Bright prospects: Comparing the H Survey with large-scale radio continuum emission

Prospects

In this section, results from the Parkes 2.4-GHz work of relevance to the AAO/UKST H survey will be examined. These are areas in which interesting comparisons between the radio and H data may be made.

Diffuse thermal emission

A section of the Parkes 2.4-GHz survey can be seen in Fig. 1, showing a region of the Galactic Plane centred on longitude . Even though this section of the survey is relatively devoid of bright objects, the image nevertheless shows the Plane to be a confused and complex place, with bright HII regions (e.g. the complex between longitude), supernova remnants (e.g. G299.2-1.5; see Duncan et al. 1997a) and extragalactic sources, (e.g. G307.0-4.2) superposed on fainter, extended emission.

This diffuse emission is very extensive, generally appearing between latitudes of and over all longitudes of the survey coincident with bright spiral arms. The images reveal condensations and other structure within this emission, much of which is known to be thermal. Both the positioning and structural characteristics of this faint component suggest an association with the brighter, ``classical'' HII regions, such that these objects are probably embedded within the more extended (and presumably lower density) regions of faint emission.

The presence of this diffuse, thermal component over most of the longitude range covered by the Parkes survey suggests that this ``core-halo'' structure is a common morphology for many HII complexes in the Galaxy (see also e.g. Lockman 1976; Anantharamaiah 1985, 1986).

 
Figure 1: Total-power image of a region of the Galactic Plane, between longitudes of and . This image clearly shows the faint, diffuse emission seen close to the Plane (generally between latitudes of ), which is apparently associated with HII complexes (see Section 4.1). Such faint emission often exhibits considerable structure. Towards higher latitudes, the faint ring arcs of a large supernova remnant can be discerned. This remnant is approximately in angular diameter (see Section 4.3). The resolution of this image is 10.4 arcmin, and the rms noise is approximately 17 mJy beam area. The grey-scale wedge is labelled in units of mJy beam area. Contour levels are: 2, 4, 10, 20 and 40 Jy beam area.

The Gum nebula

The western end of the Parkes survey, between longitudes of approximately , covers an area of the Plane occupied by the Gum nebula (Gum 1952; 1956). In this region, part of which is shown in Fig. 2, we have detected radio emission from the Gum's faint emission nebulosities (see Duncan et al. 1996 for further detail). Interestingly, this emission also contains a non-thermal, polarised component which exhibits strikingly uniform vectors over many degrees of sky. The vector orientations are also plotted on Fig. 2, showing the electric vector of the received radiation. These vector orientations exhibit an rms variation in position-angle of only . This is a very uniform distribution for an area of the Plane approximately in extent, and is unlike any other area of the Parkes survey. It is the uniformity of the vector angles over this region of the Plane which leads us to conclude that the survey is detecting large-scale, uniform, magnetic field structure associated with the Gum.

Although not shown here, the survey has also revealed another example of magnetic fields associated with H emission, near longitude (see Fig. 5 of Duncan et al. 1996). The alignment and uniformity of the vectors over this latter region again suggest that we are tracing magnetic field structure associated with this H feature, and also show that the rotation measures over this area are rather low (|RM| < 20 rad m).

Of course, such detections are only possible for nebulae with very low emission measures, otherwise the polarised radio emission will be destroyed by the depolarising effects of the ionised gas.

 
Figure 2: This image shows the polarised intensities detected over a section of the 2.4-GHz survey, dominated by the Gum nebula (see Section 4.2). The orientation of the polarisation vectors (electric vector of the received radiation) are also superposed on the image; note the uniformity in the vector angles over this large region. A vector is plotted every 16 arcmin. The resolution of this image is 10.4 arcmin, and the rms noise is approximately 11 mJy beam area. The grey-scale wedge is labelled in units of mJy beam area.

New supernova remnants

A large number of new supernova remnants (SNRs) and SNR candidates have been uncovered by the Parkes 2.4-GHz survey. A total of 22 such objects were discovered as a result of this work, ranging in angular diameter from approximately 20 arcmin to (see Duncan et al. 1997a for more information). There are many possibilities for H detections from these objects. Several examples of SNR discoveries from the survey are shown in the accompanying Figs.

In addition to the diffuse, thermal emission discussed in section 4.1, Fig. 1 shows the large remnant G304+0 ( in diameter), which appears as a series of faint, ring arcs lying symmetrically about the centre of the Fig. These arcs are mainly seen towards higher Galactic latitudes, away from the bright Plane emission.

Another new SNR, G321.3-3.8, appears in Fig. 3 as an ellipse approximately in diameter. The south-east boundary of this SNR appears as a well-defined arc of emission, whilst the north-western side is fainter and more diffuse. Although not shown here, the south-eastern edge of the remnant appears strongly polarised (Duncan et al. 1997a).

As many of these new SNRs are of large angular size, they are probably quite nearby. In particular, the largest remnants (of approximately diameter and greater) must lie on the local arm. As such, there is a high probability that many of these objects may be detected by the AAO/UKST survey. Indeed, some initial results presented by Walker (1997) appear to show an H counterpart to a section of the G304+0 remnant.

We are proceeding with radio investigations of many of these objects, including high-resolution observations of a number of the smaller SNRs with the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA).

 
Figure 3: Total-power image of one of the new supernova remnants, G321.3-3.8. This SNR appears as an elliptical region of emission, with a central minimum. The south-east side of the remnant is considerably brighter than the rest of the shell. This image has a resolution of 10.4 arcmin, and an rms noise of 17 mJy beam area. The grey-scale wedge is labelled in units of mJy beam area. Contour levels are: 200, 300, 400, 500 and 600 mJy beam area.

Outflows from HII complexes

The Parkes survey has identified a number of HII complexes along the southern Plane which seem to be associated with outflows of thermal material. The most spectacular example is the HII complex G345.0+1.5 (IC 4628). Ionised material appears to be sourced from this region, which is located some 50 pc above the Galactic Plane (Caswell & Haynes 1987), in a well collimated outflow which reaches heights of approximately 200 pc from the Plane. This feature is clearly seen in both the radio data (Fig. 4a) and previous optical observations (Laval 1972).

In other regions of the Plane (Fig. 4b) a number of bi-polar outflows of low-density, ionised gas are visible. Fig. 4(b) shows an area of the survey dominated by several bright HII complexes. To the north and south of these regions, faint, bi-polar features are visible, which correspond to decreases in the level of diffuse, background polarization. We interpret these features as resulting from depolarisation of the more distant polarised emission (originating from distances greater than those of the HII complexes), by passage through low-density, thermal material. If this interpretation is correct, then the regions which show the strongest depolarisations (closest to the HII complexes) contain thermal electrons at densities of 1 - 10 cm (see Duncan et al. 1997b for more information). At such low densities, and in the presence of other thermal emission in the vicinity, it is questionable whether these bi-polar structures could be detected by the H survey; nevertheless, this remains an exciting possibility.

Finally, we note that the improved angular resolution of the AAO/UKST H survey may uncover evidence for more outflows and unusual morphologies associated with HII regions.

 
Figure 4: Examples of HII complexes exhibiting possible ``outflow'' morphologies, discovered from the Parkes survey (see Section 4.4). At left (a), the bright HII region IC 4628 (G345.0+1.5) appears to be producing a conspicuous outflow of ionised gas, which reaches up to a latitude of . The right-hand image (b) shows a Stokes-Q map of a region of the Plane containing several bright HII complexes. Data in the vicinity of the complexes have been blanked, and appear black. Note the dark ``channels'' stretching from the HII complexes towards higher Galactic latitudes; we believe these to be indicative of faint, bi-polar outflows of low-density, thermal gas. Both of these images have a resolution of 10.4 arcmin. The rms noises are approximately (a) 17 mJy beam area and (b) 10 mJy beam area. The grey-scale wedges are labelled in units of mJy beam area. The contour levels in (a) are: 2.0, 3.2, 5, 10, 20 and 30 Jy beam area.

© Copyright Astronomical Society of Australia 1997