Probing the close environment of the supermassive black hole at the center

of the galaxy with GRAVITY

Probing Strong Gravity Prague, February 18, 2010

Perrin

Probing the close environment of the supermassive black hole at the center

of the galaxy with GRAVITY

Probing Strong Gravity Prague, February 18, 2010

Amorim, Araujo-Hauck, Bartko, Baumeister, Berger, Brandner, Carvas, Cassaing, Chapron, Choquet, Clénet, Collin, Dodds-Eden, Eckart,

Eisenhauer, Fédou, Fischer, Gendron, Genzel, Gillessen, Gräter, Hamaus, Haubois, Haug, Hippler, Hofmann, Hormuth, Houairi, Ihle, Jocou, Kellner,

Kervella, Klein, Kolmeder, Lacour, Lapeyrère, Laun, Lenzen, Lima, Moratschke, Moulin, Naranjo, Neumann, Patru, Paumard, Perraut, Perrin, Pfuhl, Rabien, Ramos, Reess, Rohloff, Rousset, Sevin, Straubmeier,

Thiel, Vincent, Wiest, Zanker-Smith, Ziegleder, Ziegler

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Mini spiral(50’’)

S star cluster(12-400 mas)

Circumnuclear disk(120’’)

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2-disk centralcluster

(0.5 pc-12.5’’)

The environment of Sgr A* Sgr A*

10 µas

Orbits of stars around Sgr A*

Schödel et al. (2002)

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Orbits of stars around Sgr A*

Schödel et al. (2002)

S2

Two ways of measuring strong GRAVITY effects around Sgr A*

1. Studying the closest star orbits inside the central 60 mas

Need to resolve star cluster.

Scale ~ 100 Rg = 1 mas resolution.

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(mas) (mas)

Relativistic precession in Schwarzschild metric

Sgr A* blinking

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Genzel et al. (2003)

Two ways of measuring strong GRAVITY effects around Sgr A*

2. Using flaring regions as test particles.

Measure flare motion.Scale ~ 1 Rg = 10 µas

accuracyTime scale = 10 min

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Genzel et al. (2003) Hot spot orbiting the ISCO or a more distant orbit.

Eckart et al. A&A 500, 935 (2009)

Sgr A* is quite dark

Long wavelengths are well suited.Stars are bright in the near-infrared (orbits) and instruments sensitive enough to allow for short exposures (flares).

How to get to the 1 mas resolution and 10 µas accuracy in

the near-infrared ?

One of Prague famous astronomers,

Tycho Brahe, found the solution:

use a large instrument

Measurement accuracy scales as the

reciprocal of the size of the instrument.

Use the 4 VLT in interferometric mode

~ 140 m

Resolution: 3 mas @ 2.2 µm(K band)

Build GRAVITY !(General Relativity viA Vlt InterferomeTrY)

Interferometric imaging in the near-

infrared worksAltair

Monnier et al. 2007

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Cep

Zhao et al. 2008

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Betelgeuse

Haubois et al. 2009

Mira

Perrin et al. in prep

Cyg

Lacour et al. 2009

Imaging the closest stars with GRAVITY

One-night observation image:

Paumard et al. (2005)

mas mas

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Point Spread Function Dirty 6-star image After deconvolution

mas mas mas

Imaging the closest stars with GRAVITY

Orbits after 15 months of observation:

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Paumard et al. (2005)

mas mas

1 mas = 100 Rg

Schwarzschild advance of pericenter is detected

Lens-Thirring and Quadrupole Precession

Testing the no-hair theorem

Orbital plane precession (precession of the angular momentum vector around the spin of the black hole)

Will (2008)

Wheeler’s “black holes have no hair” theorem: a BH is fully characterized by only three parameters: Mass M, Spin J, Electric chargeIn particular Quadrupole Q2 = -J2 / M

1 year orbit, e=0.9

Measurement of frame dragging precession may be feasible after a few years for orbits in the radial range between 0.2 mpc and 1 mpc (5 and 25 mas)Merritt et al. (2009)

Referencestar Sgr A*

The measured distance between the two interferograms is:

opd = B.S

Hence:

S = opd/B

A 5 nm accuracy on opd with a 100 m baseline yields a 10 µas accuracy on S.

S

opd0

opd = B.S

Narrow angle interferometric astrometry

Performance analysis:• errors from atmosphere, baseline, noise, pupil position, etc …• 23 µas per baseline• 13 µas with 6 baselines

Muterspaugh et al. (2006): “… the 20μas level has been demonstrated …”

110 m and 87 m baselines40 cm telescopes

The Palomar Testbed Interferometer did it !

See Frédéric Vincent’s talk

Measuring the last stable orbit

Newtonprimary GR image

totalimage

secondary GR image

Paumard et al. (2005)

Reference sources for GRAVITY near Sgr A*

2”

K ~ 9.6

Sgr A*

K ~ 15 - 18

Reference source for adaptive optics

Reference sources - IRS 16 - for interferometry (imaging and astrometry)

GRAVITY is in the design …

Adaptive Optics Wavefront Sensor

Metrology Laser Injection

Fringe Tracking Spectrometer

Science Spectrometer

IO Beam combiner

… and prototyping phase

Fibered delay line

4-telescope integrated optics beam combiner

Laser metrology system

Metrology test on VLT secondary mirror at Paranal

Where we are standing and where we are going

Preliminary Design Review took place in december 2009

Final Design Review is scheduled for June 2011

First tests at Paranal : 2014

Hopefully first results on Sgr A* in 5 years from now.