The Metagalactic Ionizing Field in the Local Group

Details The Metagalactic Ionizing Field in the Local Group The Metagalactic Ionizing Field in the Local Group

1998-06-23

arxiv.org/abs/astro-ph/9806316v1

Astronomy and Astrophysics

Astrophysics

10 pages; Latex; 1 encapsulated PS figure. Invited talk, ESO/ATNF International Workshop "Looking Deep in the Southern Sky", D

Scientific paper

We discuss the sources which are likely to dominate the ionizing field throughout the Local Group. In terms of the limiting flux to produce detectable H-alpha emission (4-10 x 10**3 phot/cm**2/s), the four dominant galaxies (M31, Galaxy, M33, LMC) have spheres of influence which occupy a small fraction (<10%) of the Local Volume. There are at least two possible sources of ionization whose influence could be far more pervasive: (i) a cosmic background of ionizing photons; (ii) a pervasive warm plasma throughout the Local Group. The COBE FIRAS sky temperature measurements permit a wide variety of plasmas with detectable ionizing fields. It has been suggested (Blitz et al. 1996; Spergel et al. 1996; Sembach et al. 1995, 1998) that a substantial fraction of high velocity clouds are external to the Galaxy but within the Local Group. Deep H-alpha detections are the crucial test of these claims and, indeed, provide a test bed for the putative Local Group corona.

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