The red supergiants & Wolf-Rayet stars of NGC 604

TL;DR

This study analyzes the stellar populations in NGC 604, revealing distinct ages for Wolf-Rayet and red supergiant stars, estimating the total stellar mass, and suggesting significant ionizing photon leakage affecting observations.

Contribution

It provides new age estimates for RSG and WR stars in NGC 604 using SED fitting and highlights the impact of RSGs on IR observations of star formation.

Findings

RSGs are older than WR stars, with ages around 12.4 Myr and 3.2 Myr respectively.

Approximately 49% of ionizing photons leak from NGC 604, affecting Hα flux measurements.

Total stellar mass of NGC 604 is estimated at about 3.8 x 10^5 solar masses.

Abstract

We study the post-main sequence stars in NGC 604, the most luminous HII region in M33. Previously, a number of Wolf-Rayet (WR) stars and one red supergiant (RSG) have been discovered. Based on broadband photometry of the region, we present evidence that is consistent with the presence of this RSG and with three more RSG candidates. Using SED fitting based on HST UVIJHK photometry we estimate the ages of the WR stars and RSGs finding that the two populations are from distinct formation episodes with ages 3.2$\pm%1.0Myrs and 12.4$\pm$2.1Myrs, respectively. The RSGs have greater extinctions towards their line of sight than the WR stars consistent with the RSGs producing large amount of dust. Using the WR and RSG populations and similar SED fits to the most massive O stars we estimate that the total stellar mass is (3.8 $\pm$ 0.6) x 10^5Msun. We find a large discrepancy between the expected H{\alpha} flux from such a massive cluster and that one observed. This suggests that 49 (+16,-19) percent of the ionizing photons produced by massive stars in NGC 604 is leaking from this HII region. We also suggest that the implications of an old RSG population mean that if NGC 604 was more distant and only observed in the infrared (IR) it would be difficult to study the youngest burst of star formation due to the contamination of RSGs.