1,089 research outputs found
The influence of Massive Black Hole Binaries on the Morphology of Merger Remnants
Massive black hole (MBH) binaries, formed as a result of galaxy mergers, are
expected to harden by dynamical friction and three-body stellar scatterings,
until emission of gravitational waves (GWs) leads to their final coalescence.
According to recent simulations, MBH binaries can efficiently harden via
stellar encounters only when the host geometry is triaxial, even if only
modestly, as angular momentum diffusion allows an efficient repopulation of the
binary loss cone. In this paper, we carry out a suite of N-body simulations of
equal-mass galaxy collisions, varying the initial orbits and density profiles
for the merging galaxies and running simulations both with and without central
MBHs. We find that the presence of an MBH binary in the remnant makes the
system nearly oblate, aligned with the galaxy merger plane, within a radius
enclosing 100 MBH masses. We never find binary hosts to be prolate on any
scale. The decaying MBHs slightly enhance the tangential anisotropy in the
centre of the remnant due to angular momentum injection and the slingshot
ejection of stars on nearly radial orbits. This latter effect results in about
1% of the remnant stars being expelled from the galactic nucleus. Finally, we
do not find any strong connection between the remnant morphology and the binary
hardening rate, which depends only on the inner density slope of the remnant
galaxy. Our results suggest that MBH binaries are able to coalesce within a few
Gyr, even if the binary is found to partially erase the merger-induced
triaxiality from the remnant.Comment: 16 pages, 13 figures, 4 tables; accepted for publication in MNRA
Hawking Radiation from Fluctuating Black Holes
Classically, black Holes have the rigid event horizon. However, quantum
mechanically, the event horizon of black holes becomes fuzzy due to quantum
fluctuations. We study Hawking radiation of a real scalar field from a
fluctuating black hole. To quantize metric perturbations, we derive the
quadratic action for those in the black hole background. Then, we calculate the
cubic interaction terms in the action for the scalar field. Using these
results, we obtain the spectrum of Hawking radiation in the presence of
interaction between the scalar field and the metric. It turns out that the
spectrum deviates from the Planck spectrum due to quantum fluctuations of the
metric.Comment: 35pages, 4 figure
Generation of human colon organoids from healthy and inflammatory bowel disease mucosa
Ulcerative colitis and Crohn's disease are chronic inflammatory bowel diseases (IBD) of unknown cause characterized by a relapsing-remitting behavior. Growing evidence supports the idea that the epithelial barrier plays a central role in the pathogenesis of IBD as well as in its evolution over time, thus representing a potential target for novel therapeutic options. In the last decade, the introduction of 3D epithelial cultures from ex vivo-expanded intestinal adult stem cells (ASCs) has impacted our ability to study the function of the epithelium in several gastrointestinal disorders, including IBD. Here, we describe in detail a reproducible protocol to generate Matrigel-embedded epithelial organoids from ASCs of non-IBD and IBD donors using small colonic biopsies, including steps for its optimization. A slightly modified version of this protocol is also provided in case surgical samples are used. With this method, epithelial organoids can be expanded over several passages, thereby generating a large quantity of viable cells that can be used in multiple downstream analyses including genetic, transcriptional, proteomic and/or functional studies. In addition, 3D cultures generated using our protocol are suitable for the establishment of 2D cultures, which can model relevant cell-to-cell interactions that occur in IBD mucosa
Hyperk\"ahler torsion structures invariant by nilpotent Lie groups
We study HKT structures on nilpotent Lie groups and on associated
nilmanifolds. We exhibit three weak HKT structures on which are
homogeneous with respect to extensions of Heisenberg type Lie groups. The
corresponding hypercomplex structures are of a special kind, called abelian. We
prove that on any 2-step nilpotent Lie group all invariant HKT structures arise
from abelian hypercomplex structures. Furthermore, we use a correspondence
between abelian hypercomplex structures and subspaces of to
produce continuous families of compact and noncompact of manifolds carrying non
isometric HKT structures. Finally, geometrical properties of invariant HKT
structures on 2-step nilpotent Lie groups are obtained.Comment: LateX, 12 page
Gravitational instability of Einstein-Gauss-Bonnet black holes under tensor mode perturbations
We analyze the tensor mode perturbations of static, spherically symmetric
solutions of the Einstein equations with a quadratic Gauss-Bonnet term in
dimension . We show that the evolution equations for this type of
perturbations can be cast in a Regge-Wheeler-Zerilli form, and obtain the exact
potential for the corresponding Schr\"odinger-like stability equation. As an
immediate application we prove that for and , the sign
choice for the Gauss-Bonnet coefficient suggested by string theory, all
positive mass black holes of this type are stable. In the exceptional case , we find a range of parameters where positive mass asymptotically flat
black holes, with regular horizon, are unstable. This feature is found also in
general for .Comment: 7 pages, 1 figure, minor corrections, references adde
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