Ejection of hypervelocity stars from the Galactic Centre by intermediate-mass black holes

Baumgardt, Holger; Gualandris, Alessia; Zwart, Simon F. Portegies

doi:10.1111/j.1365-2966.2006.10818.x

Cited by 115 publications

(167 citation statements)

References 33 publications

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“…Relaxing the assumption of a universal mass function, however, it is also possible that there are simply very few old stars at the GC. For example, Perets et al (2007) demonstrate that, for reasonable assumptions about the mass function and old-star binary fraction, the ejection rate of 1 M stars should be approximately 0.5 Myr −1 (but see Yu & Tremaine 2003), and could be substantially enhanced by the presence of a secondary massive perturber as envisioned by, e.g., Polnarev & Rees (1994), Baumgardt et al (2006), and Levin (2006) and not included in the Kenyon et al (2008) calculations. Lu et al 2010 have suggested that the HVS population is emerging from the disk at the GC, which is thought to have a top-heavy initial mass function (IMF; Bartko et al 2010).…”

Section: Discussionmentioning

confidence: 99%

Segue-2 Limits on Metal-Rich Old-Population Hypervelocity Stars in the Galactic Halo

Kollmeier¹,

Gould²,

Rockosi³

et al. 2010

ApJ

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We present new limits on the ejection of metal-rich old-population hypervelocity stars (HVSs) from the Galactic center (GC) as probed by the SEGUE-2 survey. Our limits are a factor of 3-10 more stringent than previously reported, depending on stellar type. Compared to the known population of B-star ejectees, there can be no more than 30 times more metal-rich old-population F/G stars ejected from the GC. Because B stars comprise a tiny fraction of a normal stellar population, this places significant limits on the combination of the GC mass function and the ejection mechanism for HVSs. In the presence of a normal GC mass function, our results require an ejection mechanism that is about 5.5 times more efficient at ejecting B stars compared to low-mass F/G stars.

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Section: Discussionmentioning

confidence: 99%

Segue-2 Limits on Metal-Rich Old-Population Hypervelocity Stars in the Galactic Halo

Kollmeier¹,

Gould²,

Rockosi³

et al. 2010

ApJ

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“…Gualandris et al 2004Gualandris et al , 2005Baumgardt et al 2006;Gvaramadze et al 2009;Mapelli et al 2005). It has been recently suggested that globular clusters may contain central black holes in the mass range of 10 3 −10 4 M (Miller & Hamilton 2002;Baumgardt et al 2005) which fall on top of the relation between the velocity dispersion and black hole mass seen for galaxies (e.g.…”

Section: Introductionmentioning

confidence: 99%

High-velocity stars in the cores of globular clusters: the illustrative case of NGC 2808

Lützgendorf

Gualandris

Kissler-Patig

et al. 2012

A&A

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Context. We report the detection of five high-velocity stars in the core of the globular cluster NGC 2808. The stars lie on the red giant branch and show total velocities between 40 and 45 km s −1 . For a core velocity dispersion σ c = 13.4 km s −1 , this corresponds to up to 3.4σ c . These velocities are close to the estimated escape velocity (∼50 km s −1 ) and suggest an ejection from the core. Two of these stars have been confirmed in our recent integral field spectroscopy data and we will discuss them in more detail here. These two red giants are located at a projected distance of ∼0.3 pc from the center. According to their positions on the color magnitude diagram, both stars are cluster members. Aims. We investigate several possible origins for the high velocities of the stars and conceivable ejection mechanisms. Since the velocities are close to the escape velocity, it is not obvious whether the stars are bound or unbound to the cluster. We therefore consider both cases in our analysis. Methods. We perform numerical simulations of three-body dynamical encounters between binaries and single stars and compare the resulting velocity distributions of escapers with the velocities of our stars. If the stars are bound, the encounters must have taken place when the stars were still on the main sequence. We compare the predictions for a single dynamical encounter with a compact object with those of a sequence of two-body encounters due to relaxation. If the stars are unbound, the encounter must have taken place recently, when the stars were already in the giant phase. Results. After including binary fractions and black-hole retention fractions, projection effects, and detection probabilities from Monte-Carlo simulations, we estimate the expected numbers of detections for all the different scenarios. Based on these numbers, we conclude that the most likely scenario is that the stars are bound and were accelerated by a single encounter between a binary of main-sequence stars and a ∼10 M black hole. Finally, we discuss the origin of previously discovered fast stars in globular clusters, and conclude that the case of NGC 2808 is most likely a representative case for most other detections of fast stars in globular clusters. We show that with the present analysis we are able to explain high-velocity stars in the clusters M3 and 47 Tucanae with simple dynamical encounters.

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“…Such attempts involved Smooth Particle Hydrodynamics (SPH) and Monte Carlo simulations which tried to account for the under density of giants by means of collisions with other stars and stellar remnants (Dale et al 2009;Freitag 2008). Another explanation could be the disturbance of the cusp of stars after experiencing a minor merger event or an in-spiraling of an intermediatemass black hole, which then would lead to deviations from equilibrium; hence causing a shallower power-law profile of the cusp (Baumgardt et al 2006). Merritt (2010) explains the observations by the evolution of a parsec-scale initial core model.…”

Section: Introductionmentioning

confidence: 98%

The S-star cluster at the center of the Milky Way

Sabha¹,

Eckart²,

Merritt³

et al. 2012

A&A

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Sagittarius A*, the super-massive black hole at the center of the Milky Way, is surrounded by a small cluster of high velocity stars, known as the S-stars. We aim to constrain the amount and nature of stellar and dark mass associated with the cluster in the immediate vicinity of Sagittarius A*. We use near-infrared imaging to determine the K s -band luminosity function of the S-star cluster members, and the distribution of the diffuse background emission and the stellar number density counts around the central black hole. This allows us to determine the stellar light and mass contribution expected from the faint members of the cluster. We then use post-Newtonian N-body techniques to investigate the effect of stellar perturbations on the motion of S2, as a means of detecting the number and masses of the perturbers. We find that the stellar mass derived from the K s -band luminosity extrapolation is much smaller than the amount of mass that might be present considering the uncertainties in the orbital motion of the star S2. Also the amount of light from the fainter S-cluster members is below the amount of residual light at the position of the S-star cluster after removing the bright cluster members. If the distribution of stars and stellar remnants is strongly enough peaked near Sagittarius A*, observed changes in the orbital elements of S2 can be used to constrain both their masses and numbers. Based on simulations of the cluster of high velocity stars we find that at a wavelength of 2.2 μm close to the confusion level for 8 m class telescopes blend stars will occur (preferentially near the position of Sagittarius A*) that last for typically 3 years before they dissolve due to proper motions.

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Ejection of hypervelocity stars from the Galactic Centre by intermediate-mass black holes

Cited by 115 publications

References 33 publications

Segue-2 Limits on Metal-Rich Old-Population Hypervelocity Stars in the Galactic Halo

Segue-2 Limits on Metal-Rich Old-Population Hypervelocity Stars in the Galactic Halo

High-velocity stars in the cores of globular clusters: the illustrative case of NGC 2808

The S-star cluster at the center of the Milky Way

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