ESO 215-G?009: An extreme H I-rich dwarf irregular galaxy

We present deep BVRI band images and H I line observations of the nearby, low surface brightness galaxy ESO 215-G?009, which were obtained with the Australian National University 2.3 m telescope and the Australia Telescope Compact Array, respectively. ESO 215-G?009 was selected from the HIPASS Bright Galaxy Catalog because it has the second highest H I 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 × 10 ^-3 M_⊙ yr^-1. The extended H I disk shows regular rotation (V_rot = 51 ± 8 km s^-1) and, at a column density of ∼5.0 × 10¹⁹ atoms cm^-2, can be traced out to over 6 times the Holmberg radius of the stellar component (radius μ_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 H I flux density is 122 ± 4 Jy km s^-1 within a radius of 370″. Given a Galactic extinction of A_B = 0.95 ± 0.15 mag, we derive an H I mass-to-light ratio of 22 ± 4 M_⊙/L_⊙,B for ESO 215-G?009. To our knowledge this is the highest M_HI/L_B ratio for a galaxy to be confirmed by accurate measurement to date.