  Australia Telescope Compact Array HI observations reveal the existence of
5.8x10^8 M_sun of HI gas in the central 7 kpc of the edge-on spiral galaxy NGC
3175. The detected HI and CO gas can explain why star formation, as traced by
other emission processes, is going on in the inner part of its disk. On the
other hand, the entire outer disk, beyond 3.5 kpc radius, shows no HI emission,
has a very red colour and exhibits neither radio continuum nor H-alpha
emission. This indicates that the outer part of NGC 3175 is quiescent, i.e. not
forming stars at a measurable rate. Its HI deficiency and the small extent of
the HI layer, which is confined to the boundaries of the optically visible
disk, make NGC 3175 a peculiar spiral galaxy. No intergalactic HI gas in the
NGC 3175 group was detected in our interferometric observations. Earlier Parkes
telescope single dish HI observations put an upper limit on the amount of
diffuse gas that might have been missed by the interferometer at 2x10^8 M_sun.
On DSS plates no galaxy in the NGC 3175 group of galaxies (Garcia 1993) is
close enough to it and none exhibits disturbances that could indicate a close
interaction which might have led to the stripping of large parts of its HI gas.
Thus, despite an extensive multi-wavelength investigation, the reason for the
unusual absence of HI and star formation activity in the outer disk of NGC 3175
remains an intriguing mystery.
