2005ApJ...619..779C

We have obtained HST WFPC2 observations of three very luminous but morphologically different giant H II regions (GHRs) in M101, NGC 5461, NGC 5462, and NGC 5471, in order to study cluster formation in GHRs. Images obtained in the F547M and F675W bands are used to identify cluster candidates and for photometric measurements, and images in the F656N band are used to show ionized interstellar gas. The measured colors and magnitudes are compared with the evolutionary tracks generated by the Starburst99 and Bruzual & Charlot population synthesis models to determine the ages and masses of the cluster candidates that are more luminous than M_F547M=-9.0. The brightest clusters detected in the PC images are measured and found to have effective radii of 0.7-2.9 pc. NGC 5461 is dominated by a very luminous core and has been suggested to host a super-star cluster (SSC). Our observations show that it contains three R136-class clusters superposed on a bright stellar background in a small region. This tight group of clusters may dynamically evolve into an SSC in the future, and may appear unresolved and be identified as an SSC at large distances, but at present NGC 5461 contains no SSCs. NGC 5462 consists of loosely distributed H II regions and clusters without a prominent core. It has the largest number of cluster candidates among the three GHRs studied, but most of them are faint and older than 10 Myr. NGC 5471 has multiple bright H II regions and contains a large number of faint clusters younger than 5 Myr. Two of the clusters in NGC 5471 are older than R136, but just as luminous; they may be the most massive clusters in the three GHRs studied. The fraction of stars formed in massive clusters has been estimated from the clusters' contribution to the total stellar continuum emission and from a comparison between the ionizing power of the clusters and the ionizing requirement of the associated H II regions. Both estimates show that ≲50% of massive stars are formed in massive clusters; consequently, the Hα luminosity of an H II region does not provide a sufficient condition for the existence of SSCs. The cluster luminosity functions (LFs) of the three GHRs show different slopes. NGC 5462 has the steepest cluster LF and the most loosely distributed interstellar gas, qualitatively consistent with the hypothesis that massive clusters are formed in high-pressure interstellar environments. The combined cluster LF of the three GHRs has a slope similar to the universal cluster LFs seen in starburst galaxies and nonstarburst spiral galaxies.