The mysterious H I deficiency of NGC 3175

¹ Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
² XMM-Newton Science Operations Centre, Apartado 50727, 28080 Madrid, Spain
³ Astrophysics Division, Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands
⁴ Anglo-Australian Observatory, PO Box 296, Epping, NSW 1710, Australia

Corresponding author: M. Dahlem, mdahlem@eso.org

Received: 6 June 2000
Accepted: 28 February 2001

Abstract

Australia Telescope Compact Array H I observations reveal the existence of 5.8 10⁸ of H I gas in the central 7 kpc of the edge-on spiral galaxy NGC 3175. The detected H Iand 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 H I emission, has a very red colour and exhibits neither radio continuum nor Hα emission. This indicates that the outer part of NGC 3175 is quiescent, i.e. not forming stars at a measurable rate. Its H I deficiency and the small extent of the H I layer, which is confined to the boundaries of the optically visible disk, make NGC 3175 a peculiar spiral galaxy. No intergalactic H I gas in the NGC 3175 group was detected in our interferometric observations. Earlier Parkes telescope single dish H I observations put an upper limit on the amount of diffuse gas that might have been missed by the interferometer at 2 10⁸ . On DSS plates no galaxy in the NGC 3175 group of galaxies (García [CITE]) 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 H I gas. Thus, despite an extensive multi-wavelength investigation, the reason for the unusual absence of H I and star formation activity in the outer disk of NGC 3175 remains an intriguing mystery.