The star cluster - field star connection in nearby spiral galaxies. II. Field star and cluster formation histories and their relation

Abstract

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)).Comment: 18 pages, 17 figures. A&A accepte