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 \pm 1 \ > Myr and a total stellar mass of 1.6 ^ { +1.6 } _ { -1.0 } \times 10 ^ { 5 } \ > M _ { \astrosun } 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 _ { \astrosun } and 10 ^ { 4 } \ > M _ { \astrosun } .
Their spectral properties indicate that some of these sources are embedded clusters in process of formation , which together account for \sim 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 .