Journal Article

A search for H <span class="smallCaps">i</span> 21 cm absorption in strong Mg <span class="smallCaps">ii</span> absorbers in the redshift desert

N. Kanekar, J. X. Prochaska, S. L. Ellison and J. N. Chengalur

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 1, pages 385-401
Published in print June 2009 | ISSN: 0035-8711
Published online June 2009 | e-ISSN: 1365-2966 | DOI:
A search for H i 21 cm absorption in strong Mg ii absorbers in the redshift desert

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We report results from a deep search for redshifted H i 21 cm absorption in 55 strong Mg iiλ2796 absorbers (having Wλ27960 > 0.5 Å) at intermediate redshifts, 0.58 < zabs < 1.70, with the Green Bank Telescope (GBT) and the Giant Metrewave Radio Telescope (GMRT). Nine detections of H i 21 cm absorption were obtained, all at 1.17 < zabs < 1.68, including three systems reported earlier by Gupta et al. (2007). Absorption was not detected at >3σ significance in 32 other Mg ii absorbers, with 26 of these providing strong upper limits to the H i 21 cm optical depth, τ < 0.013 per ∼10 km s−1. For the latter 26 systems, the spin temperature Ts of the absorber must be >[800 ×f] K (where f is the covering factor), if the H i column density is ≥2 × 1020 cm−2, i.e. if the absorber is a damped Lyman α system (DLA). Data on the remaining 13 systems of the sample were affected by radio frequency interference and were hence not useful.

Two of the Mg ii absorbers, at zabs∼ 1.4106 towards 2003−025 and at zabs∼ 0.9115 towards 2149+212, are known DLAs. We detect H i 21 cm absorption towards 2003−025 with the GMRT and estimate the spin temperature of the DLA to be Ts=[(905 ± 380) ×f] K. Conversely, the GBT observations of 2149+212 resulted in a non-detection of H i 21 cm absorption, yielding the 3σ limit Ts > [2700 ×f] K.

Excluding ‘associated’ systems (within 3000 km s−1 of the quasar redshift), the detection rate of H i 21 cm absorption in strong Mg iiλ2796 absorbers is per cent, at a 3σ optical depth sensitivity of ∼0.013 per 10 km s−1. Comparing the detection rates of H i 21 cm and damped Lyman α absorption in strong Mg ii absorber samples yields a detection rate of H i 21 cm absorption in DLAs of per cent, consistent with the detection rate in low-z DLAs. Since H i 21 cm absorption arises in cold neutral gas, this indicates that most gas-rich galaxies contain significant fractions of cold H i by z∼ 1.

Finally, we use the observed detection rate of H i 21 cm absorption in Mg ii absorbers to infer the cosmological mass density of neutral gas in DLAs, assuming that (1) the average H i column density in our H i 21 cm absorber sample is the same as that measured by Rao, Turnshek & Nestor in their DLA sample and (2) the detection rate of H i 21 cm absorption in the DLAs of our Mg ii sample is the same as that in known DLAs at 0.09 < zabs < 3.45. We obtain ΩGAS∼ (0.55+0.42−0.22) × 10−3, at , slightly lower than, but consistent with, the value obtained by Rao, Turnshek & Nestor from their DLA survey at similar redshifts.

Keywords: galaxies: evolution; galaxies: ISM; radio lines: galaxies

Journal Article.  12488 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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