The diffuse UV and soft X-ray background and the local interstellar medium

Abstract

The various processes by which the different phases of the interstellar medium can be studied in emission in the ultraviolet through soft X-ray region of the spectrum are reviewed, and the nature of the information that this diffuse radiation carries is outlined. The cool neutral component of the ISM can be studied through reflection of UV starlight off interstellar dust and, potentially, in H 2 fluorescence emission. This first source of diffuse UV light is intimately related to the IRAS cirrus emission in the infrared, whereas the presence of detectable H 2 Lyman and Werner bands in the UV background spectrum is a clear prediction of current theories concerning the formation and destruction of the H 2 molecule. Its detection would provide a crucial test of these theories, as well as yielding a novel method of mapping interstellar H 2. The warm (T ⋍ 10 4 − 10 5 K) ionized gas which has been observed in absorption in the disk and corona of the Galaxy may produce emission in UV coronal lines at detectable levels. The determination of the intensity of this line emission would provide unique information on the pressure and filling factor of the gas, as well as a direct means of determining whether the ionization of this phase of the ISM is maintained by collisional or photoionization processes. The hottest ( T ≈ 10 6 K) coronal plasma component of the ISM can presently only be detected through its thermal emission in soft X-rays. The observed properties of the diffuse soft X-ray background therefore provide the only information available on the pervasiveness of very hot gas in the disk and halo of the Galaxy. The soft X-ray data are consistent with the local ISM having an embedded cloud structure as predicted in supernova-dominated theories of the ISM, but definite conclusions as to the applicability of this picture on larger scales cannot be reached at present.