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The neutral atomic gas content of galaxies is usually studied in the HI 21cmemission line of hydrogen. However, at higher redshifts, we need very deepintegrations to detect HI emission. The HI absorption does not suffer from thisdependence on distance as long as there is a bright enough background radiosource. However, resolved HI absorption studies of galaxies are rare. We reportone such rare study of resolved HI absorption against the radio galaxy 3C 433at $z = 0.101$ with the VLA. The resolved kinematics of the absorber, locatedagainst the southern lobe of the 3C 433, shows that it has regular kinematicswith an HI mass $\lesssim 3.4 \times 10^{8} M_{\odot}$ for T$_{spin} =$ 100K.Our deep optical continuum and H$\alpha$ observations from the Gran TelescopioCANARIAS (GTC) show that the absorber is a faint disc galaxy in the sameenvironment as 3C 433 with a stellar mass $\sim 10^{10} M_{\odot}$ and astar-formation rate of 0.15 $M_{\odot}~yr^{-1}$ or less. For its HI mass, HIcolumn density, stellar mass, and star-formation rate, this galaxy lies wellbelow the main sequence of star-forming galaxies. Its HI mass is lower than thegalaxies studied in HI emission at $z \sim 0.1$. Our GTC imaging revealsinteresting alignments between H$\alpha$ and radio emission in the HI companionand in the host galaxy of the AGN as well as in the circumgalactic medium inbetween. This suggests that the shock ionization of gas by the propagatingradio source may happen across tens of kpc. Our work supports the potential ofstudying the HI content in galaxies via absorption in the case of a fortuitousalignment with an extended radio continuum. This allows us to trace galaxieswith low HI masses which would otherwise be missed by deep HI emission surveys.In conjunction with the deep all-sky optical surveys, the blind HI surveys withthe SKA pathfinders will be able to detect many such systems.

Disc galaxy resolved in HI absorption against the radio lobe of 3C 433: Case study for future surveys