The star cluster - field star connection in nearby spiral galaxies. II.
  Field star and cluster formation histories and their relation
  Recent studies have started to cast doubt on the assumption that most stars
are formed in clusters. Observational studies of field stars and star cluster
systems in nearby galaxies can lead to better constraints on the fraction of
stars forming in clusters. We aim to constrain the amount of star formation
happening in long-lived clusters for four galaxies through the homogeneous
study of field stars and star clusters. Using HST/ACS-WFPC2 images of the
galaxies NGC45, NGC1313, NGC5236 and NGC7793, we estimate star formation
histories by means of the synthetic CMD method. Masses and ages of star
clusters are estimated using simple stellar population model fitting. Comparing
observed and modeled luminosity functions we estimate cluster formation rates.
By randomly sampling the stellar IMF, we construct artificial star clusters and
quantify how stochastic effects influence cluster detection, integrated colors
and age estimates. Star formation rates appear to be constant over the past
10-100 Myr. The number of clusters identified per galaxy varies, with few
massive (>10^5Msun) and few old (>1Gyr) clusters. The galaxies NGC5236 and
NGC1313 show high star and cluster formation rates compare to NGC7793 and
NGC45. Stochastic sampling of the SIMF has a strong impact on estimation of
ages, colors and completeness for clusters with masses <10^4Msun, while for
high masses the effect is less pronounced. Stochasticity also makes size
measurements highly uncertain at young ages, making it difficult to distinguish
between clusters and stars based on sizes. The ratio of star formation
happening in clusters compared to the global star formation ($\Gamma$) appears
to vary for different galaxies. We find no obvious relation between $\Gamma$
and the star formation rate density within the range probed here. The $\Gamma$
values do, however, appear to correlate with the specific U-band luminosity
(T_L (U)).
