ALBERT

Description: We study the Kennicutt–Schmidt relation between average star formation rate (SFR) and average cold gas surface density in the H  i -dominated ISM of nearby spiral and dwarf irregular galaxies. We divide galaxies into grid cells varying from sub-kpc to tens of kpc in size. Grid-cell measurements of low SFRs using Hα 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 SFR at a given value of 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 H  i -dominated regions for both kinds of galaxies. The relation is offset towards longer gas consumption time-scales 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 H  i -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 stellar heating and supernova feedback set the thermal and turbulent pressure.

Description: 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 H i spectra were decomposed into multiple Gaussian components and narrow Gaussian components were identified as cold H i . In the second method, the brightness temperature ( T B  ) is used as a tracer of cold H i . 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 H i column density. We find that the star formation rate density has a power-law dependence on the H i column density, but that the slope of this power law is significantly flatter than that of the canonical Kennicutt–Schmidt relation.

Description: We present the results of H i spectral stacking analysis of Giant Metrewave Radio Telescope (GMRT) observations targeting the Cosmological Evolution Survey (COSMOS) field. The GMRT data cube contains 474 field galaxies with redshifts known from the zCOSMOS-bright 10 k catalogue. Spectra for the galaxies are co-added and the stacked spectrum allows us to make a ~3 measurement of the average H i mass. Using this average H i mass, along with the integral optical B -band luminosity of the galaxies and the luminosity density of the COSMOS field, a volume normalization is applied to obtain the cosmic H i mass density ( H i ). We find a cosmic H i mass density of H i = (0.42 ± 0.16) x 10 –3 at z ~ 0.37, which is the highest redshift measurement of H i ever made using H i spectral stacking. The value we obtained for H i at z ~ 0.37 is consistent with that measured from large blind 21-cm surveys at z = 0, as well as measurements from other H i stacking experiments at lower redshifts. Our measurement, in conjunction with earlier measurements, indicates that there has been no significant evolution of H i gas abundance over the last 4 Gyr. A weighted mean of H i from all 21-cm measurements at redshifts z 0.4 gives H i = (0.35 ± 0.01) x 10 –3 . The H i measured (from H i 21-cm emission measurements) at z 0.4 is, however, approximately half that measured from damped Lyman-α absorption (DLA) systems at z 2. Deeper surveys with existing and upcoming instruments will be critical to understand the evolution of H i in the redshift range intermediate between z ~ 0.4 and the range probed by DLA observations.

Description: We present a detailed study of the extremely isolated Sdm galaxy UGC 4722 ( M B  = –17.4) located in the nearby Lynx-Cancer void. UGC 4722 is a member of the Catalogue of Isolated Galaxies, and has also been identified as one of the most isolated galaxies in the Local Supercluster. Optical images of the galaxy however show that it has a peculiar morphology with an elongated ~14 kpc-long plume. New observations with the Russian 6-m telescope (BTA) and the Giant Metrewave Radio Telescope (GMRT) of the ionized and neutral gas in UGC 4722 reveal the second component responsible for the disturbed morphology of the system. This is a small, almost completely destroyed, very gas-rich dwarf ( M B  = –15.2, M (H i )/ L B  ~ 4.3) We estimate the oxygen abundance for both galaxies to be 12 + log (O/H) ~ 7.5–7.6 which is two to three times lower than what is expected from the luminosity–metallicity relation for similar galaxies in denser environments. The ugr colours of the plume derived from Sloan Digital Sky Survey (SDSS) images are consistent with a simple stellar population with a post starburst age of 0.45–0.5 Gyr. This system hence appears to be the first known case of a minor merger with a prominent tidal feature consisting of a young stellar population.

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 use data for faint ( M B  〉 –14.5) dwarf irregular galaxies drawn from the Faint Irregular Galaxy GMRT Survey to study the correlation between the surface densities of atomic gas ( gas,atomic ) and star formation rate ( SFR ) in the galaxies. The estimated gas-phase metallicity of our sample galaxies is Z ~ 0.1 Z . 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 per cent (9/43) of our sample galaxies, we find that the H i distribution is significantly disturbed, with little correspondence between the optical and H i 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 gas,atomic and SFR averaged over the entire star-forming disc of the galaxy. For these galaxies, we find a nearly linear relation between the star formation rate and the atomic gas density, namely ${\rm \log \Sigma _{SFR} = 0.91^{+0.23}_{-0.25}\, \log \Sigma _{gas,atomic} - 3.84^{+0.15}_{-0.19}}$ . The corresponding gas consumption time-scale 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 time-scale 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 H i distribution is disturbed or not. The mean gas consumption time-scale 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 time-scale for dwarfs is intermediate between the values of ~100 and ~2 Gyr estimated for the outer molecule-poor and inner molecule-rich regions of spiral discs.

Description: We model the observed vertical distribution of the neutral hydrogen (H  i ) in the faint ( M B  ~ –13.7 mag) edge-on dwarf irregular galaxy KK250. Our model assumes that the galaxy consists of axisymmetric, coplanar 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-fitting model is able to reproduce the observed vertical distribution of the H  i gas, as well as the observed velocity profiles. The best-fitting model has a large velocity dispersion (~22 km s –1 ) at the centre of the galaxy, which falls to a value of ~8 km s –1 by a galactocentric radius of 1 kpc, which is similar to both the scalelength 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 H  i 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 half-width at half-maximum (HWHM) of the H  i vertical distribution in KK250 is ~350 pc. For comparison the HWHM of the vertical distribution of the H  i 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.

Description: Assuming that Fast Radio Bursts (FRBs) are of extragalactic origin, we have developed a formalism to predict the FRB detection rate and the redshift distribution of the detected events for a telescope with given parameters. We have adopted FRB 110220, for which the emitted pulse energy is estimated to be E 0 = 5.4 x 10 33 J, as the reference event. The formalism requires us to assume models for (a) pulse broadening due to scattering in the ionized intergalactic medium – we consider two different models for this, (b) the frequency spectrum of the emitted pulse – we consider a power-law model E –α with –5 ≤ α ≤ 5, and (c) the comoving number density of the FRB occurrence rate n ( E , w i , z ) – we ignore the z dependence and assume a fixed intrinsic pulse width w i = 1 ms for all the FRBs. The distribution of the emitted pulse energy E is modelled through (a) a delta function where all the FRBs have the same energy E = E 0 , and (b) a Schechter luminosity function where the energies have a spread around E 0 . The models are all normalized using the four FRBs detected by Thornton et al. Our model predictions for the Parkes telescope are all consistent with the inferred redshift distribution of the 14 FRBs detected there to date. We also find that scattering places an upper limit on the redshift of the FRBs detectable by a given telescope; for the Parkes telescope, this is z ~ 2. Considering the upcoming Ooty Wide Field Array, we predict an FRB detection rate of ~0.01 to ~10 3 d –1 .

Description: We have carried out deep, high velocity resolution, interferometric Galactic H i -21 cm absorption spectroscopy towards 32 compact extragalactic 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 10 –3 per 1 km s –1 velocity channel and are thus sufficiently sensitive to detect absorption by warm neutral hydrogen with H i column densities N H i 10 20 cm –2 , spin temperatures T s  ≤ 5000 K and line widths equal to the thermal width (20 km s –1 ). H i -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 H i -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 ≥3 significance) even at a velocity resolution of 1 km s –1 is 1000 K, indicating that we are detecting the warm neutral medium along most sightlines. This is by far the largest sample of Galactic H i -21 cm absorption spectra of this quality, providing a sensitive probe of physical conditions in the neutral atomic interstellar medium.

Description: We discuss physical conditions in Galactic neutral hydrogen based on deep, high-velocity resolution interferometric H i -21 cm absorption spectroscopy towards 33 compact extragalactic radio sources. The H i -21 cm optical depth spectra have root-mean-square noise values 10 –3 per 1 km s –1 velocity channel, i.e. sufficiently sensitive to detect H i -21 cm absorption by the warm neutral medium (WNM). Comparing these spectra with H i -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 H i -21 cm absorption spectra yielded very few components with linewidths in the temperature range of stable WNM, with no such WNM components detected for 16 of the 30 sightlines. We find that some of the detected H i -21 cm absorption along 13 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 H i column density, we find that typically at least 28 per cent 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.