Diffuse Ionized Gas in Edge‐on Spiral Galaxies: Extraplanar and Outer Disk Hα Emission

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Abstract
We present Hα images of five edge-on galaxies: NGC 891, NGC 4631, NGC 4244, NGC 3003, and UGC 9242. We also analyze [S II] λλ6717, 6731 and [O III] λ5007 images of NGC 4631. For several of these galaxies, these images are the most sensitive to date. We analyze the ionized gas content, with particular attention to the diffuse ionized gas (DIG). The DIG layer in NGC 891 is traced out to at least 5 kpc from the midplane, confirming an earlier spectroscopic detection. The DIG in four of these galaxies contributes 40%-50% of the total Hα luminosity, similar to the contribution in face-on galaxies, but in NGC 891 the DIG contributes 80%-90%. This is likely due to the higher dust content in the disk of NGC 891, which obscures the H II regions but may also reflect the extraordinary prominence of the DIG layer in that galaxy. Our very deep image of UGC 9242 shows very low surface brightness emission, as low as 0.3 pc cm-6, reaching 4-5 kpc above the midplane. This galaxy also exhibits filaments near the bright Hα nucleus, an indication of a starburst superwind. In NGC 4631 we see a very large shell of emission extending 3.5 kpc into the halo. The [S II]/Hα and [O III]/Hα ratios in NGC 4631 are consistent with the ratios seen in other galaxies, and with photoionization models. There is a region on the southeast side of disk where the [O III]/(Hα + [N II]) ratio reaches over 1.0 in the DIG, which coincides with an H I supershell. We use our very deep images of NGC 3003 and UGC 9242 to search for ionized gas in the outer disks as a test of the strength of the metagalactic ionizing radiation field. We find no outer disk emission down to our 1 σ limit of 0.13 pc cm-6 on scales of 1.5 kpc in NGC 3003. Based on this limit we rule out a metagalactic ionizing radiation field stronger than 11 × 10-23 ergs cm-2 s-1 Hz-1 sr-1. There is an indication of extended disk emission in UGC 9242, which would imply a stronger radiation field, but various concerns, most importantly flat-fielding uncertainties due to foreground stars in the image, lead us to question whether this feature is real.
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