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 +/- 0.31 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-circle dot/L-circle dot. 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 isothermal 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-circle dot/L-circle dot 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 of 0.15. (C) 2016 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim