Abstract
ABSTRACT We present early science results from the First Large Absorption Survey in H i (FLASH), a spectroscopically blind survey for 21-cm absorption lines in cold hydrogen (H i) gas at cosmological distances using the Australian Square Kilometre Array Pathfinder (ASKAP). We have searched for H i absorption towards 1253 radio sources in the GAMA 23 field, covering redshifts between z = 0.34 and 0.79 over a sky area of approximately 50 deg2. In a purely blind search, we did not obtain any detections of 21-cm absorbers above our reliability threshold. Assuming a fiducial value for the H i spin temperature of Tspin = 100 K and source covering fraction cf = 1, the total comoving absorption path-length sensitive to all Damped Lyman α Absorbers (DLAs; NH i ≥ 2 × 1020 cm−2) is ΔX = 6.6 ± 0.3 (Δz = 3.7 ± 0.2) and super-DLAs (NH i ≥ 2 × 1021 cm−2) is ΔX = 111 ± 6 (Δz= 63 ± 3). We estimate upper limits on the H i column density frequency distribution function that are consistent with measurements from prior surveys for redshifted optical DLAs, and nearby 21-cm emission and absorption. By cross-matching our sample of radio sources with optical spectroscopic identifications of galaxies in the GAMA 23 field, we were able to detect 21-cm absorption at z = 0.3562 towards NVSS J224500−343030, with a column density of $N_{\rm H\,\small{I}} = (1.2 \pm 0.1) \times 10^{20}\, (T_{\rm spin}/100\, \mathrm{K})$ cm−2. The absorber is associated with GAMA J22450.05−343031.7, a massive early-type galaxy at an impact parameter of 17 kpc with respect to the radio source and which may contain a massive (MH i ≳ 3 × 109 M⊙) gas disc. Such gas-rich early types are rare, but have been detected in the nearby Universe.
Highlights
Star formation and supermassive black hole (SMBH) growth are two important processes in galaxies that influence their evolution throughout cosmic history
We do not yet understand why the global rates of star formation (e.g. Hopkins & Beacom 2006; Madau & Dickinson 2014; Driver et al 2018) and SMBH growth (e.g. Ueda et al 2003; Shankar et al 2009) both peaked at z ≈ 2, and declined by an order magnitude to this epoch
Our results show that a lower rate of detection for intrinsic 21-cm absorbers is expected from future wide-field radio-selected surveys, by how much is yet to be determined
Summary
Star formation and supermassive black hole (SMBH) growth are two important processes in galaxies that influence their evolution throughout cosmic history. Searches for 21-cm absorption in known DLAs suggest that the typical spin temperature of high column density systems is anticorrelated with their metallicity and may increase at redshifts above z = 2, beyond the peak of star formation in the Universe (Kanekar et al 2014). This would be consistent with a model whereby relatively metal-poor DLAs in the early Universe By explicitly modelling the source covering fraction as a function of angular diameter distance, Curran (2019, 2017) used literature searches for redshifted 21-cm absorption to show that the spin temperature has evolved with the star formation rate history of the Universe.
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