The nuclear star cluster of the Milky Way: proper motions and mass

By R. Schödel, David Merritt, A. Eckart. 2009

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Context. Nuclear star clusters (NSCs) are located at the photometric and dynamical centers of the majority of galaxies. They are among the densest star clusters in the Universe. The NSC in the Milky Way is the only object of this class that can be resolved into individual stars. The massive black hole Sagittarius A* is located at the dynamical center of the Milky Way NSC. Aims. In this work we examine the proper motions of stars out to distances of 1.0 pc from Sgr A*. The aim is to examine the velocity structure of the MW NSC and acquire a reliable estimate of the stellar mass in the central parsec of the MW NSC, in addition to the well-known black hole mass. Methods. We use multi-epoch adaptive optics assisted near-infrared observations of the central parsec of the Galaxy obtained with NACO/CONICA at the ESO VLT. Stellar positions are measured via PSF fitting in the individual images and transformed into a common reference frame via suitable sets of reference stars. Results. We measured the proper motions of more than 6000 stars within ~1.0 pc of Sagittarius A*. The full data set is provided in this work. We largely exclude the known early-type stars with their peculiar dynamical properties from the dynamical analysis. The cluster is found to rotate parallel to Galactic rotation, while the velocity dispersion appears isotropic (or anisotropy may be masked by the cluster rotation). The Keplerian fall-off of the velocity dispersion due to the point mass of Sgr A* is clearly detectable only at R $\la$ 0.3 pc. Nonparametric isotropic and anisotropic Jeans models are applied to the data. They imply a best-fit black hole mass of $3.6^{\rm +0.2}_{-0.4}$ $\times$ 10 6 $M_{\odot}$. Although this value is slightly lower than the current canonical value of 4.0 $\times$ 10 6 $M_{\odot}$, this is the first time that a proper motion analysis provides a mass for Sagittarius A* that is consistent with the mass inferred from orbits of individual stars. The point mass of Sagittarius A* is not sufficient to explain the velocity data. In addition to the black hole, the models require the presence of an extended mass of $0.5{-}1.5$ $\times$ 10 6 $M_{\odot}$ in the central parsec. This is the first time that the extended mass of the nuclear star cluster is unambiguously detected. The influence of the extended mass on the gravitational potential becomes notable at distances $\ga$0.4 pc from Sgr A*. Constraints on the distribution of this extended mass are weak. The extended mass can be explained well by the mass of the stars that make up the cluster.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/6fd352be6c49a6b62d1c3475d8a0b51dc950bcc7

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Citations in the corpus

SECULAR EVOLUTION OF COMPACT BINARIES NEAR MASSIVE BLACK HOLES: GRAVITATIONAL WAVE SOURCES AND OTHER EXOTICA (2012, background): “…the broken power-law density profile: ρ(r) = ρ0 ( r r0 )−γ [ 1 + ( r r0 )2 ](γ−1.8)/2 (1) with γ the inner slope index and r0 = 0.5pc. Setting ρ0 = 5.2 × 105M⊙pc−3 this gives a good fit to the observed space density at the Galactic center outside the core, normalized at 1 pc (Schödel et al. 2009).”
Galactic Searches for Dark Matter (2012, background): “Kinematic studies of the MW NSC find that the total mass contained within it can be fully-accounted for by the constituent stars of the cluster (Schödel et al., 2009).”
Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole (2018, background): “…at the Keck telescopes) have delivered more than 104 stellar motions and orbit determinations for ≈ 45 individual stars (Schödel et al. 2002; Ghez et al. 2003, 2008; Eisenhauer et al. 2005; Gillessen et al. 2009b, 2017; Schödel et al. 2009; Meyer et al. 2012; Boehle et al. 2016; Fritz et al. 2016).”
IMPROVING GALACTIC CENTER ASTROMETRY BY REDUCING THE EFFECTS OF GEOMETRIC DISTORTION (2010, methodology): “However, we do detect rotation in the plane of the Galaxy, as has been previously reported by both Trippe et al. (2008) and Schödel et al. (2009) and is shown in Figure 18, which shows that there is a preferred angle for the proper motion vectors of 25.4o± 16.3o, consistent with the angle of the…”
Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster (2009, background): “They show signs of recurring star formation, and the discovery of rotation of the NSCs of NGC 4244 (Seth et al. 2008) and of the Milky Way (Trippe et al. 2008; Schödel et al. 2009) indicates that NSCs may at least partially form by accretion of material from their host galaxy.”
DISSIPATIONLESS FORMATION AND EVOLUTION OF THE MILKY WAY NUCLEAR STAR CLUSTER (2011, methodology): “The total mass of the 12, tidally-truncated clusters sums to ∼ 1.5 × 107M⊙, roughly the observed mass of the Milky Way nuclear star cluster (Schödel et al. 2009).”
Quantifying the coexistence of massive black holes and dense nuclear star clusters (2009, background): “The nuclear star cluster has most recently been examined by Schödel et al. (2009a) and Oh et al. (2009).”
Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole (2020, background): “They determined increasingly precise motions for more than 104 stars, and orbits for ≈ 50 (Schödel et al. 2002; Ghez et al. 2003, 2008; Eisenhauer et al. 2005; Gillessen et al. 2009b; Schödel et al. 2009; Meyer et al. 2012; Boehle et al. 2016; Fritz et al. 2016; Gillessen et al. 2017).”
Intermediate mass black holes in AGN discs – I. Production and growth (2012, background): “In our own Galaxy, the distributed mass within ∼ 1pc of Sgr A* is ∼ 10 − 30% of the mass of the supermassive black hole (Schödel et al. 2009).”