Warm ionized gas in the edge-on galaxies NGC 4565 and NGC 4631

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Abstract
We present H-alpha observations of two edge-on galaxies: NGC 4565 and NGC 4631. In contrast to NGC 891, which was studied in a previous paper, neither of these galaxies shows evidence for a smooth, vertically extended, diffuse, warm ionized medium. NGC 4565 is a weak H-alpha emitter, and shows no evidence for a chimney mode in its disk. It is most likely that its young associations do not contain the numbers of OB stars necessary to create superbubbles and chimneys and power a significant halo. NGC 4631 is a strong H-alpha emitter, and shows evidence of disturbances in its distribution of H-alpha emission, presumably due to recent encounters with its companions. The lack of a significant diffuse H-alpha halo may be due to a hot, unconfined wind from the disk. Although NGC 4631 does not show classic wormlike structures as in NGC 891, there is patchy high-z emission in abundance, and a few faint high-z loops, which are probably the result of star-formation activity in the disk. We discuss the implications of our results on NGC 891, NGC 4565, and NGC 4631 for the disk-halo connection, in particular for the emerging connection between the galaxies' disk H-alpha emission and radio halo properties. Above the nuclear region of NGC 4631, there is a clear, vertical, "double-worm" structure, which we suggest is due to the breakout of a large superbubble created by an encounter-driven star formation event in the central regions. The structure coincides with a radio continuum feature seen at 2.7 GHz and possibly 8.1 GHz by Duric, Crane, and Seaquist. This nuclear star formation activity is probably a scaled-down version of similar activity in NGC 3079, M82, and NGC 253.

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