The structure of the nuclear stellar clusterof the Milky Way

andA. Eckart, R. Genzel,D. Merritt, T. Alexander, A. Sternberg,

J. Moultaka, T. Ott, C. Straubmeier, F. Kul, L. Meyer

Rainer Schödel

IAU AssemblyPragueAugust 2006

Stellar Cusps around Supermassive Black Holes

age of a galaxy nucleus > relaxation time steady state solution of Fokker-Planck equation predicts stellar cusp

• single mass: ~r -7/4 (Bahcall & Wolf, 1976; Lightman & Shapiro 1977)

• multiple masses: ~r -3/2-7/4 (depending on stellar mass)

• size of cusp ~0.1-0.2 rinfluence, BH

relaxation time in center of Milky Way a few 109 yr we expect to observe a stellar cuspGC is only such system that can be resolved into individual stars.

Amaro-Seoane et al. (2004)

cusp

collisions destroy stars

large scale cluster

distance from Sgr A* (arcsecs)

First AO results with NACO/VLT

0.1

1

10

100

0.1 1 10 1000.1

1

10

100

isothermal cluster modelbroken power law, α1 =1.37, α2=2NACO Ks≤17, correctedSHARP, K≤15, *5.5NAOS/CONICA, H≤19.8, low confusion

radius from SgrA* (arcsecs)

ρ*( R )=3x107 (R/arcsec)-1.4M pc-3

Genzel et al. 2003

Cusp

large scale cluster

New Analysis of GC Stellar Cluster• use of uniform data sets: seeing limited + AO assisted

• extent of cusp/location of break radius?

• spatially variable extinction taken into account

• Refined methods: PSF fitting with spatially variable PSF analysis of number counts and background light no binning of counts

• 2D structure of cluster, symmetry of cluster

Above all ... to convince the remaining skeptics!

640

light

yea

rsSeeing Limited Imaging

ISAAC/VLT 2.09 mFWHM ~0.4”(color image from J+2.09 m imaging)

150”

/ 1

8 lig

ht y

ears

IRS 7

Sgr A*N

EFOV of AOobservations

Point source extractionand background estimation with StarFinder(Diolaiti et al., 2000)

Background light density inner 2.5 pc

broken power-law: good fit!

AO Imaging Data

NACO/VLTadaptive opticsresolution ~0.06”

~40”40” FOV

~10000 stars with magK ≤ 17.5

IRS 16

IRS 13

IRS 7

Sgr A*

Extinction in the Central pc

IRS 16

IRS 13

IRS 7

Sgr A*

“mini-cavity”

Extinction derivedfrom narrow-bandimaging (2.00, 2.06, 2.24,& 2.27 m, = 0.06 m);assumption stars are blackbodies with T = 5000 K, AK~-1.75

AK = 1.8 - 3.6 mag(contours AK = 0.2 mag)

Outflow directed SSWfrom IRS 16/Sgr A*?(see poster by K. Muzic)

Schödel et al., subm. to A&A

2D extinction corrected density map

Adaptively smoothed:40 stars for each pixel

smoothing radius~0.5” near Sgr A*~1” near edge of image

Some “clumps” appearto be present in the cluster.One of them is the well-known co-moving groupIRS 13E.(see Maillard et al., 2004;Schödel et al., 2005)

Schödel et al., subm. to A&A

Sgr A*

Stellar surface number density inner 0.5 pc

magK= 9.75 - 17.75

single power law: no satisfying fit!

rbreak = 7”1”

broken power-law: good fit!

αcusp = 0.250.1αcluster = 0.60.1

Horizontal branch/red clump stars

magK= 14.75 - 15.75

Dominated by low mass/old stars

young/high mass stars

see also Genzel et al. (2003), Paumard et al. (2006)

Summary

1. Extinction: New method for deriving extinction Indications for outflow? No significant influence on number counts

1. Extinction: New method for deriving extinction Indications for outflow? No significant influence on number counts

2. Structure of Cluster: Clearly a broken power-law! Presence of “clumps” - relation to recent star formation?

1. Extinction: New method for deriving extinction Indications for outflow? No significant influence on number counts

2. Structure of Cluster: Clearly a broken power-law! Presence of “clumps” - relation to recent star formation?

3. Cusp: Detected with high significance; shallower than Bahcall-Wolf (~R-0.75) solution (due to mixing of different masses) Extent of cusp: ~7” (~0.25 pc) Effects of mass segregation.

1. Extinction: New method for deriving extinction Indications for outflow? No significant influence on number counts

2. Structure of Cluster: Clearly a broken power-law! Presence of “clumps” - relation to recent star formation?

3. Cusp: Detected with high significance; shallower than Bahcall-Wolf (~R-0.75) solution (due to mixing of different masses/classes) Extent of cusp: ~7” (~0.25 pc) Effects of mass segregation.Results are in good agreement with theory. Models can now be fine-tuned with the exact values for the GC cluster.

Thank you for

your attention!