Relationships between Galactic Radio Continuum and H Emission

Prospects for multi-wavelength studies of the ISM

The theory outlined above pertains to emission mechanisms per se, and does not touch on the broader question of the physical mechanisms responsible for the structure that is seen in H and the radio continuum. Reviews of these mechanisms can be found in texts (e.g. Longair 1992; Lozinskaya 1992; Kirk et al.  1994) and conference proceedings (e.g. Hollenbach and Thronson 1987). It is clear from this literature that there are several outstanding problems for which combined H and radio continuum observations would provide importance guidance to theory.

One example illustrating this point concerns the correlation of fine-scale H  structure and radio continuum structure in supernova remnants. In some remnants (e.g. HB3 - see Fesen et al.  1995; IC443 - see Duin and van der Laan 1975) there are large areas with a close correspondence between the shape and brightness distribution in the two forms of emission, suggesting that non-thermal electrons are accelerated and/or the magnetic field intensified in close proximity to the site of thermal excitation of H. In other remnants (e.g. the Cygnus loop, Lozinskaya 1992, especially pp 134-143) there is close correlation in some of the fine structure but also uncorrelated structures, suggesting a diversity of excitation mechanisms in different parts of the remnant. In yet other remnants, such as the Vela complex, there is only a small degree of correlation. The situation in Vela is illustrated in Figure 1, which compares an image made using the Molonglo Observatory Synthesis Telescope (MOST) in the radio continuum at 843 MHz with an H image made by Dr M. Bessell of Mount Stromlo Observatory using a large format CCD detector mounted behind an 0.2 nm H filter and a small camera attached to the MSSSO 0.4 m telescope.

  
Figure 1: Comparison between a radio continuum image made with the MOST at 843 MHz (top) and an H image of the same region of the Vela SNR (bottom). The object in the top left of the image is an H II region. The radio image is from Bock (1997), and the H image has been kindly provided by Dr M Bessell.

It seems likely that a basic factor in the co-existence or otherwise of H and radio continuum fine structure is the nature of the pre-existing interstellar medium: if mechanical energy from the supernova blast propagates into a low-density, hot ISM phase then little H radiation would be expected, while at the same time structuring of the non-thermal radio continuum could arise through waves, instabilities or inhomogeneities in the density or magnetic field. On the other hand, propagation into a dense, cool region would lead to H radiation from regions lying in the appropriate range of density and temperature. The question of associated non-thermal emission from such high density regions requires further research, for the available models of particle acceleration rest on the theory of collisionless shocks. Such shocks might exist close to the collisional shocks responsible for the H  emission, but are unlikely to be precisely coincident.

The Molonglo Observatory Synthesis Telescope (MOST) has completed a survey of the radio continuum emission from the southern Galactic plane () with an angular resolution of 40 arcsec and a sensitivity of about 5 mJy per beam area (Whiteoak et al.  1989; Whiteoak and Green 1996; Green and Cram 1998). This survey is ideally suited to extensive studies of the correlation between the radio continuum and optical H radiation that will become possible when the UKST H survey is under way. In addition to the existing MOST survey, a second survey has commenced exploiting the new wide-field capability of the MOST (Bock 1997; Bock et al.  1998), and this will provide additional high-resolution, low-noise panoramic radio continuum images of the southern Galactic plane. Figure 2, one of the first Galactic images made in the new MOST survey, illustrates the coverage and sensitivity of single images in the survey to fine-scale continuum emission, and suggests that comparisons between UKST H images and radio continuum images are likely to be a fruitful way to explore the Galactic ISM.

  
Figure 2: An image in the radio continuum at 843 MHz, made as part of the new MOST survey of the Galactic Plane. The small dot to the lower right indicates the size of the MOST beam.

There are clearly excellent prospects for significant progress on a number of outstanding problems in Galactic astrophysics that will emerge from multi-wavelength studies of the interstellar medium which include both H and radio continuum imaging.

© Copyright Astronomical Society of Australia 1997