uGMRT detection of associated HI 21 cm absorption at $z\approx 3.5$

Abstract

We report a uGMRT detection of HI 21-cm absorption associated with the radio source 8C 0604+728, at $z=3.52965$. The source is at the highest redshift at which associated HI 21-cm absorption has been discovered to date, surpassing earlier known absorber at $z\approx 3.39$. We estimate ultraviolet luminosities of $(3.2\pm 0.1)\times 10^{23}\mathrm{W}\mathrm{H}{\mathrm{z}}^{-1}$ and $(6.2\pm 0.2)\times 10^{23}\mathrm{W}\mathrm{H}{\mathrm{z}}^{-1}$ , and ionising photon rates of $(1.8\pm 0.1)\times 10^{56}{\mathrm{s}}^{-1}$ and $(5.0\pm 0.1)\times 10^{56}{\mathrm{s}}^{-1}$, using data at different epochs; the source shows year-scale variability in both its luminosity and photon rate. The luminosity and photon rate at later epochs are $\approx 6.2$ and $\approx 1.7$ times higher than thresholds suggested in the literature above which all the neutral hydrogen in the AGN host galaxy is expected to be ionised. The detection demonstrates that neutral hydrogen can survive in the host galaxies of AGNs with high ultraviolet luminosities. We estimate a high equivalent width ratio of $15.2$ for the $\mathrm{Ly}\alpha$ and HeII emission lines detected in the optical spectrum, that is consistent with AGN photoionisation models. However, a significant contribution from young stellar populations to the excess $\mathrm{Ly}\alpha$ flux cannot be ruled out.

7 pages, 4 Figures, one table. Accepted for publication in MNRAS, corrected few typos in section 3.3 and summary

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• HI absorption at z~0.7 against the lobe of the powerful radio galaxy PKS 0409-75
• FLASH Pilot Survey: Detections of associated 21 cm HI absorption in GAMA galaxies at 0.42 < z <1.00
• Redshift evolution of the HI detection rate in radio-loud active galactic nuclei
• Analyzing the 21cm forest with Wavelet Scattering Transform: Insight into non-Gaussian features of the 21cm forest
• HI gas playing hide-and-seek around a powerful FRI-type quasar at z$\sim$2.1
• The FAST HI 21-cm absorption blind survey. II. -- Statistic Exploration for Associated and Intervening systems