Halos, starbursts, and superbubbles in spirals

Abstract

Detectable quantities of interstellar material are present in the halo of the Milky Way galaxy and in a few edge‐on spiral galaxies, largely in the form of neutral atomic gas, warm ionized material, and cosmic rays. Theoretical and observational arguments suggest that million degree gas should be present also, so sensitive ROSAT observations have been made of the large nearby edge‐on spiral galaxies for the purpose of detecting hot extraplanar gas. Of the six brightest non‐starburst edge‐on galaxies, three exhibit extraplanar X‐ray emission: NGC 891, NGC 4631, and NGC 4565. In NGC 891, the extended emission has a density scale height of 7 kpc and an extent along the disk of 13 kpc in diameter. This component is close to hydrostatic equilibrium, has a luminosity of 4.4×1039 erg s−1, and a mass of 108 M☉. Extended and structured extraplanar hot gas is seen around the interacting edge‐on spiral NGC 4631, with X‐ray emission associated with a giant loop of Hα and HI emission; spurs of X‐ray emission extending from the disk are seen also. Hot gas is expected to enter the halo through superbubble breakout, and a search for superbubbles in normal spiral galaxies have shown that these phenomena are present, but of low surface brightness and are detected in only a few instances.Unlike the normal spiral galaxies where the gas is bound to the systems, the hot gas in starburst galaxies is being expelled. In M82, the X‐ray emission lies in loosely‐defined cones that extend to a projected radius of 6.5 kpc. The decrease of the surface brightness with position from the nucleus is consistent with adiabatic expansion of gas flowing from the central region of the galaxy. © 1995 American Institute of Physics.