University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Friday, October 4th 2019
2:30 pm:
Speaker: Robert Benjamin, University of Wisconsin-Whitewater
Subject: Discovery of Optical Emission Lines from the Inner Milky Way: Evidence for LI(N)ER-like gas

The first detection optical emission lines from the inner regions of a galaxy was in 1909 by E.A. Fath (a professor at Carleton College from the 1920s to 1950). But it was not until 1936 that a spectroscopic campaign at Mt Wilson and Lick Observatories began to turn up evidence for diffuse ionized gas in the inner parts of some galaxies using the [O II] 3727 line. Measurements of the [N II]/H-alpha ratio by M. Burbidge and G. Burbidge in the early 1960s showed that this gas was characterized by unusually high line ratios that could not be explained as dilute HII regions. Many of these systems were dubbed LINERS (low ionization nuclear emission line regions) in 1980; the ionization levels were neither consistent with a star-forming radiation field nor the radiation field of an Active Galactic Nuclus (AGN). The nearest LINER sytem known was M31. I will describe the Wisconsin H-alpha Mapper discovery of a reservoir of diffuse ionized gas in the inner Milky Way between Galactocentric radius R_G=0.25 to 1.5 kpc and the models developed to interpret these observations. We estimate a total mass ionized mass of 12±4 x 10^6 solar masses, finding that the atomic gas is at least 50% ionized. The [N II]/H-alpha ratio, which increases from 0.3 to 2.5 with Galactocentric radius is unlike anywhere else in the Milky Way, but typical of LINER systems. The ionizing flux required to maintain this gas is 10 times the estimated flux of Lyman continuum photons in the solar neighborhood (if ionized from the outside) or 5-10% of all Lyman continuum photons from the CMZ (if ionized from the inside). I will describe the implication of these results for understanding the nature of LINER systems.

Faculty Host: Thomas W. Jones

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