The Red Supergiant Content of M31 and M33

We identify red supergiants (RSGs) in our spiral neighbors M31 and M33 using near-IR (NIR) photometry complete to a luminosity limit of log L/Lo=4.0. Our archival survey data cover 5 deg^2 M31, 3 for M33, are likely spatially these massive stars. Gaia is used remove foreground stars, after which the RSGs can be separated from asymptotic giant branch (AGB) stars color-magnitude diagram. The derive effective temperatures bolometric luminosities via MARCS stellar atmosphere models. resulting H-R diagrams show superb agreement with evolutionary tracks Geneva group. census includes 6400 2850 within their Holmberg radii; by contrast, only few hundred known so far Milky Way. catalog serves as basis study RSG binary frequency being published separately, well future studies relating evolution Here we use matches between NIR-selected optical counterparts that apparent similarity reddening OB result Malmquist bias; average extinction higher than M33. As expected, distribution follows arms, while much older AGB population more uniformly spread across each galaxy's disk.