The mid-infrared properties and gas content of active galaxies over large look-back times

Abstract

Upon an expansion of all of the searches for redshifted HI 21-cm absorption (0.0021 < z 5.19), we update recent results regarding the detection of 21-cm in the non-local Universe. Specifically, we confirm that photo-ionisation of the gas is the mostly likely cause of the low detection rate at high redshift, in addition to finding that at z < 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 ultra-violet photons. From the near--infrared (3.4, 4.6 and 12 micron) 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 ionising 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 < 0.1), the range considered by the other studies.

Accepted by MNRAS

References in corpus (18)

• Probing radio source environments via HI and OH absorption
• The HI absorption "Zoo"
• A survey for redshifted molecular and atomic absorption lines I
• A Giant Metrewave Radio Telescope search for associated H{\sc i} 21\,cm absorption in GHz-peaked-spectrum sources
• Probing the gas content of radio galaxies through HI absorption stacking
• HI Absorption in High-Frequency Peaker Galaxies
• The detectability of HI 21-cm absorption in damped Lyman-alpha systems
• Correlation between X-ray and radio absorption in compact radio galaxies
• A comparative study of intervening and associated HI 21-cm absorption profiles in redshifted galaxies
• A search for HI 21cm absorption toward the highest redshift radio loud objects
• Connecting X-ray absorption and 21cm neutral hydrogen absorption in obscured radio AGN
• Giant Metrewave Radio Telescope detection of associated HI 21cm absorption at $z=1.2230$ towards TXS1954+513
• HI emission and absorption in nearby, gas-rich galaxies
• HI absorption in nearby compact radio galaxies
• Atomic and Molecular Absorption in Redshifted Radio Sources
• HI absorption towards low luminosity radio-loud AGNs of different accretion modes and WISE colours
• WISE data as a photometric redshift indicator for radio AGN
• The potential of tracing the star formation history with HI 21-cm in intervening absorption systems

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• FLASH Pilot Survey: Detections of associated 21 cm HI absorption in GAMA galaxies at 0.42 < z <1.00
• The AGN fuelling/feedback cycle in nearby radio galaxies - V. The cold atomic gas of NGC 3100 and its group
• QSO redshift estimates from optical, near-infrared and ultraviolet colours
• HI Absorption in Low-power Radio AGNs Detected by FAST
• FASHI: An untargeted survey of the 21 cm HI absorption galaxies with FAST
• Ultra-violet photo-ionisation in far-infrared selected sources