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 M_sun/yr. The extended HI disk shows regular rotation (v_rot = 51 +/- 8 km/s), and at a column density of ~5.0 x 10^19 atoms/cm^2 can be traced out to over six times the Holmberg radius of the stellar component (radius at mu_B = 26.6 mag/arcsec^2). 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 A_B = 0.95 +/- 0.15 mag, we derive an HI mass-to-light ratio of 22 +/- 4 M_sun/L_sun for ESO215-G?009. To our knowledge this is the highest ratio for a galaxy to be confirmed by accurate measurement to date.

24 Pages, 23 Figures (very low resolution for some images). Accepted for publication in AJ (September 2004). Full versions (PS and PDF) with full resolution figures are at http://www.mso.anu.edu.au/~bewarren/ESO215_Gq009/