The cluster population of the irregular galaxy NGC 4449 as seen by the
  Hubble Advanced Camera for Surveys
  We present a study of the star cluster population in the starburst irregular
galaxy NGC 4449 based on B, V, I, and Ha images taken with the Advanced Camera
for Surveys on the Hubble Space Telescope. We derive the cluster properties
such as size, ellipticity, and total magnitudes. Cluster ages and masses are
derived fitting the observed spectral energy distributions with different
population synthesis models. Our analysis is strongly affected by the
age-metallicity degeneracy; however, if we assume a metallicity of ~1/4 solar,
as derived from spectroscopy of HII regions, we find that the clusters have
ages distributed quite continuously over a Hubble time, and they have masses
from ~10^3 M_sun up to ~2 x 10^6 M_sun, assuming a Salpeters' IMF down to 0.1
M_sun. Young clusters are preferentially located in regions of young star
formation, while old clusters are distributed over the whole NGC 4449 field of
view, like the old stars (although we notice that some old clusters follow
linear structures, possibly a reflection of past satellite accretion). The high
SF activity in NGC 4449 is confirmed by its specific frequency of young massive
clusters, higher than the average value found in nearby spirals and in the LMC
(but lower than in other starburst dwarfs such as NGC 1705 and NGC 1569), and
by the flat slope of the cluster luminosity function (dN(L_V)\propto L_V^{-1.5}
dL for clusters younger than 1 Gyr). We use the upper envelope of the cluster
log(mass) versus log(age) distribution to quantify cluster disruption, and do
not find evidence for the high (90%) long-term infant mortality found by some
studies. For the red clusters, we find correlations between size, ellipticity,
luminosity and mass: brighter and more massive clusters tend to be more
compact, and brighter clusters tend to be also more elliptical.
