An HI study of the collisional ring galaxy NGC 922
  We present new atomic hydrogen (HI) observations of the collisional ring
galaxy NGC 922 obtained using the Australia Telescope Compact Array. Our
observations reveal for the first time the vast extent of the HI disc of this
galaxy. The HI morphology and kinematics of NGC 922 show that this galaxy is
not the product of a simple drop-through interaction, but has a more complex
interaction history. The integrated HI flux density of NGC 922 from our
observations is 24.7 Jy km s$^{-1}$, which is within the error of the flux
value obtained using the $64$-m Parkes radio telescope. This flux density
translates to a total HI mass of $1.1*10^{10}$ M$_{\circ}$ and corresponds to
an HI to total mass fraction (M$_{HI}$/M$_{tot}$) of approximately $0.11$. The
gaseous structures of NGC 922 are more extended to the north and include an HI
tail that has a projected physical length of $8$ kpc. Gas warps are also
evident in the velocity field of NGC 922 and are more prominent on the
approaching and the western side of the disc. In comparison with a large sample
of star-forming galaxies in the local Universe, NGC 922 possesses a high gas
fraction relative to galaxies with a similar stellar mass of ~$10^{10.4}$
M$_{\circ}$, and exhibits a high specific star formation rate.
