THE NEARBY AND EXTREMELY METAL-POOR GALAXY CGCG 269-049

摘要

We present Hubble Space Telescope (HST) and Spitzer Space Telescope images and photometry of the extremely metal-poor (Z ≈ 0.03 Z_☉) blue dwarf galaxy CGCG 269-049. The HST images reveal a large population of red giant and asymptotic giant branch stars, ruling out the possibility that the galaxy has recently formed. From the magnitude of the tip of the red giant branch, we measure a distance to CGCG 269-049 of only 4.9 ±0.4 Mpc. The spectral energy distribution of the galaxy between ～3.6-70 μm is also best fitted by emission from predominantly ～ 10 Gyr old stars, with a component of thermal dust emission having a temperature of 52 ± 10 K. The HST and Spitzer photometry indicate that more than 60% of the CGCG 269-049 stellar mass consists of stars ～ 1 -10 Gyr old, similar to other local blue dwarf galaxies. Our HST Hα image shows no evidence of a supernova-driven outflow that could be removing metals from the galaxy, nor do we find evidence that such outflows occurred in the past. Taken together with the CGCG 269-049 large ratio of neutral hydrogen mass to stellar mass (～ 10), these results are consistent with recent simulations in which the metal deficiency of local dwarf galaxies results mainly from inefficient star formation, rather than youth or the escape of supernova ejecta.