G. Bourda, P. Charlot, A. CollioudLaboratoire d’Astrophysique de

BordeauxR. Porcas

Max Planck Institut fur RadioastronomieS. Garrington

Jodrell Bank Observatory

Linking the ICRF and the future Gaia optical frame

The Gaia astrometric mission

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20092

Gaia will observe 1 billion stars and 500 000 QSOsAstrometric accuracy

V magnitude6 -

1314 15 16 17 18 19 20 mag

Parallax 8 13 21 34 55 90 155 275 as

Proper motion 5 7 11 18 30 50 80 145 as / an

Position @2015 6 10 16 25 40 70 115 205 as

Launch: 2012Preliminary catalog: ~ 2015Final catalog: 2018-2020

Which science with Gaia?

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20093

M2, Paris - 08/01/2009 F. Mignard

Stellar Systems

Reference frameFundam

ental physics

Structu

re ga

lactiq

ue

Solar s

ystem

Stellar physics

Qua

sars

& g

alax

ies

Exo-planets

Reference frame

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20094

Hartwick & Schade, 1990

With a flawless selection : GAIA will observe on ~ half of the sky

– B 16 200

– B 18 20 000

– B 20 400 000

Simulations of Gaia catalog (Slezak and Mignard)

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20095

Galactic absorption included in the visible range

G < 18 18< G < 19 19< G < 20

Automatic recognition of QSOs based on photometry

Context in 5-10 years

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20096

By 2015-2020, two extragalactic celestial reference frames will be availableICRF

Position accuracy: ICRF: σ = 250 µas ICRF-2: σ = 40 µas

Position accuracy: 16 µas ≤ σ ≤ 70 µas @ 15 ≤ V ≤ 18

Link sources must have: accurate VLBI positions no structure accurate Gaia positions V≤ 18

Linking the two frames is important • to ensure continuity of the reference frame• to register optical and radio positions with the highest accuracy

Gaia

Current status of the link (1)

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20097

~30% ICRF

Optical magnitude of ICRF sources

~10% ICRF

Astrometric source quality

Current status of the link (2)

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20098

Astrometric source quality

(SI=structure index) Distribution of SI worse for the

V<18 sources than for the 18 < V < 20 sources (Bourda et al. 2008)

Confirmed by comparison of ICRF position accuracies for the two categories

The potentially best GAIA sources for the alignment with the ICRF are not the best ICRF sources !!!

Only 10% of the current ICRF sources suitable Must find new candidates

Overview of observing program

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 20099

Sample consists of 447 sourcesTargets selected from NVSS, excluding ICRF and

VCS sourcesOptical magnitude V ≤ 18Total flux density (NVSS) ≥ 20 mJyδ ≥ -10º

Observing Strategy1.VLBI detection2.Imaging3.Accurate astrometry (for the most compact

sources)

Step 1: VLBI detection

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200910

Two 48-hr EVN experiments (S/X geodetic style @ 1 Gbps)224 sources (mostly from CLASS) observed in June 2007

(project EC025A)223 sources observed in October 2007 (project EC025B)

4 or 5-station networkEffelsberg (100m), Medicina (32m), Noto (32m), Onsala

(25m)+ Robledo (70m) for EC025B

S and X detection ratesEC025A: 96%EC025B: 82%

Overall detection rate: ~ 89 % (398 sources)

Flux density distribution (1)

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200911

All flux densities are the mean correlated flux densities over all baselines

X-band (mJy)X-band (mJy) S-band S-band (mJy)(mJy)

Flux density distribution (2)

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200912

Comparison of X-band flux density distribution in ICRF, VCS and for this project

VCS: Median = 210 mJy

ICRF: Median = 700 mJy

This project: Median = 26 mJy

Spectral index distribution

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200913

α

median = -0.34

224 CLASS

sources

Step 2: imaging

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200914

105 sources observed with global VLBI (VLBA + EVN) in March 2008 (selected from EC025A)

48-hour dual-frequency S/X @ 512 MbpsSchedule optimized for imaging

ResultsAll 105 sources successfully imaged at both X + S

bandsDynamic range: ~ 1%

X-band images (1)

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200915

Some very good link sources

X-band images (2)

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200916

Some not so good link sources…

Prospects

P. CharlotVLBA Astrometry Workshop, Socorro, 21-23 July 200917

Image the remaining 293 targets in our sampleCarry out global astrometry on the most compact

sources and get their position to better than <100 μas

Search for more candidates in the ICRF-2/VCS listsAttack the southern hemisphereIssues of core shifts

Task now officially recognized as part of the Gaia DPAC (Data Processing Analysis Consortium)

Ultimately the Gaia link sources should form the basis of ICRF-3 by 2015