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S. T. Miller, S. Veilleux (University of Maryland)
We have conducted a systematic imaging and spectroscopic survey of a complete sample of ~ 25 nearby edge-on spiral galaxies, probing the gaseous extent of the galaxies to unprecedented depths. Through these observations we place stricter constraints on the origin and the ionization process of extraplanar diffuse ionized gas (DIG). These galaxies have been observed at KPNO/CTIO using conventional narrow-band filters centered on H\alpha and flatfielded with super-skyflats in order to obtain deeper images than ever before. We have also obtained separate, ultra-deep H\alpha and [N~II]\lambda6583 images using the Taurus Tunable Filter at the AAT/WHT, allowing us to create two-dimensional [N~II]/H\alpha maps which are excellent probes of the gas excitation. While few galaxies demonstrate the extended DIG emission as seen in NGC~891, many of our targets show filaments extending out to distances on the order of a few kpcs.
The galaxies with high-\vertz\vert DIG were observed spectroscopically at the 2.1m telescope at KPNO to levels rarely reached in other observations. Our observations focussed on a spectral range (4500 -- 7300 Å) which has proven rich in emission-line diagnostics, and a detailed analysis of the line ratios as a function of \vertz\vert has been carried out. Our initial results suggest that while the main source of ionization is pure stellar radiation, other scenarios are needed to fully explain the observed line ratios.
We also compare our results to output from our photoionization models created using the program CLOUDY. We take into account the multi-phase nature of the ISM, the possible depletion of certain gas-phase abundance of metals onto dust grains, and the absorption and hardening of the stellar radiation field as it propagates through the dusty disk. This analysis allows us to determine or at least place strong constraints on the ionization parameter as a function of \vertz\vert, the reddening and density profiles, and the temperature and ionization fraction of the extraplanar material. The results of this analysis have been combined with measurements of the velocity field and line widths to place additional constraints on the origin and source of ionization of the high-\vertz\vert gas.