High angular resolution imaging of the galactic center
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High Angular Resolution Imaging of the Galactic Center. Andrea Ghez University of California Los Angeles Collaborators E. E. Becklin, G. Duchene, S. Hornstein, J. Lu, M. Morris, A.Tanner, S. Wright. Image courtesy of 2MASS. Key Questions. Astrometry (Source Position) Questions

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High Angular Resolution Imaging of the Galactic Center

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High Angular Resolution Imaging of the Galactic Center

Andrea Ghez

University of California Los Angeles


E. E. Becklin, G. Duchene, S. Hornstein, J. Lu, M. Morris, A.Tanner, S. Wright

Image courtesy of 2MASS

Key Questions

  • Astrometry (Source Position) Questions

    • Is there a supermassive black hole at the center of our Galaxy?

    • Is it associated with the unusual radio source Sgr A*?

    • What is the distance to the Galactic center (Ro)

    • Origin of young stars near black hole?

    • Is there a halo of dark matter surrounding the black hole?

  • Photometry Questions

    • Why is the black hole so dim (10-9 LEd)?

    • Does the black hole influence the appearance / evolution of the stars?

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 stars as test particles

    • F = -G Mencl m/ R

      • Impatient -> velocity dispersions (ensemble)

      • Patient -> full 3-d orbits (individual)

        I.Black HoleII.Stellar Cluster (r a r -2)


a r-1/2
















Contribution from

Luminous Matter

Evidence for

Dark Matter

Inferred Dark Matter Density From Low Angular Resolution Studies was too Small to Definitively Claim a Black Hole

  • 3x106 Mo within 105 Rsh

  • Black Hole Alternatives

    • Clusters of dark objects permitted with the inferred density of ~109 Mo/pc3

    • Fermion Ball


Inward Bound

Star closest to the center are the keys!


at center “confusion” due to high density of stars

Need high spatial resolution


Source separation 0.”1 in central 1”x1”

Direct imaging resolution 0.”4

Dynamic Range

~1000:1 (bright sources are at 1-3” and

can be near the edge)

NTT La Silla

Two Independent High Resolution Imaging Studies

Keck (10-meter)NTT (3.6-meter)

1995 - present1992 - 2001 0.”0450.”15Ghez et al. 1998, 2000 Eckart & Genzel 1996, 2002

Gezari et al. 2002Genzel et al 1997, 2000

Tanner et al. 2002

Hornstein et al 2002VLT (8-meter)

Ghez et al. 2003a,b,c2002 - present


Schodel et al. 2002, 2003

Eisenhauer et al. 2003

Genzel et al. 2003a,b

VLT Atacama, Chile

Keck Telescopes on Mauna Kea Hawaii

Light from science target

Light from reference star

Science Camera

Wavefront sensor


Diffraction-Limited Images Have Been Obtained with 2 Methods: Speckle & Adaptive Optics (AO)

Beam Splitter

Deformable Mirror

Science Camera

AO allows deeper

images & spectra!




5.”1 = 0.2pc

Speckle Imaging

Basic Building Block

A Single Short Exposure

150 milliseconds

Shift reference source: IRS 16C





Final Shift-and-Add Map

5,000 - 10,000 frames

Speckle vs. Adaptive Optics

  • AO images better for detection (finding) plus multi-l and spectra!

  • Speckle images better for astronomy (tracking)

Energy in Core

Speckle = 5%

AO = 35%

Core Size

Speckle = 0.”05

AO = 0.”11

Speckle PSF

Challenge for speckle is to minimize the effects of halo

Two passes

ID sources - high correlation threshhold to avoid false ID

Find missing sources - after sources have been tracked through multiple images, look for them in maps where they were “missing” at predicted locations with lower threshhold

Tracking Issues

  • Alignment of the images

    • Using all possible sources, we minimized the net displacement of the stars

      • Initially all sources (~200 sources)

      • Now eliminating sources with large velocities (>600 km/sec)

  • Who is who?

    • Initially only taking data once a year

    • Now 2-3 times a year and have benefit of more info about how stars are moving (tricky at closest approach)

Positional Uncertainties


depends on brightness


depends on location

(grows towards the edge)

Central 1”x1” (not shown!)

brightest stars (K=14 mag)

equal contribution



~1 milli-arcsec astrometric accuracy

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



Eckart & Genzel 1997 & Ghez et al. 1998 (shown)

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

Proper Motions Now Permit Complete Astrometric Orbital Solutions


Orbits Increase Dark Mass Density By x104, Making Black Hole Hypothesis Hard to Escape

* Dark Mass Density

Velocities: 1012Mo/pc3

Accelerations: 1013Mo/pc3

Orbits: 1017Mo/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 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)




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

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 10ee

Sgr A*



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

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)

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, 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 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)

    • Late-Type (G/K) Giant

      • Deep Carbon Monoxide (CO) absorption lines

    • Early-Type (O/B) Dwarf

      • Weak Hydrogen ( Brg) absorption lines

      • Weak Helium (He) absorption lines

    Stellar-Type Degeneracy Easily Broken with Spectroscopy


    Local Gas Makes it Difficult to Detect Weak Brg, Unless Star has Large Doppler Shift

    Local Gas


    • Local Gas has strong Brg emission lines

      • 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:

    • 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)



    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

    Black Hole

    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.

    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)

    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)

    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)


    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

    Distribution of Semi-major Axes

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

    • 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”

    • The Future

      • More orbits (# ~ t3)

      • Ro to 1% (may allow a recalibration cosmic scale distance ladder)

      • Deviations for Keleperian orbits!


    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)



    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!)

    Contribution from

    Luminous Matter

    Gas Radial Velocity


    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`

    Dark Matter

    Confirmed with


    Radial Velocity


    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)

    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

    Sgr A*


    Sgr A*

    Speckle vs. Adaptive Optics

    Adaptive Optics Imaging

    75 sec


    170 stars

    Speckle Image

    1570 sec

    Klim~16 mag

    84 stars

    AO detected twice as many sources! (extend l & spectra)

    Speckle better at astrometry in central 1”x1”

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