ESO215-G?009: An Extreme HI-Rich Dwarf Irregular Galaxy
Abstract
We present deep BVRI band images and HI line observations of the nearby, low surface brightness galaxy ESO215-G?009 which were obtained with the ANU 2.3-m Telescope and the Australia Telescope Compact Array, respectively. ESO215-G?009 was selected from the HIPASS Bright Galaxy Catalog because it has the second highest HI mass-to-light ratio of the galaxies with measured B band apparent magnitudes. We find that it is an isolated dwarf irregular galaxy with an old stellar population. We place an upper limit on the current star formation rate of ~2.5 x 10-3 Msun/yr. The extended HI disk shows regular rotation (vrot = 51 +/- 8 km/s), and at a column density of ~5.0 x 1019 atoms/cm2 can be traced out to over six times the Holmberg radius of the stellar component (radius at muB = 26.6 mag/arcsec2). After foreground star subtraction, we measure a B band apparent magnitude of 16.13 +/- 0.07 mag within a radius of 80". The HI flux density is 122 +/- 4 Jy km/s within a radius of 370". Given a Galactic extinction of AB = 0.95 +/- 0.15 mag, we derive an HI mass-to-light ratio of 22 +/- 4 Msun/Lsun for ESO215-G?009. To our knowledge this is the highest ratio for a galaxy to be confirmed by accurate measurement to date.
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