Local Volume dwarf galaxy Cassiopea I. Gas metallicity, extinction and distance

Abstract

Cas I is a LV dIrr with a wide range of suggested distances. Tikhonov (2019), using the HST images and the TRGB method, places Cas I at D = 1.6+-0.1 Mpc. Besides, he estimates the stellar metallicity of Cas I at the level of z ~ 0.0004 (Z ~ Zo/50). Such a nearby extremely low-metallicity dwarf, if real, would be a very valuable object for detailed studies. An alternative TRGB distance of Cas I, of 4.5+-0.2 Mpc, based on the deeper HST images, was presented in the EDD. It places Cas I midway between IC342 (D ~ 3.3 Mpc) and Maffei 1 (D ~ 5.7 Mpc). We wish to check the suggested extremely low metallicity of Cas I, to improve the estimate of the large MW extinction and to improve the distance estimate to Cas~I. We use the SAO 6-m telescope spectroscopy to estimate gas metallicity in two HII regions in Cas I and to derive, via their observed Balmer decrements, the independent upper limit to the value of the MW extinction. We derive values of 12+log(O/H) = 7.83+-0.1 and 7.58+-0.1 dex in two HII regions of Cas I, corresponding to Z(gas) of 5-10 times higher than Z(stars) for its stars. The measured Balmer decrements in these HII regions, result in the maximal MW extinction of A_B = 3.06+-0.14 mag in comparison to A_B = 3.69+-0.4 derived via the IR dust emission and is used in other estimates of the distance to Cas I. This reduces the original EDD distance till 4.1 Mpc. The relation of Berg et al. (2012) for the LV late-type galaxies, between 12+log(O/H) and M_B, is used to bracket M_B for Cas I. This, in turn, allows one to get an independent estimate of the distance to Cas~I, of ~1.64~Mpc, albeit with the large 1-sigma uncertainty of factor 2.17. The combination of the above distance estimates, accounting for their uncertainties, results in the probable value of D ~ 3.65 Mpc, what favours Cas I to reside in the environs of IC342.