Australia Telescope National Facility

Summary of the ``Sub-microJansky Radio Sky'' workshop
Andrew Hopkins, Ron Ekers, Carole Jackson, Lawrence Cram, Anne Green, Dick Manchester, Lister Staveley-Smith and Ray Norris, PASA, 16 (2), in press.

Next Section: Radio stars - Lawrence
Title/Abstract Page: Summary of the ``Sub-microJansky
Previous Section: Galaxy source density -
Contents Page: Volume 16, Number 2

The Galaxy - Anne Green

To explore in depth the dynamics and evolution of external galaxies, it is helpful to remember that the galaxy we see in most detail is our own. A study of the distribution and kinematics of the gaseous phases of the Galaxy gives valuable insights into star formation rates, the evolution of metallicity and the initial mass function for comparable external systems. Of course many difficulties arise because we are located within the structure.

Galactic radio emission is seen over a very wide range of angular scales. For a future telescope that is sensitive to faint structures on many scales, the Galaxy will reveal filamentary and diffuse features which may constrain some extragalactic observations. Images of the Galaxy from the Molonglo Observatory Synthesis Telescope (MOST) show much faint, extended structure (Figure 4) (Green et al. 1998). Futhermore, this is only about 10% of the total emission received, as seen in total power images such as the 10GHz data from Nobeyama (Handa et al. 1987). Deconvolution and sidelobe contamination will be factors to consider. Some of the science issues which may shape the technical requirements include:

A more detailed investigation of the radio/FIR correlation through studies of faint, relic supernova remnants (SNRs). This clarifies the contribution of localized and diffuse synchrotron emission.
Supernova Remnants: A search for old, evolved remnants will increase knowledge on dispersal of ejected stellar and circumstellar material, raise the number of pulsar/SNR associations as more older SNRs are discovered, and show relationships with large-scale Galactic magnetic fields. We should look for faint envelopes and other signs of interaction with the ISM, to study the evolution of the gaseous disk. Evolution of the particle population can be studied from spectral index variations. This is only effective if total flux densities are measured. Polarisation data is also more meaningful when the total power is observed.
Shocks: The role of shocks in Galactic ecology is important. Under restricted conditions, shock excited OH masers are found when SNRs interact with molecular clouds. Magnetic field strengths along the line of sight can be measured via Zeeman splitting of the maser lines in circular polarization. The maser features may also be correlated with the sites of gamma-ray enhancements possibly connected to cosmic ray acceleration.
HII Regions: Radio recombination lines show the distribution of ionised gas and star-forming regions in the spiral arms. Estimated distances for these regions from the radio observations cover parts of the Galaxy not available optically or with infrared telescopes because of interstellar dust extinction. It is expected that many components will be superposed along any given sight line. From theoretical considerations, the line intensities are greater at higher frequencies (in the region of 5GHz). The partially ionised medium surrounding HII clouds can also be studied provided the dynamic range and sensitivity of the observations are high.
Interstellar medium: Many diverse structures are seen in HI (chimneys and windblown bubbles for example). The 21cm line is used to study these features as well as the distribution of cool, neutral gas clouds, which are seen in absorption. The large scale structure of the neutral gas can then be related to the ionised component. The dynamical interaction between the disk and halo will also be investigated.

A telescope which can complete these projects will require a broad frequency range for spectral index work (0.5 - 5GHz). A spectral line cabability in domain near 1.7GHz (OH lines) and 5GHz (radio recombination lines) is also necessary, as is a bandwidth of several 100kms^-1 with velocity resolution <0.2kms^-1, in particular for Zeeman splitting measurements. The resolution of about

$0\hbox{$.\!\!^{\prime\prime}$}1$ which has been suggested is adequate, and the increased sensitivity to about $1\,\mu$ Jy or better will be helpful for detecting faint structures. Total flux density measurements in intensity and polarised emission are very important for mass and energy calculations of extended sources, many of which will be on the scale of degrees.

**Figure 4:** A MOST image of a portion of the galaxy. Note the dominance of the extended emission and filamentary structures due to HII regions and SNRs over the point sources, the extragalactic objects.
$\begin{figure} \centerline{\psfig{figure=G311.ps,angle=0,height=12cm}}\par\end{figure}$

Next Section: Radio stars - Lawrence
Title/Abstract Page: Summary of the ``Sub-microJansky
Previous Section: Galaxy source density -
Contents Page: Volume 16, Number 2

ASKAP

Public