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

Original Case of Central Black Holes Active Galactic Nuclei (AGN) n Emit energy at an enormous rate n Radiation unlike that normally produced by stars or gas n Variable on short time scales n Contain gas moving at extremely high speeds CENTRAL ACCRETING BLACK HOLES Cyg A Jets ~10 5 pc (galaxy 1/10 this size)

Do “normal” (non-active) galaxies have “quiet” black holes?

Milky Way is Best Place to Answer this Question n Pro - Closer (8 kpc) n Con - Obstructed View (dust) u Optical light: 1 out of every 10 billion photons emitted makes it to us (invisible!) u 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* Circum- Nuclear Disk (CND)

Sgr A* n Is unusual radio source Sgr A* coincident with black hole? u Non-thermal emission u Compact u Low-velocity VLA: J.-H. Zhao

Dynamical Proof of Black Hole n Need to show mass confined to a small volume u R sh = 3 x M BH km(M BH in units of M sun ) n Use gas/stars as test particles   = -G M encl m/ R I. Black HoleII. Stellar Cluster (  r -2 )  r -1/2 (Velocity Dispersion) 1/2 r r r Enclosed Mass BH stars r Enclosed Mass stars BH (Velocity Dispersion) 1/2

Plot from Genzel 1994 VLA 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 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) Dark Matter Confirmed with Stellar Radial Velocity Measurements Contribution from Luminous Matter Evidence for Dark Matter

However, Inferred Dark Matter Density was too Small to Definitively Claim a Black Hole n Black Hole Alternatives u Clusters of dark objects permitted with the inferred density of ~10 9 M o /pc 3 u Fermion Ball n 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) present ”0450.”15 Ghez et al. 1998, 2000 Eckart & Genzel 1996, 2002 Gezari et al. 2002Genzel et al 1997, 2000 Tanner et al Hornstein et al 2002VLT (8-meter) Ghez et al. 2003a,b,c present 0.”056 Schodel et al. 2002, 2003 Eisenhauer et al Genzel 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 sensor Computer Deformable Mirror Beam Splitter Science Camera Computer Science Camera Wavefront Sensor…. AO allows deeper images & spectra!

Spot Deviation Incoming Wave Lenslet Array Subaperture Focal Spots 2-Dimensional Detector The Shack-Hartmann Wavefront Sensor Run at Hz & deformable mirror has ~300 segments

Tremendous Progress Has Been Made With High Angular Resolution Techniques on Large Telescopes 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 (x10 3 ), Which Ruled Out Clusters of Dark Objects Eckart & Genzel 1997 & Ghez et al (shown) ~1 milli-arcsec astrometric accuracy RA DEC

Black Hole Case Strengthened by Acceleration Measurements n Accelerations provided first measurement of dark mass density that is independent of projection effects  = 3 a 2-d / (4 G R 2-d 3  n Dark mass density increased by 10x (~ M o /pc 3 ) leaving only fermion balls as BH alternative. n Center of attraction coincident with Sgr A* (±30 mas) n Minimum orbital period of 15 yrs for S0-2 inferred Ghez et al (shown), Eckart et al. 2002

Proper Motions Now Permit Complete Astrometric Orbital Solutions 1"

Orbits Increase Dark Mass Density By x10 4, Making Black Hole Hypothesis Hard to Escape * Dark Mass Density Velocities: M o /pc 3 Accelerations: M o /pc 3 Orbits: M o /pc 3 * Fermion ball hypothesis no longer works as an alternative for all supermassive black holes m ~ 50kev c -2 Mass fermion ball < 2x10 8 M o * Milky Way is now the best example of a normal galaxy containing a supermassive black hole S0-16 has smallest periapse passage R min = 90 AU = 1,000 R s 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 n Improves Estimate of Black Hole’s Properties u Mass: 3.7±0.4 x 10 6 (Ro/8kpc) 3 M o u Position: ±1.5 mas n Adds Estimate Black Hole’s Velocity on the Plane of the Sky u 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”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 ”

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

Simultaneous Orbital Solution Allows a Larger Number of Orbits to be Determined n Black hole’s properties fixed by S0-2, S0-16, & S0-19 u M, X o, Y o, V x, V y n Less curvature needed for full orbital solution for other stars  P, T o, e, i, w,  u Need only 6 kinematic variables measured (R x, R y, V x, V y, A x, A y )

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/yr 2 )

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, Stellar- Type (age/mass) is Degenerate Based on 2  m brightness (K = 13.9 to 17; M k = -3.8 to -0.9) two expected possibilities u Late-Type (G/K) Giant (cool & large; old & low mass) u Early-Type (O/B) Dwarf / Main-Sequence Star (hot & small; young & high mass)

Stellar-Type Degeneracy Easily Broken with Spectroscopy u Early-Type (O/B) Dwarf  Weak Hydrogen ( Br  ) absorption lines F Weak Helium (He) absorption lines u Late-Type (G/K) Giant F 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 |V z | < ~300 km/s F For OB stars these are the strongest lines, which are already quite weak ~a few Angstroms u For low V z 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: u Vz = to km/sec  EW(Br  = 3 Ang u EW (HeI) = 1 Ang u V rot = 170 km/sec

Digression: Addition of Spectra Also Provide a Direct Measure of Galactic Center Distance (R o ) NTT/VLT Keck VLT

Digression: R o is now largest source of mass (spin…) uncertainty Ghez et al 2003 (Keck) Eisenhauer et al (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 (shown, R=2,000), Lu et al (2004) Genzel et al (R=35) Integrated spectra: Eckart et al 1999 & Figer et al. 2000

Presence of OB Stars Raises Paradox of Youth n OB stars u Have hot photospheres (~30,000 K) u Are young (<~10 Myr) & massive (~15 M o ), assuming that they are unaltered by environment n 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

n Possible Forms of “Astronomical Botox” u Need to make stellar photosphere hot F Heated (tidally?) by black hole (e.g., Alexander & Morris 2003) No significant intensity variations as stars go through periapse F Stripped giants (e.g., Davies et al. 1998) F Accreting compact objects (e.g., Morris 1993) F Merger products (e.g., Lee 1994, Genzel et al. 2003) Are These Old Stars Masquerading as Youths?

n Past Gas Densities Would Have to Have Been Much Higher n What densities are needed? u ~10 14 cm -3 at R= 0.01 pc (apoapse distance of S0-2) n Mechanism for enhancing past gas densities u Accretion disk (e.g., Levin & Beloborodov 2003) u 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? n At larger radii, tidal forces compared to gas densities are no longer a problem n At 30 pc, young stellar clusters observed u Arches and Quintuplet (e.g., Figer et al. 2000, Cotera et al. 1999) u Massive (10 4 M o ) & Compact (0.2 pc) HST/Figer

Migration Inwards is Difficult, Due to Short Time-scales & Large Distances n Ideas u Massive binaries on radial orbits experience three body exchange with central black hole (Gould & Quillen 2003) u Cluster migration (Gerhard et al. 2000, Kim & Morris 2003, Portegies-Zwart et al 2003, McMillan et al. 2003) F Need very central condensed cluster core u 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 2x10 5 (R / 16,000 AU) 1/2 M o Distribution of Semi-major Axes

Conclusions u Dramatically improved case for black hole F Dark matter density increased to M o /pc 3 with orbits, making the Milky Way the best example of a normal galaxy containing a supermassive black hole u First detection of IR emission from accreting material F More variable than X-ray F If from non-thermal tail of e -, shocks/reconnections happening more frequently than previously thought u Direct measure of distance to GC (R o ) u Raised paradox of youth F Majority of stars in Sgr A* cluster appear to be young F Low present-day gas densities & large tidal forces present a significant challenge for star formation (none of present theories entirely satifactory) F Dynamical insight from orbits Central 1”x 1” u The Future F More orbits (# ~ t 3 ) F R o to 1% (may allow a recalibration cosmic scale distance ladder) F Deviations for Keleperian orbits!

A Few Introductions Are Necessary n Units n Why did people think there might be a black hole at the center of our Galaxy? n Galactic center environment

Astronomical Units Distances & 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 = AU ~ 3 x 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 n He I Emission-Line Stars u Massive ( M o ) post-main- sequence stars formed within the last 8 Myrs u Located at distances from the black hole of pc, which is 10x further than the Sgr A* cluster stars n Formation problem u 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