Giant Metrewave Radio Telescope detection of associated HI 21cm absorption at $z = 1.2230$ towards TXS1954+513

We have used the 610 MHz receivers of the Giant Metrewave Radio Telescope (GMRT) to detect associated HI 21cm absorption from the $z = 1.2230$ blazar TXS1954+513. The GMRT HI 21cm absorption is likely to arise against either the milli-arcsecond-scale core or the one-sided milli-arcsecond-scale radio jet, and is blueshifted by $\approx 328$ km s$^{-1}$ from the blazar redshift. This is consistent with a scenario in which the HI cloud giving rise to the absorption is being driven outward by the radio jet. The integrated HI 21cm optical depth is $(0.716 \pm 0.037)$ km s$^{-1}$, implying a high HI column density, $N_{\rm HI} = (1.305 \pm 0.067) \times ({\rm T_s/100\: K}) \times 10^{20}$ cm$^{-2}$, for an assumed HI spin temperature of 100 K. We use Nickel Telescope photometry of TXS1954+513 to infer a high rest-frame 1216 \AA\ luminosity of $(4.1 \pm 1.2) \times 10^{23}$ W Hz$^{-1}$. The $z = 1.2230$ absorber towards TXS1954+513 is only the fifth case of a detection of associated HI 21cm absorption at $z > 1$, and is also the first case of such a detection towards an active galactic nucleus (AGN) with a rest-frame ultraviolet luminosity $\gg 10^{23}$ W Hz$^{-1}$, demonstrating that neutral hydrogen can survive in AGN environments in the presence of high ultraviolet luminosities.