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The structure of the nuclear stellar cluster of the Milky Way

Rainer Schödel. The structure of the nuclear stellar cluster of the Milky Way. IAU Assembly Prague August 2006. and A. Eckart, R. Genzel,D. Merritt, T. Alexander, A. Sternberg, J. Moultaka, T. Ott, C. Straubmeier, F. Kul, L. Meyer. cusp. collisions destroy stars. large scale cluster.

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The structure of the nuclear stellar cluster of the Milky Way

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  1. Rainer Schödel The structure of the nuclear stellar cluster of the Milky Way IAU Assembly Prague August 2006 and A. Eckart, R. Genzel,D. Merritt, T. Alexander, A. Sternberg, J. Moultaka, T. Ott, C. Straubmeier, F. Kul, L. Meyer

  2. cusp collisions destroy stars large scale cluster Amaro-Seoane et al. (2004) 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 cusp GC is only such system that can be resolved into individual stars.

  3. First AO results with NACO/VLT ρ*( R )=3x107 (R/arcsec)-1.4M pc-3 Cusp large scale cluster Genzel et al. 2003 distance from Sgr A* (arcsecs)

  4. 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!

  5. Seeing Limited Imaging IRS 7 640 light years 150” / 18 light years N Sgr A* FOV of AO observations E ISAAC/VLT 2.09 m FWHM ~0.4” (color image from J+2.09 m imaging) Point source extraction and background estimation with StarFinder (Diolaiti et al., 2000)

  6. Background light density inner 2.5 pc broken power-law: good fit!

  7. AO Imaging Data IRS 7 Sgr A* IRS 13 IRS 16 NACO/VLT adaptive optics resolution ~0.06” ~40”40” FOV ~10000 stars with magK ≤ 17.5

  8. “mini-cavity” Extinction in the Central pc IRS 7 Extinction derived from narrow-band imaging (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) Sgr A* IRS 13 Outflow directed SSW from IRS 16/Sgr A*? (see poster by K. Muzic) IRS 16 Schödel et al., subm. to A&A

  9. 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” appear to be present in the cluster. One of them is the well- known co-moving group IRS 13E. (see Maillard et al., 2004; Schödel et al., 2005) Sgr A* Schödel et al., subm. to A&A

  10. Stellar surface number density inner 0.5 pc rbreak = 7”1” broken power-law: good fit! cusp = 0.250.1 cluster = 0.60.1 single power law: no satisfying fit! magK= 9.75 - 17.75

  11. Horizontal branch/red clump stars young/high mass stars Dominated by low mass/old stars magK= 14.75 - 15.75 see also Genzel et al. (2003), Paumard et al. (2006)

  12. 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.

  13. Thank you for your attention!

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