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H I Observations of SA 68-6597: The Faintest Blue Compact Dwarf Galaxy

Blue compact dwarf galaxies (BCDs) are faint (MB ≤ -17 mag), compact (R < 1 kpc), and at least qualitatively very blue galaxies, due to active star formation, and have low metallicities. Found serendipitously as part of a redshift survey of faint galaxies with the Keck Telescope (DEEP), SA 68-6597 is at a distance of 80 Mpc, and is one of the faintest (-12.4 mag), lowest metallicity (~0.05 Z☉) BCDs known. Its Hβ line width of σ = 27 km s-1 and small size, Reff ~ 190 pc, suggest that it is an extremely low mass galaxy. We have used the Arecibo telescope to measure the H I properties of SA 68-6597 in order to better constrain its total mass and its potential for future star formation. SA 68-6597 has a M = (1.4 ± 0.4) × 107 M☉ and an H I FWHM line width of 33. Combining the H I line width with an estimate of the size of the H I disk, we derive a Mdyn ≳ 3 × 107 M☉. The values M/LB = 1.0 ± 0.3 M☉/L☉, Mdyn/LB ≥ 2 M☉/L☉, and M/Mdyn ≲ 0.47 are typical for BCDs. Combining the measured star formation rate of 0.003 M☉ yr-1 with the H I mass, we derive a gas depletion timescale of 5 ± 2 Gyr. While SA 68-6597 is a fainter, lower mass, higher metallicity counterpart to other BCDs like I Zw 18 and SBS 0335-052, its H I properties suggest that it will not evolve dramatically in the near future. Given the limits on its gaseous and dynamical masses, SA 68-6597 may be able to evolve into a moderately massive dwarf spheroidal galaxy.