The star clusters in the starburst irregular galaxy NGC 1569

We examine star clusters in the irregular starburst galaxy NGC 1569 from Hubble Space Telescope images taken with filters F336W, F555W, and F814W. In addition to the two super-star clusters that are well known, we identify 45 other clusters that are compact but resolved. Integrated UVI colors of the clusters span a large range, and comparison with coeval evolutionary models suggests that the ages range from 2-3 Myr to 1 Gyr. Most of the clusters have colors consistent with ages of less than or equal to 30 Myr, placing them at the end of the recent burst of star formation. We examine the radial surface brightness profiles of four of the clusters and fit King models to three of them. The colors of the clusters are approximately constant with radius. The four clusters have half-light radii and core radii that are in the range observed in present-day globular clusters in our Galaxy. However, they are somewhat less concentrated than the average globular cluster. The two well-known super-star clusters have luminosities (and one has a known mass) that are comparable to those of typical globular clusters. The other two clusters and likely numerous others in the sample are similar to a small globular cluster and to R136 in the LMC. The conditions that produced the recent starburst, therefore, have also been those necessary for producing compact bright star clusters. We examine resolved stars in the outer parts of the super-star clusters. We find that cluster A contains many bright blue stars. Some of the blue stars are bright enough to be evolved massive stars. There is also a small population of red supergiants. Components A1 and A2 within cluster A have similar colors, and a two-dimensional color map does not offer evidence that one component is dominated by red supergiants and the other is not. The apparent contradiction of the presence of red supergiants with Wolf-Rayet stars is allowed by the evolution of massive stars or may instead be due to an age spread within cluster A. The stars that we resolve around cluster B, on the other hand, contain a small population of more normal blue massive stars and a large population of red supergiants. The presence of the red supergiants is consistent with the view that cluster B is in its red supergiant phase. The presence of the red supergiant stars in clusters A and B is also verified in near-infrared spectra, where we find strong stellar CO absorption features. The various age indicators are consistent with a picture in which cluster B is of order 10-20 Myr old, the older stars in cluster A are greater than or equal to7 Myr old. The timescale to form the holes seen in H alpha and H I is comparable to the age of cluster B.