Genuine Irregular Galaxies as a Relic of Building Blocks of Galaxies
  In order to understand nature of building blocks of galaxies in the early
universe, we investigate "genuine irregular galaxies (GIGs)" in the nearby
universe. Here, GIGs are defined as isolated galaxies without regular
structures (spheroid, bulge, disk, bar, spiral arm, and nucleus). Using the
results of two excellent studies on galaxy morphology based on the Sloan
Digital Sky Survey (SDSS), we obtain a sample of 66 irregular galaxies. We
carry out new classification of them into GIGs and non-GIGs which have regular
structure or show evidence for galaxy interaction, by using the SDSS Data
Release 10 images. We then find that a half of these irregular galaxies (33/66)
are GIGs and obtain an unambiguous sample of 33 GIGs for the first time. We
discuss their observational properties by comparing them with those of
elliptical, S0, spiral galaxies, and irregular galaxies without the GIGs. We
find that our GIGs have smaller sizes, lower optical luminosities, bluer
rest-frame optical colors, lower surface stellar mass densities, and lower gas
metallicity than normal galaxies. All these properties suggest that they are in
chemically and dynamically younger phases even in the nearby universe.
