ALBERT

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: 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.

Description: We use the measured B -band axial ratios of galaxies from an updated catalogue of Local Volume galaxies to determine the intrinsic shape of dwarf irregular galaxies (de Vaucouleurs’ morphological types 8–10). 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.

Description: We use MERLIN, VLA and VLBA observations of Galactic H  i absorption towards 3C 138 to estimate the structure function of the H  i 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 from 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 H  i 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 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 magnetohydrodynamic origin).

Description: We present a detailed study of an estimator of the H i column density, based on a combination of H i 21 cm absorption and H i 21cm emission spectroscopy. This ‘isothermal’ estimate is given by N H I , ISO = 1.823 x 10 18 [ tot x T B / [ 1 – e – tot ]dV, where tot is the total H i 21cm optical depth along the sightline and 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 H I , ISO with the true H i column density N H I . 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 i column density to the isothermal estimate, N H I / N H I , ISO , is within a factor of 2 of unity while the 68.2 per cent confidence intervals are within a factor of 3 of unity, out to high H i column densities, ≤5  x 10 23 cm –2 per 1 km s –1 channel, and high total optical depths, ≤1000. The isothermal estimator thus provides a significantly better measure of the H i 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 H i column density gas in the Milky Way.

Description: Using the Tremaine–Weinberg method, we measure the speed of the H  i bar seen in the disc of NGC 3741. NGC 3741 is an extremely gas-rich galaxy with an H i disc which extends to about 8.3 times its Holmberg radius. It is also highly dark matter dominated. Our calculated value of the pattern speed p is 17.1 ± 3.4 km s –1 kpc –1 . We also find the ratio of the corotation radius to the bar semimajor axis to be ( 1.6 ± 0.3), indicating a slow bar. This is consistent with bar models in which dynamical friction results in a slow bar in dark-matter-dominated galaxies.

Description: We report results from a programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman α absorbers (DLAs). This involved (1) H i 21 cm absorption studies of a large sample of DLAs towards radio-loud quasars, (2) very long baseline interferometric studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and the velocity widths of low-ionization metal lines. Including literature data, our sample consists of 37 DLAs with estimates of the harmonic mean spin temperature T s . We find a statistically significant (4) difference between the T s distributions in the high- z ( z  〉 2.4) and low- z ( z  〈 2.4) DLA samples. The high- z sample contains more systems with high spin temperature, T s 1000 K. The T s distributions in DLAs and the Galaxy are also significantly (6) different, with more high- T s sightlines in DLAs than in the Milky Way. The high T s values in the high- z DLAs of our sample arise due to low fractions of the cold neutral medium (CNM). Only 2 of 23 DLAs at z  〉 1.7 have T s values indicating CNM fractions 〉20 per cent, comparable to the median value (27 per cent) in the Galaxy. We tested whether the H i column density measured towards the optical quasar might be systematically different from that towards the radio core by comparing the H i column densities inferred from H i 21 cm emission studies at different spatial resolutions (15 pc-1 kpc) in the Large Magellanic Cloud. The high-resolution N H i values are, on average, larger than the smoothed ones for N H i 〉 10 21 cm –2 , but lower than the smoothed N H i estimates for N H i 〈 10 21 cm –2 . Since there are far more DLAs with low N H i values than high ones, the use of the optical N H i value for the radio sightline results in a statistical tendency to underestimate DLA spin temperatures. For 29 DLAs with metallicity estimates, we confirm the presence of an anticorrelation between T s and metallicity [ Z /H], at 3.5 significance via a non-parametric Kendall-tau test. This result was obtained with the assumption that the DLA covering factor is equal to the core fraction. However, Monte Carlo simulations show that the significance of the result is only marginally decreased if the covering factor and the core fraction are uncorrelated, or if there is a random error in the inferred covering factor. We also find statistically significant evidence for redshift evolution in DLA spin temperatures even for the DLA sub-sample at z  〉 1. Since all DLAs at z  〉 1 have angular diameter distances comparable to or larger than those of their background quasars, they have similar efficiency in covering the quasars. We conclude that low covering factors in high- z DLAs cannot account for the observed redshift evolution in spin temperatures.