pdfCLEAN, AGNs, and Interstellar Scattering Toward the Galactic Center

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Session 50 -- The Milky Way
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

[50.04] pdfCLEAN, AGNs, and Interstellar Scattering Toward the Galactic Center

T. Joseph W. Lazio, James M. Cordes (Cornell), Dale Frail (NRAO)

Observations of Sgr~A$^*$ and OH/IR stars toward the Galactic center have revealed a region of enhanced and anisotropic angular broadening. However, the radial\/ distance of the scattering material from the Galactic center is as yet unconstrained. If the scattering region is an unrelated superposition with the Galactic center and distant, e.g. 1~kpc, extragalactic sources seen through the region should be broadened to roughly double the size of the OH/IR sources, i.e. $\sim 1\arcsec$. If the scattering region is local to the Galactic center, e.g. 50~pc, and a consequence of the extreme properties of the Galactic center, extragalactic source sizes should be much\/ larger, e.g. $\sim 2$ arcmin .

We have observed 10 fields toward the Galactic center using the VLA in the BnA configuration. We identify potential AGNs in these fields using pdfCLEAN, a procedure of source detection amidst noise with a known probability distribution (Zepka et al.~1994, ApJ, 427, 438): Image pixels whose intensity deviates from the expected intensity distribution for a noise-only image indicate the presence of sources in the image.

Our current catalog has a sensitivity of 2--10 mJy and contains nearly 100 sources with fluxes between 2 and 500 mJy. A comparison against a previous Galactic plane survey with similar resolution and a sensitivity of 10--50 mJy (Zoonematkermani et al.~1990, ApJS, 74, 181; Helfand et al.~1992, ApJS, 80, 211) indicates that a significant fraction of our catalog is new.

A surprisingly large number of these sources is compact, though not all of these are AGN, as the catalog also includes ultra-compact H$\,${\sc ii} regions, supernova remnants, and possibly pulsars and transient sources. However, fields within $\sim 1\deg$ of Sgr~A$^*$ appear to show a deficit of sources. Such a deficit is consistent with a scattering region local to the Galactic center, resulting in source sizes so large as to be resolved out by our observations.

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