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26 April 2024 |
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The properties of extragalactic radio sources selected at 20 GHz | | Elaine M. Sadler
; Roberto Ricci
; Ronald D. Ekers
; J.A. Ekers
; Paul J. Hancock
; Carole A. Jackson
; Michael J. Kesteven
; Tara Murphy
; Chris Phillips
; Robert F. Reinfrank
; Lister Staveley-Smith
; Ravi Subrahmanyan
; Mark A. Walker
; Warwick E. Wilson
; Gianfranco De Zotti
; | | Date: |
16 Mar 2006 | | Abstract: | We present some first results on the variability, polarization and general properties of radio sources selected in a blind survey at 20 GHz, the highest frequency at which a sensitive radio survey has been carried out over a large area of sky. Sources with flux densities above 100 mJy in the AT20G Pilot Survey at declination -60 to -70 were observed at up to three epochs during 2002-4, including near-simultaneous measurements at 5, 8 and 18 GHz in 2003. Of the 173 sources detected, 65% are candidate QSOs, BL Lac objects or blazars, 20% galaxies and 15% faint (b > 22 mag) optical objects or blank fields.
On a 1-2 year timescale, the general level of variability at 20 GHz appears to be low. For the 108 sources with good-quality measurements in both 2003 and 2004, the median variability index at 20 GHz was 6.9% and only five sources varied by more than 30% in flux density.
Most sources in our sample show low levels of linear polarization (typically 1-5%), with a median fractional polarization of 2.3% at 20 GHz. There is a trend for fainter sources to show higher fractional polarization.
At least 40% of sources selected at 20GHz have strong spectral curvature over the frequency range 1-20 GHz. We use a radio `two-colour diagram’ to characterize the radio spectra of our sample, and confirm that the radio-source population at 20 GHz (which is also the foreground point-source population for CMB anisotropy experiments like WMAP and Planck) cannot be reliably predicted by extrapolating the results of surveys at lower frequencies. As a result, direct selection at 20 GHz appears to be a more efficient way of identifying 90 GHz phase calibrators for ALMA than the currently-proposed technique of extrapolation from all-sky surveys at 1-5 GHz. | | Source: | arXiv, astro-ph/0603437 | | Services: | Forum | Review | PDF | Favorites | |
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