@article {23801, title = {Gaia Data Release 2. The celestial reference frame (Gaia-CRF2)}, volume = {616}, year = {2018}, month = {2018/08/1}, pages = {A14}, abstract = {Context. The second release of Gaia data (Gaia DR2) contains the astrometric parameters for more than half a million quasars. This set defines a kinematically non-rotating reference frame in the optical domain. A subset of these quasars have accurate VLBI positions that allow the axes of the reference frame to be aligned with the International Celestial Reference System (ICRF) radio frame.
Aims: We describe the astrometric and photometric properties of the quasars that were selected to represent the celestial reference frame of Gaia DR2 (Gaia-CRF2), and to compare the optical and radio positions for sources with accurate VLBI positions.
Methods: Descriptive statistics are used to characterise the overall properties of the quasar sample. Residual rotation and orientation errors and large-scale systematics are quantified by means of expansions in vector spherical harmonics. Positional differences are calculated relative to a prototype version of the forthcoming ICRF3.
Results: Gaia-CRF2 consists of the positions of a sample of 556 869 sources in Gaia DR2, obtained from a positional cross-match with the ICRF3-prototype and AllWISE AGN catalogues. The sample constitutes a clean, dense, and homogeneous set of extragalactic point sources in the magnitude range G ≃ 16 to 21 mag with accurately known optical positions. The median positional uncertainty is 0.12 mas for G \< 18 mag and 0.5 mas at G = mag. Large-scale systematics are estimated to be in the range 20 to 30 μas. The accuracy claims are supported by the parallaxes and proper motions of the quasars in Gaia DR2. The optical positions for a subset of 2820 sources in common with the ICRF3-prototype show very good overall agreement with the radio positions, but several tens of sources have significantly discrepant positions.
Conclusions: Based on less than 40\% of the data expected from the nominal Gaia mission, Gaia-CRF2 is the first realisation of a non-rotating global optical reference frame that meets the ICRS prescriptions, meaning that it is built only on extragalactic sources. Its accuracy matches the current radio frame of the ICRF, but the density of sources in all parts of the sky is much higher, except along the Galactic equator.}, keywords = {astrometry, Astrophysics - Astrophysics of Galaxies, Astrophysics - Instrumentation and Methods for Astrophysics, catalogs, reference systems}, isbn = {0004-6361}, url = {https://ui.adsabs.harvard.edu/abs/2018A\&A...616A..14G}, author = {Gaia Collaboration and Mignard, F. and Klioner, S. A. and Lindegren, L. and Hern{\'a}ndez, J. and Bastian, U. and Bombrun, A. and Hobbs, D. and Lammers, U. and Michalik, D. and Ramos-Lerate, M. and Biermann, M. and Fern{\'a}ndez-Hern{\'a}ndez, J. and Geyer, R. and Hilger, T. and Siddiqui, H. I. and Steidelm{\"u}ller, H. and Babusiaux, C. and Barache, C. and Lambert, S. and Andrei, A. H. and Bourda, G. and Charlot, P. and Brown, A. G. A. and Vallenari, A. and Prusti, T. and de Bruijne, J. H. J. and Bailer-Jones, C. A. L. and Evans, D. W. and Eyer, L. and Jansen, F. and Jordi, C. and Luri, X. and Panem, C. and Pourbaix, D. and Randich, S. and Sartoretti, P. and Soubiran, C. and van Leeuwen, F. and Walton, N. A. and Arenou, F. and Cropper, M. and Drimmel, R. and Katz, D. and Lattanzi, M. G. and Bakker, J and Cacciari, C. and Casta{\~n}eda, J. and Chaoul, L. and Cheek, N. and De Angeli, F. and Fabricius, C. and Guerra, R. and Holl, B. and Masana, E. and Messineo, R. and Mowlavi, N. and Nienartowicz, K. and Panuzzo, P. and Portell, J. and Riello, M. and Seabroke, G. M. and Tanga, P. and Th{\'e}venin, F. and Gracia-Abril, G. and Comoretto, G. and Garcia-Reinaldos, M. and Teyssier, D. and Altmann, M. and Andrae, R. and Audard, M. and Bellas-Velidis, I. and Benson, K. and Berthier, J. and Blomme, R. and Burgess, P. and Busso, G. and Carry, B. and Cellino, A. and Clementini, G. and Clotet, M. and Creevey, O. and Davidson, M. and De Ridder, J. and Delchambre, L. and Dell{\textquoteright}Oro, A. and Ducourant, C. and Fouesneau, M. and Fr{\'e}mat, Y. and Galluccio, L. and Garc{\'\i}a-Torres, M. and Gonz{\'a}lez-N{\'u}{\~n}ez, J. and Gonz{\'a}lez-Vidal, J. J. and Gosset, E. and Guy, L. P. and Halbwachs, J. -L. and Hambly, N. C. and Harrison, D. L. and Hestroffer, D. and Hodgkin, S. T. and Hutton, A. and Jasniewicz, G. and Jean-Antoine-Piccolo, A. and Jordan, S. and Korn, A. J. and Krone-Martins, A. and Lanzafame, A. C. and Lebzelter, T. and L{\"o}ffler, W. and Manteiga, M. and Marrese, P. M. and Mart{\'\i}n-Fleitas, J. M. and Moitinho, A. and Mora, A. and Muinonen, K. and Osinde, J. and Pancino, E. and Pauwels, T. and Petit, J. -M. and Recio-Blanco, A. and Richards, P. J. and Rimoldini, L. and Robin, A. C. and Sarro, L. M. and Siopis, C. and Smith, M. and Sozzetti, A. and S{\"u}veges, M. and Torra, J. and van Reeven, W. and Abbas, U. and Abreu Aramburu, A. and Accart, S. and Aerts, C. and Altavilla, G. and {\'A}lvarez, M. A. and Alvarez, R. and Alves, J. and Anderson, R. I. and Anglada Varela, E. and Antiche, E. and Antoja, T. and Arcay, B. and Astraatmadja, T. L. and Bach, N. and Baker, S. G. and Balaguer-N{\'u}{\~n}ez, L. and Balm, P. and Barata, C. and Barbato, D. and Barblan, F. and Barklem, P. S. and Barrado, D. and Barros, M. and Barstow, M. A. and Bartholom{\'e} Mu{\~n}oz, L. and Bassilana, J. -L. and Becciani, U. and Bellazzini, M. and Berihuete, A. and Bertone, S. and Bianchi, L. and Bienaym{\'e}, O. and Blanco-Cuaresma, S. and Boch, T. and Boeche, C. and Borrachero, R. and Bossini, D. and Bouquillon, S. and Bragaglia, A. and Bramante, L. and Breddels, M. A. and Bressan, A. and Brouillet, N. and Br{\"u}semeister, T. and Brugaletta, E. and Bucciarelli, B. and Burlacu, A. and Busonero, D. and Butkevich, A. G. and Buzzi, R. and Caffau, E. and Cancelliere, R. and Cannizzaro, G. and Cantat-Gaudin, T. and Carballo, R. and Carlucci, T. and Carrasco, J. M. and Casamiquela, L. and Castellani, M. and Castro-Ginard, A. and Chemin, L. and Chiavassa, A. and Cocozza, G. and Costigan, G. and Cowell, S. and Crifo, F. and Crosta, M. and Crowley, C. and Cuypers, J. and Dafonte, C. and Damerdji, Y. and Dapergolas, A. and David, P. and David, M. and de Laverny, P. and De Luise, F. and De March, R. and de Souza, R. and de Torres, A. and Debosscher, J. and del Pozo, E. and Delbo, M. and Delgado, A. and Delgado, H. E. and Diakite, S. and Diener, C. and Distefano, E. and Dolding, C. and Drazinos, P. and Dur{\'a}n, J. and Edvardsson, B. and Enke, H. and Eriksson, K. and Esquej, P. and Eynard Bontemps, G. and Fabre, C. and Fabrizio, M. and Faigler, S. and Falc{\~a}o, A. J. and Farr{\`a}s Casas, M. and Federici, L. and Fedorets, G. and Fernique, P. and Figueras, F. and Filippi, F. and Findeisen, K. and Fonti, A. and Fraile, E. and Fraser, M. and Fr{\'e}zouls, B. and Gai, M. and Galleti, S. and Garabato, D. and Garc{\'\i}a-Sedano, F. and Garofalo, A. and Garralda, N. and Gavel, A. and Gavras, P. and Gerssen, J. and Giacobbe, P. and Gilmore, G. and Girona, S. and Giuffrida, G. and Glass, F. and Gomes, M. and Granvik, M. and Gueguen, A. and Guerrier, A. and Guiraud, J. and Guti{\'e}, R. and Haigron, R. and Hatzidimitriou, D. and Hauser, M. and Haywood, M. and Heiter, U. and Helmi, A. and Heu, J. and Hofmann, W. and Holland, G. and Huckle, H. E. and Hypki, A. and Icardi, V. and Jan{\ss}en, K. and Jevardat de Fombelle, G. and Jonker, P. G. and Juh{\'a}sz, A. L. and Julbe, F. and Karampelas, A. and Kewley, A. and Klar, J. and Kochoska, A. and Kohley, R. and Kolenberg, K. and Kontizas, M. and Kontizas, E. and Koposov, S. E. and Kordopatis, G. and Kostrzewa-Rutkowska, Z. and Koubsky, P. and Lanza, A. F. and Lasne, Y. and Lavigne, J. -B. and Le Fustec, Y. and Le Poncin-Lafitte, C. and Lebreton, Y. and Leccia, S. and Leclerc, N. and Lecoeur-Taibi, I. and Lenhardt, H. and Leroux, F. and Liao, S. and Licata, E. and Lindstr{\o}m, H. E. P. and Lister, T. A. and Livanou, E. and Lobel, A. and L{\'o}pez, M. and Managau, S. and Mann, R. G. and Mantelet, G. and Marchal, O. and Marchant, J. M. and Marconi, M. and Marinoni, S. and Marschalk{\'o}, G. and Marshall, D. J. and Martino, M. and Marton, G. and Mary, N. and Massari, D. and Matijevi{\v c}, G. and Mazeh, T. and McMillan, P. J. and Messina, S. and Millar, N. R. and Molina, D. and Molinaro, R. and Moln{\'a}r, L. and Montegriffo, P. and Mor, R. and Morbidelli, R. and Morel, T. and Morris, D. and Mulone, A. F. and Muraveva, T. and Musella, I. and Nelemans, G. and Nicastro, L. and Noval, L. and O{\textquoteright}Mullane, W. and Ord{\'e}novic, C. and Ord{\'o}{\~n}ez-Blanco, D. and Osborne, P. and Pagani, C. and Pagano, I. and Pailler, F. and Palacin, H. and Palaversa, L. and Panahi, A. and Pawlak, M. and Piersimoni, A. M. and Pineau, F. -X. and Plachy, E. and Plum, G. and Poggio, E. and Poujoulet, E. and Pr{\v s}a, A. and Pulone, L. and Racero, E. and Ragaini, S. and Rambaux, N. and Regibo, S. and Reyl{\'e}, C. and Riclet, F. and Ripepi, V. and Riva, A. and Rivard, A. and Rixon, G. and Roegiers, T. and Roelens, M. and Romero-G{\'o}mez, M. and Rowell, N. and Royer, F. and Ruiz-Dern, L. and Sadowski, G. and Sagrist{\`a} Sell{\'e}s, T. and Sahlmann, J. and Salgado, J. and Salguero, E. and Sanna, N. and Santana-Ros, T. and Sarasso, M. and Savietto, H. and Schultheis, M. and Sciacca, E. and Segol, M. and Segovia, JC and S{\'e}gransan, D. and Shih, I. -C. and Siltala, L. and Silva, A. F. and Smart, R. L. and Smith, K. W. and Solano, E. and Solitro, F. and Sordo, R. and Soria Nieto, S. and Souchay, J. and Spagna, A. and Spoto, F. and Stampa, U. and Steele, I. A. and Stephenson, C. A. and Stoev, H. and Suess, F. F. and Surdej, J. and Szabados, L. and Szegedi-Elek, E. and Tapiador, D. and Taris, F. and Tauran, G. and Taylor, M. B. and Teixeira, R and Terrett, D. and Teyssandier, P. and Thuillot, W. and Titarenko, A. and Torra Clotet, F. and Turon, C. and Ulla, A. and Utrilla, E. and Uzzi, S. and Vaillant, M. and Valentini, G. and Valette, V. and van Elteren, A. and Van Hemelryck, E. and van Leeuwen, M. and Vaschetto, M. and Vecchiato, A. and Veljanoski, J. and Viala, Y. and Vicente, D. and Vogt, S. and von Essen, C. and Voss, H. and Votruba, V. and Voutsinas, S. and Walmsley, G. and Weiler, M. and Wertz, O. and Wevers, T. and Wyrzykowski, {\L}. and Yoldas, A. and {\v Z}erjal, M. and Ziaeepour, H. and Zorec, J. and Zschocke, S. and Zucker, S. and Zurbach, C. and Zwitter, T.} }