Composition of the galactic center star cluster

Buchholz, Rainer; Schödel, R.; Eckart, A.

doi:10.1051/0004-6361/200811497

Cited by 174 publications

(413 citation statements)

References 80 publications

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“…One would expect from theoretical models that, if relaxed, the stellar density near the SMBH should be steeply-rising and form a Bahcall & Wolf (1976) cusp. In contrast, observations by Do et al (2009); Buchholz et al (2009); Bartko et al (2010) show that the distribution of old stars near the SMBH appears to have a core. Understanding the nuclear star cluster dynamics may therefore give useful constraints on the mechanisms by which it formed and evolved (Merritt 2010).…”

Section: Introductionmentioning

confidence: 87%

“…The minimum allowable inner slope for a spherical system with a black hole to have a positive DF is γ = 0.5. From the data it appears that the Galactic NSC instead has a core (Buchholz et al 2009;Fritz et al 2014), with the number density possibly even decreasing very close to the center (r < 0.3 pc). This is far from the expected Bahcall-Wolf cusp, indicating that the NSC is not fully relaxed.…”

Section: Evolution Of the Nscmentioning

confidence: 95%

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The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass

Chatzopoulos¹,

Fritz²,

Gerhard³

et al. 2014

Monthly Notices of the Royal Astronomical Society

177

216

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We derive new constraints on the mass, rotation, orbit structure and statistical parallax of the Galactic old nuclear star cluster and the mass of the supermassive black hole. We combine star counts and kinematic data from Fritz et al. (2014), including 2'500 line-of-sight velocities and 10'000 proper motions obtained with VLT instruments. We show that the difference between the proper motion dispersions σ l and σ b cannot be explained by rotation, but is a consequence of the flattening of the nuclear cluster. We fit the surface density distribution of stars in the central 1000 ′′ by a superposition of a spheroidal cluster with scale ∼ 100 ′′ and a much larger nuclear disk component. We compute the self-consistent two-integral distribution function f (E, L z ) for this density model, and add rotation self-consistently. We find that: (i) The orbit structure of the f (E, L z ) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v l -histograms. (ii) This requires an axial ratio near q 1 = 0.7 consistent with our determination from star counts, q 1 = 0.73 ± 0.04 for r < 70 ′′ . (iii) The nuclear star cluster is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the nuclear star cluster mass, black hole mass, and distance M * (r < 100 ′′ ) = (8.94±0.31| stat ±0.9| syst )×10 6 M ⊙ , M • = (3.86±0.14| stat ±0.4| syst )×10 6 M ⊙ , and R 0 = 8.27±0.09| stat ±0.1| syst kpc, where the estimated systematic errors account for additional uncertainties in the dynamical modeling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A * strongly reduces the degeneracy between black hole mass and Galactic centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al. (2009) gives M • = (4.23±0.14)×10 6 M ⊙ and R 0 = 8.33±0.11 kpc.

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Section: Introductionmentioning

confidence: 87%

Section: Evolution Of the Nscmentioning

confidence: 95%

The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass

Chatzopoulos¹,

Fritz²,

Gerhard³

et al. 2014

Monthly Notices of the Royal Astronomical Society

177

216

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“…Bow shocks are caused by bright emission-line stars that either have strong winds or move through the minispiral (e.g. Tanner et al 2005Tanner et al , 2006Geballe et al 2006;Viehmann et al 2006;Perger et al 2008;Buchholz et al 2009;Sanchez-Bermudez et al 2014).…”

Section: Featureless Spectramentioning

confidence: 99%

KMOS view of the Galactic centre

Feldmeier-Krause

Neumayer

Schödel

et al. 2015

A&A

Self Cite

106

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Context. The Galactic centre hosts a crowded, dense nuclear star cluster with a half-light radius of 4 pc. Most of the stars in the Galactic centre are cool late-type stars, but there are also > ∼ 100 hot early-type stars in the central parsec of the Milky Way. These stars are only 3−8 Myr old. Aims. Our knowledge of the number and distribution of early-type stars in the Galactic centre is incomplete. Only a few spectroscopic observations have been made beyond a projected distance of 0.5 pc of the Galactic centre. The distribution and kinematics of earlytype stars are essential to understand the formation and growth of the nuclear star cluster. Methods. We cover the central >4 pc 2 (0.75 sq. arcmin) of the Galactic centre using the integral-field spectrograph KMOS (VLT). We extracted more than 1000 spectra from individual stars and identified early-type stars based on their spectra. Results. Our data set contains 114 bright early-type stars: 6 have narrow emission lines, 23 are Wolf-Rayet stars, 9 stars have featureless spectra, and 76 are O/B type stars. Our wide-field spectroscopic data confirm that the distribution of young stars is compact, with 90% of the young stars identified within 0.5 pc of the nucleus. We identify 24 new O/B stars primarily at large radii. We estimate photometric masses of the O/B stars and show that the total mass in the young population is > ∼ 12 000 M . The O/B stars all appear to be bound to the Milky Way nuclear star cluster, while less than 30% belong to the clockwise rotating disk. We add one new star to the sample of stars affiliated with this disk. Conclusions. The central concentration of the early-type stars is a strong argument that they have formed in situ. An alternative scenario, in which the stars formed outside the Galactic centre in a cluster that migrated to the centre, is refuted. A large part of the young O/B stars is not on the disk, which either means that the early-type stars did not all form on the same disk or that the disk is dissolving rapidly.

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“…The flux densities were measured by aperture photometry with circular apertures of 40 mas radius and corrected for extinction, using A K = 2.46 (Scoville et al 2003;Schödel et al 2007Schödel et al , 2011Buchholz et al 2009). Possible uncertainties in the extinction of a few tenths of a magnitude do not influence the general results obtained in this paper.…”

Section: The Nir Observationsmentioning

confidence: 99%

Millimeter to X-ray flares from Sagittarius A*

Eckart

García-Marín

Vogel

et al. 2012

A&A

Self Cite

146

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Context. We report on new simultaneous observations and modeling of the millimeter, near-infrared, and X-ray flare emission of the source Sagittarius A* (SgrA*) associated with the super-massive (4 × 10 6 M ) black hole at the Galactic center. Aims. We study the applicability of the adiabatic synchrotron source expansion model and study physical processes giving rise to the variable emission of SgrA* from the radio to the X-ray domain. Methods. Our observations were carried out on 18 May 2009 using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope, the ACIS-I instrument aboard the Chandra X-ray Observatory, the LABOCA bolometer at the Atacama Pathfinder EXperiment (APEX), and the CARMA mm telescope array at Cedar Flat, California. Results. The X-ray flare had an excess 2−8 keV luminosity between 6 and 12×10 33 erg s −1 . The observations reveal flaring activity in all wavelength bands that can be modeled as the signal from an adiabatically expanding synchrotron self-Compton (SSC) component.Modeling of the light curves shows that the sub-mm follows the NIR emission with a delay of about three-quarters of an hour with an expansion velocity of about v exp ∼ 0.009c. We find source component sizes of around one Schwarzschild radius, flux densities of a few Janskys, and spectral indices α of about +1 (S (ν) ∝ ν −α ). At the start of the flare, the spectra of the two main components peak just short of 1 THz. To statistically explain the observed variability of the (sub-)mm spectrum of SgrA*, we use a sample of simultaneous NIR/X-ray flare peaks and model the flares using a synchrotron and SSC mechanism. Conclusions. These parameters suggest that either the adiabatically expanding source components have a bulk motion larger than v exp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of SgrA*. For the bulk of the synchrotron and SSC models, we find synchrotron turnover frequencies in the range of 300−400 GHz. For the pure synchrotron models, this results in densities of relativistic particles of the order of 10 6.5 cm −3 and for the SSC models, the median densities are about one order of magnitude higher. However, to obtain a realistic description of the frequency-dependent variability amplitude of SgrA*, models with higher turnover frequencies and even higher densities are required.

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Composition of the galactic center star cluster

Cited by 174 publications

References 80 publications

The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass

The old nuclear star cluster in the Milky Way: dynamics, mass, statistical parallax, and black hole mass

KMOS view of the Galactic centre

Millimeter to X-ray flares from Sagittarius A*

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