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 similar to 70% of the full H alpha 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 similar to 4300 M-circle dot and an unabsorbed (0.35-2.5 keV) X-ray luminosity of L-X = 9.3 x 10(35) erg s(-1). These values are consistent with a stellar mass-loss bubble entirely powered by similar to 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 similar to 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 similar to 1750 M-circle dot. However, mass loss from young stars cannot account for the unabsorbed X-ray luminosity of L-X = 4.8 x 10(35) 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.