  To explore the properties of extreme outer stellar disks, we obtained
ultra-deep V and GALEX UV images of 4 dwarf irregular galaxies and one Blue
Compact Dwarf galaxy and ultra-deep B images of 3 of these. Our V-band surface
photometry extends to 29.5 magnitudes arcsec^-2. We convert the FUV and V-band
photometry, along with Halpha photometry, into radial star formation rate
profiles that are sensitive to timescales from 10 Myrs to the lifetime of the
galaxy. We also compare the stellar distributions, surface brightness profiles,
and star formation rate profiles to HI-line emission maps, gas surface density
profiles, and gas kinematics. Our data lead us to two general observations:
First, the exponential disks in these irregular galaxies are extraordinarily
regular. The stellar disks continue to decline exponentially as far as our
measurements extend. In spite of lumpiness in the distribution of young stars
and HI distributions and kinematics that have significant unordered motions,
sporadic processes that have built the disks-star formation, radial movement of
stars, and perhaps even perturbations from the outside-have, nevertheless,
conspired to produce standard disk profiles. Second, there is a remarkable
continuity of star formation throughout these disks over time. In four out of
five of our galaxies the star formation rate in the outer disk measured from
the FUV tracks that determined from the V-band, to within factors of 5,
requiring star formation at a fairly steady rate over the galaxy's lifetime.
Yet, the HI surface density profiles generally decline with radius more
shallowly than the stellar light, and the gas is marginally gravitationally
stable against collapse into clouds. Outer stellar disks are challenging our
concepts of star formation and disk growth and provide a critical environment
in which to understand processes that mold galaxy disks.
