Abstract

We measure the recent star formation history (SFH) across M31 using optical images taken with the Hubble Space Telescope as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We fit the color-magnitude diagrams in similar to 9000 regions that are similar to 100 pc x 100 pc in projected size, covering a 0.5 square degree area (similar to 380 kpc(2), deprojected) in the NE quadrant of M31. We show that the SFHs vary significantly on these small spatial scales but that there are also coherent galaxy-wide fluctuations in the SFH back to similar to 500 Myr, most notably in M31's 10 kpc star-forming ring. We find that the 10 kpc ring is at least 400 Myr old, showing ongoing star formation (SF) over the past similar to 500 Myr. This indicates the presence of molecular gas in the ring over at least 2 dynamical times at this radius. We also find that the ring's position is constant throughout this time, and is stationary at the level of 1 km s(-1), although there is evidence for broadening of the ring due to the diffusion of stars into the disk. Based on existing models of M31's ring features, the lack of evolution in the ring's position makes a purely collisional ring origin highly unlikely. Besides the well-known 10 kpc ring, we observe two other ring-like features. There is an outer ring structure at 15 kpc with concentrated SF starting similar to 80 Myr ago. The inner ring structure at 5 kpc has a much lower star formation rate (SFR) and therefore lower contrast against the underlying stellar disk. It was most clearly defined similar to 200 Myr ago, but is much more diffuse today. We find that the global SFR has been fairly constant over the last similar to 500 Myr, though it does show a small increase at 50 Myr that is 1.3 times the average SFR over the past 100 Myr. During the last similar to 500 Myr, similar to 60% of all SF has occurred in the 10 kpc ring. Finally, we find that in the past 100 Myr, the average SFR over the PHAT survey area is 0.28 +/- 0.03 M-circle dot yr(-1) with an average deprojected intensity of 7.3 x 10(-4) M-circle dot yr(-1) kpc(-2), which yields a total SFR of similar to 0.7 M-circle dot yr(-1) when extrapolated to the entire area of M31's disk. This SFR is consistent with measurements from broadband estimates.