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The stellar kinematics of galactic disks are key to constraining diskformation and evolution processes. In this paper, for the first time, wemeasure the stellar age-velocity dispersion correlation in the inner 20 kpc(3.5 disk scale lengths) of M31 and show that it is dramatically different fromthat in the Milky Way. We use optical Hubble Space Telescope/Advanced Camerafor Surveys photometry of 5800 individual stars from the Panchromatic HubbleAndromeda Treasury (PHAT) survey and Keck/DEIMOS radial velocity measurementsof the same stars from the Spectroscopic and Photometric Landscape ofAndromeda's Stellar Halo (SPLASH) survey. We show that the averageline-of-sight velocity dispersion is a steadily increasing function of stellarage exterior to R=10 kpc, increasing from 30 km/s for the young upper mainsequence stars to 90 km/s for the old red giant branch stars. This monotonicincrease implies that a continuous or recurring process contributed to theevolution of the disk. Both the slope and normalization of the dispersion vs.age relation are significantly larger than in the Milky Way, allowing for thepossibility that the disk of M31 has had a more violent history than the diskof the Milky Way, more in line with cosmological predictions. We also findevidence for an inhomogeneous distribution of stars from a second kinematicalcomponent in addition to the dominant disk component. One of the largest andhottest high-dispersion patches is present in all age bins, and may be thesignature of the end of the long bar.

A clear age-velocity dispersion correlation in Andromeda's stellar disk