Star formation in the outer disks of spiral galaxies: Ultraviolet and Hα photometry

We present an analysis of ultradeep UV and Hα imaging of five nearby spiral galaxies to study the recent star formation in the outer disk. Using azimuthally averaged ellipse photometry as well as aperture photometry of individual young stellar complexes, we measure how star formation rates (SFRs) and UV and Hα colors vary with radius. We detect azimuthally averaged UV flux to 1.2-1.4 R ₂₅ in most galaxies; at the edge of the detected UV disk, the surface brightnesses are 28-29magarcsec^-2, corresponding to SFR surface densities of 3 × 10^-4 M yr^-1kpc ^-2. Additionally, we detect between 120 and 410 young stellar complexes per galaxy, with a significant number of detections out to 1.5 R ₂₅. We measure radial FUV-NUV profiles, and find that the dispersion in the UV colors of individual young stellar complexes increases with radius. We investigate how radial variations in the frequency of star formation episodes can create color gradients and increasing dispersion in the UV colors of star-forming regions, like those observed in our study. Specifically, we use recently published, high spatial and temporal resolution measurements of Σ_SFR throughout the disk of M33 to estimate the frequency of star formation episodes throughout the disk of a typical spiral galaxy. We use stellar synthesis models of these star formation histories (SFHs) to measure the variations in UV colors and find that we can replicate large dispersions in UV colors based on episodic SFHs.