NGC 604, the Scaled OB Association (SOBA) Prototype. I. Spatial Distribution of the Different Gas Phases and Attenuation by Dust

We have analyzed Hubble Space Telescope and ground-based data to characterize the different gas phases and their interaction with the massive young cluster in NGC 604, a giant H II region in M33. The warm ionized gas is made out of two components: a high-excitation, high surface brightness H II surface located at the faces of the molecular clouds directly exposed to the ionizing radiation of the central scaled OB association (SOBA); and a low-excitation, low surface brightness halo that extends to much larger distances from the ionizing stars. The cavities created by the winds and supernova explosions are filled with X-ray–emitting coronal gas. The nebular lines emitted by the warm gas experience a variable attenuation as a consequence of the dust distribution, which is patchy in the plane of the sky and with clouds interspersed among emission-line sources in the same line of sight. The optical depth at Hα as measured from the ratio of the thermal radio continuum to Hα shows a very good correlation with the total CO (1 → 0) column, indicating that most of the dust resides in the cold molecular phase. The optical depth at Hα as measured from the ratio of Hα to Hβ also correlates with the CO emission but not as strongly as in the previous case. We analyze the difference between those two measurements, and we find that ≲11% of the H II gas is hidden behind large–optical-depth molecular clouds; we pinpoint the positions in NGC 604 where that hidden gas is located. We detect two candidate compact H II regions embedded inside the molecular cloud; both are within short distance of WR/Of stars, and one of them is located within 16 pc of a red supergiant. We estimate the age of the main stellar generation in NGC 604 to be ≈3 Myr from the ionization structure of the H II region, a value consistent with previous age measurements. The size of the main cavity is smaller than that predicted by extrapolating from single-star wind-blown bubbles; possible explanations for this effect are presented.