The ionized edges of spiral galaxies

Sunyaev (1969) first argued that the HI disks of spiral galaxies truncate at a few times the optical diameter because the exponentially declining HI column eventually becomes fully ionized by the metagalactic radiation field (see also Bochkarev & Sunyaev 1975). It was realized by Bland-Hawthorn et al. (1994) that the predicted levels of emission (Maloney 1993; Dove & Shull 1994) could be reached with the `staring' technique. However, there may be alternative explanations in specific cases. In particular, Bland-Hawthorn & Maloney (1997) have shown that the young disk population could be responsible since almost all - if not all - spirals have at least some warp (Kuijken 1996; Carignan 1995, personal communication).

We can summarize possible sources of truncation in galaxies as follows. Space does not allow for a discussion of each case. The deprojected shape of the HI disk at a fixed column density is shown in the first column. The source of the truncation and the expected trend in (with increasing radius) are shown in the second and third columns respectively:

 shape of HI diskIsource of truncationItrend with increasing radius 
 axisymmetric      		 ambient radiation field                		  decreasing 
 bisymmetric       		 tides due to external galaxy           		  = 0 
 mirror symmetric  		 integral-sign warp + internal source 		  increasing 
 asymmetric        		 ram pressure from external medium      		  complex 
 

In order to explore these cases, we are currently studying a range of galaxy types in different environments. The galaxies chosen for detailed study include M31, M33, M83, NGC 628, NGC 3198, NGC 5266, Fourcade-Figueroa, and the Sculptor Group. The most detailed work has concentrated on NGC 253 (Bland-Hawthorn, Freeman & Quinn 1996), M33 and NGC 3198 (Bland-Hawthorn, Veilleux & Carignan 1996). In the case of NGC 253, we see ionized gas with 0.1 cm pc at and beyond the HI disk (Fig. 1).

  
Figure 1: The emission-line spectrum at the HI edge compared with the off-field spectrum. The difference of these spectra is shown below. Remarkably, the [NII]6548 line has a surface brightness comparable to the H surface brightness, as compared with solar-abundance HII regions where the ratio is an order of magnitude smaller. The azimuthally averaged galaxy continuum underlies the spectrum and corresponds to roughly 24 mag arcsec below [NII] falling to 25 mag arcsec below H.

The [NII]/ ratio appears to close to unity. This has enabled us to extend the rotation curve by 25% over the HI data. It is unlikely that these ratios can be explained by a quasar-produced ionizing background. Such ratios need additional heating without further ionization, e.g., from ram pressure heating as the disk moves through an external medium. For M33, we have succeeded in detecting at the HI edge at similar flux levels. The WHT/CFHT observational setups did not allow us to simultaneously observe both and [NII]. This galaxy has a very substantial HI warp and the outer regions may be seeing the central disk population (Patel & Wilson 1995).

© Copyright Astronomical Society of Australia 1997