LQAC-5: The fifth release of the Large Quasar Astrometric Catalogue

Souchay, J.; Gattano, C.; Andrei, A. H.; Souami, D.; Coelho, Bruno; Barache, C.; Taris, F.; Secrest, Nathan J.; Berthereau, A.

doi:10.1051/0004-6361/201834955

Cited by 18 publications

(23 citation statements)

References 61 publications

(55 reference statements)

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“…In this paper we have constructed an improved version of the LQAC5 catalogue (Souchay et al 2019), which we called LQAC5+, by matching its total sample of 592809 quasars with the Gaia EDR3 catalogue (Brown et al 2018) This process, with a search radius of 1", led to 416113 cross-matched objects, which correspond to 70.2 % of the totality. A lack of crossmatches in the vast majority involve objects too faint to be detected by the satellite, whose magnitude threshold is G = 21.…”

Section: Discussionmentioning

confidence: 99%

“…In the mean time, Souchay et al (2019) constructed an updated version of the Large Quasar Astrometric Catalogue (LQAC; Souchay et al 2009), LQAC-5, which consists of a compilation of all the objects duly identified as quasars by the authors of the catalogues belonging to the compilation, with criteria adapted to the different properties of the surveys involved and their observational capabilities. Readers should notice that the LQAC-5 does not consist of a mere compilation of catalogues of quasars, but it contains useful additional data such as the optimized determination of the equatorial coordinates of the objects not detected by Gaia, mainly through the Large Quasar Reference Frame LQRF (Andrei et al 2009).…”

Section: Introductionmentioning

confidence: 99%

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Quasars with large proper motions: A selection from the LQAC-5 catalogue combined with Gaia EDR3

Souchay

Secrest

Lambert

et al. 2022

A&A

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Context. Thanks to the cross-identification between the Gaia EDR3 (E-Data Release 3) catalogue and the quasars' compiled catalogue LQAC-5 (Large Quasar Astrometric Catalogue-5), accurate astrometric parameters as well as photometric measurements were available for a set of 416113 quasars. Aims. After analysing the astrometric and photometric properties of these quasars,we concentrate our study on objects characterized by significant proper motion, which contradicts the postulate that they are fixed in the celestial sphere. Methods. We derived statistical properties of the positions, parallaxes, and proper motions of the quasars cross-matched with Gaia EDR3. We carried out a specific investigation of quasars with a proper motion larger than 10 mas/y, and used colour-colour diagrams in mid-infrared to discriminate these quasars from stellar contamination. We discuss, in detail, a few cases where the abnormal proper motion could be caused by misidentifications. Results. We note that 43 % of LQAC-5 quasars were not detected by Gaia EDR3, mainly because of the cut-off magnitude at G = 21. The comparison between r(G) (Gaia) and r (SDSS) shows a normal distribution. The normalized distributions of parallaxes and proper motions are identical to that of the Gaia CRF2, as well as the distribution of the G BP − G RP colour index. We have identified 937 LQAC-5 objects (0.27 % of the sample) with Gaia EDR3 proper motion which is significant at the 5σ level. We have clearly shown that below z = 0.35, the integrated magnitude G(r) obtained from SDSS spectra is smaller than G (Gaia), leading to the statement that the quasars are generally resolved under this threshold, which should lead to a bad determination of proper motion. Moreover, to discriminate quasars from stellar contamination, we have plotted them in a mid-infrared colour-colour diagram. Finally, we have made a final selection of 41 objects with µ > 10 mas/y, satisfying drastic constraints, which makes them potential candidates for considering their proper motion as real. Conclusions. In this paper we carried out a complete analysis of the quasars of the LQAC-5 which were cross-identified with Gaia EDR3 ; this serves as an alternative and complementary study with respect to Gaia CRF2, involving a different population of quasars. We selected a set of 41 quasars with a proper motion exceeding 10 mas/y, which can be considered as very high for objects which are a priori fixed in the celestial sphere.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Quasars with large proper motions: A selection from the LQAC-5 catalogue combined with Gaia EDR3

Souchay

Secrest

Lambert

et al. 2022

A&A

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“…There have been various efforts to identify extragalactic objects in the Gaia releases, mostly via a positional cross-match to other catalogues. Souchay et al (2019), for example, cross-matched the Sloan Digital Sky Survey (SDSS) Data Release 14 quasar catalogue to GDR2, finding 399 000 counterparts within 1 in the latter. Liao et al (2019) compiled a list of 1.8 million quasars from various sources, about one million of which match to GDR2 objects to within 2 .…”

Section: Introductionmentioning

confidence: 99%

Quasar and galaxy classification in Gaia Data Release 2

Bailer‐Jones

Fouesneau

Andrae

2019

Monthly Notices of the Royal Astronomical Society

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We construct a supervised classifier based on Gaussian Mixture Models to probabilistically classify objects in Gaia data release 2 (GDR2) using only photometric and astrometric data in that release. The model is trained empirically to classify objects into three classes -star, quasar, galaxy -for G ≥ 14.5 mag down to the Gaia magnitude limit of G = 21.0 mag. Galaxies and quasars are identified for the training set by a cross-match to objects with spectroscopic classifications from the Sloan Digital Sky Survey. Stars are defined directly from GDR2. When allowing for the expectation that quasars are 500 times rarer than stars, and galaxies 7500 times rarer than stars (the class imbalance problem), samples classified with a threshold probability of 0.5 are predicted to have purities of 0.43 for quasars and 0.28 for galaxies, and completenesses of 0.58 and 0.72 respectively. The purities can be increased up to 0.60 by adopting a higher threshold. Not accounting for this expected low frequency of extragalactic objects (the class prior) would give both erroneously optimistic performance predictions and severely impure samples. Applying our model to all 1.20 billion objects in GDR2 with the required features, we classify 2.3 million objects as quasars and 0.37 million objects as galaxies (with individual probabilities above 0.5). The small number of galaxies is due to the strong bias of the satellite detection algorithm and on-ground data selection against extended objects. We infer the true number of quasars and galaxies -as these classes are defined by our training set -to be 690 000 and 110 000 respectively (±50%). The aim of this work is to see how well extragalactic objects can be classified using only GDR2 data. Better classifications should be possible with the low resolution spectroscopy (BP/RP) planned for GDR3.

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“…We were also curious about a possible dependence of the offset with redshift, as suggested in Zacharias & Zacharias (2014); Makarov et al (2017). In order to check this effect, we included in our analyses the redshift z taken from the the fifth release of the Large Quasar Astrometric Catalogue (LQAC-5; Souchay et al 2019).…”

Section: Methodsmentioning

confidence: 99%

Comparison of multifrequency positions of extragalactic sources from global geodetic VLBI monitoring program and Gaia EDR3

Liu,

Lambert,

Charlot

et al. 2021

Preprint

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Context. Comparisons of optical positions derived from the Gaia mission and radio positions measured by very long baseline interferometry (VLBI) probe the structure of active galactic nuclei (AGN) on the milli-arcsecond (mas) scale. So far, such comparisons have focused on using the S /X band (2/8 GHz) radio positions but have not taken advantage of the VLBI positions that exist at higher radio frequencies, namely K band (24 GHz) and X/Ka band (8/32 GHz). Aims. We extend previous works by considering two additional radio frequencies (K band and X/Ka band) with the aim to study the frequency dependence of the source positions and its potential connection with the physical properties of the underlying AGN. Methods. We compared the absolute source positions measured at four different wavelengths, that is, the optical position from the Gaia Early Data Release 3 (EDR3) and the radio positions at the dual S /X, X/Ka combinations and at K band, as available from the third realization of the International Celestial Reference Frame (ICRF3), for 512 common sources. We first aligned the three ICRF3 individual catalogs onto the Gaia EDR3 frame and compare the optical-to-radio offsets before and after the alignment. Then we studied the correlation of optical-to-radio offsets with the observing (radio) frequency, source morphology, magnitude, redshift, and source type. Results. The deviation among optical-to-radio offsets determined in the different radio bands is less than 0.5 mas, but there is statistical evidence that the optical-to-radio offset is smaller at K band compared to S /X band for sources showing extended structures. The optical-to-radio offset was found to statistically correlate with the structure index. Large optical-to-radio offsets appear to favor faint sources but are well explained by positional uncertainty, which is also larger for these sources. We did not detect any statistically significant correlation between the optical-to-radio offset and the redshift. Conclusions. The radio source structure might also be a major cause for the radio-to-optical offset. For the alignment of with the Gaia celestial reference frame, the S /X band frame remains the preferred choice at present.

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LQAC-5: The fifth release of the Large Quasar Astrometric Catalogue

Cited by 18 publications

References 61 publications

Quasars with large proper motions: A selection from the LQAC-5 catalogue combined with Gaia EDR3

Quasars with large proper motions: A selection from the LQAC-5 catalogue combined with Gaia EDR3

Quasar and galaxy classification in Gaia Data Release 2

Comparison of multifrequency positions of extragalactic sources from global geodetic VLBI monitoring program and Gaia EDR3

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