The mid-life crisis of the Milky Way and M31

Abstract

Upcoming next generation galactic surveys, such as Gaia and HERMES, will deliver unprecedented detail about the structure and make-up of our Galaxy, the Milky Way, and promise to radically improve our understanding of it. However, to benefit our broader knowledge of galaxy formation and evolution we first need to quantify how typical the Galaxy is with respect to other galaxies of its type. Through modeling and comparison with a large sample of galaxies drawn from the Sloan Digital Sky Survey and Galaxy Zoo, we provide tentative yet tantalizing evidence to show that both the Milky Way and nearby M31 are undergoing a critical transformation of their global properties. Both appear to possess attributes that are consistent with galaxies midway between the distinct blue and red bimodal color populations. In extragalactic surveys, such "green valley" galaxies are transition objects whose star formation typically will have all but extinguished in less than 5Gyr. This finding reveals the possible future of our own galactic home and opens a new window of opportunity to study such galactic transformations up close. © 2011. The American Astronomical Society. All rights reserved.