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