Multi-frequency Radio Astronomy as a Tool to Study Active Galaxies and Cosmology

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AbstractGuerra AST 99-05625Multi-frequency radio observations and theoretical studies of active galaxies will be carried out. The analysis of the data obtained will be used to address issues concerning the physics of active galaxies and the Big Bang cosmology. Telescopes operating at radio wavelengths at the National Radio Astronomy Observatory (NRAO) will be utilized, including the NRAO 140-foot telescope in Green Bank, West Virginia and the NRAO Very Large Array (VLA) in New Mexico. The results obtained will be used to launch more ambitious radio observing campaigns to further the same goals. This research is comprised of two components. In the first component, radio-loud active galaxies which have "inverted spectra" (i.e., are brighter at higher radio frequency than lower radio frequency) have been selected from the VLA Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey and the Green Bank six centimeter (GB6) radio catalogs. These inverted-spectrum radio sources may be a small part of a class of radio sources that have been difficult to detect up to this date. Existing radio source catalogs are biased against the detection of inverted spectrum radio sources since they are faintest at the radio frequencies used for observations to compile existing catalogs (4.8 GHz in frequency and lower). However, models of radio-loud active galaxies predict inverted-spectrum radio sources as the precursors to other classes of radio sources (e.g., flat-spectrum radio sources). Inverted-spectrum radio sources can be brightest at frequencies well above 4.8 GHz, even above 20 GHz. A class of extra-galactic radio sources that is brightest above 20 GHz would be a significant foreground contamination to cosmic microwave background (CMB) anisotropy measurements such as will be made by NASA's Microwave Anisotropy Probe (MAP), which is scheduled to be launched in 2001.The data obtained at the NRAO 140-foot telescope and other facilities will be used to place upper limits on foreground contamination of the CMB anisotropy and to study the evolution of inverted-spectrum radio sources into other classes of radio-loud active galaxies. Ultimately, these data will be used to devise optimal strategies for radio surveys of the sky in the frequency range from 15 GHz to 43 GHz. Surveys of the radio "sky" at these frequencies are the best method for obtaining data for detecting inverted-spectrum radio sources.The second component of this research project is a study of the gaseous environments of powerful extended radio sources. The environments of powerful extended radio sources appear to be gas-rich clusters of galaxies. These radio sources are detected farther, both in space and time, than galaxy clusters that are detected in the visible light or X-ray wavelengths. Thus, environments surrounding powerful extended radio sources can be used to probe cluster evolution father back in time than clusters detected by other methods.A method to measure the properties of the gas in a cluster utilizes the Sunyaev-Zel'dovich (S-Z) effect. Radio sources with a predicted S-Z signal from their environments greater than the radio emission from the active galaxy itself will be selected from the Third Cambridge Revised Catalog (3CR) sample of powerful extended radio sources. Radio maps at frequencies up to 43 GHz will be collected and used to compute the radio signal above 100 GHz from the active galaxy. These radio maps will be obtained from the VLA archives and the remainder will be obtained by observations at the VLA. The S-Z signal will be computed on the basis of the interaction of the radio source with the surrounding gas. A target list for observations of the S-Z effect will be determined from these data.