We have analyzed HST and ground-based data to characterize the different gas
phases and their interaction with the MYC in NGC 604, a GHR 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 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 SN 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 H alpha
as measured from the ratio of the thermal radio continuum to H alpha 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 H alpha as
measured from the ratio of H alpha to H beta 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 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 RSG. We estimate
the age of the main stellar generation in NGC 604 to be approx. 3 Myr from the
ionization structure of the H II region. The size of the main cavity is smaller
than the one predicted by extrapolating from single-star wind-blown bubbles.Comment: 50 pages, 15 figures. To appear in the September issue of the
Astronomical Journal. Some of the figures in this version have a very low
resolution due to the absurd limitations (for 2004) imposed on file size by
astro-ph. A full-resolution version of the figures is available at
http://www.stsci.edu/~jmai