A Cluster of Black Holes at the Galactic Center

By J. Miralda-Escudé, A. Gould. 2000

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If the stellar population of the bulge contains black holes formed in the final core collapse of ordinary stars with M ≳ 30 M☉, then about 25,000 stellar mass black holes should have migrated by dynamical friction into the central parsec of the Milky Way, forming a black hole cluster around the central supermassive black hole. These black holes can be captured by the central black hole when they randomly reach a highly eccentric orbit due to relaxation, either by direct capture (when their Newtonian peribothron is less than 4 Schwarzschild radii) or after losing orbital energy through gravitational waves. The overall depletion timescale is ~30 Gyr, so most of the 25,000 black holes remain in the central cluster today. The presence of this black hole cluster would have several observable consequences. First, the low-mass, old stellar population should have been expelled from the region occupied by the black hole cluster as a result of relaxation, implying a core in the profile of solar-mass red giants with a radius of ~2 pc (i.e., 1′). The observed central density cusp (which has a core radius of only a few arcseconds) should be composed primarily of young (≲1 Gyr) stars. Second, flares from stars being captured by supermassive black holes in other galaxies should be rarer than usually expected because the older stars will have been expelled from the central regions by the black hole clusters of those galaxies. Third, the young (≲2 Gyr) stars found at distances ~3-10 pc from the Galactic center should be preferentially on highly eccentric orbits. Fourth, if future high-resolution K-band images reveal sources microlensed by the Milky Way's central black hole, then the cluster black holes could give rise to secondary ("planet-like") perturbations on the main event.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/88a73b41345c20a426c896fac926243f660e323b

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Citations in the corpus

Doing Science with eLISA: Astrophysics and Cosmology in the Millihertz Regime (2012, background): “The problem of the presence of a dark cusp has been addressed, for the Galactic Centre, by different authors, from a semi-analytical and numerical standpoint (Freitag et al., 2006a,b, Hopman and Alexander, 2006a, Miralda-Escudé and Gould, 2000, Sigurdsson and Rees, 1997).”
Rapid and Bright Stellar-mass Binary Black Hole Mergers in Active Galactic Nuclei (2016, methodology): “Since black holes are more massive than typical stars, black holes which formed in the local neighborhood of the galactic center sink to the center due to two-body encounters in the nuclear stellar cluster (Miralda-Escudé & Gould 2000; O’Leary et al. 2009).”
The structure of the nuclear stellar cluster of the Milky Way (2006, background): “The cusp is possibly dominated by stellar black holes (see, e.g., Morris 1993; Miralda-Escudé & Gould 2000; Mouawad et al. 2005; Amaro-Seoane et al. 2004; Freitag et al. 2006; Hopman & Alexander 2006).”
MONITORING STELLAR ORBITS AROUND THE MASSIVE BLACK HOLE IN THE GALACTIC CENTER (2008, background): “• In addition to the MBH a substantial amount of mass might reside in form of a cluster of dark stellar remnants around the MBH (Morris 1993; Miralda-Escudé & Gould 2000; Muno et al. 2005; Mouawad et al. 2005; Hopman & Alexander 2006).”
Gravitational waves from scattering of stellar-mass black holes in galactic nuclei (2008, methodology): “Indeed, as many as ∼20 000 BHs are expected to have segregated into the inner ≈1 pc of the Milky Way (Morris 1993; Miralda-Escudé & Gould 2000; Freitag, Amaro-Seoane & Kalogera 2006, hereafter FAK06; Hopman & Alexander 2006b, hereafter HA06).”
Chandra X-Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy (2001, background): “The final stellar possibility that we consider for producing some fraction of the X-rays is that of a population of compact stellar objects in the entourage of the central black hole (Morris 1993; Lee 1995; Miralda-Escudé & Gould 2000).”
On the rate of black hole binary mergers in galactic nuclei due to dynamical hardening (2017, background): “Stellar-mass black hole binaries (BHB) are believed to live in galactic nuclei due to a combination of stellar evolution and dynamical friction (Morris 1993; Miralda-Escudé & Gould 2000; Antonini & Rasio 2016).”
Gravitational wave snapshots of generic extreme mass ratio inspirals (2005, background): “[12] for discussion pointing to the possibility of a “minicluster” of stellar mass black holes in the center of our galaxy, and Ref.”
AN IMPROVED DISTANCE AND MASS ESTIMATE FOR SGR A* FROM A MULTISTAR ORBIT ANALYSIS (2016, background): “In addition to the central supermassive black hole, a central cluster of stellar mass black holes and/or other compact objects has been predicted and theoretically explored (e.g., Morris 1993; Miralda-Escudé & Gould 2000; Freitag et al. 2006; Merritt 2010).”
The Effect of Mass Segregation on Gravitational Wave Sources near Massive Black Holes (2006, background): “The event rate of EMRIs has been estimated by numerous authors (Hils & Bender 1995; Sigurdsson & Rees 1997; Miralda-Escudé & Gould 2000; Freitag 2001; Ivanov 2002; Freitag 2003; Alexander & Hopman 2003; Hopman & Alexander 2005, 2006) but remains rather uncertain, in part because of the slow nature…”
Resonant Relaxation near a Massive Black Hole: The Stellar Distribution and Gravitational Wave Sources (2006, background): “…by a dynamically significant distribution of dark mass in the form of compact remnants, in particular SBHs that have sunk to the center by mass-segregation over the lifetime of the Galaxy (Morris 1993; Miralda-Escudé & Gould 2000; Hopman & Alexander 2006; Freitag, Amaro-Seoane & Kalogera 2006).”
Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits (2008, methodology): “This would allow an exploration of a possible cluster of stellar remnants surrounding the central black hole, suggested byMorris (1993),Miralda-Escudé & Gould (2000), and Freitag et al. (2006).”
The Orbital Statistics of Stellar Inspiral and Relaxation near a Massive Black Hole: Characterizing Gravitational Wave Sources (2005, unknown): “(rS/rh)1/4 (Miralda-Escudé & Gould 2000) and Ap ’ 0:2 for p ¼ 0 (Alexander & Hopman 2003).”
Starbursts near supermassive black holes: young stars in the Galactic Centre, and gravitational waves in LISA band (2006, background): “…ms is the typical mass of a star in the cusp [which is thought to be dominated by stellar-mass black holes in the central 0.1pc, see Miralda-Escude and Gould (2000)], Porb is the orbital period of the star, and βs is the numerical coefficient, found to be approximately 0.5…”
Intermediate mass black holes in AGN discs – I. Production and growth (2012, background): “We expect a dense nuclear cluster of objects to surround the supermassive black hole as a result of stellar evolution, dynamical friction, secular evolution and minor mergers (e.g. Morris 1993; Miralda-Escudé & Gould 2000; Merritt 2010).”
STRONG MASS SEGREGATION AROUND A MASSIVE BLACK HOLE (2008, background): “…et al. 2005; Gillessen et al. 2008; Ghez et al. 2008), is still 10–100 times larger than predicted by approximate theoretical estimates (Morris 1993; Miralda-Escudé & Gould 2000), FokkerPlanck calculations (Hopman & Alexander 2006b), or by the conservative drain limit (Alexander & Livio 2004),…”