The mysterious HI deficiency of NGC3175

Australia Telescope Compact Array HI observations reveal the existence of 5 .8 10(8)M(circle dot) of HI gas in the central 7 kpc of the edge-on spiral galaxy NGC3175. 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, sho ws no HI emission, has a very red colour and exhibits neither radio continu um nor H alpha emission. This indicates that the outer part of NGC3175 is q uiescent, i.e. not forming stars at a measurable rate. Its HI deficiency an d the small extent of the HI layer, which is confined to the boundaries of the optically visible disk, make NGC3175 a peculiar spiral galaxy. No inter galactic H I gas in the NGC3175 group was detected in our interferometric o bservations. Earlier Parkes telescope single dish HI observations put an up per limit on the amount of diffuse gas that might have been missed by the i nterferometer at 2 10(8) M-circle dot. On DSS plates no galaxy in the NGC31 75 group of galaxies (Garcia 1993) is close enough to it and none exhibits disturbances that could indicate a close interaction which might have led t o the stripping of large parts of its H I gas. Thus, despite an extensive m ulti-wavelength investigation, the reason for the unusual absence of H I an d star formation activity in the outer disk of NGC3175 remains an intriguin g mystery.