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Transcript of High Angular Resolution Imaging of the Galactic Center Andrea Ghez University of California Los...
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
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 = -G Mencl m/ R
Impatient -> velocity dispersions (ensemble) Patient -> full 3-d orbits (individual)
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
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
6”
Contribution from Luminous Matter
Evidence forDark Matter
Inward Bound
Star closest to the center are the keys!
BUT
at center “confusion” due to high density of stars
Need high spatial resolution
Crowding Source separation 0.”1 in central 1”x1” Direct imaging resolution 0.”4Dynamic Range ~1000:1 (bright sources are at 1-3” and can be near the edge)
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!
Speckle Imaging
Basic Building Block
A Single Short Exposure
150 milliseconds
Shift reference source: IRS 16C
5.”1 = 0.2pc
Final Shift-and-Add Map
5,000 - 10,000 frames
16NW16NE
16C
16SW
Core SizeSpeckle = 0.”05 AO = 0.”11
Energy in CoreSpeckle = 5%AO = 35%
Speckle vs. Adaptive Optics
AO images better for detection (finding) plus multi- and spectra! Speckle images better for astronomy (tracking)
Speckle PSF
Challenge for speckle is to minimize the effects of halo
Two passes ID sources - high correlation threshhold to avoid false IDFind 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
~1 milli-arcsec astrometric accuracy
RA
DE
C
Centroiding depends on brightness
Alignment depends on location
(grows towards the edge)
Central 1”x1” (not shown!)brightest stars (K=14 mag) equal contribution
Proper Motion Measurements Increased Dark Matter Density (x103), Which Ruled Out Clusters of Dark Objects
Eckart & Genzel 1997 & Ghez et al. 1998 (shown)
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
1"QuickTime™ and aGIF decompressor
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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!
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6
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!)
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
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
Speckle vs. Adaptive Optics
1” Sgr A*
Sgr A*
Speckle Image1570 sec
Klim~16 mag84 stars
Adaptive Optics Imaging75 sec
Klim~16mag170 stars
AO detected twice as many sources! (extend & spectra)Speckle better at astrometry in central 1”x1”