Optical-radio positional offsets for active galactic nuclei

Orosz, G.; Frey, S.

doi:10.1051/0004-6361/201321279

Cited by 41 publications

(44 citation statements)

References 101 publications

(122 reference statements)

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“…The SDSS source is ∼ 60 mas away from J1548+3335a, indicating that this radio component positionally coincides with the optical AGN, since SDSS sources have a positional uncertainty of ∼ 60 mas (e.g. Orosz & Frey 2013). …”

Section: Comments On Peculiar Sourcesmentioning

confidence: 84%

On the nature of bright compact radio sources atz> 4.5

Coppejans¹,

Frey²,

Cseh³

et al. 2016

Mon. Not. R. Astron. Soc.

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High-redshift radio-loud quasars are used to, among other things, test the predictions of cosmological models, set constraints on black hole growth in the early universe and understand galaxy evolution. Prior to this paper, 20 extragalactic radio sources at redshifts above 4.5 have been imaged with very long baseline interferometry (VLBI). Here we report on observations of an additional ten z > 4.5 sources at 1.7 and 5 GHz with the European VLBI Network (EVN), thereby increasing the number of imaged sources by 50 per cent. Combining our newly observed sources with those from the literature, we create a substantial sample of 30 z > 4.5 VLBI sources, allowing us to study the nature of these objects. Using spectral indices, variability and brightness temperatures, we conclude that of the 27 sources with sufficient information to classify, the radio emission from one source is from star formation, 13 are flat-spectrum radio quasars and 13 are steep-spectrum sources. We also argue that the steep-spectrum sources are off-axis (unbeamed) radio sources with rest-frame self-absorption peaks at or below GHz frequencies and that these sources can be classified as gigahertz peaked-spectrum (GPS) and megahertz peaked-spectrum (MPS) sources.

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Section: Comments On Peculiar Sourcesmentioning

confidence: 84%

On the nature of bright compact radio sources atz> 4.5

Coppejans¹,

Frey²,

Cseh³

et al. 2016

Mon. Not. R. Astron. Soc.

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“…Kovalev et al 2008;and Popović et al 2012), observational studies tend to find much larger effects (e.g. Taris et al 2011;Orosz & Frey 2013, and the surveys discussed below). Based on deep CCD imaging of the optical counterparts of ICRF sources, linked to the Hipparcos/Tycho-2 reference frame, Zacharias & Zacharias (2014) hypothesised the existence of a detrimental, astrophysical, random noise (DARN) opticalradio offset on a level of 10 mas.…”

Section: Discussion Of the Optical-radio Offsetsmentioning

confidence: 99%

GaiaData Release 1

Mignard

Klioner

Lindegren

et al. 2016

A&A

133

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Context. As part of the data processing for Gaia Data Release 1 (Gaia DR1) a special astrometric solution was computed, the so-called auxiliary quasar solution. This gives positions for selected extragalactic objects, including radio sources in the second realisation of the International Celestial Reference Frame (ICRF2) that have optical counterparts bright enough to be observed with Gaia. A subset of these positions was used to align the positional reference frame of Gaia DR1 with the ICRF2. Although the auxiliary quasar solution was important for internal validation and calibration purposes, the resulting positions are in general not published in Gaia DR1. Aims. We describe the properties of the Gaia auxiliary quasar solution for a subset of sources matched to ICRF2, and compare their optical and radio positions at the sub-mas level. Methods. Descriptive statistics are used to characterise the optical data for the ICRF sources and the optical-radio differences. The most discrepant cases are examined using online resources to find possible alternative explanations than a physical optical-radio offset of the quasars. Results. In the auxiliary quasar solution 2191 sources have good optical positions matched to ICRF2 sources with high probability. Their formal standard errors are better than 0.76 milliarcsec (mas) for 50% of the sources and better than 3.35 mas for 90%. Optical magnitudes are obtained in Gaia's unfiltered photometric G band. The Gaia results for these sources are given as a separate table in Gaia DR1. The comparison with the radio positions of the defining sources shows no systematic differences larger than a few tenths of a mas. The fraction of questionable solutions, not readily accounted for by the statistics, is less than 6%. Normalised differences have extended tails requiring case-by-case investigations for around 100 sources, but we have not seen any difference indisputably linked to an optical-radio offset in the sources. Conclusions. With less than a quarter of the data expected from the nominal mission it has been possible to obtain positions at the sub-mas level for most of the ICRF sources having an optical counterpart brighter than 20.5 mag.

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“…Second, there is no insurance that optical photocentre of ICRF2 quasars correspond to the same position as that determined from VLBI. This is subject to recent investigations (Orosz & Frey 2013).…”

Section: Recomputation Of Equatorial Coordinates From the Lqrfmentioning

confidence: 94%

“…These include the 3414 quasars of the International Celestial Reference Frame (ICRF2, Ma et al 2009;Bobholtz et al 2010), 5198 from the Very Long Baseline Array (VLBA), 1858 from the Very Large Array (VLA, Claussen 2006) and 2118 from Jodrell Bank-VLA Astrometric Survey (JVAS). Notice that the positional off-set between the radio VLBI centre and the photocentre of these quasars is subject to particular recent investigations (Orosz & Frey 2013;Zacharias & Zacharias 2014). These objects with optimal astrometric quality at radio wavelengths represent only a small part of the LQAC-2.…”

Section: Astrometric Improvementmentioning

confidence: 99%

A whole sky study of quasars known population starting from the LQAC-2 compiled catalogue

Gattano¹,

Souchay²,

Barache³

2014

A&A

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Context. Thanks to huge surveys, such as the Sloan Digital Sky Survey (SDSS), the last decade has shown a dramatic increase in the number of known quasars. In the second release of the general compiled catalogue Large Quasar Astrometric Catalogue (LQAC), 187 504 objects are recorded. Aims. From this catalogue, we carry out statistical studies dealing with several topics: the astrometric accuracy of the quasars, their spatial location, the distribution of the distance to the closest neighbour, the identification of binary quasars, the completness of catalogues at a given magnitude and the estimation of the number of quasars expected to be detected by the astrometric space mission Gaia. Methods. We analyse the astrometric improvements brought by the LQAC-2 in terms of equatorial coordinates off-sets. We plot the bi-dimensional spatial distribution of the LQAC-2 quasars according to their equatorial, galactic, and ecliptic coordinates, thus exploring the anisotropy of the distribution. We compare the observed distribution of closest neighbours with the theoretical values based on a Poisson distribution. Moreover, we perform a comparison between two catalogues, the SDSS and the 2dF inside a huge common field. By extrapolating to the whole sky we deduce the number of quasars that will be detected by Gaia. Results. We show how the equatorial, ecliptic, and galactic distribution of recorded quasars is strongly affected by the galactic extinction as well as by the deficiency of detections in the southern hemisphere. In homogenous zones covered by the SDSS survey we identify a significant excess of closest neighbours at short angular distances, with respect to the theoretical estimation, which is caused by the presence of binary quasars. Moreover, we detail the incompletness of systematic survey catalogues at any magnitude threshold, when comparing two huge surveys such as the SDSS and 2dF. Following this study we deduce the number of quasars detected by Gaia under a given magnitude threshold. For V = 20, this number should be at least 1 million objects.

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Optical-radio positional offsets for active galactic nuclei

Cited by 41 publications

References 101 publications

On the nature of bright compact radio sources atz> 4.5

On the nature of bright compact radio sources atz> 4.5

GaiaData Release 1

A whole sky study of quasars known population starting from the LQAC-2 compiled catalogue

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