Population synthesis of accreting white dwarfs: II. X-ray and UV
  emission
  Accreting white dwarfs (WDs) with non-degenerate companions are expected to
emit in soft X-rays and the UV, if accreted H-rich material burns stably. They
are an important component of the unresolved emission of elliptical galaxies,
and their combined ionizing luminosity may significantly influence the optical
line emission from warm ISM. In an earlier paper we modeled populations of
accreting WDs, first generating WD with main-sequence, Hertzsprung gap and red
giant companions with the population synthesis code \textsc{BSE}, and then
following their evolution with a grid of evolutionary tracks computed with
\textsc{MESA}. Now we use these results to estimate the soft X-ray
(0.3-0.7keV), H- and He II-ionizing luminosities of nuclear burning WDs and the
number of super-soft X-ray sources for galaxies with different star formation
histories. For the starburst case, these quantities peak at $\sim 1$ Gyr and
decline by $\sim 1-3$ orders of magnitude by the age of 10 Gyr. For stellar
ages of $\sim$~10 Gyr, predictions of our model are consistent with soft X-ray
luminosities observed by Chandra in nearby elliptical galaxies and He II
4686$\AA/\rm{H}{\beta}$ line ratio measured in stacked SDSS spectra of retired
galaxies, the latter characterising the strength and hardness of the UV
radiation field. However, the soft X-ray luminosity and
He~II~4686$\AA/\rm{H}{\beta}$ ratio are significantly overpredicted for stellar
ages of $\lesssim 4-8$ Gyr. We discuss various possibilities to resolve this
discrepancy and tentatively conclude that it may be resolved by a modification
of the typically used criteria of dynamically unstable mass loss for giant
stars.
