The UKST H Survey - Scientific Background

Expectations of the survey

The Galaxy

The Galaxy is, of course, the spiral in which we can study the star formation process in most detail. Much of this star formation occurs within the dense parts of molecular clouds and complexes and may proceed as a result of instabilities within the cloud or may be triggered by energetic events near the cloud as discussed briefly below (Sect 3).

The spatial extent and detailed morphology of the resulting HII regions, OB associations and the wide variety of structures - `shells, rings, holes, loops, bubbles, filaments, superbubbles, supershells, supergiant shells, arcs or cavities' (Tenorio-Tagle & Bodenheimer 1988) can be well studied via H imaging (eg. Rodgers et al. 1960, Brand & Zealey 1975). Because of their proximity, these structures can present very large angular sizes; Barnard's loop (probably the first structure detected in H imaging of the Galaxy) subtends (Pickering 1890) and the Gum nebula is even larger. More distant complexes or groups of HII regions, such as NGC 6334, are still of the order across, yet present fine detail on arc second scales in H images (Meaburn & White 1982). Given the interaction of these structures with their larger scale environment (Tenorio-Tagle & Palous 1987) it is clear that photographic techniques are still appropriate for imaging the large areas involved.

In addition, emission lines such as H also trace out ionised gas in general in the interstellar medium (see eg. McCray & Snow 1979, Tenorio-Tagle & Bodenheimer 1988), again providing information on star forming processes and the star formation history of a given region. The high efficiency of Tech Pan means that we should be able to detect the H background from the DIG (diffuse interstellar gas) in the Galactic Plane. Taking a strength of 5-10 Rayleighs, which seems typical, this corresponds to about 30-60 detected photons per hour per square arc second for a UKST film with an assumed 50% filter efficiency and 5% film DQE.

Of particular interest on the large scale will be the comparisons between the H emission and the distributions of other indicators of interstellar gas and star formation activity such as radio continuum (eg. Phillipps et al 1981a,b), molecular lines (Oliver, Masheder & Thaddeus 1996), neutral hydrogen (Burton 1985, Hartmann & Burton 1997), masers (eg. Cohen, Masheder & Walker 1991), dust clouds (Hartley et al 1986) or IRAS flux (eg. Wouterloot & Brand 1989). Structures of interest in the context of triggered star formation would include shells of H, HI or CO around star formation regions or H seen outside CO clouds due to background ionisation.

The high resolution available to Tech-Pan H imaging also represents a significant advance in the ability to resolve point sources from extended emission and to determine accurately the surface brightness variations in the extended regions. We will also be able to define better the sharp fronts expected around ionised gas clouds, investigate in more detail the morphology and environment of Herbig-Haro objects and search for more distant or less spatially extended examples. HH objects, being known to exhibit structural changes over time scales of years, may be studied in follow-up work.

The Magellanic Clouds

Old `supergiant' shells, up to 1000 pc across, can be seen particularly well in the external, but very near neighbour galaxies, the Large and Small Magellanic Clouds (Meaburn 1980, Hunter 1994). The perimeters of these shells appear to enclose the locations of more recent star formation, which generates further SNe and stellar winds (Dopita, Matthewson & Ford 1985), so their morphology gives information on the processes by which star formation is propagated (eg. Elmegreen & Lada 1977, Gerola & Seiden 1978). The star formation history in the Magellanic Clouds, individually or as nearby examples of irregular and dwarf galaxies, is of particular topical interest (see eg. papers in Haynes & Milne 1991).

HII regions in the SMC, along with bubbles and SN remnants have recently been surveyed by le Coarer et al. (1993) who emphasise the usefulness of homogeneous wide area, high sensitivity observations. See also the HII region catalogue of Davies, Elliott & Meaburn 1976, obtained from early UKST plates. Other discrete emission line objects in the Clouds such as planetary nebulae (Morgan & Good 1992), usually detected using objective prism plates, should also be easily detectable in the H imaging.

Clearly, too, most dynamical studies of star formation regions, in our Galaxy or the Magellanic Clouds, with their implications for the energetics of the central stars and the structure of the ambient ISM, also depend on prior deep H imaging. Meaburn (1980), in particular, has emphasised the advantages of being able to detect the faintest nebulosities (see also le Coarer et al. 1993).

External Galaxies

We should also be able to resolve HII structures in other galaxies. In this regard Hunter, Hawley & Gallagher (1993) have recently studied ionised gas outside normal HII regions in three or four nearby irregular galaxies. They find that the relation to current massive star formation is far from clear. Martinbeau, Carignan & Roy (1994) on the other hand have looked at the connection between HI and H in an irregular galaxy, while another important relationship is that between far infra-red emission and H as star formation indicators (Phillipps & Disney 1988, Devereux & Young 1993). However, any conclusions at the moment are tentative at best because of the sparseness of suitable image data.

For external galaxies in general, interest is primarily in their integrated (or if possible spatially resolved) star formation rate (SFR), with the obvious implications for galaxy evolution. Again this can most conveniently be obtained from the H emission. The most quoted reference in this area is still that of Kennicutt & Kent (1983) which contains spectroscopic H measurements for just 200 galaxies (and in fact their primary sample of detected normal field spirals contains just 77 objects, see Phillipps & Disney 1985). H imaging, as opposed to spectroscopy, can clearly have great advantages in the investigation of these problems, since a Schmidt Telescope can observe a very large number of galaxies simultaneously obtaining spatially resolved images directly. CCD narrow band imaging would also be an alternative here, giving high S/N, but this is typically limited to single objects (see eg. Aparicio & Rodriguez-Ulloa (1992) for Sextans A). Imaging external galaxies at H with the UKST has previously been limited by the relative coarseness of the older emulsions but these difficulties should now be overcome by the use of Tech-Pan. Besides measuring the integrated star formation rates, we will also be able to identify individual HII regions and star forming complexes. Kennicutt, Edgar & Hodge (1989) have discussed in detail the information on the star formation process which can be derived from HII region luminosity functions and size distributions and their positional spread across a galaxy (see also Phillipps & Edmunds 1991, Vila-Costas & Edmunds 1992).

© Copyright Astronomical Society of Australia 1997