ALBERT

Description: Damped Lyα absorbers (DLAs) are a well-studied class of absorption line systems, and yet the properties of their host galaxies remain largely unknown. To investigate the origin of these systems, we have conducted an imaging survey of 32 quasar fields with intervening DLAs between z  ~ 1.9 and 3.8, leveraging a technique that allows us to image galaxies at any small angular separation from the background quasars. In this paper, we present the properties of the targeted DLA sample, new imaging observations of the quasar fields, and the analysis of new and archival spectra of the background quasars.

Description: We present results from a survey designed to probe the star formation properties of 32 damped Lyman α systems (DLAs) at z ~ 2.7. By using the ‘double-DLA’ technique that eliminates the glare of the bright background quasars, we directly measure the rest-frame far-ultraviolet flux from DLAs and their neighbouring galaxies. At the position of the absorbing gas, we place stringent constraints on the unobscured star formation rates (SFRs) of DLAs to 2 limits of $\dot{\psi } 〈 0.09\hbox{-}0.27$ M yr –1 , corresponding to SFR surface densities sfr 〈 10 –2.6 –10 –1.5 M yr –1 kpc –2 . The implications of these limits for the star formation law, metal enrichment, and cooling rates of DLAs are examined. By studying the distribution of impact parameters as a function of SFRs for all the galaxies detected around these DLAs, we place new direct constraints on the bright end of the UV luminosity function of DLA hosts. We find that ≤13 per cent of the hosts have $\dot{\psi }\ge 2$ M yr –1 at impact parameters $b_{\rm dla} \le (\dot{\psi }/{\rm M_{\odot }\, yr^{-1}})^{0.8}+6 \,\rm kpc$ , differently from current samples of confirmed DLA galaxies. Our observations also disfavour a scenario in which the majority of DLAs arise from bright Lyman-break galaxies at distances 20 ≤ b dla 〈 100 kpc. These new findings corroborate a picture in which DLAs do not originate from highly star-forming systems that are coincident with the absorbers, and instead suggest that DLAs are associated with faint, possibly isolated, star-forming galaxies. Potential shortcomings of this scenario and future strategies for further investigation are discussed.

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 report a deep search for redshifted H  i  21 cm emission from three damped and sub-damped Lyman α absorbers (DLAs) at z 0.1 with the Green Bank Telescope (GBT). No evidence for a redshifted H  i  21 cm emission signal was obtained in the GBT spectra of two absorbers, with the data on the third rendered unusable by terrestrial interference. The non-detections of H  i  21 cm emission yield strong constraints on the H  i masses of the associated galaxies, M H i 〈 2.3 x 10 9 x ( V/100) 1/2 M for the sub-DLA at z  = 0.0830 towards J1553+3548 and M H i 〈 2.7 x 10 9 x ( V/100) 1/2 M for the DLA at z  = 0.0963 towards J1619+3342, where V is the H  i  21 cm line width, in km s –1 . This continues the trend of low H  i masses found in all low- z DLAs and sub-DLAs that have been searched for redshifted H  i  21 cm emission. Low-redshift absorbers with relatively low H  i column densities, few x 10 20 cm –2 , thus do not typically arise in massive gas-rich galaxies.

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: 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.