J/A+A/644/A17ivo://CDS.VizieR/J/A+A/644/A17doi:10.26093/cds/vizier.36440017CDSHeintz K.E.Fynbo J.P.U.Geier S.J.Moller P.Krogager J.-K.,Konstantopoulou C.De Burgos A.Christensen L.Steinhardt C.L.,Milvang-jensen B.Jakobsson P.Hog E.Arvedlund B.E.H.K.,Christiansen C.R.Hansen T.B.Henriksen P.D.Kuszon K.B.Mckenzie I.B.,Mosekjaer K.A.Paulsen M.F.K.Sukstorf M.N.Wilson S.N.Orgaard S.K.K.2021-09-06T11:53:26Z2021-02-10T14:27:10ZCDS support team

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

cds-question@unistra.frquasarsHere we explore the efficiency and fidelity of a purely astrometric selection of quasars as point sources with zero proper motions in the Gaia data release 2 (DR2). We have built a complete candidate sample including 104 Gaia-DR2 point sources, which are brighter than 20th magnitude in the Gaia G-band within one degree of the north Galactic pole (NGP); all of them have proper motions that are consistent with zero within 2{sigma} uncertainty. In addition to pre-existing spectra, we have secured long-slit spectroscopy of all the remaining candidates and find that all 104 stationary point sources in the field can be classified as either quasars (63) or stars (41). One of the new quasars that we discover is particularly interesting as the line-of-sight to it passes through the disc of a foreground (z=0.022) galaxy, which imprints both NaD absorption and dust extinction on the quasar spectrum. The selection efficiency of the zero-proper-motion criterion at high Galactic latitudes is thus =~60%. Based on this complete quasar sample, we examine the basic properties of the underlying quasar population within the imposed limiting magnitude. We find that the surface density of quasars is 20deg^-2^ (at G<20mag), the redshift distribution peaks at z~1.5, and only eight systems (13_-3_^+5^%) show significant dust reddening. We then explore the selection efficiency of commonly used optical, near-, and mid-infrared quasar identification techniques and find that they are all complete at the 85-90% level compared to the astrometric selection. Finally, we discuss how the astrometric selection can be improved to an efficiency of =~70% by including an additional cut requiring parallaxes of the candidates to be consistent with zero within 2{sigma}. The selection efficiency will further increase with the release of future, more sensitive astrometric measurements from the Gaia mission. This type of selection, which is purely based on the astrometry of the quasar candidates, is unbiased in terms of colours and intrinsic emission mechanisms of the quasars and thus provides the most complete census of the quasar population within the limiting magnitude of Gaia.2020A&A...644A..17Hhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/644/A17CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toI/345 : Gaia DR2 (Gaia Collaboration, 2018)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/644/A17https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/644/A17http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/644/A17https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/644/A17https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/644/A17http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/644/A17GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/A+A/644/A17/tableb1 (Observing log ordered by telescope and date (72 sources))https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/644/A17/tableb1?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/644/A17/tableb1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/644/A17/tableb1?GETtext/xml+votable180.050000true193.827527.0095280.005555555555555556Cone search capability for table J/A+A/644/A17/tableb2 (*Classification of the stationary sources identified as quasars)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/644/A17/tableb2?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/644/A17/tableb2?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/644/A17/tableb2?GETtext/xml+votable180.050000true192.520833326.51861110.005555555555555556Cone search capability for table J/A+A/644/A17/tableb3 (*Classification of the stationary sources identified as stars)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/A+A/644/A17/tableb3?https://vizier.iucaa.in/viz-bin/conesearch/J/A+A/644/A17/tableb3?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/A+A/644/A17/tableb3?GETtext/xml+votable180.050000true192.020833326.97669440.0055555555555555565/7037 6/10400 28126-28127https://cdsarc.cds.unistra.fr/viz-bin/moc/J/A+A/644/A17?format=asciidefaultJ/A+A/644/A17/tableb1Observing log ordered by telescope and date (72 sources)recnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource Name, GQHHMMSS+DDMMSSmeta.id;meta.maincharRAJ2000Right ascension (J2000)pos.eq.ra;meta.mainDEJ2000Declination (J2000)pos.eq.dec;meta.mainrmagr magnitude (AB)magphot.mag;em.opt.RfloatTelTelescope/Instrumentmeta.notecharObs.dateObservation datetime.epoch;obsnullableExpTimeExposure timestime.duration;obs.exposurecharJ/A+A/644/A17/tableb2*Classification of the stationary sources identified as quasarsz? This work redshiftsrc.redshiftfloatnullablerecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource Name, GQHHMMSS+DDMMSSmeta.id;meta.maincharS/NSignal-to-noisestat.snrfloatS/N-PlxMeasurement of the signal-to-noise of the proper motions and parallaxes as reported in the Gaia-DR2 cataloguestat.snrfloatu-g? u-g colour indexmagphot.color;em.opt.U;em.opt.BfloatnullableJ-K? J-K colour indexmagphot.color;em.IR.J;em.IR.KfloatnullableW1-W2? W1-W2 colour indexmagphot.color;em.IR;em.IRfloatnullablezprevPrevious redshift (1)src.redshift;pos.heliocentricfloatn_zprev[P] P for photometric redshiftmeta.notecharAVAbsorption in V bandmagphot.magfloatSurveySurveymeta.notechar_RARight Ascension (J2000) from SIMBAD (not part of the original data)degpos.eq.ra;meta.main_DEDeclination (J2000) from SIMBAD (not part of the original data)degpos.eq.dec;meta.mainJ/A+A/644/A17/tableb3*Classification of the stationary sources identified as starsrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSourceSource Name, GQHHMMSS+DDMMSSmeta.id;meta.maincharS/N-PMMeasurements of the signal-to-noise of the proper motions as reported in the Gaia-DR2 cataloguestat.snrfloatS/N-PlxMeasurements of the signal-to-noise of the parallaxes as reported in the Gaia-DR2 cataloguestat.snrfloatu-g? u-g colour indexmagphot.color;em.opt.U;em.opt.BfloatnullableJ-K? J-K colour indexmagphot.color;em.IR.J;em.IR.KfloatnullableW1-W2? W1-W2 colour indexmagphot.color;em.IR;em.IRfloatnullableTypeMK spectral typesrc.spTypechar_RARight Ascension (J2000) from SIMBAD (not part of the original data)degpos.eq.ra;meta.main_DEDeclination (J2000) from SIMBAD (not part of the original data)degpos.eq.dec;meta.main