ASTROSAT/UVIT Near and Far Ultraviolet Properties of the M31 Bulge
  AstroSat has surveyed M31 with the UVIT telescope during 2017 to 2019. The
central bulge of M31 was observed in 2750-2850 A, 2000-2400 A, 1600-1850 A,
1450-1750 A, and 1200-1800 A filters. A radial profile analysis, averaged along
elliptical contours which approximate the bulge shape, was carried out in each
filter. The profiles follow a Sersic function with an excess for the inner 8"
in all filters, or can be fit with two Sersic functions (including the excess).
The ultraviolet colours of the bulge are found to change systematically with
radius, with the center of the bulge bluer (hotter). We fit the UVIT spectral
energy distributions (SEDs) for the whole bulge and for 10 elliptical annuli
with single stellar population (SSP) models. A combination of two SSPs fits the
UVIT SEDs much better than one SSP, and three SSPs fits the data best. The
properties of the three SSPs are age, metallicity (Z) and mass of each SSP. The
best fit model is a dominant old, metal-poor (10^10 yr, log(Z/Zsun)=-2, with
Zsun the solar metallicity) population plus a 15% contribution from an
intermediate (10^9.5 yr, log(Z/Zsun)=-2) population plus a small contribution
(2%) from a young high-metallicity (10^8.5 yr, log(Z/Zsun)=-0.5) population.
The results are consistent with previous studies of M31 in optical: both reveal
an active merger history for M31.
