Ionisation of the atomic gas in redshifted radio sources

Abstract

We report the results of a survey for HI 21-cm absorption at z < 0.4 in a new sample of radio sources with the Giant Metrewave Radio Telescope. Of the 11 sources for which there are good data, we obtain zero detections, where four are expected upon accounting for the ionising photon rates and sensitivity. Adding these to the previously published values, we confirm that the non-detection of 21-cm absorption in active sources at high redshift is due to photo-ionisation of the gas rather than excitation by 21-cm photons (significant at 6.09 sigma and 2.90 sigma, respectively). We also confirm a strong correlation between the absorption strength and the reddening of the source, suggesting that dust plays a significant role in shielding the gas from the ambient ultra-violet field. An anti-correlation between the 21-cm detection rate and the radio turnover frequency is also found, which runs contrary to what is expected on the basis that the higher the turnover frequency, the more compact the source. It is, however, consistent with the hypothesis that the turnover frequency is related to the electron density, supported by a correlation between the turnover frequency and ionising photon rate.

Accepted by MNRAS

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• The HI absorption zoo: JVLA extension to $z\sim 0.4$
• QSO photometric redshifts from SDSS, WISE and GALEX colours
• uGMRT detection of associated HI 21 cm absorption at $z\approx 3.5$
• The evolution of cold neutral gas and the star formation history
• Redshift evolution of the HI detection rate in radio-loud active galactic nuclei
• Intervening or associated? Machine learning classification of redshifted H I 21-cm absorption
• Redshifts of radio sources in the Million Quasars Catalogue from machine learning
• On the scarcity of redshifted OH and millimetre-band molecular absorption
• Ultra-violet photo-ionisation in far-infrared selected sources