Hubble Space Telescope Observations of the Cometary Blue Compact Dwarf Galaxy UGC 4483: A Relatively Young Galaxy?*

摘要

We present V and I photometry of the resolved stars in the cometary blue compact dwarf galaxy UGC 4483 using Hubble Space Telescope Wide Field Planetary Camera 2 images. The resulting I versus V-I color-magnitude diagram (CMD) reaches limiting magnitudes V = 27.5 mag and I = 26.5 mag for photometric errors less than 0.2 mag. It reveals not only a young stellar population of blue main-sequence stars and blue and red supergiants but also an older evolved population of red giant and asymptotic giant branch stars. The measured magnitude I = 23.65 ± 0.10 mag of the red giant branch tip results in a distance modulus m - M = 27.63 ± 0.12, corresponding to a distance of 3.4 ± 0.2 Mpc. The youngest stars are associated with the bright H II region at the northern tip of the galaxy. The population of older stars is found throughout the low surface brightness body of the galaxy and is considerably more spread out than the young stellar population, suggesting stellar diffusion. The most striking characteristics of the CMD of UGC 4483 are the very blue colors of the red giant stars and the high luminosity of the asymptotic giant branch stars. Both of these characteristics are consistent with either (1) a very low metallicity ([Fe/H] = -2.4, like the most metal-deficient globular clusters) and an old age of 10 Gyr or (2) a higher metallicity ([Fe/H] = -1.4, as derived from the ionized gas emission lines) and a relatively young age of the oldest stellar population in UGC 4483, not exceeding ~2 Gyr. Thus, our data do not exclude the possibility that UGC 4483 is a relatively young galaxy having formed its first stars only ~2 Gyr ago.