Reaching the Oldest Stars beyond the Local Group: Ancient Star Formation in UGC 4483

Abstract

We present new WFC3/UVIS observations of UGC 4483, the closest example of a metal-poor blue compact dwarf galaxy, with a metallicity of Z ≃ 1/15 Z<SUB>⊙</SUB> and located at a distance of D ≃ 3.4 Mpc. The extremely high quality of our new data allows us to clearly resolve the multiple stellar evolutionary phases populating the color-magnitude diagram (CMD), to reach more than 4 mag deeper than the tip of the red giant branch, and to detect for the first time core He-burning stars with masses ≲2 M<SUB>⊙</SUB>, populating the red clump and possibly the horizontal branch (HB) of the galaxy. By applying the synthetic CMD method to our observations, we determine an average star formation rate over the whole Hubble time of at least $(7.01\pm 0.44)\times {10}^{-4}$ ${M}_{\odot }\,{{yr}}^{-1}$ , corresponding to a total astrated stellar mass of $(9.60\pm 0.61)\times {10}^{6}$ M<SUB>⊙</SUB>, 87% of which went into stars at epochs earlier than 1 Gyr ago. With our star formation history recovery method we find the best fit with a distance modulus of DM = 27.45 ± 0.10, slightly lower than previous estimates. Finally, we find strong evidence of an old (≳10 Gyr) stellar population in UGC 4483 thanks to the detection of an HB phase and the identification of six candidate RR Lyrae variable stars. * Based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA Contract NAS 5-26555.