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E.J. Guerra (Rowan Univ.), D.B. Haarsma, R.B. Partridge (Haverford College)
Extragalactic radio sources with spectral peaks above 4.8 GHz are difficult to detect on the basis of available surveys since the highest radio frequency survey of any substantial solid angle is the Green Bank 6 cm (GB6) survey. Until surveys at higher frequencies are performed, selecting sources with inverted spectra between 1.4 GHz and 4.8 GHz (increasing flux with increasing frequency) is the most promising method of detecting Gigahertz Peaked Spectrum (GPS) sources with peak frequencies above 4.8 GHz. Many models of radio source evolution predict the existence of such GPS sources as the precursors of other classes of radio sources. The radio spectra and other properties of these ``extreme'' GPS sources may be used to constrain such models.
Extreme GPS sources can be a significant foreground for cosmic microwave background (CMB) anisotropy experiments (such as the MAP and Planck Surveyor satellites) because a steeply rising spectrum can mimic the Rayleigh-Jeans portion of the CMB spectrum. Extrapolating radio source counts from lower frequencies is risky, since available surveys are at frequencies approximately 20 to 100 times lower than those used in CMB anisotropy experiments. The source counts and spectral indices of discrete radio sources between 20 GHz and 100 GHz are open and important questions for CMB anisotropy experiments, and necessary ingredients for foreground subtraction if results from these experiments are to be used to accurately determine cosmological parameters.
A sample was carefully selected from the cross-correlation of the VLA FIRST (February 1998 release) and GB6 surveys. For our purposes, a source is defined as inverted when \alpha1.4-4.8>0.4, where S \propto \nu\alpha. Data cuts have been enforced to minimize the effects of confused or extended sources and insure reliable spectral indices. The remaining sample contains 308 inverted-spectrum radio sources.
Observations at 21 GHz are scheduled in November 1998 at the 140-foot telescope, and preliminary results will be presented. Also, plans to use the VLA at multiple wavebands and an ongoing project to identify optical counterparts will both be discussed.