The Connection Between X-ray Binaries and Star Clusters in NGC 4449
  We present 23 candidate X-ray binaries with luminosities down to 1.8x10^36
erg/s, in the nearby starburst galaxy NGC 4449, from observations totaling 105
ksec taken with the ACIS-S instrument on the Chandra Space Telescope. We
determine count rates, luminosities, and colors for each source, and perform
spectral fits for sources with sufficient counts. We also compile a new catalog
of 129 compact star clusters in NGC 4449 from high resolution, multi-band
optical images taken with the Hubble Space Telescope, doubling the number of
clusters known in this galaxy. The UBVI,Ha luminosities of each cluster are
compared with predictions from stellar evolution models to estimate their ages
and masses. We find strong evidence for a population of very young massive,
black-hole binaries, which comprise nearly 50% of the detected X-ray binaries
in NGC 4449. Approximately a third of these remain within their parent star
clusters, which formed t < 6-8 Myr ago, while others have likely been ejected
from their parent clusters. We also find evidence for a population of somewhat
older X-ray binaries, including both supergiant and Be-binaries, which appear
to be associated with somewhat older t ~ 100-400 Myr star clusters, and one
X-ray binary in an ancient (t ~ 10 Gyr) globular cluster. Our results suggest
that detailed information on star clusters can significantly improve
constraints on X-ray binary populations in star-forming galaxies.
