A sub-kiloparsec scale view of star formation in M31

Thesis (Ph.D.)--University of Washington, 2016-08

This dissertation examines the properties of star formation in the nearest large Milky Way- like galaxy, the Andromeda Galaxy (M31). Using resolved star data from the Hubble Space Telescope obtained as part of the Panchromatic Hubble Andromeda Treasury (PHAT), I model the optical color-magnitude diagrams (CMDs) of > 9000 regions that are 100pc×100pc (projected) in size to derive the most finely spatially-resolved star formation history (SFH) of M31 to date. I find that M31’s 10 kpc star-forming ring is a long-lived feature, continually forming stars over at least the past 500 Myr. Additionally, I find that the star formation rate in M31 has decreased by a factor of 3 – 4 over the same period of time. This is strong evidence that M31 is turning off its star formation. I use these SFHs to predict the ultraviolet flux in each region. To do this, I create modeled spectral energy distributions by summing up simple stellar populations with ages and SFRs set by the SFH and convolving with the pare them with the observed FUV and NUV maps obtained by the Galaxy Evolution Explorer (GALEX) response curves to generate flux. I then create maps of this predicted flux in the far- and near-ultraviolet (FUV and NUV) and compare them with the observed FUV and NUV maps obtained by GALEX . The time resolution provided by the spatially-resolved SFHs enables very accurate modeling of the UV flux. The predicted and observed fluxes agree to within 5% in each band. I also generate maps of the intrinsic, dust-free flux and compare those to maps of GALEX FUV + Spitzer 24 μm data and I find that the synthetic maps require much more flux. This suggests a discrepancy with the 24 μm correction. This also results in an under-estimate of the FUV + 24 μm derived SFR compared to that determined from the spatially-resolved SFHs. I also explore variation of the dust attenuation curve across the disk of M31. Using GALEX observations and the predicted, dust-free UV flux, I constrain the total-to-selective attenuation, RV , and the strength of the 2175 ̊A bump relative to the Milky Way, fbump, in each of the 100 pc size regions. I model the ensemble distribution and find a mean RV of μRV = 2.94 ± 0.01 with a width of σRV = 0.71 ± 0.01. This is steeper than the Milky Way value and implies smaller dust grains are acting as absorbers. I find considerable variation in RV across the disk including a systematic decrease in RV toward the center of the galaxy. Previous measurements of RV in M31 have been limited to only a small number of sightlines. The results presented here are the largest ensemble study of RV within the Andromeda Galaxy.