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 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 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

CENTRAL ACCRETINGBLACK 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

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!)

Overview ofGalactic Center

(~100 pc)

From Genzel 1994

Overview of Galactic Center

(~ 10 pc)SgrA*

Circum-Nuclear Disk (CND)

Sgr A*

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

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 = -G Mencl m/ R

I. Black Hole II. Stellar Cluster ( r -2)

r-1/2(VelocityDispersion)1/2

r r

r

EnclosedMass BH

stars

r

EnclosedMass

stars

BH

(VelocityDispersion)1/2

Plot from Genzel 1994VLA 6 cm image of mini-spiral`

• 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)

Gas Radial Velocity Measurements

Gave 1st Hint of Dark Matter

Contribution from Luminous Matter

Evidence forDark 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)

Dark Matter Confirmed with

StellarRadial Velocity Measurements

Contribution from Luminous Matter

Evidence forDark Matter

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.

6”

Two Independent High Resolution Imaging Studies

Keck Telescopes on Mauna Kea Hawaii

Keck (10-meter) NTT (3.6-meter)1995 - present 1992 - 2001 0.”045 0.”15Ghez et al. 1998, 2000 Eckart & Genzel 1996, 2002Gezari et al. 2002 Genzel et al 1997, 2000 Tanner et al. 2002Hornstein et al 2002 VLT (8-meter)Ghez et al. 2003a,b,c 2002 - present

0.”056Schodel et al. 2002, 2003Eisenhauer et al. 2003Genzel et al. 2003a,b

NTT La Silla

VLT Atacama, Chile

Diffraction-Limited Images Have Been Obtained with 2 Methods:

Speckle & Adaptive Optics (AO) Light from science target

Light from reference star

Wavefront sensorComputer

Deformable Mirror

Beam Splitter

Science Camera

Computer

Science Camera

WavefrontSensor….

AO allows deeper images & spectra!

Spot Deviation

Incoming Wave

Lenslet Array Subaperture Focal Spots

2-Dimensional Detector

The Shack-Hartmann Wavefront Sensor

Run at 100-500 Hz & deformable mirror has ~300 segments

Tremendous Progress Has Been Made With High Angular Resolution

Techniques on Large Telescopes

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6

Motions on the Plane of the Sky Easily Measured

200 stars tracked, only central 1”x1” shown

1"

Proper Motion Measurements Increased Dark Matter Density (x103), Which Ruled Out Clusters of Dark Objects

Eckart & Genzel 1997 & Ghez et al. 1998 (shown)~1 milli-arcsec astrometric accuracy

RA

DE

C

Black Hole Case Strengthenedby Acceleration Measurements

Accelerations provided first measurement of dark mass density that is independent of projection effects

= 3 a2-d / (4 G R2-d

3 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

Proper Motions Now Permit Complete Astrometric Orbital Solutions

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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 < 2x108 Mo

* Milky Way is now the best example of a normal galaxy containing a supermassive black hole

S0-16 has smallest periapse passageRmin = 90 AU = 1,000 Rs

Ghez et al. 2002, 2003 (shown); Schoedel et al. 2002, 2003Independent solutions for 3 stars (those that have gone through periapse)

Simultaneous Orbital Solution is More Powerful than Independent

Orbital Solutions Improves Estimate of Black

Hole’s Properties 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 Velocity: 30 ±30 km/s

S0-2

S0-16

S0-19

Orbits Improve Localization of Black Hole in IR Reference by an Order of Magnitude, Assisting Searches for IR Emission Associated with Black Hole

1”Dynamical Center pinpointed to ±1.5 milli-arcsec (12 AU)

IRS 7

IRS 10ee

Sgr A*

SiO masers used to locate Sgr A* position in IR frame (±10 milli-arcsec) Reid et al. 2003

0.1”

At 3.8 m, Stellar and Dust Emission are Suppressed, Facilitating the

Detection of Sgr A*

Keck AO L’(3.8 m) 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

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

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

Simultaneous Orbital Solution Allows a Larger Number of Orbits to be

Determined

Black hole’s 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, 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 onSemi-Major Axis > ~1000 AUApoapse 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, Stellar-Type (age/mass) is Degenerate

Based on 2 m 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)

Stellar-Type Degeneracy Easily Broken with Spectroscopy

Early-Type (O/B) Dwarf Weak Hydrogen ( Br)

absorption lines Weak Helium (He)

absorption lines

Late-Type (G/K) Giant Deep Carbon

Monoxide (CO) absorption lines

Local Gas Makes it Difficult to Detect Weak Br, Unless Star has Large

Doppler Shift

Local Gas has strong Br emission lines Effects ability to detect stellar Br 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

S0-2

Local Gas

S0-1

1”

Br 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 = 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/VLTKeck

KeckVLT

Digression: Ro is now largest source of mass (spin…) uncertainty

Ghez et al 2003 (Keck)Eisenhauer et al. 2003 (NTT/VLT)

1, 2, 3 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

OB stars 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

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)

Are These Old Stars Masquerading as Youths?

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)

Are Stars Young & Formed In-Situ?

Are Stars Young, Formed at Larger Radii, & Efficiently

Migrated Inwards?

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)

HST/Figer

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

Only Cluster Shuttled Inward with Intermediate Black Hole Reproduces

Orbital Properties, but Where are They?Directions of Apoapse Vectors

Orbital limit on reflex motion (< 30 km/s) limits IMBH to 2x105 (R / 16,000 AU)1/2 Mo

Distribution of Semi-major Axes

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 1”x 1”

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The Future More orbits (# ~ t3) Ro to 1% (may allow a recalibration

cosmic scale distance ladder) Deviations for Keleperian orbits!

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

Astronomical UnitsDistances & Angles

R=1AU

D=1pc

=1”

Angles: • 1 arc-second [”]= 1/3600 degree• Atmosphere limits angular resolution of most observations to 1”

Distances: • Astronomical Unit (AU) = Earth-Sun distance• 1 parsec [pc] = distance at which 1 AU subtends 1”

1 pc = 206265 AU~ 3 x 1013 km~ 3 light years

Distance to Galactic Center = 8,000 pc

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 pcOB stars in

Sgr A* clusterBright He I emission-line stars