تسجيل الدخول

تسجيل

Small-scale structure of the ISM

المؤلف:  James Binney and Michael Merrifield

المصدر:  GALACTIC ASTRONOMY

الجزء والصفحة:  p-598

2026-11-01

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The ISM is extremely inhomogeneous. Studies of interstellar absorption along lines of sight to nearby stars indicate that the Sun is located in an irregular region of low density. In most of this region, which extends to over 100 pc from the Sun in some directions, but only  pc in others, the number density of  nuclei is  and the ionization level is high (e.g. Pounds et al. 1993). Scattered throughout this low-density region there are many clouds of denser material. The clouds have radii of a few parsecs and column densities  in the range . From these numbers one infers particle densities   and masses . The Sun is located in one of these small clouds, which occupy only a small fraction  of the volume of the Galactic disk, but contain perhaps 90% of the mass of the ISM near the Sun. In the clouds the temperature is ~ 100 K, while the temperature in the low-density material is ~ 10 000 K.

At distances of order  from the Sun there are large volumes of much hotter plasma. This plasma must have a temperature  because it is rich in highly ionized species such as , and emits soft X-rays. The plasma must be located more than  from the Sun because in X-ray maps made with the ROSAT observatory, nearby, cool interstellar clouds show up as dark filaments on a bright background; these clouds lie in front of the X-ray emitting plasma.

Regions filled by million-degree plasma are thought to form when gas that has been accelerated to velocities in excess of   by supernovae and fast winds from hot, young stars, crashes into essentially stationary gas. This collision gives rise to a shock front that thermalizes the kinetic energy of the high-velocity gas. Outflows that are driven by many young stars and supernovae often give rise in this way to long thin regions of million-degree plasma, which are called worms. The largest worms extend from the plane right up into the Galactic halo, and are conspicuous features of 21-cm maps of the nearby Galaxy (Heiles 1984).