Revisiting the Faint Blue Galaxy Problem

Abstract

Abstract We have investigated whether the pure luminosity evolution (PLE) model in a flat universe with cosmological constant $\Lambda$ can explain the faint blue galaxy (FBG) problem in light of recent observations of a morphologically divided number count, redshift distribution, and color distribution. While the original FBG problem means an inconsistency between the total number count and redshift distribution at around $B > 22\,\mathrm{mag}$, we consider here the FBG problem in a broader context. In particular, we pay attention to an evolution model of irregular galaxies, of which predictions from the standard model disagree with recent observations. In addition to the standard 'S model' with slow star formation, we introduce a non-standard 'R model' with rapid star formation for irregular galaxies, and investigate the properties of the sum of these models (S+R model). The observed total number count and redshift distribution in the $B$-band can be well reproduced by the PLE model in a $\Lambda$-dominated universe. As for irregular galaxies, while the S model underproduces the observed count, the S+R model can account for the observation. Considering that the observed color distribution of faint irregular galaxies is very broad, we propose a possibility that irregular galaxies are composed of at least two populations.