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
Key Questions
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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?
Original Case of Central Black Holes
Active Galactic Nuclei (AGN)
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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
CENTRAL ACCRETING
BLACK HOLES
Cyg A Jets
~105 pc (galaxy 1/10 this size)
Do “normal” (non-active) galaxies
have “quiet” black holes?
Milky Way is Best Place to Answer this
Question
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Pro - Closer (8 kpc)
Con - Obstructed View (dust)
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Optical light: 1 out of every 10 billion photons emitted makes it to
us (invisible!)
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Near Infrared light: 1 out of every 10 photons emitted makes it to us
(visible!)
Overview of
Galactic Center
(~100 pc)
From Genzel 1994
Overview of
Galactic Center
(~ 10 pc)
SgrA*
CircumNuclear
Disk (CND)
Sgr A*
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Is unusual radio
source Sgr A*
coincident with black
hole?
 Non-thermal
emission
 Compact
 Low-velocity
VLA: J.-H. Zhao
Dynamical Proof of Black Hole
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Need to show mass confined to a small volume
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Rsh = 3 x MBH km (MBH in units of Msun)
Use gas/stars as test particles
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BH
F = -G Mencl m/ R
II. Stellar Cluster (r a r -2)
I. Black Hole
(Velocity
Dispersion)1/2
a r-1/2
(Velocity
Dispersion)1/2
r
Enclosed
Mass
r
stars
BH
r
Enclosed
Mass
stars
r
Gas Radial Velocity
Measurements
Gave 1st Hint of
Dark Matter
Contribution from
Luminous Matter
Evidence for
Dark Matter
• HI rotation along Galactic Plane
Plot from Genzel 1994
VLA 6 cm image of mini-spiral`
(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)
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)
However, Inferred Dark Matter
Density was too Small to Definitively
Claim a Black Hole
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Black Hole Alternatives
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Clusters of dark objects
permitted with the inferred
density of ~109 Mo/pc3
Fermion Ball
High spatial resolution
techniques needed to make
further progress.
6”
Two Independent
High Resolution Imaging Studies
Keck (10-meter)
1995 - present
0.”045
Ghez et al. 1998, 2000
Gezari et al. 2002
Tanner et al. 2002
Hornstein et al 2002
Ghez et al. 2003a,b,c
Keck Telescopes on Mauna Kea Hawaii
NTT La Silla
NTT (3.6-meter)
1992 - 2001
0.”15
Eckart & Genzel 1996, 2002
Genzel et al 1997, 2000
VLT (8-meter)
2002 - present
0.”056
Schodel et al. 2002, 2003
Eisenhauer et al. 2003
Genzel et al. 2003a,b
VLT Atacama, Chile
Diffraction-Limited Images Have
Been Obtained with 2 Methods:
Speckle & Adaptive Optics (AO)
Light from
science target
Light from
reference star
a dna ™emiTkciuQ
rosserpmoced FIG
.erutcip siht ees ot dedeen era
Beam Splitter
Deformable
Mirror
Science Camera
Computer
Wavefront
Wavefront
sensor
Sensor….
AO allows deeper
images & spectra!
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
Tremendous Progress Has Been Made
With High Angular Resolution
Techniques on Large Telescopes
6
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Motions on the Plane of the Sky
Easily Measured
200 stars tracked, only central 1”x1” shown
1"
DEC
RA
Proper Motion Measurements Increased
Dark Matter Density (x103), Which
Ruled Out Clusters of Dark Objects
~1 milli-arcsec astrometric accuracy
Eckart & Genzel 1997 & Ghez et al. 1998 (shown)
Black Hole Case Strengthened
by Acceleration Measurements
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Ghez et al. 2000 (shown), Eckart et al. 2002
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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
Proper Motions Now Permit Complete
Astrometric Orbital Solutions
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1"
Orbits Increase Dark Mass Density By
x104, Making Black Hole Hypothesis
Hard to Escape
S0-16 has smallest periapse passage
Rmin = 90 AU = 1,000 Rs
* 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 < 2x108 Mo
Ghez et al. 2002, 2003 (shown);
Schoedel et al. 2002, 2003
Independent solutions for 3 stars
(those that have gone through periapse)
* Milky Way is now the best
example of a normal galaxy
containing a supermassive
Simultaneous Orbital Solution is
More Powerful than Independent
Orbital Solutions
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S0-2
Improves Estimate of Black
Hole’s Properties
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S0-16
S0-19
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Mass: 3.7±0.4 x 106 (Ro/8kpc)3 Mo
Position: ±1.5 mas
Adds Estimate Black Hole’s
Velocity on the Plane of the
Sky
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Velocity: 30 ±30 km/s
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”
Dynamical Center pinpointed to
±1.5 milli-arcsec (12 AU)
1”
At 3.8 mm, Stellar and Dust Emission
are Suppressed, Facilitating the
Detection of Sgr A*
QuickTime™ and a
GIF decompressor
are needed to see this picture.
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)
Factor of 4 Intensity Change
Over 1 week and Factor of 2
Change in 40 minutes
Similarity of Flaring Time-scales
Suggests IR and X-ray Originate
From Same Mechanism
Chandra / Baganoff et al. 2001
Flaring from non-thermal tail of
high energy electrons
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Models
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Physical Process
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Markoff et al 2001
Yuan et al. 2003
Shocks
Magnetic reconnection
Emission Mechanism
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IR Synchrotron
X-Ray Self-Synchrotron
Compton or synchrotron
IR variability suggests
electrons are accelerated
much more frequently
than previously thought
Simultaneous Orbital Solution Allows
a Larger Number of Orbits to be
Determined
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Black hole’s properties
fixed by S0-2, S0-16, &
S0-19
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M, Xo, Yo, Vx, Vy
Less curvature needed
for full orbital solution
for other stars
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P, To, e, i, w, W
Need only 6 kinematic
variables measured (Rx,
Ry, Vx, Vy, Ax , Ay)
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 (> 2 mas/yr2)
Lower Limit on
Semi-Major Axis > ~1000 AU
Apoapse Distance > ~2000 AU
No selection effect against detecting K<16 mag with A<1000 AU
Possible Bias in Distribution of
Apoapse Directions
Other angle - inclination - appears random
With Only Imaging Data, StellarType (age/mass) is Degenerate
Based on 2 mm brightness (K = 13.9 to 17; Mk = -3.8 to -0.9)
two expected possibilities
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Late-Type (G/K) Giant (cool & large; old & low mass)
Early-Type (O/B) Dwarf / Main-Sequence Star (hot & small; young &
high mass)
Stellar-Type Degeneracy Easily
Broken with Spectroscopy
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Late-Type (G/K) Giant
 Deep Carbon
Monoxide (CO)
absorption lines
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Early-Type (O/B) Dwarf
 Weak Hydrogen ( Brg)
absorption lines
 Weak Helium (He)
absorption lines
Local Gas Makes it Difficult to Detect
Weak Brg, Unless Star has Large
Doppler Shift
Local Gas
S0-2
S0-1
1”
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Local Gas has strong Brg emission lines
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Effects ability to detect stellar Brg absorption lines if |Vz| < ~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
Brg in OB Stars in Sgr A* Cluster
Detected as They Go Through
Closest Approach
Example of S0-2:
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Vz = +1100 to -1500 km/sec
EW(Br g) = 3 Ang
EW (HeI) = 1 Ang
Vrot = 170 km/sec
Digression: Addition of Spectra Also
Provide a Direct Measure of Galactic
Center Distance (Ro)
NTT/VLT
Keck
Keck
VLT
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
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, R=2,000), Lu et al (2004)
Genzel et al. 1997 (R=35)
Integrated spectra: Eckart et al 1999 & Figer et al. 2000
Presence of OB Stars Raises
Paradox of Youth
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OB stars
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Have hot photospheres (~30,000 K)
Are young (<~10 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.
Black Hole
Are These Old Stars Masquerading
as Youths?
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Possible Forms of “Astronomical Botox”
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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?
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Past Gas Densities Would Have to Have Been
Much Higher
What densities are needed?
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~1014 cm-3 at R= 0.01 pc (apoapse distance of S0-2)
Mechanism for enhancing past gas densities
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Accretion disk (e.g., Levin & Beloborodov 2003)
Colliding cloud clumps (e.g., Morris 1993, Genzel et al.
2003)
Are Stars Young, Formed at Larger
Radii, & Efficiently Migrated
Inwards?
HST/Figer
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At larger radii, tidal forces compared to gas densities
are no longer a problem
At 30 pc, young stellar clusters observed
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Arches and Quintuplet (e.g., Figer et al. 2000, Cotera et al. 1999)
Massive (104 M ) & Compact (0.2 pc)
Migration Inwards is Difficult, Due
to Short Time-scales & Large
Distances
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Ideas
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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
Only Cluster Shuttled Inward with
Intermediate Black Hole Reproduces
Orbital Properties, but Where are They?
Distribution of Semi-major Axes
Directions of Apoapse Vectors
Orbital limit on reflex motion (< 30 km/s) limits
IMBH to 2x105 (R / 16,000 AU)1/2 Mo
Conclusions
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Dramatically improved case for black
hole
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QuickTime™ and a
GIF decompressor
are needed to see this picture.
First detection of IR emission from
accreting material
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Central 1”x 1”
The Future
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More orbits (# ~ t3)
Ro to 1% (may allow a recalibration
cosmic scale distance ladder)
Deviations for Keleperian orbits!
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
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
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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
A Few Introductions Are
Necessary
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Units
Why did people think there might be a black hole
at the center of our Galaxy?
Galactic center environment
Astronomical Units
Distances & Angles
Angles:
• 1 arc-second [”]= 1/3600 degree
• Atmosphere limits angular
resolution of most observations to 1”
R=1AU 1 pc = 206265 AU
~ 3 x 1013 km
~ 3 light years
D=1pc
Distances:
• Astronomical Unit (AU) =
Earth-Sun distance
• 1 parsec [pc] = distance at
which 1 AU subtends 1”
Distance to Galactic Center
= 8,000 pc
Q=1”
Sgr A* Cluster Stars Amplifying
a Problem Originally Raised by
the He I Emission Line Stars
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He I Emission-Line Stars
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Massive (20-100 Mo) post-mainsequence 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
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Required gas densities are not as
severe, but still not found at 0.1 pc
Bright He I
emission-line stars
OB stars in
Sgr A* cluster