We report an ASKAP search for associated HI 21-cm absorption against bright
radio sources from the Molonglo Reference Catalogue (MRC) 1-Jy sample. The
search uses pilot survey data from the ASKAP First Large Absorption Survey in
\hi (FLASH) covering the redshift range 0.42<z<1.00. From a sample of 62
MRC 1-Jy radio galaxies and quasars in this redshift range we report three new
detections of associated HI 21-cm absorption, yielding an overall detection
fraction of 1.8%−1.5%+4.0%. The detected systems comprise two radio
galaxies (MRC 2216−281 at z=0.657 and MRC 0531−237 at z=0.851) and one
quasar (MRC 2156−245 at z=0.862). The MRC 0531−237 absorption system is
the strongest found to date, with a velocity integrated optical depth of 143.8±0.4kms−1. All three objects with detected HI 21-cm
absorption are peaked-spectrum or compact steep-spectrum (CSS) radio sources,
classified based on our SED fits to the spectra. Two of them show strong
interplanetary scintillation at 162 MHz, implying that the radio continuum
source is smaller than 1 arcsec in size even at low frequencies. Among the
class of peaked-spectrum and compact steep-spectrum radio sources, the HI
detection fraction is 23%−13%+22%. This is consistent within
1σ with a detection fraction of ≈42%−15%+21% in
earlier reported GPS and CSS samples at intermediate redshifts (0.4<z<1.0). All three detections have a high 1.4 GHz radio luminosity, with MRC
0531−237 and MRC 2216−281 having the highest values in the sample, >27.5WHz−1. The preponderance of extended radio sources in our sample
could partially explain the overall low detection fraction, while the effects
of a redshift evolution in gas properties and AGN UV luminosity on the neutral
gas absorption still need to be investigated.Comment: 28 pages, 9 figures and 7 Tables. Submitted to MNRA