Relationships between Galactic Radio Continuum and H Emission

Observational considerations

Three major effects ensure that the ratio derived above does not usually correspond to the observed ratio of the flux density in the radio continuum to the brightness of H: (a) the extinction of H emission by grains, (b) the contribution of non-thermal radio frequency radiation to the detected radio flux density, and (c) the differences between the responses of optical and radio telescopes to extended sources, most importantly the failure of synthesis radio telescopes to detect structural components with large angular scales.

The general extinction of the galaxy follows a relationship of the form , where b is the galactic latitude. This implies that there is at least 1 magnitude of extinction (in V) if . In addition, there can be local enhancements of extinction in star-forming regions which are often associated with H emission regions. The extinction implies that the observed H flux from many emitting regions, especially the more distant ones, is severely attenuated. Although there are problems as noted above, it is possible to attempt to correct for this attenuation using observations of the ``Balmer decrement'' (the ratios of fluxes in the lines of the Balmer series) provided that (1) the attenuation is not too great, (2) the frequency dependence of the grain attenuation is known and (3) the intrinsic ratio of the Balmer line fluxes can be reliably predicted.

Non-thermal radio radiation from the Galaxy is due to synchrotron emission from relativistic electrons. The synchrotron emissivity from an electron population with an energy distribution per unit volume is given by (e.g., Ginzberg and Syrovatskii 1965)

Here, is a dimensionless constant of order unity, and is the component of the magnetic field perpendicular to the line of sight. The power law index characterizes much of the Galactic relativistic electron population in the energy range responsible for synchrotron emission in the radio continuum, so that the spectrum of the synchrotron radiation varies as . Consequently, the spectrum of the radio continuum can distinguish thermal and non-thermal radiation, since thermal radiation is either approximately flat () in optically thin regions, or varying as in optically thick regions. Another distinction between thermal and non-thermal radio continuum radiation is that, provided that the magnetic field of the source region is sufficiently ordered, and that depolarizing effects are small along the line of sight, synchrotron radiation is polarized while thermal Bremsstrahlung is not.

To a first approximation Galactic radio continuum radiation consists of two non-thermal components and two thermal components. The two main non-thermal components are: (1) a disk of emission coinciding with the plane of the Galaxy which produces an all-sky brightness peaking towards |b| = 0 and and (2) localized emission from supernova remnants dotted throughout the Galaxy, but concentrated towards the plane and towards the inner Galaxy. The former component arises from diffuse cosmic ray electrons and the general Galactic magnetic field, while the latter arises from electrons accelerated, and magnetic fields disturbed, by the supernova. The two main thermal components are: (1) diffuse emission highly concentrated towards the plane and the centre, and (2) discrete sources associated with H II regions of various types. The former component presumably arises in the diffuse warm ISM, while the latter is predominantly associated with star-forming regions.

In view of the spectrum of non-thermal radiation, and the flat spectrum of optically thin thermal radiation, the ratio of thermal to non-thermal radio continuum emission tends to increase with increasing radio frequency. At ``low'' radio frequencies ( 1 GHz) non-thermal emission dominates the diffuse Galactic radiation field, while at ``high'' radio frequencies ( 10 GHz) the diffuse thermal component tends to dominate (Handa et al.  1987). The task of distinguishing between thermal and non-thermal discrete sources (i.e., between H II regions and supernova remnants in the simplest case) is often straightforward, but in many complex regions the separation is very difficult and frustrates attempts to decypher the observed structures in terms of evolutionary trends, triggering mechanisms and physical relationships. Additional observational material, such as the presence of shock-excited OH maser emission, can help elucidate the relationships between thermal and non-thermal processes (e.g. Frail et al.  1996).

The role of the telescope in ``filtering'' H or radio continuum radiation can be crucial. For optical observations made with an H filter (either digitally or photographically) the telescope will respond to all angular scales in the field which are larger than the angular resolution, although the radiation will be detected only where it rises above the system noise (including the general emission of the night sky). Optical spectroscopy (or Fabry-Perot interferometry) can better distinguish H against the sky background and can also provide kinematic data from the Doppler shift of the line.

Radio frequency images made by scanning with a single dish are analogous in many respects to optical observations made through a filter, although the angular resolution of the largest single dish radio telescopes (10 arcmin for a 100 m telescope at 1 GHz) is far poorer than that of quite modest optical telescopes with sufficient collecting power to attain adequate signal-to-noise ratios in H  (e.g. 0.5 arcsec for an 0.25 m telescope).

The parallels between optical and radio imaging are less direct for observations are made with a radio interferometer. It is true that an interferometer, like a single dish telescope, possesses an lower limit to the angular scales it can resolve in the image. This correspond to the largest spacings of the interferometer. Interferometers generally have longer maximum spacings than those of single dishes, so interferometer images usually have higher angular resolution. However, in comparing optical (or single-dish radio) images and images made with interferometers, it is important to recognize that the normal mode of operation of an interferometer does not measure visibilities on spacings smaller than the distance between the closest elements. This has the effect of forming a spatially high-pass filtered image with zero mean, in which source structure on large angular scales is not detected. The resulting image can be quite misleading unless this fact is recognized.

In particular, the general diffuse non-thermal radiation of the Galactic plane, which has a brightness far greater than much of the small scale emission from H II regions and supernova remnants, is simply not detected by most interferometers. Thus, although the ridge of radio continuum emission lying towards the Galactic centre has a total flux density of well over 1000 Jy at 1.4 GHz (in ), the total flux density in ``resolved'' objects whose VLA images have been published (ie, Sgr A, B, C, D, E and other complexes) is much less than 100 Jy (cf. Handa et al.  1987; Salter and Brown 1988; Liszt 1988). Comparisons between radio interferometer and optical images must be made with particular attention to the absence of low angular scales in the former.

© Copyright Astronomical Society of Australia 1997