The outer regions of disk galaxies show a drop-off in optical and Halpha emission, suggesting a radius beyond which stars do not form. This has been assumed to owe to a critical surface density, corresponding to this radius, below which star formation does not take place. GALEX recently challenged this picture with the discovery that 30% of disk galaxies show UV emission, indicating star formation, out to 2-3 times the optical radius of the galaxy. This indicates that stars are forming well beyond the threshold in Halpha. It has been suggested that local over-densities in outer HI disks drive star formation at large radii when they exceed the star formation threshold density. We run smooth particle hydrodynamics simulations of disk galaxies with extended gas disks to test whether over-densities owing to spiral structure in the outer disk can reproduce the observed star formation. We indeed find that spiral density waves from the inner disk propagate into the outer gas disk and raise local gas regions above the star formation density threshold, yielding features similar to those observed. Because the amount of star formation is low, we expect to see little optical emission in outer disks, as observed. Our results indicate that Type I XUV disks can be simulated simply by adding an extended gas disk to an isolated galaxy and evolving it with fiducial star formation parameters. This illustrates that a star formation threshold density is not equivalent to a star formation threshold radius.

Recent studies by GALEX show that galaxy disks are more extended than previously thought. Thirty percent of disk galaxies have UV emission, indicating young stars, beyond their optical extent. N-Body simulations of structure formation in a LCDM universe indicate that inner disks form first and their outskirts form gradually as gas is accreted. UV emission in outer disks may be star formation in recently accreted gas and therefore the most recent signature of disk formation. This means local over-densities in extant gas disks allow star formation at low rates in outer disks for many gigayears. Therefore, UV emission in outer disks may be star formation in gas that has long been present in the galaxy. To distinguish between these two scenarios (each of which is probably explains some outer disk star formation), detailed observations of the stellar populations of outer disks are necessary. We are currently using data from the SINGs legacy project of the Spitzer Space Telescope to analyze the infrared properties of outer disk objects. By analyzing the relative amount of emission in the Halpha, NUV, FUV, 3.6 micron, 8 micron, and 24 micron bands, we will put constraints on the ages and dust properties of outer disk objects. We will compare the results to theories describing the star formation history of outer disks.

The outer regions of galactic disks have been a topic of increased interest since GALEX surveys discovered that nearly 30% of galaxies show UV emission, indicating star formation, beyond their optical extent. While the young stellar populations of outer disks are increasingly well studied in the UV and Halpha, little is known about the older stellar population of outer disks. We have obtained 5 hours on Spitzer/IRAC to image the outskirts of the famous XUV (extended UV emission) disk NGC 4625 to the depth required to measure its 3.6 micron profile. In combination with existing GALEX UV imaging, Halpha imaging, HI maps and metallicity measurements, we will then constrain outer disks' star formation histories and evolution. We will calculate stellar mass, UV-3.6 micron colors, gas fractions and specific star formation rates as a function of radius and compare them to theoretical predictions.

We have taken a new look at the canonical merger remnant and luminous infrared galaxy (LIRG) NGC 6240 using multi-wavelength archival data and new Spitzer/IRAC data. NGC 6240 is a unique merger remnant that has double nuclei detectable in the optical and x-ray and is on the border between LIRG and ULIRG classes. We use photometry to analyze the distributions of the stars and dust in NGC 6240 and to search for AGN indicators in the nucleus. We use these results to compile a consistent picture of the structure of NGC 6240 and discuss possibilities for the future of the remnant.