The Galactic Center a unique laboratory Stefan Gillessen, Reinhard Genzel, Frank Eisenhauer, Thomas Ott, Katie Dodds-Eden, Oliver Pfuhl, Tobias Fritz
The GC is highly obscured Radio IR X-ray
Extremely dense star cluster 30 = 4 lightyears Schödel+ 2006 (ISAAC, VLT)
The central 20 : Seeing limited
AO 1
AO 2
Really a big step forward: AO
Strehl ratio 40% NACO, HKL color composite
Diffractionlimited images
100 stars in the central arcsecond S2
Observed Tracks
Observed Tracks
Observed Tracks
Observed Tracks
Observed Tracks
Result of 16 years monitoring
Currently: 30 orbits known S2 20 stars shown, Gillessen+ 2009
Stars move on Keplerian orbits Real Data (!) Model
2008 1992 S2: the showcase star AO, 8/10m pose err: < 500µas Speckle 3.5m pos err: 2 mas VLT & Keck data suitably combined (Gillessen et al. 2009, ApJL, 707, 114) 2002 Speckle 10m pos err: 1 mas period: 15.9 years semi major axis: 125 mas eccentricity 0.88 M = 4.30 ± 0.06 ± 0.35 x 10 6 M R 0 = 8.28 ± 0.15 ± 0.30 kpc
Radial velocity data & fit
M = 4 10 6 M in 100 AU 30 AU
The radio view Sgr A*
Sgr A* and mass coincide to within 2mas Reid+ 2007
Sgr A* must be very heavy perfectly linear motion reflex motion of Sun (~200 km/s) intrinsic motion gal. l : -7.2 ± 8.5 km/s gal. b: -0.4 ± 0.9 km/s Sgr A* is much heavier than surrounding stars > 4 x 10 5 M Reid 2007, 2009
1.3mm VLBI
Sgr A* is very small: r < 4 Rs 1 AU Shen et al. 2005 Bower et al. 2006 Doeleman et al. 2008 (VLBI) Schwarzschild radius: 10 µas
Not yet real imaging but a size measurement SMT-CARMA JCMT-CARMA SMT-JCMT Falcke et al. 2000, Gammie et al., Broderick et al.,...
The dream for the future Broderick & Loeb 2006
Sgr A* is a MBH Compact mass from stellar orbits: 4 x 106 M Radio Sgr A* coincides with the mass Radio Sgr A* moves in a straight line Radio Sgr A* is < 1 AU
Mass and R 0 are highly correlated pure astrometry: M ~ R 3 astrometry + radial velocities: M ~ R 2.0 only radial velocities: M ~ R 0
How well do we know that potential is that of a point mass? Measured fraction of mass inside of S2 orbit that is not pointlike (100 AU < r < 2000 AU) Theory: Drain Limit Number of XRBs: (Alexander & Livio 2004) (Muno 2005) Theory: Dynamical Modeling 98% of 4 million suns in 100 AU (Cosmological) Dark Matter (Hopman & Alexander 2006) (Gnedin & Primack 2004, Vasiliev & Zelnikov 2008)
How sensitive are we to relativistic effects? Wrong: For given orbit compare Keplerian & Relativistic data Right: For given data compare Keplerian & Relativistic fit
Zucker et al. 2006 Special relativistic effects for S2 easily observable with today s technique Romer effect Transverse Doppl Gravitational reds Spectroscopy error band
Astrometric deviations are much harder to detect x (µas) Dec < 0.5 mas RA best fitting Keplerian orbit minus best fitting relativistic orbit t (yr) Schwarzschild correction to 1/r potential
Rubilar & Eckart (2001) Another way to look at it: Measure pericenter shift explicitely expected: Δω = 0.22 per revolution (16 years) measured: Δω = 0.84 in 18 years
Surprisingly, the S-stars are young Eisenhauer+ 2005
The spectrum of S2 really is that of an ordinary main sequence B2 star Martins+ 2008
S-stars: A Paradox of Youth Ghez+ 2003 Star formation so close the MBH impossible Stars are too young to have migrated from further out
For r > 1 : Hard to measure accelerations r < 1 r > 1 x y vx vy vz a 2D (a, e, i, ω, Ω, t)
The traces for the young, clockwise moving stars intersect in one point orientation of orbital angular momentum Bartko+ 2009 Lu+ 2009
(Most of) the CW moving O/WR-stars revolve in a disk
The disk has a topheavy IMF Bartko+ 2010
Disk orbits have <e> 0.4 Bartko+ 2009
cumulative PDF Eccentricities: S-stars Disk stars 1.0 17 early-type S-stars: 6 disk stars: 0.8 0.6 3.8 0.8 cpdf e e = 0.34 ± 0.18 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 eccentricity
Orbital planes: S-stars disk stars
Two paradoxes of Youth O/WR stars B stars 1 < R < 10 age 6 Myr R< 1 age 10 8 yr
Portegies-Zwart+ 2005 Idea I: Cluster in-spiral Gerhard 2001 Problems: To spiral in within 6 Myr, cluster mass would exceed stellar mass seen by far large IMBH would be required surface density profile of disk is too steep Where are the B-stars?
Bonnel & Rice 2008, Hobbs & Nayakshin 2008 Idea II: In-situ formation in infalling gas cloud More promising: critical density for star formation is reached easily moderate eccentricities IMF gets top-heavy warps possible
Two paradoxes of Youth O/WR stars B stars 1 < R < 10 age 6 Myr R< 1 age 10 8 yr
The S-stars puzzle is hard In-situ formation Fast transport Rejuvination Critical density ~ M/R 3 2 10 11 g/cm 3 (for R = 0.5 ) Core of clump in molecular cloud 10 6 /cm 3 2 10 18 g/cm 3 cosmic pool game fast relaxation processes Migration from O/WR star disks? Stars are actually old but look young stripping of giants, S-stars are the hot cores Spectrum of S2
Currently a Hills-like mechanism seems to be preferred Massive Perturbers Scattering of field binaries into near loss cone orbits due to Massive Perturbers Tidal break-up of binaries at pericenter passage Hills 1988 Fast Relaxation of orbit to match observed properties Resonant? Migration Formation of B-stars in (former) disks Interactions to increase <e> 2 nd disk, stellar cusp, IMBH Interactions to lower <a> planetary migration Fast Relaxation IMBH?
Both scenarios direct stars to large <e> orbits within few Myr Massive Perturbers Migration Perets, Hopman & Alexander 2007 Löckmann, Baumgardt & Kroupa (2008)
Both resonant and IMBH-aided relaxation are fast enough Massive Perturbers Migration Alexander 2008 Merritt, Gualandris, Mikkola 2009
The eccentricity distribution might be the clue Migration Perets+ 2009 More orbits Massive Perturbers?
Dynamics of the star cluster Schödel+ 2009
The old stars: missing towards the center? Buchholz+ 2009, Do+ 2009, Bartko+ 2010
Standard game : σ mass
Cluster rotation: In v rad and proper motion
The next puzzle: Sgr A* should be bright - but is not Limit: Eddington luminosity radiation pressure = gravitation S2 L = x 5 x 10 44 erg/s = x 10 11 L pos of Sgr A* S17
Sgr A* is dim at all wavelengths: ~ 10-8 < 200 L
Sgr A* does not have a surface Accretion flow models: Accretion rate is well-determined from radio data: ~ 5 x 10-8 M /yr radio flux: 1% - 0.01% of the gravitational binding energy is released before surface / horizon is reached > 99% of the energy reaches surface / horizon Assume a surface: Accretion powered source would have to approach black body in equilibrium Size of surface limited by submm measurement Broderick & Narayan 2009 Such a small object with given infall rate would shine in the M/NIR M/NIR-Observations in quiescence constrain allowable surface
Observational constraint: Only 0.4% of the energy can be released from a surface Broderick & Narayan 2009 Allowed
Sometimes, SgrA* flares up in the NIR Typically one flare per night Lasts ~ 90 min Much redder than the stars Genzel et al. 2003, Eisenhauer et al. 2005 Gillessen et al. 2006, Hornstein et al. 2007 Dodds-Eden et al. 2009
Sgr A* is a source that undergoes bursts Dodds-Eden et al. in prep.
Continuous variability & a tail of flares Dodds-Eden et al. in prep.
Sgr A* is the only strongly polarized source in the GC Sum of two polarizations Difference of two polarizations Trippe 2007
Flares are synchrotron emission of transiently heated electrons
Flares often are quasi-periodic 16.6.2003 3.4.2007 Minute-timescale variability: size < 2 light min = 30 µas
Basic picture: Orbiting hot spot Magnetic field winds up Reconnection event Energy from B heats electrons
Sgr A* has spin parameter > 0.5 if QPO is due to orbital dynamics allowed shortest QPO observed: 17min not allowed Genzel+ 2003
Simultaneous X-ray flares happen on the same timescale Dodds-Eden et al. 2009, Porquet et al. 2009 Eckart et al. 2006
X-ray origin is under discussion IC SSC seed photon region < 0.1 R S B > 3000G synchrotron synchr. + cool IR color wrong Dodds-Eden et al. 2009
Dodds-Eden et al. arxiv/1005.0389 Energy for flares might come from B 2 new idea : no submm flares but submm dips
Limits and beyond
What is limiting astrometry today?
Fritz+ 2009 S2 like stars: Distortions Fainter stars: Halo noise
Halo noise
Assume, we continue what we are doing. How well do we do then? NACO: Astrometry with 300 µas SINFONI: Spectroscopy with 15 km/s
2020: 3σ detection of GR precession possible
Imagine we could zoom in further Expected in central 100 mas: -- ~5 stars -- K = 17..19 mag Orbital Period: 1 year Precession: ~ few per year
The next step in angular resolution: NIR-Interferometry
Simulations show: VLTI can observe such stars Cleaned image Simulated Image PSF: 4 UTs, K, 9 hrs
A factor 15 more powerful than the VLT VLT (8m): R = 50 mas Δx = 150 µas VLTI (120m): R = 3 mas Δx = 10 µas
Flares orbit with amplitudes > 50µas 60 µas
GR effects dominate inner accretion zone
1 flare observed shows wobble, 10 flares coadded show GR 1 flare 10 flares
Dual feed, 4-telescope, adaptive optics assisted, fringe tracking beam combiner instrument
Concept for GRAVITY is advanced Object Phase reference Wavefront reference Telescope #1 2 FoV Metrology fringes on secondary Metrology Telescope #2 Metrology camera Starlight Star separator Switchyard 2 FoV Delay line MACAO DM IR wavefront sensor Tip Tilt Guider dopd control Beam Combiner Instrument IO Beam combiner Spectrometer A D Fiber coupler Polarization control OPD control Phase Shifter Metrology Laser B C
GRAVITY guaranteed to be a success We know: Flare must be a small region in space Flare must be close to event horizon Everything moves with nearly c Flares last an hour travelled path: independent of exact model
The physics perspective: Tests of GR Psaltis 2004
Dynamical tests: Low curvature, low mass Dynamical Tests
LIGO: Supernovae & gravitational waves
LISA: SMBH mergers
Fuzzy laboratory Difficult to get to dynamics submm-shadow of MBH in GC Falcke et al. 2000
VLTI in GC: GRAVITY 2 effects in stellar orbits
VLTI in GC: GRAVITY Lense-Thirring precession of central cusp stars
Flares at last stable orbit VLTI in GC: GRAVITY