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C.-H. R. Chen, Y.-H. Chu (Univ. of Illinois at Urbana-Champaign), K. E. Johnson (Univ. of Virginia)
Giant HII regions (GHRs), with high concentrations of massive stars, are an excellent laboratory to study modes of massive star formation and a potential host of super-star clusters (SSCs). We have used HST WFPC2 images in the F547M and F675W bands to study cluster formation in three very luminous but morphologically different GHRs in M101 - NGC5461, NGC5462, and NGC5471.
NGC5461 has been suggested to host a SSC in its luminous core. Our observations show that it contains three R136-class clusters superposed on a small, bright stellar background. This tight group of clusters may dynamically evolve into an SSC in the future, and may appear unresolved at large distances and be identified as an SSC. NGC5462 has loosely distributed HII regions and clusters without a prominent core. It has the largest number of clusters among the three GHRs, but most of them are faint and older than 10 Myr. NGC5471 has multiple bright HII regions and many faint clusters younger than 5 Myr. It has two clusters older than R136, but just as luminous; they may be the most massive clusters in the three GHRs.
The fraction of stars formed in massive clusters is estimated from the clusters' contribution to the total stellar continuum emission and a comparison of the clusters' ionizing power to the ionization requirement of the associated HII regions. Both estimates show that < ~ 50% of stars are formed in massive clusters. The cluster luminosity functions (CLFs)of the three GHRs show different slopes. NGC5462 has the steepest CLF and most loosely distributed interstellar gas, qualitatively consistent with the hypothesis that massive clusters are formed in high-pressure interstellar environments. The combined CLF of these three GHRs is similar to those in starburst galaxies and normal spiral galaxies.
This research is supported by grants STScI GO-6829.01-95A and GO-9934.01-A.
The author(s) of this abstract have provided an email address for comments about the abstract: c-chen@astro.uiuc.edu
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.