Thee b ac o e at t e ce te o ou black hole at the center ... · red clump depth according to stellar types • Young and blue stars form 10 y (stars ar B a cusp at the center •

Chaire Galaxies et Cosmologie

The black hole at the center of our e b ac o e at t e ce te o ouGalaxy

Françoise Combes

Our galaxy: the Milky WayBlack hole

100 000 light-years

Images in optical, infra-redg

Bar

Peanut-shapeBulge

2MASS

Zoom towards the galactic center in infraredZoom towards the galactic center, in infrared3 107 stars/pc3

Eckart et al

Shows a density cusp towards the Center50°, dust lane 300 bright stars, in 1 light-year

Galactic nucleus – in Radio

Size 4°=560pc=1700ly

Sagittarius A (Sgr A*) defines the Galactic center. It is a bright radio source (VLA) SNR = Supernova remnant

Galactic center CompositeGalactic center – Composite

The Galactic center in X-rays infrared RadioThe Galactic center in X-rays, infrared, RadioVery complex structure, filaments, supernovae bubbles, molecular

clouds, star formation?clouds, star formation?X-rays come from binaries, SNR, but also hot diffuse gas

6

Galactic center – in X-rays

Size 400 x 900 ly SgrA*

Color=energyRed= lowBlue= high Size 400 x 900 lyBlue high

The Galactic center in X-rays, by the Chandra satellite Hundreds of white dwarfs, neutron stars, stellar black holes + hot gas

(qq106°) which partly escapes 7

Astrometry and proper motions t th l ti tat the galactic center

1 light year 20 light days

VLT- adaptive optics

Adaptive optics effectsAdaptive optics effects

Corrects the effects of atmosphericturbulence

0.24ly

Galactic nucleus – in Infrared

Ra= GM/Vb2

= 0.5pc 0.05pc0 15l=0.15ly

=55 light days

The stars rapidly rotating around a point mass, with Keplerianorbits. Sagittarius A*

Animation of stars motions, in the center of the Milky Way

A complete orbit: S2p

19922010

1992

M= 4.30(±0.20)stat(0.30)sys x106 M

R = 8 28 (±0 15) (±0 29) kpc

2001SgrA*

R0= 8.28 (±0.15)stat(±0.29)sys kpc

2002

2001SgrA

Ghez et al. 2008, Gillessen et al. 2009a,b

Infrared flare of the Galactic black hole

1.7microns, NACO, VLT, 30min, May 2003

Discovery of a gas cloud in 2011Discovery of a gas cloud in 2011m

/s)

esse

(km

Vite

Distance to the black hole in arcsec (=0.1 ly)

Orbit of the gas5cloud (10-5 Mo)

137 years of period

How did the gas arrive there?

Destruction of a star by tidal forces?

Simulations of thestretch of the gas cloudst etc o t e gas c oudon its orbit

The last images (26 March 2015)g ( )

The cloud has survived the pericenter in May 2014The cloud has survived the pericenter in May 2014 There exists a star at the center

Interaction and merger with Andromeda

Perspectives for the Milky Way…In a few Gyrs

Merger of the black holes

gravitationnalwaves

Around SgrA*, star clusters• DBXX-YY Star clusters in the galactic center, in X-ray

CNR: circumnuclear ringMi i i l 60M2-3pc in radius

HCN in orangeI i d i

Mini-spiral 60M

Cavity 200M

CNR 106M

20

Ionizd gas in greenInclination of 20° /plane

7 104 cm-3

300K

Blue : ionized diffuse gasBlue : ionized diffuse gasCC: central cavitySupernova remnant (SNR) and halo

I d th d l lIn red, the dense moleculargas, in CNR and a seriesof clouds (EC, SC),of clouds (EC, SC),

MR: Molecular RidgeNR North Ridge, etc.

Star formationand feedback

21

and feedback

From Ferrière (2012)

Radio emission in the galactic centergAlthough L= 10-9 LEddington, SgrA* is a radio source

22Mini-spiral of ionized gas

Non-thermal radio filaments, synchrotron, yFilaments can be very thin, often perpendicular to theGalactic plane but not alwaysGalactic plane, but not alwaysDuring 20 years, the magnetic field was imagined poloïdal

But the formation of young stars is dominating

23

Processusocessus• Galactic B field, reconnections,• Ejection of ionized gas, magnetized by the black hole • Interactions with molecular clouds

• In fact, mean B field is weak, but strong turbulence, strong vorticity, g y

• Simulations: life-time of vortex 106-7 years, re-amplification of B field

• Characteristic scale 10pc

24Federrath 2015

Zoom inside the molecular ringg

25Genzel et al 2010

The paradox of young stars p y gclose to the black hole

1 light year

1

IRS16 SW (Ofpe/LBV)

The paradox of young stars close to the black hole

0

1close to the black hole

2.04 2.06 2.08 2.10 2.12 2.14 2.16 2.18 2.20

wavelength (m)

0

0.1

IRS16SE2 (WN5/6)

-0.1

2.05 2.10 2.15 2.20 2.25 2.30 2.35 2.40

wavelength (m)

1 0

0.9

1.0

1 light year2.10 2.15 2.20

The paradox of young stars close to the black holeclose to the black hole

~180 OB stars in the central parsec !

They could account for the fluxes in FIR, UV & EUV of the galactic center and for the excitation/ionisation of the HII region of SgrA West g g

1 light year

The paradox of young stars close to the black holeclose to the black hole

They show anThey show an ordered motion

1 light year

The paradox of young stars close to the BHTo form stars, one needs a cloud dense enough not to bedistroyed by tidal forces

nRoche ~6 1010 (R/0.1pc)-3 cm-3

But the gas density is much lower!

Formation in a dense accretion diskStars rejuvenated by collision?Migration after formation far from center?Migration, after formation far from center?

Two disks, warped and thick, or a more complex structure?

Mass versus distance at the centerEckart & Genzel 1999: the nuclear star cluster is not sufficient• Today M(BH) = 4 106M (Chatzopoulos et al 2015)Today M(BH) 4 10 M (Chatzopoulos et al 2015)

31

Distribution of stars at the center• One observes the stellar

cusp predicted by theoryS cusp predicted by theory (Young 1980) in r-7/4

• But with variations

100

csec

-2)

S-starsred clumpdepth

according to stellar types• Young and blue stars form

10

y (s

tars

arc

B

ga cusp at the center

• But not the old and red 1

ace

dens

ity

O/WR late

ones0.1

tella

r sur

fa

AGB • Old stars 106 M

• Young1.5 104 M

0.01

1 10

st AGB

32

1 10

distance from SgrA* (arcseconds)

High velocity stars• Passage of a binary star close to the BH• Prediction of Hills (1988) 1st observation in 2005 by W BrownPrediction of Hills (1988), 1 observation in 2005 by W. Brown

Stars at V > 1000km/sescape the Galaxy

Not to be confused withthe « runaway » starscoming from the diskcoming from the disk( ~400km/s)

Hyper Velocity Stars = HVSHyper Velocity Stars = HVS

33Hills 1988, Yu & Tremaine 2003, Brown 2005, 2015 

High velocity stars• Capture processus of Hills, ejection rate of 10-4/yr (Zhang 2013)

Rbt: binary tidal radius

Rt tidal radius for anindividual star

Proba of 75% of ejectionwith high velocity of the binaryY

(AU

)

of 2x 3 MoSeparation a= 0.5AU

For a peri-apse of 30 AU

(r) = 8/r2 per kpc3

34

(r) 8/r per kpc

X (AU)

HVS: hyper velocity starsyp y

Predicted ejection rate 10-4 /yr ~103 HVS in 100 kpc

Brown et al. 2005, 2006, 2008, 2010, Hills 1989, Yu & Tremaine 2003

p

Different processus produce different distributions of HVS

36

Is SgrA* the actual center of the Galaxy? VLBI precise positionningVLBI precise positionning

Here is seen the Sun motion, ,with respect to QSO SgrA* is fixed, at rest

Milli-arcsec scale

The Sun rotation around theGalactic center is in 200 MyryThe motion is seen in a fewweeks

37The position of Sagittarius A* in the sky at various epochs with respect to remote quasars, Reid et al 2004, 2009

Is SgrA* really compact? VLBI at several wavelengths 5 to 43 GHzVLBI at several wavelengths, 5 to 43 GHz

The image of SgrA* isg gsmoothed, due to the ionizedmedium along the line of sight

Effect varies in -2

At high fréquency, the scatteringIs less5 -43 GHz corresponds to6cm -0.7 cm

38The shape of Sagittarius A* in the sky at various wavelengths

Size of SgrA* at =1.3mmg

I d h b d• In red, the observed size follows thescattering in -2

Size (mas)

scattering in -2

I i t i i iIn green, intrinsic size, which begins to dominate at 1.3mmdominate at 1.3mm

Size=35 as = 0 3 AUSize 35 as 0.3 AU~size of the black hole

shadowshadow

39Wavelength (cm)

Size of SgrA* and variabilityg y

Si d• Size corrected from scattering -2

Si 0 1 1A

Major axis

Minor axisSize 0.1 mas = 1AU,

150 millions of km

The shadow of the BH, about 5 times Rs =about 5 times Rs = 40 millions km

Close to the BH!Close to the BH!

40

Shadow of the SgrA* black hole

• Different models of emission around SgrA*( d 1)• (supposed << 1)

• The shadow d t h tcorresponds to photons

swallowed by the BH (image by the lens of(image by the lens of the horizon)

• The size of the shadow is always comparable

• Falcke et al 200041

Visibility of the SgrA* black holeGas in freefall

mm-VLBIModel i=45° 0.6 mm 1.3mm

a=12

falll

Em~r-2

Falcke et al 2000Unit GM/c2 = 3as

x y

42

a=0Em=cst

GRAVITY at VLTI• The instrument will become operationnal in 2016

I f d i f b d K (2 2 ) b 4 fi d UT f 8• Infrared interferometry, band K (2.2) between 4 fixed UT of 8m, and the mobile auxiliary telescopes AT (1.8m)

• Eq i alent to an instr ment of base 180m• Equivalent to an instrument of base 180m • 5as precision, for K=15 in a few minutes

E l ti f l ti i ti t ll bit d b i ht t th l t• Exploration of relativistic stellar orbits and bright spots on the last stable orbit, etc…

43

Scenarios to probe with GRAVITY

M i i f l i h• Magnetic reconnection of clumps in the jet

• Bright spots in orbit around the BH• Bright spots in orbit around the BH• Fluctuations of the disk

44Hamaus 2008

Combination of thebeams by opticalfibres in thefibres in thetunnels

45

Event Horizon Telescope EHTEHT

• EHT is the combination of millimetric telescopes with ALMA• EHT is the combination of millimetric telescopes, with ALMAOperating in VLBI allover the world

d l f h li h i di d f bl k h lModel of the light ring, predicted for a Kerr black holea~0.9 to 0.94, i=50-60°The ring becomes an arc: deflection of light rays and relativistic

46

The ring becomes an arc: deflection of light rays, and relativisticDoppler boost (Ricarte & Dexter 2015)

Spectrum of SgrA* with jetSpectrum of SgrA with jet• The synchrotron emission becomes optically thick, at low

frequency

Falcke et al 2011

47

a c e et a 0

Spectrum of SgrA* and ADAF model • ADAF model for the quiet state of SgrA*, Yuan et al (2003)• Synchrotron (--) Compton - -) + free-free = Total (______)Synchrotron ( ) , Compton . ), + free free Total ( )

21cm 100opt UV X ADAF= Ad ection21cm 100opt UV X ADAF= AdvectionDominated Accretion Flow10-5-10-6 M/yr

flare

Inside the Bondi radius

RB ∼ 105RS ≈ 0.04 pc ≈ 1’’, B Swhere the gas thermal energy is equal to its potential energy in the gravitational field of the t e g a tat o a e d o t eblack hole

With outflow: RIAF

48

With outflow: RIAFRadiatively Inefficient Accretion Flow

Spectrum during a flare• Spectrum of SgrA* multiplied by 100 in NIR and X-rays, but

inchanged in radioNIR f h i i f l h d i• NIR comes from synchrotron emission, from electrons heated in a transient fashion, as well as X-rays (at top)

• C ld l f I C t th ( iddl )• Could also come from Inverse Compton, e- on the mm (middle)• Or Synchrotron Self Compton (e- on themselves, bottom)

49

Flares in X-rays and NIR of SgrA*• Origin is still mysterious• Matter ejection (jet) bright knots on the last stable orbit orMatter ejection (jet), bright knots on the last stable orbit, or

fluctuations on the accretion disk• Ponti et al (2015) 80 flares in X-rays( ) y• An outburst during the few months after the passage of G2 at

pericenterp• Increase by a factor 3.7 of the luminosity of SgrA* in 2013-14 • These variations of flares and luminosity is typical of quiescent y yp q

black holes

50

Flares at various • Simultaneity, or time delay: X-ray and NIR simultaneous, Radio delay• Flares more frequent in NIR. Duration typically 30min

Fl bFlares can beinterpreted as anadiabatic expansion

Pure synchrotron emissionwith n= 106.5 cm-3

+Synchrotron Self-ComptonSSC 107.5 cm-3

Turn over at 300-400GHzLorentz factor = 103

51Eckart et al 2012

Model of flare, with bright spots, g p• Simulation of light rays, and lens effects

Flux, i=70° Flux, i=90°

R 1LSOR=1LSO1.21.52 0 LSO2.0 LSO

Phase

Kerr a=0.52LSO

52

i=70°

Hamaus 2008

Correlation Radio-X and flares• An X flare per day, but with

no radiono radio• Is there a barrier for LX?

• Could be checked with the other low luminosity AGNother low luminosity AGN

53Markoff 2005

Discovery of reflected super-luminic Fe Kline

54

55Ponti et al 2010

Variability over 10 yearsy y• The emission of Fe K line varies in the

« Bridge » but not in some neighboring« Bridge », but not in some neighboring clouds

• Swept region 2 arcmin~ 15 ly in aSwept region 2 arcmin~ 15 ly in a time-scale of 2-3 yrs vapp=5c

• Not inside, since it should be ~1/r2Not inside, since it should be 1/r

FeKFeK

56Ponti et al 2010

Gas reservoirsGas reservoirsaround the center

L = 10-8 LE ?L x 160 in aflareflare

6pc

57300pc

6pc

Rodriguez-Fernandez & Combes 2008

Power increase by a factor 1000, during at least 10 yrsL/LE= 10-5 instead of 10-8

Front of light emitted 400 yrs ago

L/LE 10 instead of 10

F t f li ht itt dFront of light emitted100 yrs ago

L(SgrA*) = 1.4 1039 erg/s

58Sun100 yrs ago= 1036 erg/s today

Chandra: blue, 1 point = 300s Influence of G2? Due to better statistics

59XMM: red, after subtraction of point sources, cf the magnetar SGR 1745-2900

Ponti et al 2015

The strong flares end of 2014, are they due to G2? Here the epochs without observation have been suppressed

60

Black hole at the center of our galaxyStellar orbits, V > 1000km/s, Mass of the hole ~4 106 MoCloud G2, or gas enveloppe around a star?

Multi-wavelength environment, NIR & X-ray, binariesand compact objects diff se gasand compact objects, diffuse gas

VLBI radio mm, close to the black hole shadow, Kerr or not ?VLBI radio mm, close to the black hole shadow, Kerr or not ?Measure of the spin & relativistic effects. Test of gravity- EHT

GRAVITY in near-IR. Approaching the horizon

Still many questions: multi wavelength spectrumStill many questions: multi-wavelength spectrumOrigin of flares, last stable orbit, or jet?

61Paradox of the young stars (10Myr), HVS…