Stellar Disk Evolution of Nearby Dwarf Galaxies

Abstract

We have studied multiband surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies. Our data include Galaxy Evolution Explorer (GALEX) FUV/NUV, UBV, H\(\alpha \), and Spitzer 3.6 \(\upmu \)m images. These galaxies constitute the majority of the LITTLE THINGS survey (Local Irregulars That Trace Luminosity Extremes—The H i Nearby Galaxy Survey, http://www.lowell.edu/users/dah/littlethings/index.html). By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation (SF) histories, we derived the stellar mass surface density distributions and the SF rate (SFR) averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr, and a Hubble time. We find that, for \({\sim }80\) % (27 galaxies) of our sample galaxies, radial profiles (at least in the outer part) at shorter wavelengths, corresponding to younger stellar populations, have shorter disk scale lengths than those at longer wavelengths, corresponding to older stellar populations. This indicates that the star-forming disk has been shrinking. In addition, the radial distributions of the stellar mass surface density are well described as piece-wise exponential profiles, and \({\sim }80\) % of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of SFR in the outer parts compared to that in the inner disks gives a natural explanation for the down-bending stellar mass surface density profiles. Within the inner disks, our sample galaxies on average have constant ratios of recent SFR to stellar mass with radius. Nevertheless, \({\sim }35\) % (12 galaxies, among which 7 have baryonic mass \(\lesssim \)10\(^{8}\) M\(_{\odot }\)) of the sample exhibit negative slopes across the observed disk, which is in contrast with the so-called “inside-out” disk growth scenario suggested for luminous spiral galaxies. The tendency of SF to become concentrated toward the inner disks in low-mass dwarf irregular galaxies is interpreted as a result of their susceptibility to environmental effects and regulation through stellar feedback.