Optical Spectra of Flat-Spectrum ICRF Radio Sources

TL;DR

This paper reports spectroscopic redshifts for 120 quasars and radio galaxies from the ICRF, enhancing the celestial reference frame and confirming the similarity of VLBI sources to the broader radio-loud quasar population.

Contribution

Provides new spectroscopic redshifts for ICRF sources, supporting improved celestial reference frame accuracy and population analysis.

Findings

Redshift distribution aligns with larger radio-loud quasar samples.

Identified high-redshift quasars (z>4) among radio sources.

Enhanced the optical identification of ICRF sources.

Abstract

Continuing our program of spectroscopic observations of ICRF sources, we present redshifts for 120 quasars and radio galaxies. Data were obtained with five telescopes: the 3.58m ESO New Technology Telescope (NTT), the two 8.2m Gemini telescopes, the 2.5m Nordic Optical Telescope (NOT), and the 6.0m Big Azimuthal Telescope (BTA) of the Special Astrophysical Observatory in Russia. The targets were selected from the International VLBI Service for Geodesy & Astrometry (IVS) candidate International Celestial Reference Catalogue which forms part of an observational VLBI program to strengthen the celestial reference frame. We obtained spectra of the potential optical counterparts of more than 150 compact flat-spectrum radio sources, and measured redshifts of 120 emission-line objects, together with 19 BL Lac objects. These identifications add significantly to the precise radio--optical frame tie to be undertaken by Gaia, due to be launched in 2013, and to the existing data available for analysing source proper motions over the celestial sphere. We show that the distribution of redshifts for ICRF sources is consistent with the much larger sample drawn from FIRST and SDSS, implying that the ultra-compact VLBI sources are not distinguished from the overall radio-loud quasar population. In addition, we obtained NOT spectra for five radio sources from the FIRST and NVSS catalogs, selected on the basis of their red colors, which yielded three quasars with z>4.