THE OUTER DISKS OF DWARF IRREGULAR GALAXIES

Abstract

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