AAS 199th meeting, Washington, DC, January 2002
Session 81. High Angular Resolution: Low Frequency Radio Astronomy
Special Session Oral, Tuesday, January 8, 2002, 2:00-3:30pm, Georgetown East

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[81.03] Can We See Through the Interstellar Medium at Low Radio Frequencies?

T. J. W. Lazio (NRL)

Propagation effects due to the Galactic interstellar plasma often have a strong wavelength dependence, so that they become increasingly important at frequencies below 1000 MHz. These effects can distort either the phase of the propagating wave front, as in interstellar scattering, or its amplitude, as in free-free absorption. We discuss recent observational progress from the VLA and VLBA and future possibilities with the Low Frequency Array (LOFAR) and Square Kilometer Array (SKA):

1. A key scientific project of LOFAR will be to produce a three-dimensional map of the synchrotron emissivity in the Galaxy. The adopted strategy is to use HII regions, which become opaque at frequencies below 100 MHz, as "walls" against which to compare the foreground and background emission. An initial start to this effort uses observations toward the Galactic center with the NRL-NRAO 74 MHz system on the VLA.

2. Sub-arcminute resolution imaging of the Galactic supernova remnant W49B at 74 MHz has shown morphological differences with its appearance at higher frequencies. We show that these differences are unlikely to be internal and attribute the absorption to extrinsic free-free absorption, the presence of which has already been inferred from the low-frequency turnover in the integrated continuum spectrum and from the detection of radio recombination lines toward W49B. Our observations suggest that the absorber is a complex of classical HII regions surrounded by low-density HII gas. We identify this low-density gas as an extended HII region envelope (EHE).

3. Interpretations of existing interstellar scattering observations have assumed that the angle subtended by the scattering region is much larger than the angular broadening it produces. This assumption may be violated at low frequencies. VLBA observations at 330 MHz are being used to probe for this anomalous interstellar scattering, both in our Galaxy and in other galaxies.

Basic research in radio astronomy at the NRL is supported by the Office of Naval Research.

The author(s) of this abstract have provided an email address for comments about the abstract: lazio@rsd.nrl.navy.mil

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