  About half of nearby galaxies have a central surface brightness >1 magnitude
below that of the sky. The overall properties of these low-surface-brightness
galaxies (LSBGs) remain understudied, and in particular we know very little
about their massive black hole population. This gap must be closed to determine
the frequency of massive black holes at z=0 as well as to understand their role
in regulating galaxy evolution. Here we investigate the incidence and intensity
of nuclear, accretion-powered X-ray emission in a sample of 32 nearby LSBGs
with the Chandra X-ray Observatory. A nuclear X-ray source is detected in 4
galaxies (12.5%). Based on an X-ray binary contamination assessment technique
developed for normal galaxies, we conclude that the detected X-ray nuclei
indicate low-level accretion from massive black holes. The active fraction is
consistent with that expected from the stellar mass distribution of the LSBGs,
but not their total baryonic mass, when using a scaling relation from an
unbiased X-ray survey of normal galaxies. This suggests that their black holes
co-evolved with their stellar population. In addition, the apparent agreement
nearly doubles the number of galaxies available within ~100 Mpc for which a
measurement of nuclear activity can efficiently constrain the frequency of
black holes as a function of stellar mass. We conclude by discussing the
feasibility of measuring this occupation fraction to a few percent precision
below 1e10 solar masses with high-resolution, wide-field X-ray missions
currently under consideration.
