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Session 67 - ISM in Galaxies.
Display session, Wednesday, January 15
A very important discovery of recent X-ray satellites has been the detection of a hot interstellar medium (ISM) in early-type galaxies. In case of `isolated' early-type galaxies, the typical mass of this hot coronal gas component is a few percent of the luminous mass, its temperature is of order 10^7 K, and its electron density in the central regions is of order 10^-2 cm^-3. This hot medium is a hostile environment for dust grains:\ thermal sputtering destroys the grains on a typical timescale of only 10^7 yr (Draine amp; Salpeter 1979, ApJ 231, 77). Hence, the ISM in these objects is not expected to contain any significant amount of dust. However, recent studies have shown that about 50% of all bright elliptical galaxies have been detected by IRAS at 100 \mum (Knapp et al.\ 1989, ApJS 60, 329), indicating the presence of 10^4 - 10^7 M_ødot of dust. Thus, the presence of dust in elliptical galaxies is now beyond dispute.
However, significant controversy has remained concerning the relationship between the different components of the ISM of these galaxies, in particular whether galaxy interactions or cooling-flows dictate that interplay (see, e.g., Sparks et al.\ 1989, ApJ 345, 153; Fabian et al.\ 1994, ApJ 425, 40). E.g., X-ray-emitting early-type galaxies have been shown to often exhibit H\alpha emission (e.g., Trinchieri amp; Di Serego Alighieri 1991, AJ 101, 1647). This can be understood in terms of the cooling-flow scenario in which the H\alpha emission is due to gas cooling down from the hot component; however, it can as well be due to cool ISM having been accreted during a galaxy interaction, in which case the excess X-ray emission at the H\alpha-emitting filaments is due to excess cooling of the hot gas through heating by electron conduction of dust grains associated with the ionized gas (de Jong et al.\ 1990, Aamp;A 232, 317; Goudfrooij et al.\ 1994, Aamp;AS 105, 341). The presence of dust associated with the ionized gas filaments is crucial to this controversy, in view of the short destruction time for dust grains.
We will show and interpret results of deep X-ray (ROSAT HRI) and optical (ESO NTT, V amp; I band) imaging of NGC 5846, a giant elliptical featuring H\alpha+[N\,\sc ii] emission. The data show that the X-ray emission peaks at the H\alpha-emitting regions, which are in turn associated with a conspicuous dust lane.
Program listing for Wednesday