Ongoing Massive Star Formation in NGC 604

TL;DR

This study combines multi-wavelength observations and Bayesian analysis to investigate ongoing massive star formation in NGC 604, revealing multiple star formation episodes and embedded clusters contributing significantly to the region's stellar mass.

Contribution

It provides a detailed multi-method analysis of NGC 604, demonstrating ongoing massive star formation and identifying embedded clusters, which advances understanding of star formation processes in such regions.

Findings

Massive star formation is still ongoing in NGC 604.

Multiple star formation bursts occurred in the last few million years.

Embedded clusters contribute about 8% of the total stellar mass.

Abstract

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.