Abstract: I will discuss three independent lines of evidence that point to M31's stellar disk being thick: (1) measured trends in the reddening of red giant stars as a function of sky position caused by the thin layer of interstellar dust indicate that the stellar disk is geometrically thick, with a scale height of ~ 1 kpc; (2) red giant stars in the disk are observed to have a high line-of-sight velocity dispersion (~ 50–70 km/s) in our group's SPLASH survey; and (3) the observed chemical abundance trend in [alpha/Fe] vs. [Fe/H] space is similar to that of the MilkyWay thick disk, not the Milky Way thin disk. I will also present a comparison of the resolved stellar population kinematics of M31 and M33 as a function of stellar age and galactocentric radius and what they may be telling us about the dynamical heating of these two galactic disks.

About the Speaker: Prof Puragra (Raja) GuhaThakurta is a Distinguished Professor of Astronomy and Astrophysics at UCSC. He is theFaculty Director/Founder of Creating Equity in STEAM (CrEST) and the founder of the CrEST programs StS, PyaR, SIP, and the Global SPHERE Network. He and his students conduct research in observational astrophysics – their areas of interest include the structure, dynamics, formation, and evolution of the stellar halos and stellar disks of galaxies, galactic archaeology, dark matter, dwarf galaxies, chemical enrichment, and rare phases of stellar evolution. This research makes extensive use of the Keck telescopes, Hubble Space Telescope, James Webb Space Telescope, Lick Observatory Shane telescope, Canada-France-Hawaii Telescope, Subaru Telescope, etc. Beyond the CrEST programs, he has served as an instructor for the COSMOS program for many years and currently serves as an instructor for the Project for Inmate Education at Santa Cruz County Jail. He was awarded the AAS Education Prize in 2025. In his imaginary spare time, Raja enjoys spending time with his family, art (detailed portraiture), and playing/watching a variety of sports.

We look forward to your active participation.