Scientific Goals

Radio frequency observations of the Galactic Plane are particularly valuable because they are unaffected by obscuring dust and high stellar densities. The MOST can image in detail large-scale extended structure and operating at 843 MHz, it is well-placed to detect nonthemal emission. A -wide strip centred on the Galactic Equator ( of the proposed new survey) has been observed with the MOST over the past decade (Green et al. 1996). This strip will be repeated providing a crucial second-epoch data base for use in studies of long-term radio variability. Figure 1 shows a mosaic from the first-epoch Galactic Plane survey to illustrate the variation of intensities and angular-scale typically seen in MOST observations. Some of the scientific goals of the new survey include:

• New identification of supernova remnants (SNRs). In the first-epoch survey, there were 20 previously unidentified remnants. Given the typical scale-height of SNRs, it is estimated that an additional 20-30 candidates will be found.
• Investigation of the large-scale Galactic structure, particularly the filamentary emission seen near complex HII regions.
• A systematic study of the population of small-diameter Galactic sources as viewed by the MOST. This sample may include planetary nebulae, compact HII regions, pulsars and young SNRs. Preliminary work has been done by Whiteoak (1993).
• A program to monitor various types of transient sources. Although variability at 843 MHz is not a common phenomenon, these sources are often of great astrophysical significance. At present long term radio variability is not well-understood. Time-scales will be monitored from years down to a few minutes.

Crucial to achieving these goals is the ability to determine the nature of the objects detected by the survey. Spectral index estimates can be made by comparing the MOST data with results from the 1.4 GHz NVSS survey currently in progress at the Very Large Array (Condon et al. 1993). This will be possible for observations north of . For the remaining of the survey there are no large-scale radio frequency data bases available with the sub arcmin resolution necessary to avoid confusion effects close to the Galactic Plane. However,it has been found that the principal emission mechanism for Galactic objects can be determined as thermal or nonthermal by comparing their significantly different strengths of m intensity relative to their 843 MHz flux densities (Whiteoak and Green 1996). Thermal sources are relatively much stronger sources of infrared emission. Structural morphology for extended sources can also give a strong indication of the nature of the object.

For the small-diameter sources, the expected Galactic source density can be calculated by subtracting the contribution from the nearly isotropic distribution of background galaxies. From deep surveys away from the Galactic Plane (Subrahmanya and Mills 1987), the density of extragalactic objects is expected to be 37 sources/square degree ( mJy). Individual source identifications will require additional data at infrared, radio and (where possible) optical frequencies.

As well as investigating the Galactic sources, this survey will also contain much valuable data on the population of extragalactic objects located behind the Plane. Because of the obscuring Galaxy, identification of these sources may be difficult. For extended sources (angular diameter ) classical radio galaxy morphologies such as double-lobe, head-tail and wide-angle tail will be recognisable. Large spiral galaxies will be less easy to identify as their structure is likely to be centrally peaked with diameters of only a few minutes of arc. To determine which of the continuum sources are most probably spiral galaxies, the tight correlation between the far infrared and radio emission from these objects can be used. The relation for m and 843 MHz data is shown here in Figure 2 (Sadler 1996). The line of best fit gives a value of 100 for the ratio between the emission at the two frequencies.

To test the viability of the above methods in identifying extragalactic objects, a pilot study of a small area () of the first-epoch survey was made (Juraszek 1995). For this region, centred on , the B-band extinction is about 4.5 magnitudes, which limits the optical identification sensitivity. An investigation of source densities and the effectiveness of this galaxy identification process is continuing. Preliminary results are promising.

 
Figure 2: Plot showing IRAS 60m / radio 843 MHz correlation for spiral galaxies. The line of best fit represents a ratio estimated at 100. Triangles are upper limits at 843 MHz.

Connection with HI Multibeam Survey

A major project to conduct an HI search for galaxies, out to a distance of 14,000 km/s, is in progress using the new multibeam receiver package at the Parkes telescope (Staveley-Smith 1996). A strip along the Galactic Plane will receive particular attention. Correlation of the MOST 843 MHz continuum data with the HI detections from the multibeam survey will give new information on the radio properties of gas-rich galaxies. It is expected that there will be MOST sources () with positional accuracy of , per Parkes beam, taken as 15'. Estimations show that there will be a detectable MOST source in the radio continuum for the vast majority of the new HI detections. To identify which of the continuum sources correspond to galaxy candidates, the results will be correlated with the IRAS database. Some additional information may be gained from the shape of the HI profile, which varies in a systematic way depending on the radial displacement of the galaxy from the Parkes pointing centre (Henning 1996). The particular profile shape will enable us to predict which MOST source is most likely the counterpart.

© Copyright Astronomical Society of Australia 1997