J/A+A/526/A102ivo://CDS.VizieR/J/A+A/526/A102doi:10.26093/cds/vizier.35260102CDSBourda G.Collioud A.Charlot P.Porcas R.W.Garrington S.T.2017-10-12T13:00:03Z2011-01-06T08:05:38ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frvery-long-baseline-interferometryinterferometryradio-sourcesquasarsactive-galactic-nucleibl-lacertae-objectsThe European space astrometry mission Gaia, to be launched by 2012, will construct a dense optical QSO-based celestial reference frame which will need to be linked to the International Celestial Reference Frame (ICRF; the IAU fundamental frame), with the highest accuracy. However, it has been found that only 10% of the ICRF sources (70 sources) are suitable to establish this link. The remaining sources are not useful either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the accuracy of this alignment, we have developed a program of VLBI observations based on three steps to detect, image and measure astrometric positions of weak extragalactic radio sources, with bright optical counterparts, from a sample of 447 candidate sources. The experiments devoted to VLBI detection, carried out with the European VLBI Network (EVN) in June and October 2007, were very successful, with 398 sources detected at both S- and X-bands. From these, 105 sources were observed in March 2008 with a global VLBI array (EVN and VLBA; Very Long Baseline Array) for imaging their VLBI structures. All sources were successfully imaged in both bands and about 50% (47 sources) were found to be point-like on VLBI scales. These images are available at http://www.obs.u-bordeaux1.fr/BVID/GC030/. VLBI positions of these sources will be measured accurately in future astrometric experiments.2011A&A...526A.102Bhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/526/A102CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toJ/A+A/520/A113 : 398 extragalactic radio sources VLBI detection (Bourda+, 2010)http://www.obs.u-bordeaux1.fr/BVID/GC030/ : Bordeaux VLBI Image Databasehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/526/A102https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/526/A102http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/526/A102https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/526/A102https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/526/A102http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/526/A102GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/526/A102/table3 (List of the 105 optically-bright, weak extragalactic radio sources, observed during GC030 at X- and S-bands)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/526/A102/table3?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/526/A102/table3?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/526/A102/table3?GETtext/xml+votable180.050000true1.5960633312.598071110.0055555555555555566/696 2549 4733 4923 5035 5065 5072 5268 5303 5312 5336 5352 5637 5648 5957 6015 6067 6234 6244 6436 6499 6519 6628 6764 7220 7656 7902 8525 8578 8673 8918 9402 9490 9501 9637 9664 9796 10148 10198 10202 10280 10337 10348 10395 10800 10843 10938 10978 11045 11209 11557 11692 11703 11734 11765 11991 11993 12057 12164 12722 13071 13092 13175 13603 13664 13705 15100 15271 15306 16184 16323 17601 17843 17990 18027 19655 19803 20019 20141 20184 22014 22789 23011 23369 25542 26072 26198 26798 26833 27472 27524 27717 27787 28248 28369 28450 28575 32159 32230 32261 32410 32461 32621 32715https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/526/A102?format=asciiRadiodefaultJ/A+A/526/A102/table3List of the 105 optically-bright, weak extragalactic radio sources, observed during GC030 at X- and S-bandsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintIERSIERS designation (HHMM+DDd)meta.ref;pos.framecharBand[XS] Band of observation: X=8.4GHz, S=2.3GHzGHzinstr.bandpass;obsfloatnullableFluxTotal flux density integrated from the imagemJyphot.flux.density;em.radiofloatalphaS/X spectral index {alpha}_SX_spect.indexfloatSIContinuous structure index (1)meta.code;spect.indexfloataBeam major axis (2)masphys.angSize.smajAxisfloatbBeam minor axis (2)masphys.angSize;instr.beamfloatphiBeam major axis position angle (2)degpos.posAngfloatPeakPeak brightness (in mJy/beam)mJyphot.flux.density;em.radiofloatRMSRMS of the residuals of the final hybrid image (in mJy/beam)mJyinstr.det.noisefloatFCL%First contour level: percentage of the peak%phot.flux.density;em.radiofloatFCLFirst contour level: value (in mJy/beam)mJyphot.flux.density;em.radiofloatSimbadSimbad column added by the CDSmeta.ref.urlchar_RAPositions from paper I, Bourda et al. 2010, Cat. J/A+A/520/A113 (right ascension part)degpos.eq.ra;meta.main_DEPositions from paper I, Bourda et al. 2010, Cat. J/A+A/520/A113 (declination part)degpos.eq.dec;meta.main