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