The fact that we now have all maps scaled to the same 22 µm diameter in Fig. 5 allows us to form the median edge-on galaxy at L-band. That is, we can explore what the typical edge-on spiral galaxy looks like in the radio continuum. We have done this for 30 of our 35 galaxies, excluding NGC 660 (too distorted since it is a merger), NGC 4438 (presence of plume/bubble), NGC 4594 and NGC 5084 (large beam), and UGC 10288 (emission dominated by a background AGN).
We formed the median of the 30 galaxies after having converted the brightness to units of Jy/pixel. The result is shown in Fig. 6 and takes advantage of the collective sensitivity of all 30 galaxies. In the image, we also plot a sample 22 µm contour (red) that corresponds to the scaling of the radio data.
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Figure 6. The median edge-on spiral galaxy in L-band, made from stacking 30 of the galaxies in Fig. 5. The red ellipse is a sample 22 µm contour that corresponds to the scaling of the radio data and thus represents the disk radial extent. The beam shown is the average beam of the 30 galaxies. |
Fig. 6 shows the spectacular radio extent of the typical spiral galaxy (note that the average beam is much smaller than the displayed radio extent), predicted half a century ago by Ginzburg & Syrovatsky (1961) (see their Fig. 1, p. 18). The galaxy has the appearance of a slightly flattened ellipsoid and reveals that cosmic rays and magnetic fields not only permeate the galaxy disk itself, but extend far above and below the disk, as has been discussed by e.g. Haverkorn & Heesen (2012), Krause (2009). We note that the red ellipse is not affected by the larger beams of the radio images and should not be used, at this point, to make conclusions about the radial distribution of the emission compared to the 22 µm emission. On the other hand, the conclusion of a broadscale halo is robust; we have checked the contribution of the disk emission to the apparent vertical radio extent by examining the lower inclination galaxies in the sample, finding that the disk made a negligible contribution. If galaxies that have lower inclination are removed, the halo remains, suggesting that the vertical distribution is not a result of a projected disk. In addition, the halo extends beyond what would be expected from beam smoothing.
The galaxy scale heights (see Dumke et al. 1995, for method) and magnetic field extents will be discussed further in future papers.