Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 476, Issue 3, Pages 3580-3590Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/sty443
Keywords
galaxies: active; galaxies: ISM; quasars: absorption lines; infrared: galaxies; radio lines: galaxies; ultraviolet: galaxies
Categories
Funding
- Victoria Doctoral Scholarship
- National Aeronautics and Space Administration
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Upon an expansion of all of the searches for redshifted HI 21-cm absorption (0.002 1 <= z <= 5.19), we update recent results regarding the detection of 21-cm in the non-local Universe. Specifically, we confirm that photo-ionization of the gas is the mostly likely cause of the low detection rate at high redshift, in addition to finding that at z less than or similar to 0.1 there may also be a decrease in the detection rate, which we suggest is due to the dilution of the absorption strength by 21-cm emission. By assuming that associated and intervening absorbers have similar cosmological mass densities, we find evidence that the spin temperature of the gas evolves with redshift, consistent with heating by ultraviolet photons. From the near-infrared (lambda = 3.4, 4.6 and 12 mu m) colours, we see that radio galaxies become more quasar-like in their activity with increasing redshift. We also find that the non-detection of 21-cm absorption at high redshift is not likely to be due to the selection of gas-poor ellipticals, in addition to a strong correlation between the ionizing photon rate and the [3.4] - [4.6] colour, indicating that the UV photons arise from AGN activity. Like previous studies, we find a correlation between the detection of 21-cm absorption and the [4.6] - [12] colour, which is a tracer of star-forming activity. However, this only applies at the lowest redshifts (z less than or similar to 0.1), the range considered by the other studies.
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