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%