Before going into details of the properties of diffuse X-ray emission of
dwarf galaxies, it may be fruitful to check a few of the assumptions and
theoretical predictions in smaller, better controlled, nearby systems, like
the superbubbles in the Magellanic Clouds.
Fig. 1 shows an
H
image of the
superbubble N51D in the Large Magellanic Cloud (LMC).
The structure is a fairly typical example of its class
(Meaburn 1980).
It shows a complete shell of ionized gas with a diameter of about 100 pc
centered on one large OB association.
Such a superbubble distinguishes itself physically from a (~ 1 - 50 pc)
bubble around a single star
(Weis & Duschl
1998)
and a kpc-sized
supergiant shell (sometimes also called supershell)
(Meaburn 1980;
Chu 1995)
around a large complex of OB associations.
|
Figure 1.
H |
N51D expands with a velocity of ~ 35 km s-1
(Lasker 1980;
de Boer & Nash 1982)
into the surrounding medium. Despite of some larger loop-like
extensions of the
H shell to the
north and the south-west (as visible in
Fig. 1), this
superbubble appears to be still a closed volume. The difference in surface
brightness between the eastern and western section is partly due a
different density of the surrounding gas into which the superbubble
expands. The
gas is ionized by the OB association inside, which contains one the
UV brightest stars in the LMC, the WC5+O8Iaf binary Sk-67 104 (= HD 36402).
A detailed study of the OB association
(Oey & Smedley 1998)
yields an age of
3 Myr and determines from the observed stellar content the total
energy input of the stars to the superbubble. N51D emits diffuse
X-ray emission
(Chu & Mac Low 1990)
which is brighter near the eastern
rim of the superbubble. Fig. 1 shows the
H image of N51D
overlayed
with contours of the X-ray emission derived from an ROSAT PSPC archival data
(Bomans et al. 2001a,
in prep).
While some X-ray emission inside N51D seems to stem from point sources
(most probably X-ray binaries), diffuse emission is clearly present inside
the cavity delineated by the
H shell. The
ROSAT data confirm the
surface brightness enhancement at the bright, probably denser, eastern
boundary of N51D first detected using EINSTEIN
(Chu & Mac Low
1990),
but also show considerable substructure here, not following one-to-one
the H emission
(Bomans et al. 2001a, in prep.).
The temperature of the
hot gas is about 5 × 106 K, when fitting spectra with
collisional equilibrium models (e.g.
Raymond & Smith
1977).
The X-ray surface brightness
and luminosity of N51D does not fit well to the predictions of the
Weaver et
al. (1977)
model of an expanding superbubble and require an
additional source of X-ray emission, which can be provided by supernova
explosions of
massive stars in the OB association, meaning inside the superbubble.
As soon as the shock wave hits the dense shell wall, hot gas with high
density
and therefore surface brightness is produced, a process we may well observe
right now at the eastern inner shell wall
(Chu & Mac Low 1990,
Bomans et al. 2001a, in prep.).
It is interesting to note here, that the loop at
the south-western edge of N51D is a separate diffuse X-ray region, which
is tempting to be identified with a beginning outflow, similar
to the one in the LMC superbubble N44
(Chu et al. 1993).
With the availability of an HI synthesis map of the LMC
(Kim et al. 1998),
even some informations about the surrounding neutral gas density and
topology can be derived.
Therefore in the case of N51D all critical parameters for the evolutions
of a bubble can be observationally determined or at least estimated.
The crucial determination of the metallicity of the diffuse hot gas is still missing. The current information on superbubbles does not give reliable metallicities or how metal-enriched and more metal-poor gas is distributed in the bubble and along the shell walls, much less than giving details on the actual mixing processes. ASCA observations of a sample of LMC supernova remnants prove at least the first assumption: the hot gas is indeed metal-enriched by the supernova explosion, but it shows also that mixing of hot metal-rich and cool metal-poor gas starts right away (Hughes et al. 1998).