AAS 195th Meeting, January 2000
Session 97. Gas and Stars In and Near Galaxies
Oral, Friday, January 14, 2000, 2:00-3:30pm, Regency VII

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[97.02] The Properties and Ionization Source of Diffuse Ionized Gas in Galactic Disks

Charles G. Hoopes (NMSU)

Diffuse ionized gas (DIG) is the dominant form of ionized gas and an important component of the ISM in spiral galaxies. We present a study of the properties of the DIG in face-on and edge-on galaxies, and show that a significant fraction of the DIG is ionized by OB stars outside of HII regions. We used deep H\alpha images of 5 edge-on galaxies to examine the properties of extraplanar DIG. The H\alpha halo of NGC 891 can be traced at least 5 kpc from the plane, and UGC 9242 has a halo which extends almost as far, but at much fainter surface brightness. We set upper limits on the emission from faint DIG in the outer disk, which in turn limits the strength of the metagalactic radiation field at low redshift.

Using far-ultraviolet (FUV) images from the Ultraviolet Imaging Telescope, FUV and optical images from HST, and groundbased H\alpha images of M33, we test whether DIG is ionized by OB stars outside of HII regions. The H\alpha/FUV ratio in the DIG is consistent with ionization by field OB stars. FUV and optical photometry of stars was used to determine their spectral types and ionizing luminosity. Comparing the total ionizing luminosity from stars in the DIG with the H\alpha emission, we find that field stars can account for at least 40% of the DIG ionization. This surprising result shows that field stars are important ionization sources, and reduces the amount of leakage necessary from HII regions.

We used optical spectroscopy to probe the physical state of the DIG. A search for the HeI line in M33 allows us to set limits on the contribution of O7 and hotter stars to the ionization of the DIG, closing the loop on the FUV analysis. In M51 we discuss the spectral properties of the DIG and a possible outflow cone in NGC 5195. In NGC 4631 we examine the behavior of the DIG spectrum with distance from the plane.

This project is supported by the NSF through grant AST96-17014, by NASA through grant AR-07546.01-96A from the STScI and grant NAG5-2426, and by a Cottrell Scholar Award from Research Corporation.

The author(s) of this abstract have provided an email address for comments about the abstract: choopes@nmsu.edu

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