ESO 215-G?009: An Extreme Hi-Rich Dwarf Irregular Galaxy

We present deep BVRI band images and HI line observations of the nearby, lowsurface brightness galaxy ESO215-G?009 which were obtained with the ANU 2.3-mTelescope and the Australia Telescope Compact Array, respectively. ESO215-G?009was selected from the HIPASS Bright Galaxy Catalog because it has the secondhighest HI mass-to-light ratio of the galaxies with measured B band apparentmagnitudes. We find that it is an isolated dwarf irregular galaxy with an oldstellar population. We place an upper limit on the current star formation rateof ~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 betraced 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, wemeasure a B band apparent magnitude of 16.13 +/- 0.07 mag within a radius of80". The HI flux density is 122 +/- 4 Jy km/s within a radius of 370". Given aGalactic extinction of A_B = 0.95 +/- 0.15 mag, we derive an HI mass-to-lightratio of 22 +/- 4 M_sun/L_sun for ESO215-G?009. To our knowledge this is thehighest ratio for a galaxy to be confirmed by accurate measurement to date.