OUTSIDE-IN SHRINKING OF THE STAR-FORMING DISK OF DWARF IRREGULAR GALAXIES

We have studied multi-band surface brightness profiles of a representative sample of 34 nearby dwarf irregular galaxies. Our data include Galaxy Evolution Explorer (GALEX) FUV/NUV, UBV, and Ha and Spitzer 3.6 mu m images. These galaxies constitute the majority of the LITTLE THINGS survey (Local Irregulars That Trace Luminosity Extremes The H I Nearby Galaxy Survey). By modeling the azimuthal averages of the spectral energy distributions with a complete library of star formation histories, we derived the stellar mass surface density distributions and the star formation rate averaged over three different timescales: the recent 0.1 Gyr, 1 Gyr, and a Hubble time. We find that, for similar to 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 similar to 80% of the galaxies have steeper mass profiles in the outer disk than in the inner region. The steep radial decline of the star formation rate 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 star formation rate to stellar mass with radius. Nevertheless, similar to 35% (12 galaxies, among which 7 have baryonic mass less than or similar to 10(8) M-circle dot) 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 star formation 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.