2018MNRAS.476.2432G

We present a detailed study of the quasar-galaxy pair: J1243+4043-UGC 07904. The sight line of the background quasar ( z_q = 1.5266) passes through a region of the galaxy (z_g = 0.0169) at an impact parameter of 6.9 kpc with high metallicity (0.5 Z_☉) and negligible dust extinction. We detect H I 21-cm absorption from the foreground galaxy at arcsecond and milliarcsecond scales. For typical cold neutral medium (CNM) temperatures in the Milky Way, this 21-cm absorber can be classified as a damped Lyα absorber (DLA). We infer the harmonic mean spin temperature of the gas to be ∼400 K and for a simple two-phase medium we estimate the CNM fraction to be f_CNM = 0.27. This is remarkably consistent with the CNM fraction observed in the Galaxy and less than that of high-redshift DLAs. The quasar exhibits a core-jet morphology on milliarcsecond scales, corresponding to an overall extent of ∼9 pc at z_g. We show that the size of CNM absorbing clouds associated with the foreground galaxy is >5 pc and they may be part of cold gas structures that extend beyond ∼35 pc. Interestingly, the rotation measure of quasar J1243+4043 is higher than any other source in samples of quasars with high-z DLAs. However, we do not find any detectable differences in rotation measures and polarization fraction of sight lines with or without high-z (z >= 2) DLAs or low-z (z <= 0.3) 21-cm absorbers. Finally, the foreground galaxy UGC 07904 is also part of a galaxy group. We serendipitously detect H I 21-cm emission from four members of the group, and an ∼80 kpc long H I bridge connecting two of the other members. The latter, together with the properties of the group members, suggests that the group is a highly interactive environment.