THE EFFECTS OF EPISODIC STAR FORMATION ON THE FUV - NUV COLORS OF STAR FORMING REGIONS IN OUTER DISKS

We run stellar population synthesis models to examine the effects of a recently episodic star formation history (SFH) on UV and H alpha colors of star forming regions. Specifically, the SFHs we use are an episodic sampling of an exponentially declining star formation rate (SFR; tau model) and are intended to simulate the SFHs in the outer disks of spiral galaxies. To enable comparison between our models and observational studies of star forming regions in outer disks, we include in our models sensitivity limits that are based on recent deep UV and H alpha observations in the literature. We find significant dispersion in the FUV - NUV colors of simulated star forming regions with frequencies of star formation episodes of 1 x 10(-8) to 4 x 10(-9) yr(-1). The dispersion in UV colors is similar to that found in the outer disk of nearby spiral galaxies. As expected, we also find large variations in L-H alpha / L-FUV. We interpret our models within the context of inside-out disk growth, and find that a radially increasing tau and decreasing metallicity with an increasing radius will only produce modest FUV - NUV color gradients, which are significantly smaller than what is found for some nearby spiral galaxies. However, including moderate extinction gradients with our models can better match the observations with steeper UV color gradients. We estimate that the SFR at which the number of stars emitting FUV light becomes stochastic is similar to 2 x 10(-6) M-circle dot yr(-1), which is substantially lower than the SFR of many star forming regions in outer disks. Therefore, we conclude that stochasticity in the upper end of the initial mass function is not likely to be the dominant cause of dispersion in the FUV - NUV colors of star forming regions in outer disks. Finally, we note that if outer disks have had an episodic SFH similar to that used in this study, this should be taken into account when estimating gas depletion timescales and modeling chemical evolution of spiral galaxies.