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Masers and Galactic Structure: µas Astrometry with the VLBA

Mark Reid Harvard-Smithsonian CfA Andreas Brunthaler & Karl Menten Germany Xu Ye & Zheng Xing-Wu China Lincoln Greenhill CfA Luca Moscadelli Italy


Talk Outline
· · Locating Sgr A*(IR) with masers Proper motion of Sgr A*: Sgr A* is very massive measured value for 0/R0 Star Formation Parallaxes and Proper Motions: Locations of Spiral Arms Rotation Curve (dark matter) Spiral Density Wave Theory

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Where is the Galactic Center?


Where is the Galactic Center?
· · · Av ~ 30 mag; AK ~3 mag Many bright young stars But, no obvious AGN (Sgr A*)

Where is Sgr A*?


Where is Sgr A* (IR)
How to transfer Sgr A* (radio) to IR ? (Menten, Reid, Genzel, Eckart 1997) Radio: Red Giant masers + SgrA* IR: Red Giants (no SgrA*)

Reid, Menten, et al (2007)


Red Giant SiO Maser Stars


Where is Sgr A* (IR)
Use radio "grid";solve for
· · · IR plate scale IR plate rotation IR (low order) distortions

· Sgr A* very dark · Allows accurate orbit measurements


Stellar Orbits
· · · M = 4 x 106 Msun R < 100 AU Sgr A* = focal position (+/-10 mas) Den. > 10
16

Msun/pc

3

Ghez et al / Genzel et al


Enclosed Mass vs Radius
· Mencl ~ 4 x 106 Msun for 0.001 < R < 0.1 pc IRS 9 anomalous: V = 370 km/s & R > 0.33 pc If bound to central mass Mencl > 5 x 106 Msun Possible explanations: 1) plunging orbit from >9 pc 2) cluster of dark remnants 3) Ro ~ 9 kpc

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VLBA: Proper Motion of Sgr A*
· · · · · Observe at 43 GHz (7mm) Rapidly switch: SgrA*, QSO1, SgrA*, QSO2 Calibrate atmosphere Relative positions (0.1 mas) Expect 6 mas/yr along Plane

90 cm VLA image: Lazio et al


Proper Motion of Sgr A*
· · Sgr A* moves along Gal. Plane 6.379 (+/- 0.024) mas/yr = 241 km/s / 8.0 kpc Removing Solar Motion... = 235 km/s / 8.0 kpc Can see 7.2 km/s motion of Sun perpendicular to Gal. Plane! Could define Galactic Plane Now: HI / Sun in plane New: LSR orbit/ SgrA* in plane Sgr A*'s peculiar motion perp. to Plane 0.4 +/- 0.9 km/s Next observations in 2 weeks; could reduce uncertainty to +/- 0.5 km/s
Reid & Brunthaler (2004)

· ·

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Must Sgr A* be a SMBH?
Object
M 87 NGC 4258 Sgr A* Sgr A* SMBH

Density
(M

Method
HST: VLBA : H2O S2's orbit

Mass & Radius
3x109 Msun in 7 pc 4x107 Msun in 0.1 pc 4x106 Msun in 45 AU

2 x 106 1 x 1010 2 x 10 7 x 10
17

sun/pc

3)

21

proper motion 4x105 Msun in 0.5 AU Rsch 4x106 Msun in 0.08 AU

2 x 10

25

(10 µas @ 8 kpc) VLBI (eg, SMA-ALMA-LMT-CARMA...) @ 1 mm -> 20 uas


A Decade of Astrometric Improvements
Astrometric accuracy of VLBA: 1995: ~ 1 mas 2005: ~ 0.05 mas (calibrator -- target 1o separation) Sgr A* best results ~ 0.1 mas low Declination ... low elevations interstellar scattering ... short baselines With longer baselines & higher Dec sources can achieve 0.05 ­ 0.02 mas positions


Parallax & Proper Motion
· Observe target against a distant background source Wait for Earth to move Measure change in position of target relative to background


· ·


Parallax Signatures
· High Dec (Q2) vs Low Dec (Q1) 1) Parallax signature changes 2) NS beam degrades @ low Dec · Symmetric sampling 5 epochs / 1.0 yr (Quadrant 2) 4 epochs / 1.0 yr (Quadrant 1)


W3C (IRS5)

W3(OH)


Parallax & Proper Motion of W3OH
CH3OH masers

Massive young (O7) star Ultra-compact HII region OH & CH3OH masers Masers move slowly (~3 km/s) & have long lives (~10 yrs)

Xu, Reid, Zheng & Menten (2006)


Parallax Measurement
9 compact maser "spots" 3 background sources Position accuracy ~0.05 mas 5 parameter fit: = 0.512 (0.010) mas D = 1.95 (0.04) kpc (100 x better than Hipparcos !) xo , µ yo , µ
x y

µ's accurate to < 1 km/s

Xu, Reid, Zheng & Menten (2006)


W3 Sun

G.C.

Georgelin & Georgelin (1976)


Milky Way Spiral Structure
Before (kinematic distances) After (Trigonometric Parallaxes)


Spiral Structure
· Trigonometic Parallaxes of 12 GHz methanol masers, plus VY CMa (SiO) and Orion-Trapezium (cont.) Many are preliminary, based on 3-epochs only

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Spiral Structure
· Kinematic Distances vs. Trigonometic Parallaxes (blue vs. cyan) Kinematic distances can be very unreliable Large non-circular motions

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Spiral Structure
· Trigonometic Parallaxes to come soon (green)


Projects in Progress:
· · Galactic structure & kinematics Local Group orbits, galaxy evolution, dark matter Orion young stars (Trapezium) Hyper-luminous stars (eg, VY CMa) Ro from Sgr B2 H2O parallax BH spin: GRS1915+105 distance
McClintock, Reid, et al

· · · ·


Extragalactic Proper Motions
Parallax accuracy: 10% at 10 kpc not good enough for galaxies Proper motion: same techniques as Parallax, but accuracy ~ T-3/2 M33 project (A. Brunthaler thesis) 1) see spin (van Maanen) 2) see motion