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 _ { F 547 M } = -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 \lesssim 50 % of massive stars are formed in massive clusters ; consequently , the H \alpha 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 non-starburst spiral galaxies .