The Chandra ACIS survey of M33 (ChASeM33): Investigating the hot ionized medium in NGC 604

NGC 604 is the largest H II region in M33, second only within the Local Group to 30 Dor, and is important as a laboratory for understanding how massive young stellar clusters interact with the surrounding interstellar medium. Here we present deep (300 ks) X-ray imagery of NGC 604 obtained as part of the Chandra ACIS Survey of M33 (ChASeM33), which shows highly structured X-ray emission covering ∼70% of the full Hα extent of NGC 604. The main bubbles and cavities in NGC 604 are filled with hot (kT = 0.5 keV) X-ray-emitting gas, and X-ray spectra extracted from these regions indicate that the gas is thermal. For the western part of NGC 604 we derive an X-ray gas mass of ∼4300 M_⊙ and an unabsorbed (0.35-2.5 keV) X-ray luminosity of L_X = 9.3 × 10³⁵ erg s^_1. These values are consistent with a stellar mass-loss bubble entirely powered by ∼200 OB stars. This result is remarkable because the standard bubble model tends to underpredict the luminosity of X-ray-bright bubbles and usually requires additional heating from SNRs. Given a cluster age of ∼3 Myr, it is likely that the massive stars have not yet evolved into SNe. We detect two discrete spots of enhanced and harder X-ray emission, which we consider to be fingerprints from a reverse shock produced by a supersonic wind after it collided with the shell wall. In the eastern part of NGC 604 the X-ray gas mass amounts to ∼1750 M_⊙. However, mass loss from young stars cannot account for the unabsorbed X-ray luminosity of L_X = 4.8 × 10³⁵ erg s^-1. Off-center SNRs could produce the additional luminosity. The bubbles in the east seem to be much older and were most likely formed and powered by young stars and SNe in the past. A similar dichotomy between east and west is seen in the optical, implying that a massive wall of neutral and ionized gas shields the dynamically quiescent east from the actively star-forming west.