A search for H i 21 cm absorption in strong Mg ii absorbers in the redshift desert

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, τ3σ < 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.

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