo. Emission from three other sources is detected in the VLA observations, with only one of these sources having an optical counterpart. This group of HI sources appears to be part of complex "W", believed to lie in the background of the Virgo cluster. While several HI cloud complexes have been found in and around the Virgo cluster, it is unclear whether the ram pressure and galaxy harassment processes that are believed to be responsible for the creation of such clouds in a cluster environment are relevant at the location of this cloud complex. The extremely low metallicity of the gas, ~ 1/40 solar, also makes it unlikely that the sub-DLA consists of material that has been stripped from a galaxy. Thus, while our results have significantly improved our understanding of the host of this sub-DLA, the origin of the gas cloud remains a mystery, Comment: accepted for publication in MNRAS

We study the Kennicutt-Schmidt relation between average star formation rate and average cold gas surface density in the Hi dominated ISM of nearby spiral and dwarf irregular galaxies. We divide the galaxies into grid cells varying from sub-kpc to tens of kpc in size. Grid-cell measurements of low SFRs using H-alpha emission can be biased and scatter may be introduced because of non-uniform sampling of the IMF or because of stochastically varying star formation. In order to alleviate these issues, we use far-ultraviolet emission to trace SFR, and we sum up the fluxes from different bins with the same gas surface density to calculate the average $\Sigma_{SFR}$ at a given value of $\Sigma_{gas}$. We study the resulting Kennicutt-Schmidt relation in 400 pc, 1 kpc and 10 kpc scale grids in nearby massive spirals and in 400 pc scale grids in nearby faint dwarf irregulars. We find a relation with a power law slope of 1.5 in the HI-dominated regions for both kinds of galaxies. The relation is offset towards longer gas consumption timescales compared to the molecular hydrogen dominated centres of spirals, but the offset is an order-of-magnitude less than that quoted by earlier studies. Our results lead to the surprising conclusion that conversion of gas to stars is independent of metallicity in the HI dominated regions of star-forming galaxies. Our observed relations are better fit by a model of star formation based on thermal and hydrostatic equilibrium in the ISM, in which feedback driven turbulence sets the thermal pressure., Comment: 11 pages, 7 figures, 5 tables. Accepted for publication in MNRAS Main Journal. For the definitive version visit http://mnras.oxfordjournals.org/

We model the observed vertical distribution of the neutral hydrogen (HI) in the faint (M_B ~ -13.7 mag) edge-on dwarf irregular galaxy KK250. Our model assumes that the galaxy consists of axi-symmetric, co-planar gas and stellar discs in the external force-field of a spherical dark matter halo, and in vertical hydrostatic equilibrium. The velocity dispersion of the gas is left as a free parameter in the model. Our best fit model is able to reproduce the observed vertical distribution of the HI gas, as well as the observed velocity profiles. The best fit model has a large velocity dispersion (~ 22 km/s) at the centre of the galaxy, which falls to a value of ~ 8 km/s by a galacto-centric radius of ~ 1 kpc, which is similar to both the scale-length of the stellar disc, as well as the angular resolution of the data along the radial direction. Similarly we find that the thickness of the HI disc is also minimum at ~ 1 kpc, and increases by about a factor of ~ 2 as one goes to the centre of the galaxy or out to ~ 3 kpc. The minimum intrinsic HWHM of the HI vertical distribution in KK250 is ~ 350 pc. For comparison the HWHM of the vertical distribution of the HI in the solar neighbourhood is ~ 70-140 pc. Our results are hence consistent with other observations which indicate that dwarf galaxies have significantly puffier gas discs than spirals., Comment: 7 pages, 4 figures, Published in MNRAS

We use data for faint (M_B > -14.5) dwarf irregular galaxies drawn from the FIGGS survey to study the correlation between the atomic gas density (Sigma_gas,atomic) and star formation rate (Sigma_SFR) in the galaxies. The estimated gas phase metallicity of our sample galaxies is Z ~ 0.1 Z_sun. Understanding star formation in such molecule poor gas is of particular importance since it is likely to be of direct relevance to simulations of early galaxy formation. For about 20% (9/43) of our sample galaxies, we find that the HI distribution is significantly disturbed, with little correspondence between the optical and HI distributions. We exclude these galaxies from the comparison. We also exclude galaxies with very low star formation rates, for which stochastic effects make it difficult to estimate the true star formation rates. For the remaining galaxies we compute the Sigma_gas,atomic and Sigma_SFR averaged over the entire star forming disk of the galaxy. For these galaxies we find a nearly linear relation between the star formation rate and the atomic gas surface densities. The corresponding gas consumption timescale is ~ 10 Gyr, i.e. significantly smaller than the ~ 100 Gyr estimated for the outer regions of spiral galaxies. We also estimate the gas consumption timescale computed using the global gas content and the global star formation rate for all galaxies with a reliable measurement of the star formation rate, regardless of whether the HI distribution is disturbed or not. The mean gas consumption timescale computed using this entire gas reservoir is ~ 18 Gyr, i.e. still significantly smaller than that estimated for the outer parts of spirals. The gas consumption timescale for dwarfs is intermediate between the values of ~ 100 Gyr and ~ 2 Gyr estimated for the outer molecule poor and inner molecule rich regions of spiral disks., Comment: 11 pages (+1 as appendix), 10 Figures (+1 in appendix), 2 Tables. Accepted for publication in MNRAS Main Journal. For the definitive version visit http://mnras.oxfordjournals.org/

We present first results of the study of a set of exceptional HI sources identified in the 40% ALFALFA extragalactic HI survey catalog alpha.40 as being both HI massive (M_HI > 10^10 Msun) and having high gas fractions for their stellar masses: the HIghMass galaxy sample. We analyze UV- and optical-broadband and Halpha images to understand the nature of their relatively underluminous disks in optical and to test whether their high gas fractions can be tracked to higher dark matter halo spin parameters or late gas accretion. Estimates of their star formation rates (SFRs) based on SED-fitting agree within uncertainties with the Halpha luminosity inferred SFRs. The HII region luminosity functions have standard slopes at the luminous end. The global SFRs demonstrate that the HIghMass galaxies exhibit active ongoing star formation (SF) with moderate SF efficiency, but relative to normal spirals, a lower integrated SFR in the past. Because the SF activity in these systems is spread throughout their extended disks, they have overall lower SFR surface densities and lower surface brightness in the optical bands. Relative to normal disk galaxies, the majority of HIghMass galaxies have higher Halpha equivalent widths and are bluer in their outer disks, implying an inside-out disk growth scenario. Downbending double exponential disks are more frequent than upbending disks among the gas-rich galaxies, suggesting that SF thresholds exist in the downbending disks, probably as a result of concentrated gas distribution., Comment: 66 pages, 17 figures, 3 tables; accepted for publication in ApJ; higher resolution figures available via http://www.asiaa.sinica.edu.tw/~shan/HIghMass_140728.pdf

We use MERLIN, VLA and VLBA observations of Galactic \HI absorption towards 3C~138 to estimate the structure function of the \HI opacity fluctuations at AU scales. Using Monte Carlo simulations, we show that there is likely to be a significant bias in the estimated structure function at signal-to-noise ratios characteristic of our observations, if the structure function is constructed in the manner most commonly used in the literature. We develop a new estimator that is free from this bias and use it to estimate the true underlying structure function slope on length scales ranging $5$ to $40$~AU. From a power law fit to the structure function, we derive a slope of $0.81^{+0.14}_{-0.13}$, i.e. similar to the value observed at parsec scales. The estimated upper limit for the amplitude of the structure function is also consistent with the measurements carried out at parsec scales. Our measurements are hence consistent with the \HI opacity fluctuation in the Galaxy being characterized by a power law structure function over length scales that span six orders of magnitude. This result implies that the dissipation scale has to be smaller than a few AU if the fluctuations are produced by turbulence. This inferred smaller dissipation scale implies that the dissipation occurs either in (i) regions with densities $\gtrsim 10^3 $cm$^-3$ (i.e. similar to that inferred for "tiny scale" atomic clouds or (ii) regions with a mix of ionized and atomic gas (i.e. the observed structure in the atomic gas has a magneto-hydrodynamic origin)., Comment: 8 Figures, 1 table. Accepted for publication in Monthly notices of royal astronomical society

We discuss physical conditions in Galactic neutral hydrogen based on deep, high velocity resolution interferometric HI 21 cm absorption spectroscopy towards 33 compact extra-galactic radio sources. The HI 21 cm optical depth spectra have root-mean-square noise values \lesssim 10^{-3} per 1 km/s velocity channel, i.e., sufficiently sensitive to detect HI 21 cm absorption by the warm neutral medium (WNM). Comparing these spectra with HI 21 cm emission spectra from the Leiden-Argentine-Bonn (LAB) survey, we show that some of the absorption detected on most sightlines must arise in gas with temperatures higher than that in the stable cold neutral medium (CNM). A multi-Gaussian decomposition of 30 of the HI 21 cm absorption spectra yielded very few components with line widths in the temperature range of stable WNM, with no such WNM components detected for sixteen of the thirty sightlines. We find that some of the detected HI 21 cm absorption along thirteen of these sightlines must arise in gas with spin temperatures larger than the CNM range. For these sightlines, we use very conservative estimates of the CNM spin temperature and the non-thermal broadening to derive strict upper limits to the gas column densities in the CNM and WNM phases. Comparing these upper limits to the total HI column density, we find that typically at least 28% of the gas must have temperatures in the thermally unstable range (200-5000 K). Our observations hence robustly indicate that a significant fraction of the gas in the Galactic interstellar medium has temperatures outside the ranges expected for thermally stable gas in two-phase models., Comment: 18 pages, 8 figures. Accepted for publication in MNRAS. The definitive version will be available at http://mnras.oxfordjournals.org/

We have carried out deep, high velocity resolution, interferometric Galactic HI 21 cm absorption spectroscopy towards 32 compact extra-galactic radio sources with the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). The optical depth spectra for most sources have root mean square noise values \lesssim 10^{-3} per 1 km/s velocity channel and are thus sufficiently sensitive to detect absorption by warm neutral hydrogen with HI column densities N_HI \gtrsim 10^{20} cm^{-2}, spin temperatures T_s \leq 5000 K, and line widths equal to the thermal width (20 km/s). HI 21 cm absorption was detected against all background sources but one, B0438-436. The spectra of sources observed separately with GMRT and WSRT show excellent agreement, indicating that spectral baseline problems and contamination from HI 21 cm emission are negligible. This paper presents the absorption spectra, the emission spectra along neighbouring sightlines from the Leiden-Argentine-Bonn survey, and the derived spin temperature spectra. On every sightline, the maximum spin temperature detected (at \ge 3 sigma significance) even at a velocity resolution of 1 km/s is \gtrsim 1000 K, indicating that we are detecting the warm neutral medium along most sightlines. This is by far the largest sample of Galactic HI 21 cm absorption spectra of this quality, providing a sensitive probe of physical conditions in the neutral atomic ISM., Comment: 14 pages, 5 figures. Accepted for publication in MNRAS. The definitive version will be available at http://mnras.oxfordjournals.org/

We use the measured B band axial ratios of galaxies from an updated catalog of Local Volume galaxies to determine the intrinsic shape of dwarf irregular galaxies (de Vacouleurs' morphological types 8, 9 and 10). We find that the shapes change systematically with luminosity, with fainter galaxies being thicker. In particular, we divide our sample into sub-samples and find that the most luminous dwarfs (-19.6 < M_B < -14.8) have thin discs (thickness ~ 0.2), with the disc being slightly elliptical (axial ratio ~ 0.8). At intermediate luminosity, viz. -14.8 < M_B < -12.6, the galaxies are still characterized by elliptical discs (axial ratio ~ 0.7), but the discs are somewhat thicker (thickness ~ 0.4). The faintest dwarfs, viz. those with -12.6 < M_B < -6.7 are well described as being oblate spheroids with an axial ratio ~ 0.5. The increasing thickness of the stellar discs of dwarf irregulars with decreasing luminosity is compatible with the increasing ratio of velocity dispersion to rotational velocity with decreasing galaxy size., Comment: 5 pages, 6 figures, 2 tables. Accepted for publication in MNRAS Letters. For the definitive version visit http://mnrasl.oxfordjournals.org/

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., Comment: 6 pages, 4 figure; accepted for publication as a Letter in MNRAS

We present the results from a search for HI emission from a sample of newly discovered dwarf galaxies in the M81 group. HI is detected in three galaxies, all of which are classified as BCDs. The HI masses of these galaxies are ~ 10^6 M_sun, making these some of the lowest mass BCDs known. For these three galaxies FUV images (from GALEX) and H-alpha images (from the Russian 6m BTA telescope) are available.The H-alpha emission is very faint, and, in principle could be produced by a single O star. Further, in all cases we find offsets between the peak of the FUV emission and that of the H-alpha emission. Offsets between the most recent sites of star formation (i.e. those traced by H-alpha) and the older sites (i.e. those traced by FUV) would be natural if the star formation is stochastic. In spite of the expectation that the effects of mechanical feedback from star formation would be most directly seen in the smallest galaxies with low gravitational potentials, we only see tentative evidence of outflowing HI gas associated with the star forming region in one of the galaxies., Comment: 9 pages, 5 figures, 5 tables. Accepted for publication in MNRAS. The definitive version is available at www.blackwell-synergy.com

We study the radio - far-infrared (FIR) correlation in a sample of faint dwarf irregular galaxies using NVSS data for 1.4 GHz radio flux, Spitzer MIPS 70 um data for FIR flux, and GALEX FUV data to estimate the star formation rates (SFR). Since our target galaxies are extremely faint, we stack images of many galaxies together to estimate the average radio and FIR fluxes. We find that for a given SFR both 70 um and 1.4 GHz fluxes are low compared to the calibration for large spirals. Nonetheless, the ratio of 70 um to 1.4 GHz flux agrees within errorbars with that seen for large galaxies. The radio-FIR correlation thus appears to be the result of a 'conspiracy'. We use the SFR to estimate the non-thermal fraction of the 1.4 GHz radio emission and find it to be around 50%, much smaller than the 90% typical for spirals. We also estimate the equipartition magnetic field and find it to be ~ 2 microgauss, about five times smaller than that typical for spirals., Comment: 5 pages, 1 figure, 3 tables. Accepted for publication in MNRAS Letters. The definitive version is available at www.blackwell-synergy.com

We study the relationship between the gas column density (Sigma_HI) and the star formation rate surface density (Sigma_SFR) for a sample of extremely small (M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear stochasticity in the relation between the gas column density and star formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star formation, but the fraction of gas with ongoing star formation decreases as the gas column density decreases, and falls to about 50% at Sigma_HI ~ 3 M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency is at least a factor of ~ 2 smaller than typical of star forming regions in spirals. We also find that the ratio of H-alpha emission to FUV emission increases with increasing gas column density. This is unlikely to be due to increasing dust extinction because the required dust to gas ratios are too high. We suggest instead that this correlation arises because massive (i.e. H-alpha producing) stars are formed preferentially in regions with high gas density., Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS Letters. The definitive version is available at www.blackwell- synergy.com

Galaxy harassment is an important mechanism for the morphological evolution of galaxies in clusters. The spiral galaxy NGC 4254 in the Virgo cluster is believed to be a harassed galaxy. We have analyzed the power spectrum of HI emission fluctuations from NGC 4254 to investigate whether it carries any imprint of galaxy harassment. The power spectrum, as determined using the 16 central channels which contain most of the HI emission, is found to be well fitted by a power law $P(U)=AU^{\alpha}$ with $\alpha\ =-\ 1.7\pm 0.2$ at length-scales $1.7 \, {\rm k pc}$ to $ 8.4 \, {\rm kpc}$. This is similar to other normal spiral galaxies which have a slope of $\sim -1.5$ and is interpreted as arising from two dimensional turbulence at length-scales larger than the galaxy's scale-height. NGC 4254 is hence yet another example of a spiral galaxy that exhibits scale-invariant density fluctuations out to length-scales comparable to the diameter of the HI disk. While a large variety of possible energy sources like proto-stellar winds, supernovae, shocks, etc. have been proposed to produce turbulence, it is still to be seen whether these are effective on length-scales comparable to that of the entire HI disk. On separately analyzing the HI power spectrum in different parts of NGC 4254, we find that the outer parts have a different slope ($ \alpha = -2.0\pm0.3$) compared to the central part of the galaxy ($\alpha = -1.5\pm0.2$). Such a change in slope is not seen in other, undisturbed galaxies. We suggest that, in addition to changing the overall morphology, galaxy harassment also effects the fine scale structure of the ISM, causing the power spectrum to have a steeper slope in the outer parts., Comment: 6 figures, 1 table, accepted for publication in MNRAS letters

We determine the intrinsic axial ratio distribution of the 'gas' disks of extremely faint M_B > -14.5 dwarf irregular galaxies. We start with the measured (beam corrected) distribution of apparent axial ratios in the HI 21cm images of dwarf irregular galaxies observed as part of the Faint Irregular Galaxy GMRT Survey (FIGGS). Assuming that the disks can be approximated as oblate spheroids, the intrinsic axial ratio distribution can be obtained from the observed apparent axial ratio distribution. We use a couple of methods to do this, and our final results are based on using Lucy's deconvolution algorithm. This method is constrained to produce physically plausible distributions, and also has the added advantage of allowing for observational errors to be accounted for. While one might a priori expect that gas disks would be thin (because collisions between gas clouds would cause them to quickly settle down to a thin disk), we find that the HI disks of faint dwarf irregulars are quite thick, with mean axial ratio ~ 0.6. While this is substantially larger than the typical value of ~ 0.2 for the 'stellar' disks of large spiral galaxies, it is consistent with the much larger ratio of velocity dispersion to rotational velocity (sigma/v_c) in dwarf galaxy HI disks as compared to that in spiral galaxies. Our findings have implications for studies of the mass distribution in and the Tully - Fisher relation for faint dwarf irregular galaxies, where it is often assumed that the gas is in a thin disk., Comment: 5 pages, 4 figures. Minor changes in revised version. The definitive version is available at http://www.blackwell-synergy.com.

We estimate the power spectrum of HI intensity fluctuations for a sample of 8 galaxies (7 dwarf and one spiral). The power spectrum can be fitted to a power law P_HI(U) = A U^\alpha for 6 of these galaxies, indicating turbulence is operational. The estimated best fit value for the slope ranges from ~ -1.5 (AND IV, NGC 628, UGC 4459 and GR 8) to ~ -2.6 (DDO 210 and NGC 3741). We interpret this bi-modality as being due to having effectively 2D turbulence on length scales much larger than the scale height of the galaxy disk and 3D otherwise. This allows us to use the estimated slope to set bounds on the scale heights of the face-on galaxies in our sample. We also find that the power law slope remains constant as we increase the channel thickness for all these galaxies, suggesting that the fluctuations in HI intensity are due to density fluctuations and not velocity fluctuations, or that the slope of the velocity structure function is ~ 0. Finally, for the four galaxies with "2D turbulence" we find that the slope \alpha correlates with the star formation rate per unit area, with larger star formation rates leading to steeper power laws. Given our small sample size this result needs to be confirmed with a larger sample., Comment: 24 pages 10 figures and 2 tables. Accepted for publication in Mon. Not. of Royal Astron. Soc

We have measured the HI power spectrum of the nearly face-on spiral galaxy NGC 1058 from radio-interferometric observations using a visibility based estimator. The power spectrum is well fitted by two different power laws $P(U)=AU^{\alpha}$, one with $\alpha =- 2.5\pm 0.6$ at small length-scales $(600 {\rm pc} {\rm to} 1.5 {\rm kpc})$ and another with $\alpha =- 1.0\pm 0.2$ at large length-scales $(1.5 {\rm kpc} {\rm to} 10.0 {\rm kpc})$. We interpret this change in the slope of the power spectrum as a transition from 3D turbulence at small length-scales to 2D turbulence in the plane of the galaxy's disk at large length-scales. We use the observed break in the power spectrum to estimate the galaxy's scale-height, which we find to be $ 490 \pm 90 $ pc., Comment: Accepted for publication in Mon. Not. of Royal Astron. Soc. Letters