Faint Dwarf Galaxies in Hickson Compact Group 90 (1608.05082v1)

Abstract: We report the discovery of a very diverse set of five low-surface brightness (LSB) dwarf galaxy candidates in Hickson Compact Group 90 (HCG 90) detected in deep U- and I-band images obtained with VLT/VIMOS. These are the first LSB dwarf galaxy candidates found in a compact group of galaxies. We measure spheroid half-light radii in the range $0.7!\lesssim! r_{\rm eff}/{\rm kpc}! \lesssim! 1.5$ with luminosities of $-11.65!\lesssim! M_U! \lesssim! -9.42$ and $-12.79!\lesssim! M_I! \lesssim! -10.58$ mag, corresponding to a color range of $(U!-!I)0!\simeq!1.1!-!2.2$ mag and surface brightness levels of $\mu_U!\simeq!28.1\,{\rm mag/arcsec^2}$ and $\mu_I!\simeq!27.4\,{\rm mag/arcsec^2}$. Their colours and luminosities are consistent with a diverse set of stellar population properties. Assuming solar and 0.02 Z$\odot$ metallicities we obtain stellar masses in the range $M_|{Z\odot} \simeq 10^{5.7-6.3} M_{\odot}$ and $M_|{0.02\,Z\odot}!\simeq!10^{6.3-8}\,M_{\odot}$. Three dwarfs are older than 1 Gyr, while the other two significantly bluer dwarfs are younger than $\sim 2$ Gyr at any mass/metallicity combination. Altogether, the new LSB dwarf galaxy candidates share properties with dwarf galaxies found throughout the Local Volume and in nearby galaxy clusters such as Fornax. We find a pair of candidates with $\sim!2$ kpc projected separation, which may represent one of the closest dwarf galaxy pairs found. We also find a nucleated dwarf candidate, with a nucleus size of $r_{\rm eff}!\simeq!46!-!63$ pc and magnitude M${U,0}=-7.42$ mag and $(U!-!I)_0!=!1.51$ mag, which is consistent with a nuclear stellar disc with a stellar mass in the range $10^{4.9-6.5}\,M\odot$.