The spin temperature of high-redshift damped Lyman α systems

Abstract 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 Ts. We find a statistically significant (4σ) difference between the Ts 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, Ts ≳ 1000 K. The Ts distributions in DLAs and the Galaxy are also significantly (≈6σ) different, with more high-Ts sightlines in DLAs than in the Milky Way. The high Ts 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 Ts 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 NH i values are, on average, larger than the smoothed ones for NH i > 1021 cm−2, but lower than the smoothed NH i estimates for NH i < 1021 cm−2. Since there are far more DLAs with low NH i values than high ones, the use of the optical NH 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 Ts and metallicity [Z/H], at 3.5σ 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. ISM: evolution, galaxies: high-redshift, quasars: absorption lines, radio lines: ISM INTRODUCTION Quasar absorption spectra offer the possibility of selecting galaxies by their absorption signatures, and thus obtaining samples of high-z galaxies without a bias towards the most luminous systems. The highest H i column density absorbers detected in quasar spectra are the damped Lyman α systems (DLAs). With H i column densities |$N_{{\rm H}\,{\small {I}}}\ge 2 \times 10^{20}$| cm−2 (Wolfe, Gawiser & Prochaska 2005), similar to values seen in sightlines through the Milky Way and nearby gas-rich galaxies, DLAs have long been identified as the high-redshift counterparts of normal galaxies in the local Universe. The nature of galaxies that give rise to DLAs at different redshifts, and their typical size, mass, kinematic structure and physical conditions are all important ingredients for understanding galaxy evolution. The Sloan Digital Sky Survey (SDSS; Abazajian et al. 2009) has resulted in the detection of a vast number of DLAs at high redshifts, with nearly 7000 candidate absorbers now known at z > 2.2 (e.g. Prochaska, Herbert-Fort & Wolfe 2005; Noterdaeme et al. 2009, 2012b; Prochaska & Wolfe 2009). Unfortunately, contamination from the background quasars has made it very difficult to identify the host galaxy of the DLAs in optical images (e.g. Warren et al. 2001; but see Fumagalli et al. 2010). The low sensitivity of today's radio telescopes has meant that one cannot image the DLA hosts in the standard radio H i 21 cm and CO emission lines that have been used for detailed studies of the kinematics and dynamics of nearby galaxies. And, even following a number of recent studies, only a handful of high-z DLAs have been detected in Hα or Lyα emission (e.g. Møller, Fynbo & Fall 2004; Fynbo et al. 2010, 2011; Noterdaeme et al. 2012a; Péroux et al. 2012), with typical star formation rates (SFRs) ≲ few M⊙ per year (Péroux et al. 2012), even for high-metallicity absorbers. Thus, despite much effort over the last three decades, relatively little information has so far been gleaned from emission studies of DLAs. Detailed absorption studies remain our primary source of information about the absorbers. Around 200 DLAs have measured metallicities, elemental abundances and kinematics, from high-resolution optical echelle spectroscopy (e.g. Pettini et al. 1994, 1997, 1999, 2008; Prochaska et al. 2003a, 2007; Dessauges-Zavadsky et al. 2004; Khare et al. 2004; Ledoux et al. 2006; Petitjean, Ledoux & Srianand 2008; Penprase et al. 2010; Cooke et al. 2011; Battisti et al. 2012). These studies have yielded interesting results. For example, mean DLA metallicities have been shown to increase with decreasing redshift, as expected from models of galaxy evolution, although low-metallicity DLAs are quite common even at low redshifts (Prochaska et al. 2003b; Kulkarni et al. 2005, 2010; Rafelski et al. 2012). A positive correlation has been found between the metallicity and both the velocity spreads of low-ionization metal lines (Wolfe & Prochaska 1998; Ledoux et al. 2006) and the rest equivalent width of the Si ii λ1526 line (Prochaska et al. 2008a). This has been interpreted as evidence for a mass–metallicity relation in DLAs, similar to that seen in emission-selected high-z galaxies (e.g. Tremonti et al. 2004; Savaglio et al. 2005; Erb et al. 2006; Neeleman et al. 2013). Molecular hydrogen (H2) absorption, along with C i absorption, has been detected in about a dozen DLAs, with strong upper limits on the molecular fraction in ∼80 per cent of the observed systems (e.g. Levshakov & Varshalovich 1985; Ge & Bechtold 1997; Ge, Bechtold & Kulkarni 2001; Ledoux, Petitjean & Srianand 2003; Noterdaeme et al. 2008; Jorgenson, Wolfe & Prochaska 2010; Milutinovic et al. 2010). This has provided information on local conditions in the molecular phase, including estimates of the number density, temperature and strength of the ultraviolet (UV) radiation field (e.g. Srianand et al. 2005), along with measurements of the microwave background temperature at different redshifts (e.g. Noterdaeme et al. 2011). While absorption spectra (...truncated)