4 research outputs found
The need for a second black hole at the Galactic center
Deep infra-red observations and long-term monitoring programs have provided
dynamical evidence for a supermassive black hole of mass 3.e6 solar masses
associated with the radio source Sagitarrius A* at the center of our Galaxy.
The brightest stars orbiting within 0.1 parsecs of the black hole appear to be
young, massive main sequence stars, n spite of an environment near the black
hole that is hostile to star formation. We discuss mechanisms by which stars
born outside the central parsec can sink towards the black hole and conclude
that the drag coming from plausible stellar populations does not operate on the
short timescales required by the stellar ages. We propose that these stars were
dragged in by a second black hole of mass of 1.e3-1.e4 solar masses, which
would be classified as an intermediate-mass black hole. We discuss the
implications for the stellar populations and the kinematics in the Galactic
center. Finally we note that continued astrometric monitoring of the central
radio source offers the prospect for a direct detection of such objects.Comment: 5 pages, 2 postscript figures, submitted to ApJ letters The
introduction section has been updated since submission to Ap
On the Origin of Nuclear Star Clusters in Late Type Spiral Galaxies
A large fraction of bulgeless disk galaxies contain young compact stellar
systems at their centers, in spite of the local gravitational stability of
these disks. We evaluate two contrasting hypotheses for the origin of the
nuclear star clusters in late-type disk galaxies. The clusters could not have
migrated from distant eccentric locations in the disk. Instead they must have
formed in situ, requiring radial transport of gas toward the center of the
disk. This transport could be a consequence of the development of the
magnetorotational instability in the differentially rotating warm neutral
medium. We evaluate the rate of gas transport into the disk center and find
that it is sufficient to support continuous star formation in that location.
Enhanced stellar surface brightness in the inner few hundred parsecs and the
formation of a compact stellar system in the central few parsecs are
unavoidable in dark matter halos with divergent density profiles. We illustrate
our conclusions on a model of the nearest late-type disk galaxy M33.Comment: 4 pages, ApJ Letters, in pres
Consequences of gravitational radiation recoil
Coalescing binary black holes experience an impulsive kick due to anisotropic
emission of gravitational waves. We discuss the dynamical consequences of the
recoil accompanying massive black hole mergers. Recoil velocities are
sufficient to eject most coalescing black holes from dwarf galaxies and
globular clusters, which may explain the apparent absence of massive black
holes in these systems. Ejection from giant elliptical galaxies would be rare,
but coalescing black holes are displaced from the center and fall back on a
time scale of order the half-mass crossing time. Displacement of the black
holes transfers energy to the stars in the nucleus and can convert a steep
density cusp into a core. Radiation recoil calls into question models that grow
supermassive black holes from hierarchical mergers of stellar-mass precursors.Comment: 5 pages, 4 figures, emulateapj style; minor changes made; accepted to
ApJ Letter
The Link between Warm Molecular Disks in Maser Nuclei and Star Formation near the Black Hole at the Galactic Center
The discovery of hundreds of young, bright stars within a parsec from the
massive black hole at the center of the Galaxy presents a challenge to star
formation theories. The requisite Roche densities for the gravitational
collapse of gas clouds are most naturally achieved in accretion disks. The
water maser sources in Keplerian rotation in the nuclei of NGC4258, NGC1068,
and the Circinus galaxy indicate the presence of warm, extended, molecular
accretion disks around black holes similar in mass to the one at the Galactic
center. Here we argue that the current conditions in the maser nuclei, and
those near the Galactic center, represent two consecutive, recurrent phases in
the life cycle of the nucleus of a typical gas-rich spiral bulge. The warm
molecular disks that give rise to the observed maser emission fragment into
stellar-size objects. The stellar masses, their orbital geometry, and the total
number of stars thus formed are consistent with the values identified at the
Galactic center. The stars tend to form in compact groups resembling the IRS 13
complex that dominates the stellar light in the neighborhood of the black hole.Comment: ApJ Letters, in pres