From Gaia frame to ICRF-3 3? Univ. Nice Sophia-Antipolis & Observatory of the Côte de Azur 1
Outline Status of Gaia a QSOs with Gaia The Gaia-CRF From Gaia-CRF to ICRF-3 2
Gaia quick fact sheet Main goal : astrometry and photometric survey to V = 20 ~ 10 9 sources stars, QSOs, Solar system, galaxies Accuracy in astrometry : 25 µas @ V = 15 for parallax 10 µas V < 13 Regular scan of sky over 5 yrs each source observed about ~80 times internal autonomous detection system Launch in June 2013 from Kourou Current integration of the P/L and S/M advancing smoothly 3
Observation of QSOs QSOs will be observed like stars point like sources They will be detected t d in the Sky Mapper when G < 20 Sky coverage and astrometric performances will be the same as for stars of same magnitude no specific colour problems are expected They will be observed about 80 times distributed more or less regularly during the mission Astrometric and photometric processing will be standard global solution for astrometry epoch photometry at each transit 4
QSOs distribution with Gaia Based on the simulation used in the DPAC Universe model about 400,000 observable to G = 20 na s 2011, Vienn G<18 18< G < 19 19< G < 20 Slezak & Mignard, 2007 5
Latest QSO compilation Catalogue Veron-Cety & Veron V13 (2010) Plots in galactic coordinates similar results with LQAC (Souchay et al., 2008) V < 20 - # 115,000 AGN QSO 6
How many new QSOs Density in local deep surveys used to compute the expected number of QSOs per bin of magnitude On subtracts the number in the current general catalogue assumption: QSOs have a uniform distribution on the celestial sphere b > 25 B # new QSOs < 17 0 17-18 15,000 18-19 100,000 19-20 300,000000 20-20.5 200,000 17-20 415,000 Bourda, Mignard, Andrei, 2009 7
QSO magnitude distribution with Gaia Cumulated distribution over G magnitude based on simulation by Slezak & Mignard, 2007 unrealistic drop at V ~ 20 8
Astrometric Accuracy: QSOs 2011, Vienna 9
Accuracy of the frame : Simulation Assumptions : QSOs observed with the same accuracy as a normal star Additional noise included d for source jitter : 20 µas/yr - larger than σ pos for V < 16 QSOs have no peculiar transverse s motion Observed PM reveal the global rotation of the GAIA sphere μ cosδ = μ α δ ω sin δ cosα + ω sin δ sin α ω cosδ x = ω sin α + ω cosα x y y z Space and magnitude distribution from existing catalogues Inversion of the weighted normal matrix 10
Global rotation Pattern solved with three parameters (ω x, ω y, ω z ) na s 2011, Vienn 11
Main feature of the Gaia Frame ICRF directly in the visible today radio Between 20,000 et 50,000 primary sources 295 Inertiality < 0.5 muas/yr 20 muas/yr? 12
Transverse motions So far no systematic transverse motion detected QSOs have fixed comoving coordinates If V t ~ H 0 D µ ~ 10 µas/yr VLBI in 20 yrs with σ pos ~ 1 mas µ < 50 µas but sub-mas structure instabilities i (P. Charlot, 2003) Other sources : microlensing i P = 10-6 (Belokurov) only a handful matter ejection, superluminous motion Variable galactic aberration Macrolensing P = 10-2 (Mignard, 2003) long timescale Accelerated motion in the local group 13
Gaia-CRF 2011, Vienna 14
Terminological aspect The Gaia-CRF (*) will consist of : Defining sources the clean subset of QSOs used to fix the frame A larger set of secondary QSOs The Gaia-SRF (for stellar reference frame) About one billion stars with proper motions easy access thanks to its density will degrade with time (*) GCRF : too close to Geocentric Celestial Reference Frame 15
Gaia Stellar Reference Frame Degradation with time due to proper motion uncertainty na s 2011, Vienn 16
Gaia Astrometry Global astrometry with full sky coverage 1 billion stars 500,000000 QSOs expected to G = 20 20,000 to G = 18 The solution is brought to ICRS paradigm QSOs have no global motion beyond the cosmic acceleration pattern The Gaia Celestial l Reference Frame (Gaia-CRF) will become a standard all the positions and proper motions will be referred to the Gaia-CRF solar system object astrometry will be given in this frame density will be large with 25,000 stars/deg 2 17
Gaia- CRF Gaia will provide simultaneously : a realisation of a primary frame with the bright QSOs a dense optical version to G =20, degrading di very slowly l But when published by the Gaia community this will just be a consistent t solution of the Gaia data processing will supersede immediately any other optical celestial frames many works will refer to the Gaia-CRF BUT... neither IAU or IERS are involved in this process 18
IAU Standards Astronomical community Gaia a catalogue a Gaia community users Gaia-CRF ICRF 19
Gaia-CRF ICRF-3 Gaia-CRF must be compatible with ICRF-2 within the uncertainty of the latter Rotation: done automatically with a much larger number of QSOs Orientation: must be done explicitly with an alignment procedure same pole, same origin G. Bourda & P. Charlot talk Assessment of the optical-radio offsets Then the final realisation must be perfectly documented An IERS or IAU WG involving Gaia DPAC and international external experts should be formed competence in radio and visible physics of QSOs Finally an IAU resolution should follow to turn the GCRF into ICRF-3 20
Will the Gaia-CRF have an Epoch? 2011, Vienna 21
QSO Proper Motion and Epoch The radio ICRF is not associated to an epoch defining QSOs have fixed celestial coordinates they are not epoch dependant the define axis direction 'for ever' time is no involved in the process A stellar reference frame is defined at a particular epoch defining stars coordinates come with their proper p motion the PMs are part of the fundamental catalogue each star comes with a particular PM and its uncertainty with N stars, there are 2N parameters needed the system degrades due to the limited uncertainty of the PMs accuracy in position and annual PM are similar 22
QSO Proper Motion and Epoch What about the Gaia-CRF QSOs have a systematic proper motion of ~ 4 muas/yr Here But these are not individual PM, but the result of a systematic pattern only 3 parameters are required to maintain the system the accuracy should be close to ~ 0.5 muas/yr Individual positions of the primary sources will have an accuracy of ~ 80 muas degradation will be very very slow Therefore : the Gaia-CRF will have an epoch attached to it but it has very different meaning as for a stellar reference frame How to avoid it: take the origin at the galactic centre! this is for the future 23
2011, Vienna 24 SF2A SF2A 2011-24
Plot in equatorial coordinates amplitude ~ 4 µas/yr Secular Drift on QSOs na s 2011, Vienn galactic centre μ α μ δ cosδ Γ Γ x y = sin α + cosα c c Γ Γ x = sin δ cosα c c y sin δ sin α + Γ c z cosδ 25