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Galactic and Extragalactic Surveys

Radio Recombination Lines with the MWA

In the MWA frequency band, both Hydrogen and Carbon Radio Recombination Lines (RRL) provide unique temperature and density probes of partially ionized regions of the Interstellar Medium. The study of these lines and their application to transition layers in HII regions and molecular clouds has been limited by the resolution of single dish surveys and by high brightness temperature limits imposed by interferometers. With its compact configuration and large collecting area, the MWA will improve RRL map resolutions by an order of magnitude and enable RRL observations in tens of hours that have been impossible to carry out with existing arrays. At the lower end of the band, Carbon Recombination Lines appear in absorption against the bright Galactic Background, making simultaneous high resolution absorption/emission RRL studies toward the Galactic Center feasible.

Studies of the Interstellar Medium

An exciting data product of the MWA will be a cube of diffuse Galactic polarized emission and its corresponding Faraday rotation across the band, enabling studies of magnetic fields and turbulence in the local ISM with unprecedented detail. The broad continuous bandwidth will allow the technique of "rotation measure synthesis'' to be applied, in which a spectral cube of polarization position angle is Fourier transformed to yield the polarized signal as a function of Faraday depth. This will allow a three-dimensional dissection of magnetized gas, which will reveal the shells, filaments and sheets in which ordered magnetic fields are distributed throughout the local ISM.

This is a view of a section of the Galactic plane in linear polarization, at a frequency of 1.4 GHz, from Gaensler et al., Ap.J. 549, 959 (2001) - data taken by the ATCA. The complex structure is caused by differential Faraday rotation in an inhomogeneous magnetoionic component of the interstellar medium. In effect, this is a "shadow" cast by the ISM against the background synchrotron radiation of the galaxy. The MWA, operating at much lower frequencies, will probe the ISM much closer to home, in much greater detail.

Surveys of Supernova Remnants

Star formation theories predict the existence of 1200 – 3000 Galactic supernova remnants (SNRs) in the Milky Way. Only 265 SNRs are known. The detection of SNRs in the galaxy will allow a better handle on the production and energy density of Galactic cosmic rays, a better understanding nucleosynthesis in the ISM, turbulence, the ISM energy budget and triggered star formation. In addition, there exists the possibility of finding SNRs with unique properties. Since SNRs are spatially extended (~0.2-2 deg) & non-thermal (Sal0.5), the optimal survey strategy is with low-frequency interferometers.

Overlay of three images of a section of the galactic plane showing several newly detected SNRs. The blue is a VLA 90 cm image, the green is a Bonn 11 cm image and the red corresponds to MSX 8μm data (Brogan 2006).

At low frequencies, SNRs spectra also show spectral curvature – this information can be used to probe shock acceleration and the foreground free-free absorption. Resolved spectral index maps at low frequencies provide information on the foreground absorbing clouds and the ejecta associated with SNRs.

Cosmic Ray Tomography

H II regions become optically thick at low frequencies and appear as “absorption holes” because the smooth synchrotron background is resolved out. This allows the diffuse synchrotron emission to be separated into foreground and background components. Since distances to H II regions are known, a 3D map of the Galactic cosmic ray distribution can be obtained. When this is combined with data on the diffuse gamma-ray background, it can result in a large-scale 3D map of Galactic magnetic fields.

Figure - Galactic Center at 74 MHz; Image courtesy of NRAO/AUI and N.E. Kassim, Naval Research Laboratory