CONSTRAINING THE DARK CUSP IN THE GALACTIC CENTER BY LONG-PERIOD BINARIES

Massive black holes (MBHs) in galactic nuclei are believed to be surrounded by a high-density stellar cluster, whose mass is mostly in hard-to-detect faint stars and compact remnants. Such dark cusps dominate the dynamics near the MBH: a dark cusp in the Galactic center (GC) of the Milky Way would s...

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Bibliographic Details
Published in:The Astrophysical journal 2014-01, Vol.780 (2), p.1-13
Main Authors: Alexander, Tal, Pfuhl, Oliver
Format: Article
Language:English
Subjects:
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
Binary stars
BLACK HOLES
Cusps
DENSITY
DETECTION
EMISSION
EVAPORATION
GALAXY NUCLEI
GENERAL RELATIVITY THEORY
GRAVITATIONAL WAVES
High density
LIMITING VALUES
Lists
MASS
MILKY WAY
Orbitals
Spirals
Star clusters
STARS
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Summary:Massive black holes (MBHs) in galactic nuclei are believed to be surrounded by a high-density stellar cluster, whose mass is mostly in hard-to-detect faint stars and compact remnants. Such dark cusps dominate the dynamics near the MBH: a dark cusp in the Galactic center (GC) of the Milky Way would strongly affect orbital tests of general relativity there; on cosmic scales, dark cusps set the rates of gravitational wave emission events from compact remnants that spiral into MBHs, and they modify the rates of tidal disruption events, to list only some implications. A recently discovered long-period massive young binary (with period P sub(12) [lap] 1 yr, total mass M super(12) ~ O(100 M sub([middot in circle])), and age T super(12) ~ 6 x 10 super(6) yr), only ~0.1 pc from the Galactic MBH, sets a lower bound on the stellar two-body relaxation timescale there, mint sub(rix) [is proportional to] (P sub(12)/M sub(12)) super(2/3)T sub(12) ~ 10 sub(7)yr, and, correspondingly, an upper bound on the stellar number density, max n sub([sstarf]) ~ few x 10 super(8)/[left angle bracket]M super(2) sub([sstarf])[right angle bracket]pc super(-3) ([left angle bracket]M super(2)[sstarf][righ t angle bracket] super(1/2) is the rms stellar mass), based on the binary's survival against evaporation by the dark cusp. However, a conservative dynamical estimate, the drain limit, implies t sub(rlx) > O(10 super(8) yr). Such massive binaries are thus too short-lived and tightly bound to constrain a dense relaxed dark cusp. We explore here in detail the use of longer-period, less massive, and longer-lived binaries (P sub(12) ~ few yr, M sub(12) ~ 2-4 M sub([middot in circle]), T sub(12) 10 super(8)-10 super(10) yr), presently just below the detection threshold, for probing the dark cusp and develop the framework for translating their future detections among the giants in the GC into dynamical constraints.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/780/2/148