Reaching the oldest stars beyond the Local Group: ancient star formation
  in UGC 4483
  We present new WFC3/UVIS observations of UGC 4483, the closest example of a
metal-poor blue compact dwarf galaxy, with a metallicity of $Z \simeq 1/15\
Z_{\odot}$ and located at a distance of $D \simeq 3.4$ 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 $\lesssim 2$ M$_{\odot}$,
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
$(7.01\pm0.44) \times 10^{-4}$ $\mathrm{M_{\odot}/yr}$, corresponding to a
total astrated stellar mass of $(9.60\pm0.61)\times 10^6$ $\mathrm{M_{\odot}}$,
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 = $27.45\pm0.10$, slightly lower than previous estimates. Finally, we
find strong evidence of an old ($\gtrsim 10$ Gyr) stellar population in UGC
4483 thanks to the detection of an HB phase and the identification of six
candidate RR Lyrae variable stars.
