The star cluster - field star connection in nearby spiral galaxies I.
  Data analysis techniques and application to NGC 4395
  It is generally assumed that a large fraction of stars are initially born in
clusters. However, a large fraction of these disrupt on short timescales and
the stars end up belonging to the field. Understanding this process is of
paramount importance if we wish to constrain the star formation histories of
external galaxies using star clusters. We attempt to understand the relation
between field stars and star clusters by simultaneously studying both in a
number of nearby galaxies. As a pilot study, we present results for the
late-type spiral NGC 4395 using HST/ACS and HST/WFPC2 images. Different
detection criteria were used to distinguish point sources (star candidates) and
extended objects (star cluster candidates). Using a synthetic CMD method, we
estimated the star formation history. Using simple stellar population model
fitting, we calculated the mass and age of the cluster candidates. The field
star formation rate appears to have been roughly constant, or to have possibly
increased by up to about a factor of two, for ages younger than $\sim$300 Myr
within the fields covered by our data. Our data do not allow us to constrain
the star formation histories at older ages. We identify a small number of
clusters in both fields. Neither massive ($>10^5$ M$_\odot$) clusters nor
clusters with ages $\geq1$ Gyr were found in the galaxy and we found few
clusters older than 100 Myr. Based on our direct comparison of field stars and
clusters in NGC 4395, we estimate the ratio of star formation rate in clusters
that survive for $10^7$ to $10^8$ years to the total star formation to be
$\Gamma\sim0.03$. We suggest that this relatively low $\Gamma$ value is caused
by the low star formation rate of NGC 4395.
