The Precious Set of Radio-optical Reference Frame Objects in the Light of Gaia DR2 Data

Abstract

We investigate a sample of 3413 International Celestial Reference Frame (ICRF3) extragalactic radio-loud sources with accurate positions determined by VLBI in the S/X band, mostly active galactic nuclei (AGN) and quasars, which are cross-matched with optical sources in the second Gaia data release (Gaia DR2). The main goal of this study is to determine a core sample of astrometric objects that define the mutual orientation of the two fundamental reference frames, the Gaia (optical) and the ICRF3 (radio) frames. The distribution of normalized offsets between the VLBI sources and their optical counterparts is non-Rayleigh, with a deficit around the modal value and a tail extending beyond the 3σ confidence level. A few filters are applied to the sample in order to discard double cross-matches, confusion sources, and Gaia astrometric solutions of doubtful quality. Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) and Dark Energy Survey (DES) stacked multicolor images are used to further deselect objects that are less suitable for precision astrometry, such as extended galaxies, double and multiple sources, and obvious misidentifications. After this cleaning, 2643 quasars remain, of which 20% still have normalized offset magnitudes exceeding 3, or a 99% confidence level. We publish a list of 2119 radio-loud quasars of prime astrometric quality. The observed dependence of binned median offset on redshift shows the expected decline at small redshifts, but also an unexpected rise at z ∼ 1.6, which may be attributed to the emergence of the C IV emission line in the Gaia's G band. The Gaia DR2 parallax zeropoint is found to be color-dependent, suggesting an uncorrected instrumental calibration effect.

Evidence of theGaia–VLBI position differences being related to radio source structure

“…In the Gaia DR1, there were only a small percentage of sources with normalized arc lengths >3 (Mignard et al 2016;Petrov & Kovalev 2017a). In the Gaia DR2, the fraction of these sources increases to more than 10% (Petrov & Kovalev 2017b;Gaia Collaboration 2018b;Petrov et al 2019;Makarov et al 2019). By deselecting objects mostly based on the optical properties, Makarov et al (2019) still found that 20% of the sources had normalized arc lengths >3.…”

“…In the Gaia DR2, the fraction of these sources increases to more than 10% (Petrov & Kovalev 2017b;Gaia Collaboration 2018b;Petrov et al 2019;Makarov et al 2019). By deselecting objects mostly based on the optical properties, Makarov et al (2019) still found that 20% of the sources had normalized arc lengths >3. The factors causing these position differences between A&A 647, A189 (2021) optical and radio are still uncertain, even though there are a variety of possible astrophysical explanations (Makarov et al 2019;Plavin et al 2019a;Kovalev et al 2020).…”

“…By deselecting objects mostly based on the optical properties, Makarov et al (2019) still found that 20% of the sources had normalized arc lengths >3. The factors causing these position differences between A&A 647, A189 (2021) optical and radio are still uncertain, even though there are a variety of possible astrophysical explanations (Makarov et al 2019;Plavin et al 2019a;Kovalev et al 2020). For instance, Kovalev et al (2017) and Petrov et al (2019) suggested that the main cause of the position differences is optical structure, the optical jets at milliarcsecond scales.…”

All-sky visible and near infrared space astrometry

“…Experience in comparing astrometry at different wavelengths has been gained from cross matching the optical Gaia-CRF and the radio ICRF2/ICRF3. The socalled radio-optical offsets have been detected and are being extensively discussed in the literature [16,150]. Although these offsets are very important for the discussion of individual objects, they are significant for a small fraction of sources and also uncorrelated from one source to another.…”

Towards a robust estimation of orientation parameters between ICRF and $Gaia$ celestial reference frames

“…Karbon & Nothnagel (2019) compared several radio source position catalogues with Gaia DR2 and used X > 4.2 following Mayer (2018). Makarov et al (2019) compared ICRF3 and Gaia CRF2 and used X > 4 and X > 3 criteria. Liu et al (2020) compared ICRF1, ICRF2 and ICRF3 with Gaia CRF2 using two criteria: X > X0 = √ 2 ln N , where N is the number of sources, and D > 10 mas.…”