gaia dr2 astrometry - international astronomical union · gaia dr2 astrometry l. lindegren1 j. hern...
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Gaia DR2 astrometry
L. Lindegren1 J. Hernandez2 A. Bombrun2 S. Klioner3
U. Bastian4 M. Ramos-Lerate2 A. de Torres2 H. Steidelmuller3
C. Stephenson2 D. Hobbs1 U. Lammers2 M. Biermann4
1Lund Observatory, Lund2European Space Agency/ESAC, Madrid
3Lohrmann Observatorium, Dresden4Astronomisches Rechen-Institut, Heidelberg
IAU 30 GA – Division A: Fundamental AstronomyVienna, 2018 August 27
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 1 of 24
Main references
A18 F. Arenou et al. (2018):Gaia DR2: Catalogue validation (link)
L18 L. Lindegren et al. (2018):Gaia DR2: The astrometric solution (link)
+ New material
An extended version of this presentation is on the ESA web page
Gaia → Gaia Data → Data Release 2 → Known issues:www.cosmos.esa.int/web/gaia/dr2-known-issues
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 2 of 24
Outline
1 Random and systematic errors
2 Quality indicators
3 Spurious and anomalous parallaxes
4 Conclusions and outlook
An extended version of this presentation is on the ESA web page
Gaia → Gaia Data → Data Release 2 → Known issues:www.cosmos.esa.int/web/gaia/dr2-known-issues
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 3 of 24
Formal uncertainty in parallax
A B C D E0 2 4 6 8 10 12 14 16 18 20 22
G magnitude
0:005
0:01
0:02
0:05
0:1
0:2
0:5
1
2
5
Par
alla
xunce
rtai
nty
[mas
]
Regimes of G :
A: Too bright
B: Partly saturated(unreliable)
C: Detector andcalibration limited
D: Photon limited
E: Too faint(not published)
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 4 of 24
Formal uncertainty in parallax
A B C D E0 2 4 6 8 10 12 14 16 18 20 22
G magnitude
0:005
0:01
0:02
0:05
0:1
0:2
0:5
1
2
5
Par
alla
xunce
rtai
nty
[mas
]
Regimes of G :
A: Too bright
B: Partly saturated(unreliable)
C: Detector andcalibration limited
D: Photon limited
E: Too faint(not published)
Formal uncertainties in Gaia DR2 were estimated from the internalconsistency of measurements and do not represent the total error
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 5 of 24
Random and systematic errors
A useful model for the total (external) error in parallax for source i is
$DR2i −$true
i = ri + s(α, δ,G ,C , . . . ) (1)
Random error ri :
On average zero, uncorrelated between different sources
Formal uncertainty σi is a (possibly underestimated) estimate of itsstandard deviation: σr = kσi with correction factor k & 1.0
Systematic error s:
May depend on several variables (position, magnitude, colour, . . . )
Same for sources with sufficiently similar position, magnitude, etc
Mean value is the parallax zero point $0
Variance is σ2s
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 6 of 24
Random and systematic errors
In this model the external (total) uncertainty becomes
σext =√k2σ2i + σ2s (2)
Astrophysical applications using likelihood or Bayesian methodsrequire the probability density of the total error ei = $DR2
i −$truei
Most conservative assumption:ei is Gaussian with mean value $0 and standard deviation σext
External data must be used to “calibrate” the modelby estimating $0, k and σs (see next slides)
Values may depend on the sample used
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 7 of 24
Parallax zero point ($0)
The zero point $0 is the expected measured parallax for a sourceat infinity; it should thus be subtracted from the catalogue value.
As a global average, $0 ≡ 〈s〉 ' −0.03 mas, but:
s definitely depends on (α, δ)
s probably depends of G
s may depend of C = GBP − GRP
the dependence is probably multivariate, s(α, δ,G ,C , . . . )
No general recipe can be givenfor the correction of the zero point
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 8 of 24
Systematics s(α, δ) on large scales
QSO parallaxes smoothed by a Gaussian beam (σ = 3.7◦)(only | sin b | > 0.2 shown)
¡0:15
¡0:10
¡0:05
0
0:05
0:10
0:15
Par
alla
x[m
as]
0
0:1
0:2
0:3
0:4
0:5
0:6
0:7
0:8
0:9
1:0
Par
alla
x[m
as]
Gaussian beam Equatorial projection
Mean value = −0.030 mas, RMS of smoothed values = 0.020 mas
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 9 of 24
Systematics s(α, δ) on small scales
Quasi-periodic patterns imprinted by the Gaia scanning law
10 deg
Galactic bulge area Large Magellanic Cloud
Median parallax [mas] Median parallax [mas]
Characteristic period ' 0.6 deg, RMS variation ' 0.02–0.04 mas(A18, Figs. 12–13)
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 10 of 24
Spatial covariance function V$(θ)
θ = 0 to 180◦ θ = 0 to 7◦
0 20 40 60 80 100 120 140 160 180
Angle [deg]
¡400
¡200
0
200
400
600
800Cov
aria
nce
ofpar
alla
xer
ror
[¹as
2]
0 1 2 3 4 5 6 7
Angle [deg]
¡500
0
500
1000
1500
2000
2500
Cov
aria
nce
ofpar
alla
xer
ror
[¹as
2]
(L18, Fig. 14)
V$(θ) is a statistical description of the systematic error s(α, δ, . . . )on different scales, equivalent to an angular power spectrum
The total variance is V$(0) = σ2s , from which σs = 0.043 mas
V$(θ) and Vµ(θ) make it possible to estimate the systematicuncertainty of the mean parallax or proper motion of a cluster(see the extended version for details)
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 11 of 24
Parallax systematics vs. magnitude
6 8 10 12 14 16 18 20 22G magnitude
¡0:15
¡0:10
¡0:05
0
0:05
0:10
0:15Par
alla
xze
ropoi
nt
[mas
]
A18
Riess et al: (2018)
Stassun & Torres (2018)
Zinn et al: (2018)
QSOs
−0.03 masHST
HIP ( )
VLBI
A more negative zero point may apply to sources brighter than the QSOs
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 12 of 24
Estimating k and σs
Ratio external/internal uncertainty
6 8 10 12 14 16 18 20 22G magnitude
0:8
1:0
1:2
1:4
1:6
1:8
2:0
2:2
2:4
Ext
ernal=i
nte
rnal
unce
rtai
nty
ratio
A18
Riess et al: (2018)
QSOs
1.08
HST
HIP
VLBI
1.20
k and σs estimatedfrom σext/σi vs. G :
Quasars (blue):
k = 1.08σs = 0.043 mas
Bright stars (red):
k = 1.08 (assumed)σs = 0.021 mas
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 13 of 24
A tentative external “calibration”
Ratio external/internal uncertainty
6 8 10 12 14 16 18 20 22G magnitude
0:8
1:0
1:2
1:4
1:6
1:8
2:0
2:2
2:4
Ext
ernal=i
nte
rnal
unce
rtai
nty
ratio
A18
Riess et al: (2018)
QSOs
Tentative model
HST
HIP
VLBI
1.08
σext =√k2σ2i + σ2s
Faint (G & 13):
k = 1.08σs = 0.043 mas
Bright (G . 13):
k = 1.08σs = 0.021 mas
The model may be too pessimistic for G ' 13 to 15
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 14 of 24
Systematics in proper motions
Main points:
Systematics exist on large and small scales similar to the parallax
For faint sources the reference frame is effectively non-rotating
For G . 12 the proper motions have a significant (∼0.15 mas yr−1)rotation bias
For details see the extended version atwww.cosmos.esa.int/web/gaia/dr2-known-issues
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 15 of 24
Quality indicators for the astrometry
precision
reliability
consistency
Precision: parallax_error, pmra_error, pmdec_error, etc. → OK
Reliability: visibility_perods_used (≥ 6 for full solutions) → OK
Consistency (goodness of fit to the 5-parameter model):
. astrometric_n_bad_obs_al
. astrometric_gof_al
. astrometric_chi2_al
. astrometric_excess_noise
. astrometric_excess_noise_sig
→ not recommended
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 16 of 24
Renormalised Unit Weight Error
Recommended GoF indicator for Gaia DR2 astrometry
Not given directly in the Gaia Archive
Can be computed from the quantities:
χ2 = astrometric_chi2_al
N = astrometric_n_good_obs_al
G = phot_g_mean_mag
C = bp_rp (if available)
Unit weight error UWE =√χ2/(N − 5)
Renormalised unit weight error RUWE = UWE/u0(G ,C )
u0(G ,C ) is an empirical normalisation factor, provided as alookup table on the ESA Gaia DR2 Known issues page
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 17 of 24
Normalisation factor u0(G ,C )
This is essentially the “typical” UWE for a given magnitude and colour
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 18 of 24
HRD filtered by UWE and RUWE
UWE < 1.96 RUWE < 1.40
¡1 0 1 2 3 4 5 6
Colour index GBP¡ GRP [mag]
¡4
¡2
0
2
4
6
8
10
12
14
16
18
20
22
Abso
lute
mag
nitude
G+
5lo
g 10($=10
0m
as)
¡1 0 1 2 3 4 5 6
Colour index GBP¡ GRP [mag]
¡4
¡2
0
2
4
6
8
10
12
14
16
18
20
22
Abso
lute
mag
nitude
G+
5lo
g 10($=10
0m
as)
very blue
very red
very bright
Limits chosen to retain the same number of sourcesFiltering by RUWE gives a cleaner HRD
Blue dots are sources missing in the left diagram
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 19 of 24
Spurious parallaxes
Gaia DR2 contains some parallaxes that are horrendously wrong
Source ID G parallax RUWE
4062964299525805952 19.63 1851.88 ± 1.29 1.444065202424204492928 19.88 1847.43 ± 1.87 1.014051942623265668864 19.35 1686.27 ± 1.47 1.634048978992784308992 19.78 1634.28 ± 1.97 1.50
......
......
4089303169338901632 20.35 −1621.17 ± 1.83 0.924059697925504813440 20.76 −1706.70 ± 1.99 1.174052499285375616384 20.00 −1787.00 ± 1.45 1.244090728411324689792 20.00 −1856.58 ± 2.72 1.72
The really big errors (> 1′′) are probably cross-matching errorscausing spurious parallax solutions – these are typically faint sources
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 20 of 24
Anomalous parallaxes
Tail of negative parallaxes HRD for $ > 10 mas
¡200 ¡180 ¡160 ¡140 ¡120 ¡100 ¡80 ¡60 ¡40 ¡20 0
Parallax [mas]
1
2
5
10
20
50
100
200
500
1000
2000
5000
1e4
2e4
Cou
nt
per
mas
¡1 0 1 2 3 4 5 6
Colour index GBP¡ GRP [mag]
¡4
¡2
0
2
4
6
8
10
12
14
16
18
20
22
Abso
lute
mag
nitude
G+
5lo
g 10($=1
00m
as)
In the HRD most sources between blue lines have parallax errors >10 mas
Sources with anomalous parallaxes (wrong by ±10 to ±100 mas)are usually partially resolved doubles (ρ ' 0.2–1′′, ∆G < 2 mag)
⇒ need dedicated processing (future releases)
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 21 of 24
Conclusions and outlook
This talk focused on peculiarities and deficiencies in Gaia DR2
Knowing about them will help users make optimum use of the data
Conversely, feedback from users will help us to understand the data
Future releases will benefit from the accumulated insight
This should not obscure the tremendous advances made:
Gaia DR2 = A giant leap for astronomy!
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 22 of 24
Formal uncertainty in parallax
¡2 0 2 4 6 8 10 12 14 16 18 20 22
Magnitude
0:002
0:005
0:01
0:02
0:05
0:1
0:2
0:5
1
2
5
10
20Par
alla
xunce
rtai
nty
[mas
]
Hipparcos (105)
Gaia DR2 (109)
Gaia 5 yr (109)Gaia 10 yr (109)
Gaia DR1 (106)
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 23 of 24
Thank you!
For more information please check the extended version at
ESA Gaia → Gaia Data → Data Release 2 → Known issueswww.cosmos.esa.int/web/gaia/dr2-known-issues
Lindegren et al., 2018 Aug 27 Gaia DR2 astrometry, slide 24 of 24