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 (dIrrs). Our data include GALEX
FUV/NUV, UBV, H\alpha, and Spitzer 3.6 \mum images. These galaxies constitute
the majority of the LITTLE THINGS survey. 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 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
have shorter disk scale lengths than those at longer wavelengths. 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 SF 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 SF rate to stellar mass with radius. Nevertheless, \sim 35% (12
galaxies, among which 7 have baryonic mass < 10^8 M\odot) of the sample exhibit
negative slopes across the observed disk, which is in contrast with the
"inside-out" disk growth scenario suggested for luminous spiral galaxies. The
tendency of SF to become concentrated toward the inner disks in low mass dIrrs
is interpreted as a result of their susceptibility to environmental effects and
regulation through stellar feedback.
