TY - JOUR
T1 - The spin temperature of high-redshift damped lyman α systems
AU - Kanekar, N.
AU - Prochaska, J. X.
AU - Smette, A.
AU - Ellison, S. L.
AU - Ryan-Weber, E. V.
AU - Momjian, E.
AU - Briggs, F. H.
AU - Lane, W. M.
AU - Chengalur, J. N.
AU - Delafosse, T.
AU - Grave, J.
AU - Jacobsen, D.
AU - Bruyn, A. G.
PY - 2014/2
Y1 - 2014/2
N2 - 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) HI 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 HI column density measured towards the optical quasar might be systematically different from that towards the radio core by comparing the HI column densities inferred from HI 21 cm emission studies at different spatial resolutions (15 pc-1 kpc) in the Large Magellanic Cloud. The high-resolution NHI values are, on average, larger than the smoothed ones for NHI > 10-21 cm-2, but lower than the smoothed NHI estimates for NHI < 1021 cm-2. Since there are far more DLAs with low NHI values than high ones, the use of the optical NHI 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 onlymarginally 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.
AB - 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) HI 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 HI column density measured towards the optical quasar might be systematically different from that towards the radio core by comparing the HI column densities inferred from HI 21 cm emission studies at different spatial resolutions (15 pc-1 kpc) in the Large Magellanic Cloud. The high-resolution NHI values are, on average, larger than the smoothed ones for NHI > 10-21 cm-2, but lower than the smoothed NHI estimates for NHI < 1021 cm-2. Since there are far more DLAs with low NHI values than high ones, the use of the optical NHI 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 onlymarginally 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.
KW - Absorption lines-radio lines
KW - Evolution-galaxies
KW - High-redshift-quasars
KW - ISM
KW - Ism
UR - http://www.scopus.com/inward/record.url?scp=84894076737&partnerID=8YFLogxK
U2 - 10.1093/mnras/stt2338
DO - 10.1093/mnras/stt2338
M3 - Article
SN - 0035-8711
VL - 438
SP - 2131
EP - 2166
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
IS - 3
ER -