A High‐Resolution VLA and Optical Study of NGC 604

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Abstract
The radio structure of the supergiant H II region NGC 604 in the dwarf spiral galaxy M33 has been observed at 3.6 cm with a resolution of ~2'' (~8 pc) down to an rms noise level of ~15 μJy. At least six radio components can be distinguished embedded in a more extended halo. Each radio component requires ionizing photon fluxes in the range 6-44 times that of Orion A, and the entire nebula requires about 350 times that of Orion A. The rms electron densities are in the range of a few to ~100 cm-3, and the total ionized gas mass is ~3×106 M☉. Even though the photon fluxes required to maintain ionization are large, the radiation density in each radio component is substantially less than that in Orion A due to the much larger size of the radio components in NGC 604. From a comparison of radio and Hα images provided by D. Hunter, we have derived the optical depth at Hα over the face of the nebula. Visual extinctions are greatest toward the bright radio components (2.8 to 1.7 mag) and 0.5 mag over the rest of the nebula. The increased extinction toward the bright radio components implies that the dust is physically associated with these components and requires that the dust is present in the ionized gas. The dust is probably mostly leftover from the natal clouds from which the clusters formed, although some enhancement may have occurred via photoionization of accretion disks associated with the cluster stars and/or molecular cloudlets that have been overrun by the ionization front of the nebula.