Discovery of a large HI ring around the quiescent galaxy AGC 203001
  Here we report the discovery with the Giant Metrewave Radio Telescope of an
extremely large ($\sim$115 kpc in diameter) HI ring off-centered from a massive
quenched galaxy, AGC 203001. This ring does not have any bright extended
optical counterpart, unlike several other known ring galaxies. Our deep $g$,
$r$, and $i$ optical imaging of the HI ring, using the MegaCam instrument on
the Canada-France-Hawaii Telescope, however, shows several regions with faint
optical emission at a surface brightness level of $\sim$28 mag/arcsec$^2$. Such
an extended HI structure is very rare with only one other case known so far --
the Leo ring. Conventionally, off-centered rings have been explained by a
collision with an "intruder" galaxy leading to expanding density waves of gas
and stars in the form of a ring. However, in such a scenario the impact also
leads to large amounts of star formation in the ring which is not observed in
the ring presented in this paper. We discuss possible scenarios for the
formation of such HI dominated rings.
