The mysterious HI deficiency of NGC 3175

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.