As with many topics dependent on optical astronomy, the introduction of CCDs into mainstream research in the early 1990s propelled the study of the WIM to a new level. The increased sensitivity and linearity allowed new investigations of the morphology and extent of faintly emitting gas in the Milky Way (Section 4), providing a new view of the relationship between H ii regions and the diffuse gas. The WIM in other spiral galaxies could also be studied to similar physical extents from the plane as our galaxy, showing that this component of our ISM was ubiquitous in similar galaxies.
Deep imaging and spectroscopy of the diffuse gas of many spiral and
irregular star-forming galaxies provided a welcome global view in the
early 1990s. Since the fainter emitting gas in other galaxies was not
necessarily known a priori to be similar to the WIM of the
Milky Way, extragalactic researchers called it "diffuse ionized gas"
(DIG). Today it is generally recognized that most of the DIG emission
traces the WIM component in a galaxy.
H
imaging of edge-on spirals
reveals a large, extended, diffuse ionized component that generally
matches the inferred distribution of the Milky Way from our interior
vantage point (Figure 1). Complementary
observations of face-on galaxies show diffuse emission following star
formation and the general structure of the spiral arms. Spectroscopy of
the diffuse gas in a variety of galaxy types and orientations replicate
general trends in the Milky Way but exhibit variation in metallicity and
excitation present in such a larger observational sample.
 |
Figure 1. The WIM in NGC 891. A continuum-subtracted
H |
The deluge of observations from these initial studies provided a global view that further reinforced the WIM as a distinct component of the ISM in spirals:
luminosity
emitted from spiral galaxies comes from their WIM, not from their
H ii regions. The
power requirement is quite large and on scale with that required for the
Milky Way.
vary inversely with
H intensity, as
in our galaxy (e.g.,
Otte et al. 2002,
Otte et al. 2001,
Collins & Rand 2001,
Tüllmann & Dettmar 2000,
Golla et al. 1996).
Rossa & Dettmar (2003a,
2003b)
completed one of the most comprehensive
H imaging surveys of
spirals to date. Their study shows a clear link between infrared tracers
of active star formation and the existence of an extended ionized layer
in a galaxy. Such a relationship suggests that either the source
responsible for powering the star formation or a byproduct of the
formation itself is responsible for maintaining the WIM layer. Although
this link is consistent with ionization by massive stars, it does not
rule out a role played by other sources such as supernovae.
Evidence for a closer link between the massive stars and the WIM has
come from the work of
Ferguson et al. (1996)
and
Zurita, Rozas, & Beckman
(2000) .
In these studies of the ionized gas in moderately
inclined to face-on spirals they trace lower-limit WIM-to-total
H luminosity fractions
of 25 - 50% over the whole disk. Furthermore, they show that these
fractions are roughly constant as a function of radius for the galaxies
in their samples despite the decreasing star-formation rate per unit
area. In their very sensitive observations,
Zurita et al. (2000)
observe that enhanced diffuse emission near
H ii regions supports
that LyC flux is leaking from these sites. This impression persists
despite the fact that the extremely high contrast and sharp boundary
between an
H ii region and the
"background" suggests that the ionized region should be photon bounded
(i.e., the flux has been completely absorbed).
Zurita et al. (2002)
take this notion one step further and attempt to model the DIG of
NGC 157 beginning with the actual
H ii region
distribution and luminosities as well as the actual
H i distribution based
on 21 cm observations. They are able to examine a variety of escape
fraction scenarios and compare the results to their deep
H observations. While
the results do not quite match the resolution of their optical
observations due to limitations in the 21 cm data, the general agreement
lends additional support to a "leaky"
H ii region hypothesis.