The Galactic Center a unique laboratory

Transcription

1 The Galactic Center a unique laboratory Stefan Gillessen, Reinhard Genzel, Frank Eisenhauer, Thomas Ott, Katie Dodds-Eden, Oliver Pfuhl, Tobias Fritz

2 The GC is highly obscured Radio IR X-ray

3

4 Extremely dense star cluster 30 = 4 lightyears Schödel (ISAAC, VLT)

5 The central 20 : Seeing limited

6 AO 1

7 AO 2

8 Really a big step forward: AO

9 Strehl ratio 40% NACO, HKL color composite

10 Diffractionlimited images

11 100 stars in the central arcsecond S2

12 Observed Tracks

13 Observed Tracks

14 Observed Tracks

15 Observed Tracks

16 Observed Tracks

17 Result of 16 years monitoring

18 Currently: 30 orbits known S2 20 stars shown, Gillessen+ 2009

19 Stars move on Keplerian orbits Real Data (!) Model

20 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

21 Radial velocity data & fit

22 M = M in 100 AU 30 AU

23 The radio view Sgr A*

24 Sgr A* and mass coincide to within 2mas Reid+ 2007

25 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

26 1.3mm VLBI

27 Sgr A* is very small: r < 4 Rs 1 AU Shen et al Bower et al Doeleman et al (VLBI) Schwarzschild radius: 10 µas

28 Not yet real imaging but a size measurement SMT-CARMA JCMT-CARMA SMT-JCMT Falcke et al. 2000, Gammie et al., Broderick et al.,...

29 The dream for the future Broderick & Loeb 2006

30 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

31 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

32 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)

33 How sensitive are we to relativistic effects? Wrong: For given orbit compare Keplerian & Relativistic data Right: For given data compare Keplerian & Relativistic fit

34 Zucker et al Special relativistic effects for S2 easily observable with today s technique Romer effect Transverse Doppl Gravitational reds Spectroscopy error band

35 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

36 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

37 Surprisingly, the S-stars are young Eisenhauer+ 2005

38 The spectrum of S2 really is that of an ordinary main sequence B2 star Martins+ 2008

39 S-stars: A Paradox of Youth Ghez Star formation so close the MBH impossible Stars are too young to have migrated from further out

40 For r > 1 : Hard to measure accelerations r < 1 r > 1 x y vx vy vz a 2D (a, e, i, ω, Ω, t)

41 The traces for the young, clockwise moving stars intersect in one point orientation of orbital angular momentum Bartko Lu+ 2009

42 (Most of) the CW moving O/WR-stars revolve in a disk

43 The disk has a topheavy IMF Bartko+ 2010

44 Disk orbits have <e> 0.4 Bartko+ 2009

45 cumulative PDF Eccentricities: S-stars Disk stars early-type S-stars: 6 disk stars: cpdf e e = 0.34 ± eccentricity

46 Orbital planes: S-stars disk stars

47 Two paradoxes of Youth O/WR stars B stars 1 < R < 10 age 6 Myr R< 1 age 10 8 yr

48 Portegies-Zwart 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?

49 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

50 Two paradoxes of Youth O/WR stars B stars 1 < R < 10 age 6 Myr R< 1 age 10 8 yr

51 The S-stars puzzle is hard In-situ formation Fast transport Rejuvination Critical density ~ M/R g/cm 3 (for R = 0.5 ) Core of clump in molecular cloud 10 6 /cm 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

52 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?

53 Both scenarios direct stars to large <e> orbits within few Myr Massive Perturbers Migration Perets, Hopman & Alexander 2007 Löckmann, Baumgardt & Kroupa (2008)

54 Both resonant and IMBH-aided relaxation are fast enough Massive Perturbers Migration Alexander 2008 Merritt, Gualandris, Mikkola 2009

55 The eccentricity distribution might be the clue Migration Perets More orbits Massive Perturbers?

56 Dynamics of the star cluster Schödel+ 2009

57 The old stars: missing towards the center? Buchholz+ 2009, Do+ 2009, Bartko+ 2010

58 Standard game : σ mass

59 Cluster rotation: In v rad and proper motion

60 The next puzzle: Sgr A* should be bright - but is not Limit: Eddington luminosity radiation pressure = gravitation S2 L = x 5 x erg/s = x L pos of Sgr A* S17

61 Sgr A* is dim at all wavelengths: ~ 10-8 < 200 L

62 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% % 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

63 Observational constraint: Only 0.4% of the energy can be released from a surface Broderick & Narayan 2009 Allowed

64 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 Gillessen et al. 2006, Hornstein et al Dodds-Eden et al. 2009

65 Sgr A* is a source that undergoes bursts Dodds-Eden et al. in prep.

66 Continuous variability & a tail of flares Dodds-Eden et al. in prep.

67 Sgr A* is the only strongly polarized source in the GC Sum of two polarizations Difference of two polarizations Trippe 2007

68 Flares are synchrotron emission of transiently heated electrons

69 Flares often are quasi-periodic Minute-timescale variability: size < 2 light min = 30 µas

70 Basic picture: Orbiting hot spot Magnetic field winds up Reconnection event Energy from B heats electrons

71 Sgr A* has spin parameter > 0.5 if QPO is due to orbital dynamics allowed shortest QPO observed: 17min not allowed Genzel+ 2003

72 Simultaneous X-ray flares happen on the same timescale Dodds-Eden et al. 2009, Porquet et al Eckart et al. 2006

73 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

74 Dodds-Eden et al. arxiv/ Energy for flares might come from B 2 new idea : no submm flares but submm dips

75 Limits and beyond

76 What is limiting astrometry today?

77 Fritz S2 like stars: Distortions Fainter stars: Halo noise

78 Halo noise

79 Assume, we continue what we are doing. How well do we do then? NACO: Astrometry with 300 µas SINFONI: Spectroscopy with 15 km/s

80 2020: 3σ detection of GR precession possible

81 Imagine we could zoom in further Expected in central 100 mas: -- ~5 stars -- K = mag Orbital Period: 1 year Precession: ~ few per year

82 The next step in angular resolution: NIR-Interferometry

83 Simulations show: VLTI can observe such stars Cleaned image Simulated Image PSF: 4 UTs, K, 9 hrs

84 A factor 15 more powerful than the VLT VLT (8m): R = 50 mas Δx = 150 µas VLTI (120m): R = 3 mas Δx = 10 µas

85 Flares orbit with amplitudes > 50µas 60 µas

86 GR effects dominate inner accretion zone

87 1 flare observed shows wobble, 10 flares coadded show GR 1 flare 10 flares

88 Dual feed, 4-telescope, adaptive optics assisted, fringe tracking beam combiner instrument

89 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

90 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

91 The physics perspective: Tests of GR Psaltis 2004

92 Dynamical tests: Low curvature, low mass Dynamical Tests

93 LIGO: Supernovae & gravitational waves

94 LISA: SMBH mergers

95 Fuzzy laboratory Difficult to get to dynamics submm-shadow of MBH in GC Falcke et al. 2000

96 VLTI in GC: GRAVITY 2 effects in stellar orbits

97 VLTI in GC: GRAVITY Lense-Thirring precession of central cusp stars

98 Flares at last stable orbit VLTI in GC: GRAVITY

99