FLASH Early Science – discovery of an intervening HI 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 Kapińska, 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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9 Citations (Scopus)


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 NHI=(1.2±0.1)×1020(Tspin/100K) 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
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Early online date9 Apr 2020
Publication statusPublished - May 2020


  • Galaxies: evolution
  • Galaxies: high redshift
  • Galaxies: ISM
  • Galaxies: strucutre
  • Radio: lines galaxies


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