Unveiling a Supermassive Black Hole at the Center of Our Galaxy

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Unveiling a Supermassive Black Hole at the Center
of Our Galaxy

• Andrea Ghez
• University of California Los Angeles
• Collaborators (UCLA/Caltech/Keck)
• E. E. Becklin, G. Duchene, S. Hornstein, D. Le
  Mignant, J. Lackey/Lu, K. Matthews, M.
  Milosavljevic , M. Morris, S. Samir, B. T.
  Soifer, A.Tanner, D. Thompson, N. Weinberg, S.
  Wright

Image courtesy of 2MASS
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Key Questions

• Is there a supermassive black hole at the center
  of our Galaxy?
• Is it associated with the unusual radio source
  Sgr A?
• Why is it so dim (10-9 LEd)?
• What is the distance to the Galactic center (Ro)
• Is there a halo of dark matter surrounding the
  black hole?
• When and where are the stars born?
• Does the black hole influence the appearance /
  evolution of the stars?

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Original Case of Central Black HolesActive
Galactic Nuclei (AGN)

• Emit energy at an enormous rate
• Radiation unlike that normally produced by stars
  or gas
• Variable on short time scales
• Contain gas moving at extremely high speeds

Cyg A Jets 105 pc (galaxy 1/10 this size)
CENTRAL ACCRETING BLACK HOLES
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Do normal (non-active) galaxies have quiet
black holes?
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Milky Way is Best Place to Answer this Question

• Pro - Closer (8 kpc)
• Con - Obstructed View (dust)
• Optical light 1 out of every 10 billion photons
  emitted makes it to us (invisible!)
• Near Infrared light 1 out of every 10 photons
  emitted makes it to us (visible!)

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Overview ofGalactic Center(100 pc)
From Genzel 1994
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Overview of Galactic Center( 10 pc)
SgrA
Circum- Nuclear Disk (CND)
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Sgr A

• Is unusual radio source Sgr A coincident with
  black hole?
• Non-thermal emission
• Compact
• Low-velocity

VLA J.-H. Zhao
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Dynamical Proof of Black Hole

• Need to show mass confined to a small volume
• Rsh 3 x MBH km (MBH in units of Msun)
• Use gas/stars as test particles
• F -G Mencl m/ R
• I. Black Hole II. Stellar Cluster (r a r -2)

BH
a r-1/2
(Velocity Dispersion)1/2
(Velocity Dispersion)1/2
r
r
stars
Enclosed Mass
Enclosed Mass
stars
BH
r
r
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Contribution from Luminous Matter
Gas Radial Velocity Measurements Gave 1st Hint
of Dark Matter
Evidence for Dark Matter

• HI rotation along Galactic Plane
  (eg. Rougoor Oort 1960 Ooort 1977 Sinha 1978)
• Circumnuclear disk/ring rotation (e.g.,
  Gatley et al. 1986 Guesten et al. 1987)
• Ionized streamers in mini-spiral (e.g.,
  Serabyn Lacy 1985 Serabyn et al. 1987)

Plot from Genzel 1994 VLA 6 cm image of
mini-spiral
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Dark Matter Confirmed with Stellar Radial
Velocity Measurements
Contribution from Luminous Matter
Evidence for Dark Matter

• Integrated stellar light
  (e.g., McGinn et al. 1989 Sellgren et al. 1990)
• Individual Stars (OH/IR, giants, He I) (e.g.,
  Linquist et al. 1992 Haller et al. 1995 Genzel
  et al. 1996)

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However, Inferred Dark Matter Density was too
Small to Definitively Claim a Black Hole

• Black Hole Alternatives
• Clusters of dark objects permitted with the
  inferred density of 109 Mo/pc3
• Fermion Ball
• High spatial resolution techniques needed to make
  further progress.

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Two Independent High Resolution Imaging Studies
NTT La Silla
Keck (10-meter) NTT
(3.6-meter) 1995 - present 1992 - 2001
0.045 0.15 Ghez et al. 1998, 2000
Eckart Genzel 1996, 2002 Gezari et al.
2002 Genzel et al 1997, 2000 Tanner et al.
2002 Hornstein et al 2002 VLT (8-meter) Ghez
et al. 2003a,b,c 2002 - present
0.056 Schodel et al. 2002,
2003 Eisenhauer et al. 2003 Genzel et
al. 2003a,b
VLT Atacama, Chile
Keck Telescopes on Mauna Kea Hawaii
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Diffraction-Limited Images Have Been Obtained
with 2 Methods Speckle Adaptive Optics (AO)
Beam Splitter
Deformable Mirror
Science Camera
AO allows deeper images spectra!
Wavefront Sensor.
Computer
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The Shack-Hartmann Wavefront Sensor
Lenslet Array
Subaperture Focal Spots
Incoming Wave
2-Dimensional Detector
Spot Deviation
Run at 100-500 Hz deformable mirror has 300
segments
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Tremendous Progress Has Been Made With High
Angular Resolution Techniques on Large Telescopes
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Motions on the Plane of the Sky Easily Measured
200 stars tracked, only central 1x1 shown
1"
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Proper Motion Measurements Increased Dark Matter
Density (x103), Which Ruled Out Clusters of Dark
Objects
RA
DEC
1 milli-arcsec astrometric accuracy
Eckart Genzel 1997 Ghez et al. 1998 (shown)
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Black Hole Case Strengthenedby Acceleration
Measurements

• Accelerations provided first measurement of dark
  mass density that is independent of projection
  effects
• r 3 a2-d / (4 G R2-d3)
• Dark mass density increased by 10x ( 1013
  Mo/pc3) leaving only fermion balls as BH
  alternative.
• Center of attraction coincident with Sgr A (30
  mas)
• Minimum orbital period of 15 yrs for S0-2
  inferred

Ghez et al. 2000 (shown), Eckart et al. 2002
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Proper Motions Now Permit Complete Astrometric
Orbital Solutions
1"
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Orbits Increase Dark Mass Density By x104, Making
Black Hole Hypothesis Hard to Escape
Dark Mass Density Velocities 1012
Mo/pc3 Accelerations 1013 Mo/pc3
Orbits 1017 Mo/pc3 Fermion ball
hypothesis no longer works as an alternative
for all supermassive black holes m 50kev
c-2 Mass fermion ball lt 2x108 Mo Milky Way
is now the best example of a normal galaxy
containing a supermassive black hole
S0-16 has smallest periapse passage Rmin 90 AU
1,000 Rs
Ghez et al. 2002, 2003 (shown) Schoedel
et al. 2002, 2003
Independent solutions for 3 stars (those that
have gone through periapse)
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Simultaneous Orbital Solution is More Powerful
than Independent Orbital Solutions

• Improves Estimate of Black Holes Properties
• Mass 3.70.4 x 106 (Ro/8kpc)3 Mo
• Position 1.5 mas
• Adds Estimate Black Holes Velocity on the Plane
  of the Sky
• Velocity 30 30 km/s

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Orbits Improve Localization of Black Hole in IR
Reference by an Order of Magnitude, Assisting
Searches for IR Emission Associated with Black
Hole
SiO masers used to locate Sgr A position in IR
frame (10 milli-arcsec) Reid et al. 2003
IRS 7
IRS 10ee
Sgr A
0.1
1
Dynamical Center pinpointed to 1.5 milli-arcsec
(12 AU)
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At 3.8 mm, Stellar and Dust Emission are
Suppressed, Facilitating the Detection of Sgr A
Keck AO L(3.8 mm) images (Ghez et al. 2003,
ApJLett, in press, astro-ph/0309076) NIR results
fromVLT (Genzel et al. 2003, Nature)
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Factor of 4 Intensity Change Over 1 week and
Factor of 2 Change in 40 minutes
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Similarity of Flaring Time-scales Suggests IR and
X-ray Originate From Same Mechanism
Chandra / Baganoff et al. 2001
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Flaring from non-thermal tail of high energy
electrons

• Models
• Markoff et al 2001
• Yuan et al. 2003
• Physical Process
• Shocks
• Magnetic reconnection
• Emission Mechanism
• IR Synchrotron
• X-Ray Self-Synchrotron Compton or synchrotron
• IR variability suggests electrons are accelerated
  much more frequently than previously thought

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Simultaneous Orbital Solution Allows a Larger
Number of Orbits to be Determined

• Black holes properties fixed by S0-2, S0-16,
  S0-19
• M, Xo, Yo, Vx, Vy
• Less curvature needed for full orbital solution
  for other stars
• P, To, e, i, w, W
• Need only 6 kinematic variables measured (Rx, Ry,
  Vx, Vy, Ax , Ay)

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Eccentricities Are Consistent with an Isotropic
Distribution
While there are many highly eccentric systems
measured, there is a selection effect We only
measure orbits for stars with detectable
acceleration (gt 2 mas/yr2)
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Lower Limit onSemi-Major Axis gt 1000 AUApoapse
Distance gt 2000 AU
No selection effect against detecting Klt16 mag
with Alt1000 AU
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Possible Bias in Distribution of Apoapse
Directions
Other angle - inclination - appears random
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With Only Imaging Data, Stellar-Type (age/mass)
is Degenerate

• Based on 2 mm brightness (K 13.9 to 17 Mk
  -3.8 to -0.9) two expected possibilities
• Late-Type (G/K) Giant (cool large old low
  mass)
• Early-Type (O/B) Dwarf / Main-Sequence Star (hot
  small young high mass)

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Stellar-Type Degeneracy Easily Broken with
Spectroscopy
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Local Gas Makes it Difficult to Detect Weak Brg,
Unless Star has Large Doppler Shift
Local Gas
S0-1
1

• Local Gas has strong Brg emission lines
• Effects ability to detect stellar Brg absorption
  lines if Vz lt 300 km/s
• For OB stars these are the strongest lines, which
  are already quite weak a few Angstroms
• For low Vz sources, lack of CO is evidence that
  they are young

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Brg in OB Stars in Sgr A Cluster Detected as
They Go Through Closest Approach

• Example of S0-2
• Vz 1100 to -1500 km/sec
• EW(Br g) 3 Ang
• EW (HeI) 1 Ang
• Vrot 170 km/sec

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Digression Addition of Spectra Also Provide a
Direct Measure of Galactic Center Distance (Ro)
NTT/VLT
Keck
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Digression Ro is now largest source of mass
(spin) uncertainty
Ghez et al 2003 (Keck) Eisenhauer et al. 2003
(NTT/VLT)
1, 2, 3s contours
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The Majority of Stars in the Sgr A Cluster are
Identified as OB Stars Through Their Lack of CO
Lack
Individual spectra Gezari et al. 2002 (shown,
R2,000), Lu et al (2004) Genzel et al. 1997
(R35) Integrated spectra Eckart et al 1999
Figer et al. 2000
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Presence of OB Stars Raises Paradox of Youth

• OB stars
• Have hot photospheres (30,000 K)
• Are young (lt10 Myr) massive (15 Mo), assuming
  that they are unaltered by environment
• The Problem
• Existing gas in region occupied by Sgr A cluster
  is far from being sufficiently dense for
  self-gravity to overcome the strong tidal forces
  from the central black hole.

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Are These Old Stars Masquerading as Youths?

• Possible Forms of Astronomical Botox
• Need to make stellar photosphere hot
• Heated (tidally?) by black hole (e.g., Alexander
  Morris 2003)
• No significant intensity variations as stars go
  through periapse
• Stripped giants (e.g., Davies et al. 1998)
• Accreting compact objects (e.g., Morris 1993)
• Merger products (e.g., Lee 1994, Genzel et al.
  2003)

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Are Stars Young Formed In-Situ?

• Past Gas Densities Would Have to Have Been Much
  Higher
• What densities are needed?
• 1014 cm-3 at R 0.01 pc (apoapse distance of
  S0-2)
• Mechanism for enhancing past gas densities
• Accretion disk (e.g., Levin Beloborodov 2003)
• Colliding cloud clumps (e.g., Morris 1993, Genzel
  et al. 2003)

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Are Stars Young, Formed at Larger Radii,
Efficiently Migrated Inwards?
HST/Figer

• At larger radii, tidal forces compared to gas
  densities are no longer a problem
• At 30 pc, young stellar clusters observed
• Arches and Quintuplet (e.g., Figer et al. 2000,
  Cotera et al. 1999)
• Massive (104 Mo) Compact (0.2 pc)

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Migration Inwards is Difficult, Due to Short
Time-scales Large Distances

• Ideas
• Massive binaries on radial orbits experience
  three body exchange with central black hole
  (Gould Quillen 2003)
• Cluster migration (Gerhard et al. 2000, Kim
  Morris 2003, Portegies-Zwart et al 2003, McMillan
  et al. 2003)
• Need very central condensed cluster core
• Variation on cluster migration - clusters with
  intermediate mass black holes, which scatter
  young stars inward (Hansen Milosavljevic 2003)

From New Scientist
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Only Cluster Shuttled Inward with Intermediate
Black Hole Reproduces Orbital Properties, but
Where are They?
Directions of Apoapse Vectors
Distribution of Semi-major Axes
Orbital limit on reflex motion (lt 30 km/s) limits
IMBH to 2x105 (R / 16,000 AU)1/2 Mo
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Conclusions

• Dramatically improved case for black hole
• Dark matter density increased to 1017 Mo/pc3
  with orbits, making the Milky Way the best
  example of a normal galaxy containing a
  supermassive black hole
• First detection of IR emission from accreting
  material
• More variable than X-ray
• If from non-thermal tail of e-,
  shocks/reconnections happening more frequently
  than previously thought
• Direct measure of distance to GC (Ro)
• Raised paradox of youth
• Majority of stars in Sgr A cluster appear to be
  young
• Low present-day gas densities large tidal
  forces present a significant challenge for star
  formation (none of present theories entirely
  satifactory)
• Dynamical insight from orbits

Central 1x 1

• The Future
• More orbits ( t3)
• Ro to 1 (may allow a recalibration cosmic scale
  distance ladder)
• Deviations for Keleperian orbits!

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A Few Introductions Are Necessary

• Units
• Why did people think there might be a black hole
  at the center of our Galaxy?
• Galactic center environment

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Astronomical UnitsDistances Angles

• Angles
• 1 arc-second 1/3600 degree
• Atmosphere limits angular
  resolution of most observations to 1

1 pc 206265 AU 3 x 1013 km 3 light
years Distance to Galactic Center 8,000 pc
R1AU
D1pc

• Distances
• Astronomical Unit (AU) Earth-Sun distance
• 1 parsec pc distance at which 1 AU subtends
  1

Q1
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Sgr A Cluster Stars Amplifying a Problem
Originally Raised by the He I Emission Line Stars

• He I Emission-Line Stars
• Massive (20-100 Mo) post-main-sequence stars
  formed within the last 8 Myrs
• Located at distances from the black hole of 0.1 -
  0.5 pc, which is 10x further than the Sgr A
  cluster stars
• Formation problem
• Required gas densities are not as severe, but
  still not found at 0.1 pc

OB stars in Sgr A cluster
Bright He I emission-line stars