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Gaia DR2 and Beyond: Contributions to the Local
Distance Scale
Stefano Casertano, STScI and the SH0ES Team
TheFirstSteptotheHubbleConstant:CepheidCalibra6on
H0=73.53 +/- 1.62, Km s-1 Mpc-1
(Riess et al. 2018)
2.2% total uncertainty
19 Calibrations
300 SNe
MW Parallaxes + 3 anchors
1
2
3
*NEW*
Five Independent Sources of Geometric Cepheid Calibrations
Independent Geometric Source H0 NGC 4258 H20 Masers: Humphreys et al 2013, Riess et al 2016 (2.6%)
72.3
LMC 8 Late Detached Eclipsing Binaries: Pietzrynski et al. 2013 (2.5%)
72.0
Milky Way 10 HST FGS Short P Parallaxes: Benedict et al. 2007 --also Hipparcos (Van Leeuwen et al 2007) (2.2%)
76.2
Milky Way 8 HST WFC3 SS Long P Parallaxes: Riess et al. 2018 (3.3%)
75.7
Milky Way 50 Gaia+HST, Long P Parallaxes: Riess et al. 2018 (3.3%)
73.7
Consistent Results, Different Systematics
NEW
NEW
Gaia
Observe two fields on the sky 106.5 deg apart (the “basic angle”)
Cover the whole sky ~70 times (on average) over 5 years
Global solution (sphere reconstruction): Iteratively solve for astrometry, attitude, calibration
parameters (Lindegren+ 2012)
DR2 Parallax Uncertainties (Lindegren+18) Median error ~ 40 µas (4% @ 1 kpc) for selected MW Cepheids
Selected MW Cepheids
Median, 10-90%
QSO parallaxes in DR2 (Median -29 µas) However: Basic angle is not stable (discovered in commissioning): Variation ~ 2 mas (expected 7 µas)
Parallax offset
Data Release 2 (April 2018): 22 months of data, 26% of extended mission, 3.2e11 position measurements
MilkyWayCepheidsinGaiaDR2
0 20 59 139 297 615 1244 2496 5024 10024 19980
08/08/2013
01/27/201608/09/201508/07/2014
08/07/2012 01/26/2013
Fast Scans 7.5”/s exp time~0.01 sec Error individual Cepheid mean D<1%
F555W F814W F160W
Selected 50 Benchmark MW Cepheids - HST Photometry - Long Periods - well characterized IR light curve
Confirm offset in parallax -50 µas - larger than QSO
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 54
ParallaxZeropointOffset-anaddi6veterm
Lindegren et al (official DR2 release) uses quasars to find offset, varies with mag, color, etc. Cepheids are brighter and redder
ParallaxOffsetandH0
𝛼=1.008 +/- 0.033 --inconsistent with Planck+LCDM at 2.9 𝜎 Zp=-0.046 mas +/- 0.013 (+/- 0.006 of 𝛼= =1) -evidence errors are larger -3.8 𝜎 tension with Planck16 using prior ladder
Solve for additive (offset) and multiplicative (scale = Cepheid calibration) simultaneously
TheImpactofConstrainingtheGaiaDR2Parallaxoffset
𝛼=1.017 +/- 0.013 Full distance ladder H0=73.83 +/- 1.48 -4.3 𝜎 tension with Planck16 (if you double Zinn et al errors à 4.2 𝜎 tension )
Using Zinn et al. 2018 prior on DR2 Parallax offset (Kepler astero- seismology to measure radii for 3000+ Red Giantsà good match to Cepheids
Parallax difference; binned if > 5 objects
4 6 8 10 12 14 16 18Mean G magnitude
-0.3
-0.2
-0.1
-0.0
0.1
0.2
0.3
Astr
omet
ric m
inus
pho
tom
etric
par
alla
x (m
as)
9/97/7
8/811/11
15/1626/26
33/3728/29
34/3521/23
39/4154/57
35/3850/52
44/4442/44
38/4243/48
36/3719/20
6/6Clipped/total in bin
Offset from Cepheidswith HST photometry
600 MW Fundamental Mode Cepheids Gaia DR2 parallax, period, photometry
(recalibrated to HST photometry)
Weareop6mis6caboutGaia…
“early” “late”
• 4 times more data: 22 -> 84 months • Reduced parallax-proper motion covariance • Improved understanding of correlations • Better treatment of bright objects (G < 12 requires
gating) • Full self-calibration at focal plane level
• Currently only implemented as global calibration parameters
• Magnitude- and color-dependent systematics should decrease significantly
• QSO will determine the parallax offset • Cepheid zero-point calibration should decrease to
0.4% (limited by intrinsic scatter) Next step: DR3 (early 2021?)
Parallax correlation
in LMC
(3.5’ scale) E
arly vs late parallax solution
Single-observation
residuals
2.4% total uncertainty
Wherewecouldbein~3years
σ=1.7%
σ=1.3%
σ=0.4%
More HST σ=0.85%
Gaia+HST σ=1.0%
Gaia σ=1.5%
Gaia + HST σ=0.4%
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