The Brightest Stars of Irregular and Low-Mass Spiral Galaxies

Abstract

A search for a correlation between the luminosities of the brightest stars and luminosities of their host galaxies was carried out on archival Hubble Space Telescope (HST) $F606W$ or $F555W$ (V) and $F814W$ (I) images of 150 nearby galaxies. The sample contains only galaxies with ongoing star formation and with known distances we derived with the TRGB-method. We correlated the average absolute luminosities of the three brightest blue and the three brightest red stars with the luminosity of a host. We find a linear relation for both the blue and the red stars in irregular and low-mass spiral galaxies. Their scatters are sufficiently small (${{0.}^{{\text{m}}}}4$) to make these relations useful for distance determination for low-mass galaxies. We found that all 31 dwarf galaxies $({{M}_{B}} > - {{13}^{{\text{m}}}})$ in our sample lack bright massive stars $({{M}_{V}}{\text{(BS)}} < - {{7.}^{{\text{m}}}}0)$, probably due to the physical conditions that prevent their birth. For galaxies with higher an average luminosity in the range $- {{18}^{{\text{m}}}} < {{M}_{B}} < - {{13}^{{\text{m}}}}$, there is an asymmetry in the distribution of the number of galaxies relative to the linear dependence, indicating an increase in the fraction of galaxies with bright stars.