  The neutral atomic gas content of galaxies is usually studied in the HI 21cm
emission line of hydrogen. However, at higher redshifts, we need very deep
integrations to detect HI emission. The HI absorption does not suffer from this
dependence on distance as long as there is a bright enough background radio
source. However, resolved HI absorption studies of galaxies are rare. We report
one such rare study of resolved HI absorption against the radio galaxy 3C 433
at $z = 0.101$ with the VLA. The resolved kinematics of the absorber, located
against the southern lobe of the 3C 433, shows that it has regular kinematics
with 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 Telescopio
CANARIAS (GTC) show that the absorber is a faint disc galaxy in the same
environment as 3C 433 with a stellar mass $\sim 10^{10} M_{\odot}$ and a
star-formation rate of 0.15 $M_{\odot}~yr^{-1}$ or less. For its HI mass, HI
column density, stellar mass, and star-formation rate, this galaxy lies well
below the main sequence of star-forming galaxies. Its HI mass is lower than the
galaxies studied in HI emission at $z \sim 0.1$. Our GTC imaging reveals
interesting alignments between H$\alpha$ and radio emission in the HI companion
and in the host galaxy of the AGN as well as in the circumgalactic medium in
between. This suggests that the shock ionization of gas by the propagating
radio source may happen across tens of kpc. Our work supports the potential of
studying the HI content in galaxies via absorption in the case of a fortuitous
alignment with an extended radio continuum. This allows us to trace galaxies
with 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 with
the SKA pathfinders will be able to detect many such systems.
