THE PANCHROMATICHUBBLEANDROMEDA TREASURY. XI. THE SPATIALLY RESOLVED RECENT STAR FORMATION HISTORY OF M31

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 ∼9000 regions that are ∼100 pc × 100 pc in projected size, covering a 0.5 square degree area (∼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 ∼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 over the past ∼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 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 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 star formation starting ∼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 ∼200 Myr ago, but is much more diffuse today. We find that the global SFR has been fairly constant over the last ∼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 ∼500 Myr, ∼60% of all SF occurs 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 yr −1 with an average deprojected intensity of 7.3 × 10 −4 M yr −1 kpc −2 , which yields a total SFR of ∼0.7 M yr −1 when extrapolated to the entire area of M31's disk. This SFR is consistent with measurements from broadband estimates.

Emission-line stars in M31 from the SPLASH and PHAT surveys

“…This is consistent with the prolonged star formation (SF), spanning ∼ 0.5-1 Gyrs, predicted to have occurred in this long-lived feature (e.g. Dalcanton et al 2012;Lewis et al 2015). The blue Hα stars seem to show a relative increase compared to the SPLASH sample at the inner and outer most regions, consistent with the 5-kpc ring 15-kpc ring respectively.…”

“…The blue Hα stars seem to show a relative increase compared to the SPLASH sample at the inner and outer most regions, consistent with the 5-kpc ring 15-kpc ring respectively. It has been shown the 15-kpc ring has had a relatively recent SF burst, starting ∼ 80 Myrs ago (Lewis et al 2015), consistent with the increased number of younger Hα stars.…”

“…5, we do see that the majority of the T-MS stars lie at R deproj < 12 kpc. This could be due to more evolved MS stars existing in this region, consistent with slower and more extended SF (Lewis et al 2015), but could also relate to increased dust that reddens the MS stars. As shown in the top panel of Fig.…”

“…The main features seen in this northeast quadrant of M31 are the 5-, 10-, and 15-kpc star forming rings (Lewis et al 2015). These three main features, which are covered by the SPLASH survey, are at approximately α(2000) = 11.…”

The properties, origin and evolution of stellar clusters in galaxy simulations and observations

“…al. 2012) survey provides high resolution maps of the star formation history in M31 (Lewis et al 2015), with similar style maps to those presented here in their Figures 5 and 7. The structure of M31 is dominated by a ring, although given the inclination of M31, this structure could plausibly be a spiral arm (Gordon et al 2006).…”