The First Large Absorption Survey in H i (FLASH): I. Science goals and survey design

Abstract

We describe the scientific goals and survey design of the First Large Absorption Survey in H i (FLASH), a wide field survey for 21-cm line absorption in neutral atomic hydrogen (H i) at intermediate cosmological redshifts. FLASH will be carried out with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope and is planned to cover the sky south of $\delta \approx +40\,\deg$ at frequencies between 711.5 and 999.5 MHz. At redshifts between $z = 0.4$ and $1.0$ (look-back times of 4 – 8 Gyr), the H i content of the Universe has been poorly explored due to the difficulty of carrying out radio surveys for faint 21-cm line emission and, at ultra-violet wavelengths, space-borne searches for Damped Lyman- $\alpha$ absorption in quasar spectra. The ASKAP wide field of view and large spectral bandwidth, in combination with a radio-quiet site, will enable a search for absorption lines in the radio spectra of bright continuum sources over 80% of the sky. This survey is expected to detect at least several hundred intervening 21-cm absorbers and will produce an H i-absorption-selected catalogue of galaxies rich in cool, star-forming gas, some of which may be concealed from optical surveys. Likewise, at least several hundred associated 21-cm absorbers are expected to be detected within the host galaxies of radio sources at $0.4 < z < 1.0$ , providing valuable kinematical information for models of gas accretion and jet-driven feedback in radio-loud active galactic nuclei. FLASH will also detect OH 18-cm absorbers in diffuse molecular gas, megamaser OH emission, radio recombination lines, and stacked H i emission.

The ALMA REBELS Survey: The Cosmic H i Gas Mass Density in Galaxies at z ≈ 7

“…At intermediate redshifts (z ∼ 0.4-1), ρ HI can be constrained by stacking the 21 cm signal from a large ensemble of galaxies (e.g., Lah et al 2007;Kanekar et al 2016;Chowdhury et al 2020) or via observations of the 21 cm feature in absorption toward radio-bright continuum sources (e.g., Grasha et al 2020;Allison et al 2022). At z  2 and up to z ≈ 5, the H I gas mass density has primarily been constrained via the Lyα feature from intervening absorbers toward bright background quasars (see Péroux & Howk 2020, and references therein).…”

FLASH pilot survey: detections of associated 21 cm H i absorption in GAMA galaxies at 0.42 < z < 1.00

“…We here define the fields covered by the 6 pointings as 6 GAMA sub-fields. The aim of the deeper 6 hr repeat observations was to test whether significant numbers of absorption lines towards faint radio continuum would be missed in the 2 hr observations planned for the full FLASH survey (Allison et al 2022).…”

“…The First Large Absorption Survey in HI (FLASH; Allison et al 2022) is a wide-field survey for H absorption that will cover the entire southern sky in the redshift range 0.42 < 𝑧 < 1.00, enabled by the Australian SKA Pathfinder (ASKAP) advantages of wide field of view and 288 MHz instantaneous bandwidth. Before the full operation of the 36-antenna ASKAP, early FLASH results using a sub-array of ASKAP dishes have demonstrated the capability of this telescope for H absorption studies (Allison et al 2015(Allison et al , 2017Moss et al 2017;Allison et al 2020;Sadler et al 2020;Mahony et al 2021).…”

The technological and scientific development of ASKAP

“…EMU will provide a deep radio continuum survey at slightly lower frequencies (centred on 944 MHz) expecting to detect ~70 million galaxies (Norris et al 2021a). FLASH searches for intervening and associated HI absorption lines against distant bright continuum sources and expects to detect several hundred in each (Allison et al 2022), while VAST aims to detect highly variable and transient radio sources (Murphy et al 2021). CRAFT uses a specially designed mode to detect and localize fast transient radio sources such as FRBs (Bannister et al 2019).…”