DIFFUSE ILNIZED GAS IN EDGE-ON SPIRAL GALAXIES: EXTRAPLANAR AND OUTER DISK Ha EMISSION

摘要

We present Ha images of five edge-on galaxies: NGC 891,NGC 4631, NGC3003, and UGC9242. We also analyze[sⅡ]λλ6717,6731 and [OⅢ]λ 5007 images of NGC4631. 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 Ha luminosity, similar to the contribution in face-on galaxies, but in NGC 891 the DIG contributes 80/100-90/100.This is likely due to the higher dust content in the disk of NGC 891, which obscures the H Ⅱregions 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 Ha nucleus, an indication of a starburst superwind. In NGC4631 we see a very large shell of emission extending 3.5 kpc into the halo. the [SⅡ ]Ha and[OⅢ]/Ha rations 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 [OⅢ]/(Ha+[NⅡ]) ratio reaches over .in　the DIG, which coincides with an H I supershell. We use our very deep images of NGC3003 and UGC9242 to search for ionized gas in the outer disks as a test of the strength of the metagalactic ionizing radiation field. We find no out