VLBA Parallaxes Across the Milky Way and the Distance to NGC 4258

Mark J. Reid

Harvard-Smithsonian Center for Astrophysics

Trigonometric Parallax

1 A.U.

D(parsecs) = 1 / A(arcseconds)

1 parsec = 3.26 light years = 205,000 A.U.

d(kpc) = 1 / p(mas)

Nearest stars: d = 1 pc p = 1”

Center of Milky Way: d ~ 10 kpc p ~ 0.1 mas

Other galaxies d ~ 1 Mpc p ~ 1 as

dp

Friedrich Wilhelm Bessel

• Used Fraunhofer’s last telescope

measured first stellar parallax:

61 Cygni: p = 0.314” in 1838

(p = 0.287”)

Friedrich Wilhelm Bessel (1838)

p = 0.25” ± 0.03”

p = 0.37” ± 0.03”

HIPPARCOS(HIgh Precision PARallax COllecting Satellite)

118,000 stellar parallaxes

~ 0.001 arcseconds

10% accuracy at 100 pc…

mapped solar neighborhood

(Perryman et al 1997 A&A 323 49)

Pleiades Distance Problem

• Hipparcos: D = 120.2 ± 1.9 pc

• Ground based D = 131 135 pc• HST (3 parallaxes): D = 134.6 ± 3.1 pc

• VLBA (4 parallaxes): D = 136.2 ± 1.2 pc Carl Melis et al (in prep)

uncertainty dominated by cluster depth (2.3pc), not measurement accuracy

Very Long Baseline Interferometry: VLBA, VERA & EVN

• Radio waves “see” through galaxy

• Can “synthesize” telescope the size of the Earth

Fringe spacing (eg, VLBA):

f~/D ~ 1 cm / 8000 km = 250 as

Centroid Precision:

0.5 f / SNR ~ 10 as

Systematics:

path length errors ~ 2 cm (~2 )

shift position by ~ 2f ~ 500 as

Relative positions (to QSOs):

~ 1 deg (0.02 rad)

cancel systematics: f ~ 10 as

What does the Milky Way look like?

Hipparcos range

Gaia range (± 10 to 20 as); but cannot see through dust in Galactic plane

VLBI range (± 5 to 20 as): can “see” through plane to massive star forming regions that trace spiral structure

The BeSSeL Survey Team

M. Reid, T. Dame (CfA)

K. Menten, A. Brunthaler, B. Zhang, Y. Wu, M. Sato, Y. Choi, A. Sanna (MPIfR)

X-W Zheng, Y. Xu (Nanjing)

L. Moscadelli (Arcetri)

A. Bartkiewicz (Torun)

K. Hachisuka (Shanghai)

K. Rygl (Netherlands)

Parallax for W 49N H2O masers

p = 90 ± 7 as (D=11.1 ± 0.9 kpc) Zhang+ (2013)

Mapping Spiral Structure

• Parallaxes: VLBA, EVN & VERA >100 parallaxes for massive young stars • Tracing most spiral arms, eg… Outer arm traced

Perseus arm “gap”Local arm significant

• Model entire data set: R0 = 8.34 ± 0.16 kpc 0 = 240 ± 8 km/s

R ~ 0

Reid et al 2014

Local Group Proper Motions

• 1920s van Maanen claimed to see M33 spin!

• mas/yr motions Spiral nebulae nearby (Galactic)

• Hubble argued more distant (extra-galactic)

• van Maanen’s error not found

M33

Extragalactic Proper Motions• van Maanen experiment:

M33/19 – M33/IC133 masers

VLBA: x = 30 +/- 2, y = 10 +/- 5 as/yr

HI: vx = 106 +/-20, vy = 35 +/- 20 km/s

D = 750 +/- 50 +/- 140 kpc

M33

Brunthaler+ 2006

v

• Next: M31 (Andromeda) 5 masers

higher rotation speed

NGC 4258 • Seyfert 2 galaxy

• H2O masers in edge-on warped diskRadius 0.15 -> 0.30 pc

Rotation speed 1000 km/s Period 1000 years M 4 x 107 Msun

• Decade of VLBA and GBT observation

• Geometric model Distance

• With HST observations, recalibrate Cepheid PL relationHerrnstein, Greenhill, Moran, Miyoshi…

NGC 4258: Accretion Disk (r ~ 0.1 pc)

Modeling Disk Dynamics

Humphreys et al (2013)

A = V2 / r & r = D

M r V2

Center of P-V diagram

VLBA map

Very small

Modeling Disk Dynamics

Estimating H0

• DNGC4258 = 7.60 ± 0.17 ± 0.20 Mpc (total ±3%)

Extremely accurate, direct distance to a galaxy.

• H0 = Vrec/ D very uncertain (Vrec = Vobs – Vpec)

But, very useful for re-calibrating Cepheids…

H0,NGC4258 = 72.0 ± 3.0 km/s/Mpc (see Riess+ 2011,12)

H0,NGC4258 = 70.6 ± 3.3 km/s/Mpc (see Efstathiou 2014)

• Established H2O megamaser method for distances.

Observe similar AGN disks in galaxies where Vpec << Vrec

and estimate H0 directly without standard candels.