FLASH early science - Discovery of an intervening H I 21-cm absorber from an ASKAP survey of the GAMA 23 field

J. R. Allison, E. M. Sadler, S. Bellstedt, L. J.M. Davies, S. P. Driver, S. L. Ellison, M. Huynh, A. D. Kapinska, E. K. Mahony, V. A. Moss, A. S.G. Robotham, M. T. Whiting, S. J. Curran, J. Darling, A. W. Hotan, R. W. Hunstead, B. S. Koribalski, C. D.P. Lagos, M. Pettini, K. A. PimbbletM. A. Voronkov

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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 NH I = (1.2 ± 0.1) × 1020 (Tspin/100 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.

Original languageEnglish
Pages (from-to)3627-3641
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 2020


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