Faint Dwarf Galaxies in Hickson Compact Group 90

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 $10^{4.9-6.5}\,M_\odot$.