Extraplanar Emission-line Gas in Edge-on Spiral Galaxies. I. Deep Emission-line Imaging

The extraplanar diffuse ionized gas (eDIG) in 17 nearby, edge-on disk galaxies is studied using deep Taurus Tunable Filter Hα and [N II] λ6583 images and conventional interference filter Hα + [N II] λ6548, 6583 images which reach flux levels generally below ∼ 1 × 10−17 erg s−1 cm−2 arcsec−2. [N II] λ6583/Hα excitation maps are available for 10 of these objects. All but one galaxy in the sample exhibit eDIG. The contribution of the eDIG to the total Hα luminosity is relatively constant, of order 12 ± 4%. The Hα scale height of the eDIG derived from a two-exponential fit to the vertical emission profile ranges from 0.4 kpc to 17.9 kpc, with an average of 4.3 kpc. This average value is noticeably larger than the eDIG scale height measured in our Galaxy and other galaxies. This difference in scale height is probably due in part to the lower flux limits of our observations. The ionized mass of the extraplanar component inferred by assuming a constant filling factor of 0.2 and a constant pathlength through the disk of 5 kpc ranges from 1.4 × 107 M⊙ to 2.4 × 108 M⊙, with an average value of 1.2 × 108 M⊙. Under these same assumptions, the recombination rate required to keep the eDIG ionized ranges from 0.44 to 13 × 106 s−1 per cm−2 of the disk, or about 10 to 325% of the Galactic value. A quantitative analysis of the topology of the eDIG confirms that several galaxies in the sample have a highly structured eDIG morphology. The distribution of the eDIG emission is often correlated with the locations of the H II regions in the disk, supporting the hypothesis that the predominant source of ionization of the eDIG is photoionization from OB stars located in the H II regions. A strong correlation is found between the IR (or FIR) luminosities per unit disk area (basically a measure of the star formation rate per unit disk area) and the extraplanar ionized mass, further providing support for a strong connection between the disk and eDIG components in these galaxies. The excitation maps confirm that the [N II]/Hα ratios are systematically higher in the eDIG than in the disk. Although photoionization by disk OB stars is generally able to explain these elevated [N II]/Hα ratios, a secondary source of ionization appears to be needed when one also takes into account other line ratios; more detail is given in a companion paper (our Paper II). Subject headings: diffuse radiation – galaxies: halos – galaxies: ISM – galaxies: spiral – galaxies: structure