Ongoing Massive Star Formation in NGC 604
  NGC 604 is the second most massive H II region in the Local Group, thus an
important laboratory for massive star formation. Using a combination of
observational and analytical tools that include Spitzer spectroscopy, Herschel
photometry, Chandra imaging, and Bayesian Spectral Energy Distribution fitting,
we investigate the physical conditions in NGC 604, and quantify the amount of
massive star formation currently taking place. We derive an average age of 4
+/- 1 Myr and a total stellar mass of 1.6 (+1.6)(-1.0) x 10^5 M_sun for the
entire region, in agreement with previous optical studies. Across the region we
find an effect of the X-ray field on both the abundance of aromatic molecules
and the [Si II] emission. Within NGC 604 we identify several individual bright
infrared sources with diameters of about 15 pc and luminosity weighted masses
between 10^3 M_sun and 10^4 M_sun. Their spectral properties indicate that some
of these sources are embedded clusters in process of formation, which together
account for ~8% of the total stellar mass in the NGC 604 system. The variations
of the radiation field strength across NGC 604 are consistent with a sequential
star formation scenario, with at least two bursts in the last few million
years. Our results indicate that massive star formation in NGC 604 is still
ongoing, likely triggered by the earlier bursts.
