Distance to Irregular Galaxy Cas 1 Located near the Milky Way

Abstract

We conducted the stellar photometry using the archive images of the Hubble Space Telescope of the dwarf irregular galaxy Cas 1 located near the Milky Way. Blue supergiants detected in the obtained Hertzsprung-Russell diagram are concentrated in star-forming regions. We found that the red star branch consists of numerous AGB stars and a small number of brighter red supergiants. In the luminosity function of the peripheric red giants and AGB stars selected, the TRGB discontinuity is observed with I = 23.^m67 which corresponds to the beginning of the red giant branch. Using the TRGB method, we determined the distance modulus to the Cas 1 (m - M) = 26.01 ± 0.08 which corresponds to the distance D = 1.6 ± 0.1 Mpc. This estimate shows that the galaxy is near the Local Group and isolated from other galaxies. The absolute luminosity of Cas 1 based on the new distance measurement is M_V = -14.2, i.e. it is definitely a dwarf galaxy. The position of the red giant branch of AGB stars in the Hertzsprung-Russell diagram and measured metallicity of red giants shows that the galaxy Cas 1 has a very low metallicity [Fe/H] = -2.8.