Stellar dynamics in the Galactic centre: Proper motions and anisotropy

By R. Genzel, C. Pichon, A. Eckart, O. Gerhard, T. O. Mpe, U. Basel, O. Strasbourg. 2000

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We report a new analysis of the stellar dynamics in the Galactic Centre, based on improved sky and line-of-sight velocities for more than 100 stars in the central few arcseconds from the black hole candidate SgrA*. The main results are as follows. (1) Overall, the stellar motions do not deviate strongly from isotropy. For those 32 stars with a determination of all three velocity components, the absolute, line-of-sight and sky velocities are in good agreement, consistent with a spherical star cluster. Likewise the sky-projected radial and tangential velocities of all 104 proper motion stars in our sample are also consistent with overall isotropy. (2) However, the sky-projected velocity components of the young, early-type stars in our sample indicate significant deviations from isotropy, with a strong radial dependence. Most of the bright He i emission-line stars at separations from 1 to 10 arcsec from SgrA* are on tangential orbits. This tangential anisotropy of the He i stars and most of the brighter members of the IRS 16 complex is largely caused by a clockwise (on the sky) and counter-rotating (line of sight, compared to the Galaxy), coherent rotation pattern. The overall rotation of the young star cluster may be a remnant of the original angular momentum pattern in the interstellar cloud from which these stars were formed. (3) The fainter, fast-moving stars within ≈1 arcsec of SgrA* may be largely moving on radial or very elliptical orbits. We have so far not detected deviations from linear motion (i.e., acceleration) for any of them. Most of the SgrA* cluster members are also on clockwise orbits. Spectroscopy indicates that they are early-type stars. We propose that the SgrA* cluster stars are those members of the early-type cluster that happen to have small angular momentum, and thus can plunge to the immediate vicinity of SgrA*. (4) We derive an anisotropy-independent estimate of the Sun–Galactic Centre distance between 7.8 and 8.2 kpc, with a formal statistical uncertainty of ±0.9 kpc. (5) We explicitly include velocity anisotropy in estimating the central mass distribution. We show how Leonard–Merritt and Bahcall–Tremaine mass estimates give systematic offsets in the inferred mass of the central object when applied to finite concentric rings for power-law clusters. Corrected Leonard–Merritt projected mass estimators and Jeans equation modelling confirm previous conclusions (from isotropic models) that a compact central mass concentration (central density ≥1012.6 M⊙ pc−3) is present and dominates the potential between 0.01 and 1 pc. Depending on the modelling method used, the derived central mass ranges between 2.6×106 and 3.3×106 M⊙ for R⊙=8.0 kpc.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/8f1f6bec87b9cbc5fba0341dbe5140971f8186b7

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Citations in the corpus

Rotation Curve and Mass Distribution in the Galactic Center - From Black Hole to Entire Galaxy (2013, background): “The inner straight dashed line represents the central massive black hole of mass 3.6×106M⊙ (Genzel et al. 2000; Ghez et al. 2005; Gillesen et al. 2009).”
The M•-σ Relation for Supermassive Black Holes (2000, background): “The solid line has a slope of 1. of km s~1 for the Galaxy based on the velocityp e \ 75 dispersion between 50A and 500A (Kent 1992 ; Genzel et al. 2000).”
On the Relationship between Radio Emission and Black Hole Mass in Galactic Nuclei (2001, background): “…A* in the Galactic center, whose mass has been determined through proper motions and radial velocities of individual stars (Eckart & Genzel 1997 ; Ghez et al. 1998 ; Genzel et al. 2000), all the stellar-based masses come from integrated spectroscopy, a few from the ground, but largely from HST .”
A Geometric Determination of the Distance to the Galactic Center (2003, methodology): “In the time since 1993 Genzel et al. (2000) reported a primary (statistical parallax) distance, Ro=8.0±0.9 kpc (statistical error bar), based on a statistical comparison of proper motions and line-of-sight velocities of stars in the central 0.5 pc of the Galaxy.”
The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, methodology): “The statistical parallax of the nuclear star cluster (NSC) obtained by comparing stellar PMs and LOS velocities has been used as a third direct estimate of R0 (Genzel et al. 2000, Trippe et al. 2008, Do et al. 2013).”
The Need for a Second Black Hole at the Galactic Center (2003, background): “To test this, we have examined the existing velocity data (Genzel et al. (2000);A. Ghez,private communication) for He-line stars with three measured velocity components.”
The nuclear star cluster of the Milky Way: proper motions and mass (2009, background): “Studies of stellar dynamics have provided striking evidence for the existence of a supermassive black hole at the dynamical center of the MW NSC (see Eckart & Genzel 1996; Ghez et al. 2000; Genzel et al. 2000).”
A Cluster of Black Holes at the Galactic Center (2000, methodology): “(1) From the model Ðt of Genzel et al. (2000) to the velocity dispersion data, we Ðnd that the total mass within r0\ 1.8pc is 2 (see their Fig.”
The Slope of the Black Hole Mass versus Velocity Dispersion Correlation (2002, unknown): “For comparison, Ghez et al. (1998) find (2.4 ± 0.2) × 10 6 M ⊙ , and Genzel et al. (2000) find (2.6–3.3)×10 6 M ⊙ .”
Stellar Orbits around the Galactic Center Black Hole (2003, background): “While roughly 100 stars have reported proper-motion values in the earlier works of Ghez et al. (1998) and Genzel et al. (2000), only 18 of these have S=N > 5 and half of them have S=N < 3, making the velocity dispersion bias term nonnegligible.”
Black Hole Mass Estimates from Reverberation Mapping and from Spatially Resolved Kinematics (2000, methodology): “However, measurements in our Galaxy (Genzel et al. 1997, 2000; Ghez et al. 1998) and in NGC 4258 (Greenhill et al. 1996) probe a small region comparable to that probed by reverberation mapping in other galaxies and find no suggestion of any dark mass in addition to a BH. 3.2.”
Rotation Curves of Spiral Galaxies (2000, background): “Proper-motion studies in the near infrared show that the velocity dispersion of stars within the central 1 pc increases toward the center, indicating the existence of a massive black hole of mass 3× 106M⊙• (Genzel et al. 1997, 2000; Ghez et al. 1998).”
STRONG MASS SEGREGATION AROUND A MASSIVE BLACK HOLE (2008, background): “…orbiting the Galactic MBH (Eisenhauer et al. 2005; Ghez et al. 2005) and the accumulating data on their distribution and dynamics (Alexander 1999; Genzel et al. 2000, 2003; Schödel et al. 2003, 2007), as well as by the prospects of detecting gravitational waves (GW) from extreme mass ratio…”
An Overview of the Performance and Scientific Results from the Chandra X‐Ray Observatory (2001, methodology): “The mass of the black hole at Sgr A* has been determined from stellar motion to be M, (Genzel et al. 2000).”
Stellar Dynamics in the Central Arcsecond of Our Galaxy (2003, methodology): “…the present work, which is based on more data in the region within 100 of Sgr A*. (3) The LM mass estimates in Figure 3 of Schödel et al. (2002) and Figure 17 of Genzel et al. (2000) were corrected downward by 5%– 10% because they assumed a power-law slope of ¼ 1:8 for the inner stellar cluster.”
Stellar orbits near Sagittarius A (2002, methodology): “This is of the order of both the mean measurement uncertainty and the mean differences in proper motion velocities between the SHARP/NTT and NIRC/Keck data sets (with a time difference of about 1 yr between central epochs; Eckart & Genzel 1996; Ghez et al. 1998, 2000; Genzel et al. 2000).”
Chandra X-Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy (2001, methodology): “After decades of controversy, measurements of stellar dynamics have confidently established that the nucleus of the Milky Way harbors a supermassive black hole (SMBH) with a mass M 2:6 106 M (Genzel et al. 1997, 2000; Ghez et al. 1998, 2000; Schödel et al. 2002).”
THE LEGACY OF SCUPOL: 850 μm IMAGING POLARIMETRY FROM 1997 TO 2005 (2009, unknown): “Publication(s) Emission Publication(s) L183 SC/PC 43 0.15 Ward-Thompson et al. (1999) N Ward-Thompson et al. (2000); Crutcher et al. (2004) ρ Oph A SFR 44 0.139 Mamajek (2008) N Tamura (1999); Green et al. (2004) Holland et al. (1996); Greaves et al. (1994) ρ Oph C SFR 45 0.139 Mamajek (2008) Y ρ Op”