Introduction 1811

Looking at the sky on a clear and dark night, the first and most impressive view is the myriad of stars twinkling on the firmament and emitting light from inconceivable distances. The space between them seems to be empty. Apart from other, apparently moving objects, such as comets and planets, it was thought for a long time that the universe was infinite and static, and Newton considered the stars to represent an absolute frame of reference. Progress in astronomy is often driven by technology. The increasing use of telescopes in the seventeenth and eighteenth centuries revealed structures hitherto unseen. Messier, hunting for comets, discovered similarly looking patches on the sky that were not moving and decided therefore to make a compilation (published in several catalogues of star clusters and nebulae until 1781), to avoid further confusion. Among them Andromeda (Messier 31, or M31 for short) and Orion (M42) are the most famous ones.1 The physical nature of the nebulae became only clear with the advent of spectroscopy in the nineteenth century, its main proponents being Fraunhofer, who discovered absorption lines in the spectrum of the sun, and Bunsen and Kirchhoff who associated spectral lines in the laboratory with light emitted by specific atoms. By then it had become evident that the space between the stars, the so-called interstellar medium (ISM), is by no means empty.

The history of the ISM is generally thought to commence with the discovery of stationary Call lines in the spectrum of the spectroscopic binary system 8 Orio-nis [24]. There remained, however, a shadow of doubt, until it could be demonstrated unambiguously that these lines are interstellar, because they increase in strength with distance, and their velocity is only half of the value expected from Galactic rotation [43]. Further highlights in ISM research are the discovery of interstellar extinction by Trumpler (1930), who found that open star clusters become brighter with increasing distance, and, therefore, concluded an overestimate of the distance due to the interstellar extinction of starlight by about 1 magnitude per kiloparsec [70].

1 Incidentally, M 1 is the famous Crab Nebula, a supernova remnant - and a typical object of the hot Interstellar Medium.

Following a suggestion of Oort, the hyperfine structure transition of HI - because of the scarcity of atomic collisions in the ISM with an average density of — 1cm-3 -had been predicted to be observable in the 21 cm wavelength [72]. This was obser-vationally confirmed by Oort and collaborators a few years later. Thus in the first half of the twentieth century the ISM consisted of cold neutral hydrogen, and warm ionized gas surrounding O and B stars in so-called HII regions with sizes calculated on the basis of ionization equilibrium by [65].

The birth of the hot ISM (HIM) had to await the launch of rockets and satellites, equipped with sensitive high energy radiation detectors. Remarkably, the first diffuse (apparently all-sky) background radiation ever discovered, i.e., before the cosmic microwave background, was in X-rays in 1962 [22], and its soft component (below 2keV) in 1968 [5]. The interpretation of the measurements remained ambiguous, in particular the possible sources and their spatial distribution as we shall see below. Therefore, initially, the hallmark of the HIM became the OVI resonance line in the UV, discovered in absorption toward background stars with the Copernicus satellite, launched in 1972 as a result of Spitzer's insight into astrophys-ical plasmas and his promotion for mounting telescopes on satellites. It turned out that the widespread OVI line was not attributable to the circumstellar environment, because the column density increased with distance, thus leading to the establishment of the hot phase of the ISM [29,49,78]. Attempts to link this to the soft X-ray background (SXRB) mentioned above were doomed to fail, because the excitation temperatures were too different: collisionally ionized Ovi traces gas of —3 x 105K, whereas the SXRB monitors —106 K gas. In collisional ionization equilibrium (CIE) this difference is impossible to reconcile, since at SXRB temperatures the ion fraction of OVII is dominating OVI by more than two orders of magnitude.

It became soon clear that a new "phase"2 of the ISM had been discovered, which should dominate by volume, and due to the implied temperature range (2 x 105 < T < 2 x 106 K) must be generated by shock heating in supernova remnants (SNRs). It was shown that a supernova rate of 1 per 50 years is sufficient to maintain a large scale Galactic tunnel network [13]. However, as we shall see below, the Ovi line primarily traces older SNRs or superbubbles, the latter being generated by successive explosions in OB associations. It is, therefore, the emission in soft X-rays that gives us direct information on the structure and evolution of the hot ISM. To monitor the soft X-ray sky, a program launching sounding rockets, equipped with proportional counters and narrow bandpass filters, was initiated at the University of Madison, Wisconsin. The Wisconsin Survey (1983) [37] showed an anisotropic patchy distribution in the low energy, so-called C-band (0.16 - 0.284 keV), increasing in flux from disk to pole by about a factor of 2-3, not much different from the B-band (0.13 - 0.188 keV) and the Be-Band (0.07 - 0.111 keV). This is remarkable, as soft X-ray photons are very sensitive to photoelectric absorption from intervening neutral ISM (proportional to energy E-3), with the cross-section between the latter two bands differing by a factor of about 6. These findings led to the so-called displacement [52] and Local Hot Bubble model [67]. The idea is that the solar system is

2 Somehow loosely defined as the regions of stability in the pressure-temperature diagram.

surrounded by a bubble, filled with hot plasma with a temperature of about —106 K and a density of n — 5 x 10-3 cm-3 [58], displacing HI. Such a local HI void does indeed exist [20], and has similar extensions to the Local Bubble (LB; for recent observations cf. [32]).

A major break-through in the hot ISM research came with the launch of the ROSAT satellite [69], with its fast X-ray resolving optics [1] and the low noise PSPC detector (Position Sensitive Proportional Counter) [44]. It is fair to say that the ROSAT PSPC All-Sky Survey is still today an invaluable source of data showing the distribution of hot gas in the Galaxy. The data base is unrivalled in completeness and still complements the large observatories Chandra and XMM-Newton in the energy range 0.1 - 0.3 keV, in which they are not very sensitive.

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