ar X iv : a st ro - p h / 04 06 01 0 v 1 1 J un 2 00 4 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 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 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 M⊙ yr . The extended H i disk shows regular rotation (vrot = 51 ± 8 km s ), and at a column density of ∼ 5.0 × 10 atoms cm can be traced out to over six times the Holmberg radius of the stellar component (radius at μB = 26.6 mag arcsec ). 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 within a radius of 370. Given a Galactic extinction of AB = 0.95± 0.15 mag, we derive an H i mass-to-light ratio of 22± 4 M⊙/L⊙,B for ESO215–G?009. To our knowledge this is the highest MHI/LB ratio for a galaxy to be confirmed by accurate measurement to date. Subject headings: galaxies: individual (ESO215–G?009) — galaxies: irregular — galaxies: dwarf — galaxies: evolution — galaxies: photometry — galaxies: ISM — galaxies: kinematics and dynamics