Journal Article

On the detectability of H <span class="smallCaps">i</span> 21 cm in Mg <span class="smallCaps">ii</span> absorption systems

S. J. Curran

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 402, issue 4, pages 2657-2665
Published in print March 2010 | ISSN: 0035-8711
Published online March 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.16080.x
On the detectability of H i 21 cm in Mg ii absorption systems

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We investigate the effect of two important, but of neglected, factors which can affect the detectability of H i 21-cm absorption in Mg ii absorption systems. The effect of line-of-sight geometry on the coverage of the background radio flux and any possible correlation between the 21-cm line strength and the rest-frame equivalent width of the Mg ii 2796 Å line, as is seen in the case of damped Lyman α absorption systems (DLAs). Regarding the former, while the observed detection rate at small angular diameter distance ratios (DAabs/DAQSO > 0.8) is a near certainty (P > 0.9), for an unbiased sample, where either a detection or a non-detection is equally likely, at DAabs/DAQSO≥ 0.8 the observed detection rate has only a probability of P≲ 10−15 of occurring by chance. This ≳8σ significance suggests that the mix of DAabs/DAQSO values at zabs≲ 1 is correlated with the mix of detections and non-detections at low redshift, while the exclusively high values of the ratio (DAabs/DAQSO∼ 1) at zabs≳ 1 contribute to the low detection rates at high redshift.

In DLAs, the correlation between the 21-cm line strength and the Mg ii equivalent width (Wλ2796r) is dominated by the velocity spread of the 21-cm line. This has recently been shown not to hold for Mg ii systems in general. However, we do find the significance of the correlation to increase when the Mg ii absorbers with Mg i 2852 Å equivalent widths of Wλ2852r > 0.5Å are added to the DLA sample. This turns out to be a subset of the parameter space where Mg ii absorbers and DLAs overlap and the fraction of Mg ii absorbers known to be DLAs rises to 50 per cent (Rao, Turnshek & Nestor 2006). We therefore suggest that the width of the 21-cm line is correlated with Wλ2796r for all systems likely to be DLAs and note a correlation between Wλ2852r (Mg i) and NH i, which is not apparent for the singly ionized lines. Furthermore, the 21-cm detection rate at DAabs/DAQSO < 0.8 rises to ≳90 per cent for absorbers with Wλ2852r > 0.5Å and large values of DAabs/DAQSO may explain why the absorbers which have similar values of Wλ2796r to the detections remain undetected. We do, however, also find the neutral hydrogen column densities of the non-detections to be significantly lower than those of the detections, which could also contribute to their weak absorption. Applying the correlation to yield column densities for the Mg ii absorbers in which this is unmeasured, we find no evidence of a cosmological evolution in the neutral hydrogen column density in the absorbers searched for in 21-cm.

Keywords: galaxies: high-redshift; galaxies: ISM; quasars: absorption lines; cosmology: observations; radio lines: galaxies

Journal Article.  6377 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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