Faint Radio Sources in the NOAO Bootes Field. VLBA Imaging and Optical Identifications

Abstract

As a step toward investigating the parsec-scale properties of faint extragalactic radio sources, the Very Long Baseline Array (VLBA) was used at 5.0 GHz to obtain phase-referenced images of 76 sources in the NOAO Bootes field. These 76 sources were selected from the FIRST catalog to have peak flux densities above 10 mJy at 5 arcsec resolution and deconvolved major diameters of less than 3 arcsec at 1.4 GHz. Fifty-seven of these faint radio sources were identified with accretion-powered radio galaxies and quasars brighter than 25.5 mag in the optical I band. On VLA scales at 1.4 GHz, a measure of the compactness of the faint sources (the ratio of the peak flux density from FIRST to the integrated flux density from the NVSS catalog) spans the full range of possibilites arising from source-resolution effects. Thirty of the faint radio sources, or 39 +9/-7%, were detected with the VLBA at 5.0 GHz with peak flux densities above 6 sigma ~2 mJy at 2 mas resolution. The VLBA detections occur through the full range of compactness ratios. The stronger VLBA detections can themselves serve as phase-reference calibrators, boding well for opening up much of the radio sky to VLBA imaging. For the adopted cosmology, the VLBA resolution correponds to 17 pc or finer. Most VLBA detections are unresolved or slightly resolved but one is diffuse and five show either double or core-jet structures; the properties of these latter six are discussed in detail. Three VLBA detections are unidentified and fainter than 25.5 mag in the optical I band; their properties are highlighted because they likely mark optically-obscured active nuclei at high redshift.

arXiv comment30 pages, 6 figures, ID tweaks, to appear in the September 2005 issue of AJ

References in corpus (16)

• First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters
• Sub-milliarcsecond Imaging of Quasars and Active Galactic Nuclei III. Kinematics of Parsec-Scale Radio Jets
• The Cosmic Lens All-Sky Survey: I. Source selection and observations
• The VLBA Calibrator Survey - VCS1
• Spectroscopic Redshifts to z > 2 for Optically Obscured Sources Discovered with the Spitzer Space Telescope
• The IRAC Shallow Survey
• The Third VLBA Calibrator Survey
• Limits on the cosmological abundance of supermassive compact objects from a search for multiple imaging in compact radio sources
• Kinematic Age Estimates for 4 Compact Symmetric Objects from the Pearson-Readhead Survey
• The Bologna Complete Sample of Nearby Radio Sources
• VLBA Observations Of A Sample Of Nearby FR I Radio Galaxies
• Spitzer Observations of Optically "Invisible" Radio and X-Ray Sources: High Redshift AGN
• Deep VLBI Imaging of Faint Radio Sources in the NOAO Bootes Field
• VLBA Imaging Polarimetry of Active Galactic Nuclei - An Automated Approach
• 172 ks Chandra Exposure of the LALA Boötes Field: X-ray Source Catalog
• VLBA Astrometry of Compact Radio Sources in the Spitzer First-Look Survey

Cited by in corpus (3)

• Deep multi-frequency radio imaging in the Lockman Hole using the GMRT and VLA: I. The nature of the sub-mJy radio population
• mJIVE-20: a survey for compact mJy radio objects with the Very Long Baseline Array
• Parsec-Scale Localization of the Quasar SDSS J1536+0441A, a Candidate Binary Black Hole System