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Session 48 - Spiral & Field Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
Diffuse Ionized Gas (DIG) is an important component of the interstellar medium, containing most of the mass of ionized gas. The volume filling factor of DIG for the Milky Way is about 0.2 in the disk. This diffuse gas (n_e\approx0.2 cm^-3) is detected as faint emission in H\alpha and forbidden lines outside of HII regions. Previous observations have found that the [SII](6717+6731ÅH\alpha intensity ratio of the DIG is higher than that of HII regions, making it possible to distinguish the two types of emission through narrow-band imaging in these lines. Although edge-on spirals are being extensively studied for the presence of DIG in the halo, few studies have been done of DIG in face-on spiral galaxy disks. We present the results of a study of three Sculptor Group galaxies: NGC55 (an edge-on SBm galaxy), NGC253 (an inclined starburst Sc galaxy), and NGC300 (a face-on Sd galaxy). Narrow-band H\alpha and [SII] images were analyzed to characterize the extent and nature of the DIG. The images show the presence of DIG in all three galaxies, and the [SII]/H\alpha ratio images confirm the enhancement of [SII] emission in the diffuse gas. We find that the distribution of DIG in the disk is strongly correlated with that of HII regions in the spiral arms. We measure the H\alpha luminosity for each galaxy, and find that in all three the DIG contributes 30-40% of the total. This result is consistent with studies of DIG in other nearby galaxies, and is surprising given the markedly different nature of these galaxies. We also investigate the outflow cone from the starburst nucleus of NGC253, and find that the [SII]/(H\alpha+[NII]) ratio is intermediate between that of HII regions and the DIG, possibly indicating a mixture of photoionization and shock ionization in the outflow.
