Optical Spectra of Flat-Spectrum ICRF Radio Sources

• Main Authors: Titov, Oleg, Stanford, Laura M, Johnston, Helen M, Pursimo, Tapio, Hunstead, Richard W, Jauncey, David L, Maslennikov, K, Boldycheva, A
• Format: Journal Article
• Language: English
• Published: 14-05-2013
• Subjects: Physics - Cosmology and Nongalactic Astrophysics
• DOI: 10.48550/arxiv.1305.3017

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.