Ionized Hydrogen at Large Galactocentric Distances

J. Bland-Hawthorn, PASA, 14 (1), 64.

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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 tex2html_wrap_inline223 (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 tex2html_wrap_inline223 decreasing

bisymmetric tides due to external galaxy tex2html_wrap_inline223 = 0

mirror symmetric integral-sign warp + internal source tex2html_wrap_inline223 increasing

asymmetric ram pressure from external medium tex2html_wrap_inline223 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 tex2html_wrap_inline223tex2html_wrap_inline353 0.1 cmtex2html_wrap_inline251 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]tex2html_wrap_inline2416548 line has a surface brightness comparable to the Htex2html_wrap_inline249 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 tex2html_wrap_inline361 24 mag arcsectex2html_wrap_inline255 below [NII] falling to 25 mag arcsectex2html_wrap_inline255 below Htex2html_wrap_inline249.

The [NII]tex2html_wrap_inline369/tex2html_wrap_inline215 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 tex2html_wrap_inline215 at the HI edge at similar flux levels. The WHT/CFHT observational setups did not allow us to simultaneously observe both tex2html_wrap_inline215 and [NII]. This galaxy has a very substantial HI warp and the outer regions may be seeing the central disk population (Patel & Wilson 1995).

Next Section: Further Experiments
Title/Abstract Page: Ionized Hydrogen at Large
Previous Section: Cosmic ionizing background
Contents Page: Volume 14, Number 1

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