Sgr A*

1

Must Sgr A be a Super-Massive Black Hole?
Mark J. Reid Harvard-Smithsonian CfA Andreas
Brunthaler MPIfR/JIVE
2
Sgr A
Karl Menten Reinhard Genzel Thomas Ott Rainer
Schoedel Andreas Eckart

• Where is it?
• Does it move?

Andreas Brunthaler
3
Discovery of Galactic Center

• Shapley (1918)
• Globular Clusters point to
• Galactic Center
• Ro 13 kpc

4
Early Radio Observations
Sgr A
Piddington Minnett (1951) Haddock, Mayer
Sloanaker (1954) McGee Bolton (1954)
5
History of Sgr A

• Balick Brown (1974) Discover Sgr A
• Intense Sub-Arcsecond Structure
• Lo et al (1985) Sgr A lt 20 AU
• Backer et al (1993) lt 3 AU
• Rogers et al (1994) lt 1 AU

6
VLA Images of Sgr A
Sgr A
J-H Zhao
7
IR Image of GC

• 1-3 um image
• Where is Sgr A?

8
6 Questions

• Is Sgr A at the center of the stellar cluster?
• Is the stellar cluster tied to Sgr A?
• Is Sgr A at the dynamical center of the Galaxy?
• Does Sgr A have a peculiar motion?
• Does Sgr A have all the mass sensed by stars?
• Could exotic dark matter dominate the G. C. mass?

9
Radio/IR frame alignment

• Use stars visible in both Radio and IR
• Red Giants with masers
• Compare Radio and IR positions
• Solve for IR plate scale rotation
• Align IR with Radio to find Sgr A

10
VLA positions for stars

• 7 SiO masers lt12 of SgrA
• 0.1 1 Jy
• Positions to 3 mas
• mas/yr motions in few years

11
VLBA proper motions
VLBA proper motions
(Reid et al 2003)

• Sub-mas positions
• mas/yr motions in 3 months

12
Stellar positions motions

• 7 SiO maser stars
• positions 3 mas
• motions 100 km/s

13
VLT with Adaptive Optics

• 3-color 1.5 - 3 um
• 8.2 m VLT telescope
• CONICA (IR camera)
• NAOS (adaptive optics)
• 60 mas resolution

14
Where was Sgr A in 1995

• 15 mas accuracy
• Between stars
• S1, S2 S3
• Sgr A lt 9 mJy

15
Where was Sgr A in 2002

• Sgr A position 10 mas
• Reid et al (2003)
• Star S2
• seen at pericenter passage
• V 5000 km/s !!
• Orbit determined
• Schoedel et al (2002)

16
S2s orbit

• 15 year period
• e 0.87
• Pericenter only 15 mas
• from Sgr A !
• (120 AU 17 l-h)

(Schoedel et al 2002)
17
Stellar Orbits

• 6 stars with orbits
• Enclosed mass 4 x 106 Msun lt
  100 AU radius Schoedel etal 02,03 Ghez et
  al 03
• Sgr A within 100 AU Reid et al 03

18
Question 1

• Is Sgr A at the center of the stellar cluster?
• Yes to better than 10 mas

19
Radio/IR frame alignment

• Compare Radio(SiO) and IR positions
• Solve for IR plate scale rotation
• Align IR with Radio to find Sgr A
• Compare Radio(SiO) and IR velocities
• SiO velocities relative to Sgr A
• Tie IR velocities to Sgr A

20
Velocity Alignment

• East,North proper motions in mas/yr
• Star Radio Infrared
  Difference
• IRS 9 3.6,2.4 2.0,0.5 1.6 (0.7),1.9
  (1.2)
• IRS 7 -1.6,-4.5 -0.8,-3.6 -0.8 (1.0),-0.9
  (3.5)
• IRS 12 -0.8,-2.8 -3.3,-0.8 2.4 (0.5),-2.0
  (0.8)
• IRS 10 0.2,-2.1 0.1,-2.2 0.1 (0.4),0.1
  (1.0)
• Unweighted mean (sem) 0.8
  (0.8),-0.25(1.0)

1 mas/yr 40 km/s

• Central star cluster moves with Sgr A to 70
  km/s

21
Question 2

• Is the stellar cluster tied to Sgr A?
• Yes to better than 70 km/s

22
Proper Motion of Sgr A

• Suns Galactic Orbit
• 225 Myr period
• 220 km/s _at_ 8.0 kpc
• 6 mas/yr

23
Sgr As apparent motion

• Relative to 2 Quasars
• Suns Galactic Orbit
• 220 km/s at 8.0 kpc
• 6 mas/yr in Gal plane

(Kassim, Frail Briggs)
24
Project History

• 1979 Proposal to US VLBI Network
• To Study Feasibility of Detecting Proper
  Motion of the Galactic Center
• 15 GHz OVRO, HRAS, GB, Haystack
• Failed
• Scatter broadened Sgr A
• Limited sensitivity
• Needed VLBA !

25
Sgr As Apparent Motion

• Moves mostly along
• Galactic Plane
• Slight deviation from VSUN

Gal Plane
Best Fit
26
Eastward Motion

• Sgr A drifts smoothly
• QSOs dont move

27
Northward Motion

• Sgr A drifts smoothly
• QSO dont move
• NB error bars larger than eastward positions

28
Sgr A motion Galactic Coords

• Motion in Galactic Plane
• Qo/Ro 29.4 /- 0.9 km/s/kpc
• Compare to
• (A-B)/Ro 27.2 /- 0.9 km/s/kpc
• (Feast Whitelock 1997)
• Motion out of Galactic Plane
• Very small

29
Question 3

• Is Sgr A at the dynamical center of the Galaxy?
• Yes to within our knowledge of Qo/Ro

30
Sgr A motion toward Galactic Pole

• Solar Motion 7 km/s
• Sgr As peculiar motion
• -0.4 /- 0.8 km/s

31
Question 4

• Does Sgr A have a peculiar motion?
• No less than 1.6 km/s (out of Plane)

32
Estimating Sgr As Mass

• Chatterjee, Hernquist Loeb (2002)
• mass estimator a ltenergygt
• Mlim G M(R) m / R V2
• Set M(R) 4 x 106 for R 100 AU
• m 3 Msun
• V lt 1.6 km/s
• get M gt 3.5 x 106 Msun

33
If Sgr A is not a SMBH

• 1) Tight binary black hole
• Decays by Grav Radiation
• 2) Theoretically flexible
• eg, density power law r 1/rb

34
If Sgr A is not a SMBH

• V gtgt 1 km/s for any dark matter distribution,
  unless Mdark 0
• Would be easily observed, except

35
If Sgr A is not a SMBH

• Accel gtgt 10-2 cm/s2
• Would be easily observed, EXCEPT

36
Sgr A orbital period

• Measured V ( A) not sensitive to Plt16 yrs
• Essentially all dark mass distributions give
  Plt16 yrs
• But still detectable

37
Sgr A orbital excursions

• Would easily see excursions gt 4 AU
• Sgr A either
• i) 4 x 106 Msun
• ii) bound within
• 4 AU

2 AU
4 AU
38
Stars orbiting Sgr A

• Consider star orbiting a massive object
• MV mv
• Add in large number of stars random
  fluctuations give
• MV2 mv2

39
Effect of Bound Stars on Sgr A

• Recipe
• Put 4 x 106 stars in computer
• Place SMBH/dark mass at center of mass of system
• Solve Keplers Eq for each star
• Calculate COM after 8 years
• Determine position then velocity of SMBH
• Repeat as needed to get Vrms

• Standard IMF
• eccentricities from 0 to 0.99

40
Sgra A must be massive

• Compare simulated systems with trial
  measurements
• Sgr As mass (Msun)
• Best est. 2 x 106
• 90 conf. 0.3 x 106
• Very conservative calc. Ignored effects of DM,
  clumping, stars gt 4 pc

41
Sgra A must be massive

• Compare simulated systems with trial
  measurements
• Sgr As mass (Msun)
  Maximum liklihood lower limit
  1.2 x 106 90 confidence lower limit
  0.2 x 106
• Very conservative calculation ignored effects
  of Possible dark cluster of stellar
  remnants, Stellar clumping/collective
  effects, Stars beyond 2 pc

42
Effects of Stellar Remnants

• 10 of mass lt100 AU in stellar remnants
• Plummer distribution of Mouawad et al 2004
• Vz(SgrA) gt 0.3 km/s for 0.01 lt a lt 2 pc

43
Question 5

• Does Sgr A have all the gravitational mass?
• Yes well, more than 30 of mass

44
Exotic Dark Matter ?

• Could exotic dark matter dominate the G.C. mass?
• Not likely less than 30 of mass
  dropping
• Tied radiative source to the mass
• Fermion ball cant give Sgr As SED
• First time 106 Msun tied directly to an AGN

45
6 Questions 6 Answers

• Is Sgr A at the center of the stellar cluster?
• Yes to within 10 milli-arcsec
• Is the stellar cluster tied to Sgr A?
• Yes to within 70 km/s
• Is Sgr A at the dynamical center of the Galaxy?
• Yes to within our knowledge of Qo/Ro
• Does Sgr A have a peculiar motion?
• No less than 2 km/s
• Does Sgr A have all the mass sensed by stars?
• Yes (gt30)
• Could exotic dark matter dominate the G. C. mass?
• No (lt70 and dropping cant give
  observed SED)

46
Must Sgr A be a SMBH?

• Object Density Method Mass Radius
• (Msun/pc3)
• M 87 2 x 106 HST
  3x109 Msun in 7 pc
• NGC 4258 7 x 109 VLBA H2O
  3x107 Msun in 0.1 pc
• Sgr A 8 x 1015 S2s orbit
  4x106 Msun in 0.001 pc
• Sgr A 2 x 1022 Sgr As
  p.m. 1x106 Msun in 0.5 AU
• SMBH 2 x 1025 Rsch
  4x106 Msun in 0.08 AU
• VLBI (eg, SMA-ALMA-LMT-CARMA) _at_ 1 mm -gt 20 uas

10 uas _at_ 8 kpc
47
The Utimate Proof/Prize

• Image SMBH with resolution RSch
• Show all of the mass is contained within 3RSch
• See how accretion disk, black hole, and jets
  work