Diffuse Ionized Gas in a Sample of Edge‐on Galaxies and Comparisons with Hiand Radio Continuum Emission

Abstract

We present Hα images of four edge-on galaxies: NGC 5775, NGC 3044, NGC 4183, and NGC 3556. Our goals are twofold: first, to continue the study of the occurrence and physical properties of extraplanar diffuse ionized gas (DIG), and second, to compare extraplanar Hα, H I, and radio continuum emission. The DIG of NGC 5775 is found to be concentrated in numerous bright filaments extending as high as 6 kpc off the midplane. NGC 3044, on the other hand, has a bright, smooth region of DIG above the central part of the disk with some faint filaments at higher galactocentric radius. Vertical profile fits to the extraplanar emission show that both NGC 5775 and NGC 3044 have DIG layers more extended than the Reynolds layer. In contrast, NGC 4183 shows very little extraplanar DIG. Profile fits suggest that it is very unlikely that the DIG layer of NGC 4183 is as extended as the Reynolds layer. The correlation between levels of star formation and prominence of DIG layers for these galaxies is consistent with previous observations. A prediction of the model of the ISM is that extraplanar Hα features should have H I and radio continuum counterparts. Comparisons among these three tracers have revealed, for the first time, correlations on the scale of individual extraplanar filaments in a galactic disk. NGC 5775 shows correlations between Hα filaments, H I shells, and radio continuum spurs that are consistent with a chimney model. However, features identified as possible chimneys in NGC 5775 show radio spectral indices steeper than would be expected if cosmic rays were swept into the halo via galactic winds. Calculations of cosmic-ray propagation scale lengths within the DIG filaments demonstrate that diffusion may indeed be responsible for transporting cosmic rays through chimneys marked by DIG filaments. Correlated emission in these three tracers is also seen in NGC 3044, where extraplanar H I and radio continuum features appear to be associated with the galaxy's central region of extraplanar DIG, as well as filaments above the disk. Such correlations are not as prominent in NGC 3556, where extraplanar DIG detection is hampered by the galaxy's relatively low inclination.