Andromeda IV, a solitary gas-rich dwarf galaxy
  Observations are presented of the isolated dwarf irregular galaxy And IV made
with the Hubble Space Telescope Advanced Camera for Surveys and the Giant
Metrewave Radio Telescope in the 21 cm HI line. We determine the galaxy
distance of $7.17\pm0.31$ Mpc using the Tip of Red Giant Branch method. The
galaxy has a total blue absolute magnitude of -12.81 mag, linear Holmberg
diameter of 1.88 kpc and an HI-disk extending to 8.4 times the optical Holmberg
radius. The HI mass-to-blue luminosity ratio for And IV amounts
$12.9~M_{\odot}/L_{\odot}$. From the GMRT data we derive the rotation curve for
the HI and fit it with different mass models. We find that the data are
significantly better fit with an iso-thermal dark matter halo, than by an NFW
halo. We also find that MOND rotation curve provides a very poor fit to the
data. The fact that the iso-thermal dark matter halo provides the best fit to
the data supports models in which star formation feedback results in the
formation of a dark matter core in dwarf galaxies. The total mass-to-blue
luminosity ratio of $162~M_{\odot}/L_{\odot}$ makes And IV among the darkest
dIrr galaxies known. However, its baryonic-to-dark mass ratio
($M_{gas}+M^*)/M_T = 0.11$ is close to the average cosmic baryon fraction,
0.15.
