Abstract
Stellar photometry of 70 irregular galaxies was performed using archival Hubble Space Telescope images. The resulting Hertzsprung–Russell diagrams were used to distinguish the red supergiant and giant branches. The distances to the galaxies and the metallicity of the red giants were determined using the TRGB method. The \((V-I)\) color index of the supergiant branch at the luminosity level \(M_{I}=-7\overset{\textrm{m}}{.}0\) was chosen as the metallicity index of red supergiants. The measurements showed that no galaxy exhibits a metallicity inversion between young and old stars, which would occur if clouds of the low-metallicity intergalactic gas fall onto the galaxy. This means that the infall of intergalactic clouds, if it occurred, did not significantly affect the evolution of the stellar population of galaxies over the past billions years. Over billions of years of evolution, each galaxy underwent random processes that could alter the metallicity of its interstellar medium and stars. However, the linear relationship we have found between the metallicities of young and old stars in galaxies of different masses indicates that the bulk of galaxies’ metal accumulation has occurred in the distant past. Modern processes are increasing the metallicity of galaxies but at a significantly slower rate than during galaxy formation. Otherwise, the linear relationship between the metallicities of stars of different ages in galaxies of different masses and star formation histories would not be observed.
