840 research outputs found
Supermassive Black Hole Formation at High Redshifts via Direct Collapse: Physical Processes in the Early Stage
We use numerical simulations to explore whether direct collapse can lead to
the formation of SMBH seeds at high-z. We follow the evolution of gas within DM
halos of 2 x 10^8 Mo and 1 kpc. We adopt cosmological density profiles and
j-distributions. Our goal is to understand how the collapsing flow overcomes
the centrifugal barrier and whether it is subject to fragmentation. We find
that the collapse leads either to a central runaway or to off-center
fragmentation. A disk-like configuration is formed inside the centrifugal
barrier. For more cuspy DM distribution, the gas collapses more and experiences
a bar-like perturbation and a central runaway. We have followed this inflow
down to ~10^{-4} pc. The flow remains isothermal and the specific angular
momentum is efficiently transferred by gravitational torques in a cascade of
nested bars. This cascade supports a self-similar, disk-like collapse. In the
collapsing phase, virial supersonic turbulence develops and fragmentation is
damped. For larger initial DM cores the timescales become longer. In models
with more organized initial rotation, a torus forms and appears to be supported
by turbulent motions. The evolution depends on the competition between two
timescales, corresponding to the onset of the central runaway and off-center
fragmentation. For less organized rotation, the torus is greatly weakened, the
central accretion timescale is shortened, and off-center fragmentation is
suppressed --- triggering the central runaway even in previously `stable'
models. The resulting SMBH masses lie in the range 2 x 10^4 Mo - 2 x 10^6 Mo,
much higher than for Population III remnants. We argue that the above upper
limit appears to be more realistic mass. Corollaries of this model include a
possible correlation between SMBH and DM halo masses, and similarity between
the SMBH and halo mass functions, at time of formation.Comment: 20 pages, 15 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
Supermassive Black Hole Seed Formation at High Redshifts: Long-Term Evolution of the Direct Collapse
We use cosmological adaptive mesh refinement (AMR) code Enzo zoom-in
simulations to study the long term evolution of the collapsing gas within dark
matter (DM) halos at high redshifts. This direct collapse process is a leading
candidate for rapid formation of supermassive black hole (SMBH) seeds at high
z. To circumvent the Courant condition at small radii, we have used the sink
particle method, and focus on the evolution on scales ~0.01-10 pc. The collapse
proceeds in two stages, with the secondary runaway happening within the central
10 pc, and with no detected fragmentation. The sink particles form when the
collapsing gas requires additional refinement of the grid size at the highest
refinement level. Their mass never exceeds ~10^3 Mo, with the sole exception of
the central seed which grows dramatically to ~ 2 x 10^6 Mo in ~2 Myr,
confirming the feasibility of this path to the SMBH. The time variability of
angular momentum axis in the accreted gas results in the formation of two
misaligned disks --- a small inner disk, and a more massive, outer disk which
is inclined by ~45^o to the inner disk. The self-gravity of these disks is
heavily diluted --- both disks lie within the Roche limit of the central seed.
While the inner disk is geometrically thin and weakly asymmetric, the outer
disk flares due to turbulent motions as a result of the massive inflow along a
pair of penetrating filaments. The geometry of inflow via filaments determines
the dominant and secondary Fourier modes in this disk --- these modes have a
non-self-gravitational origin. We do not confirm that m=1 is a principal mode
that drives the inflow in the presence of a central massive object. While the
positions of the disks depend on the scale chosen to break the self-similar
collapse, the overall configuration appears to be generic, and is expected to
form when the central seed becomes sufficiently massive.Comment: 14 pages, 11 figures, MNRAS, in press, typos correcte
Creation and characterization of vector vortex modes for classical and quantum communication
Vector vortex beams are structured states of light that are non-separable in
their polarisation and spatial mode, they are eigenmodes of free-space and many
fibre systems, and have the capacity to be used as a modal basis for both
classical and quantum communication. Here we outline recent progress in our
understanding of these modes, from their creation to their characterization and
detection. We then use these tools to study the propagation behaviour of such
modes in free-space and optical fibre and show that modal cross-talk results in
a decay of vector states into separable scalar modes, with a concomitant loss
of information. We present a comparison between probabilistic and deterministic
detection schemes showing that the former, while ubiquitous, negates the very
benefit of increased dimensionality in quantum communication while reducing
signal in classical communication links. This work provides a useful
introduction to the field as well as presenting new findings and perspectives
to advance it further
Direct Collapse to Supermassive Black Hole Seeds with Radiative Transfer: Isolated Halos
Direct collapse within dark matter (DM) halos is a promising path to form
supermassive black hole (SMBH) seeds at high redshifts. The outer part of this
collapse remains optically thin, and has been studied intensively using
numerical simulations. However, the innermost region of the collapse is
expected to become optically thick and requires us to follow the radiation
field in order to understand its subsequent evolution. So far, the adiabatic
approximation has been used exclusively for this purpose. We apply radiative
transfer in the flux-limited diffusion (FLD) approximation to solve the
evolution of coupled gas and radiation, for isolated halos. For direct collapse
within isolated DM halos, we find that (1) the photosphere forms at ~10^{-6} pc
and rapidly expands outward. (2) A central core forms, with a mass of ~1 Mo,
supported by thermal gas pressure gradients and rotation. (3) Growing thermal
gas and radiation pressure gradients dissolve it. (4) This process is
associated with a strong anisotropic outflow, and another core forms nearby and
grows rapidly. (5) Typical radiation luminosity emerging from the photosphere
encompassing these cores is ~5 x 10^{37}-5 x 10^{38} erg/s, of order the
Eddington luminosity. (6) Two variability timescales are associated with this
process: a long one, which is related to the accretion flow within the central
~10^{-4}-10^{-3} pc, and ~0.1 yr, which is related to radiation diffusion. (7)
Adiabatic models have been run for comparison and their evolution differs
profoundly from that of the FLD models, by forming a central
geometrically-thick disk. Overall, an adiabatic equation of state is not a good
approximation to the advanced stage of direct collapse, mainly because the
radiation in the FLD is capable of escaping due to anisotropy in the optical
depth and associated gradients.Comment: 19 pages, 17 figures, MNRAS, in press; correcting typo
Direct Collapse to Supermassive Black Hole Seeds with Radiation Transfer: Cosmological Halos
We have modeled direct collapse of a primordial gas within dark matter halos
in the presence of radiative transfer, in high-resolution zoom-in simulations
in a cosmological framework, down to the formation of the photosphere and the
central object. Radiative transfer has been implemented in the flux-limited
diffusion (FLD) approximation. Adiabatic models were run for comparison. We
find that (a) the FLD flow forms an irregular central structure and does not
exhibit fragmentation, contrary to adiabatic flow which forms a thick disk,
driving a pair of spiral shocks, subject to Kelvin-Helmholtz shear instability
forming fragments; (b) the growing central core in the FLD flow quickly reaches
~10 Mo and a highly variable luminosity of 10^{38}-10^{39} erg/s, comparable to
the Eddington luminosity. It experiences massive recurrent outflows driven by
radiation force and thermal pressure gradients, which mix with the accretion
flow and transfer the angular momentum outwards; and (c) the interplay between
these processes and a massive accretion, results in photosphere at ~10 AU. We
conclude that in the FLD model (1) the central object exhibits dynamically
insignificant rotation and slower than adiabatic temperature rise with density;
(2) does not experience fragmentation leading to star formation, thus promoting
the fast track formation of a supermassive black hole (SMBH) seed; (3)
inclusion of radiation force leads to outflows, resulting in the mass
accumulation within the central 10^{-3} pc, which is ~100 times larger than
characteristic scale of star formation. The inclusion of radiative transfer
reveals complex early stages of formation and growth of the central structure
in the direct collapse scenario of SMBH seed formation.Comment: 19 pages, 16 figures, MNRAS, accepted for publicatio
Prospectus, August 4, 2010
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The Odd One Out
I looked in disbelief at the kid who’d just snatched my backpack. I’d known him since fi rst grade, and he’d earned his reputation as a cocky prankster. We were 13 now. But we were friends, classmates… Scholars’ Bowl teammates, for Pete’s sake. Th is was a joke, surely. “Say it! Or your backpack’s going in the highway.” He mock-threw it toward the 5 p.m. traffi c in front of our high school, and I lunged for it, grabbing a strap. We wrestled over the $10 Wal-Mart bag for a few moments before he begrudgingly released it
Supermassive Black Hole Formation at High Redshifts via Direct Collapse in a Cosmological Context
We study the early stage of the formation of seed supermassive black holes via direct collapse in dark matter (DM) haloes, in the cosmological context. We perform high-resolution zoom-in simulations of such collapse at high z. Using the adaptive mesh refinement code enzo, we resolve the formation and growth of a DM halo, until its virial temperature reaches ∼104 K, atomic cooling turns on, and collapse ensues. We demonstrate that direct collapse proceeds in two stages, although they are not well separated. The first stage is triggered by the onset of atomic cooling, and leads to rapidly increasing accretion rate with radius, from Ṁ~0.1M⊙yr−1 at the halo virial radius to few M⊙ yr−1, around the scale radius Rs ∼ 30 pc of the NFW DM density profile. The second stage of the collapse commences when the gas density takes precedence over the DM density. This is associated with the gas decoupling from the DM gravitational potential. The ensuing collapse approximates that of an isothermal sphere with Ṁ(r) ~ const. We confirm that the gas loses its angular momentum through non-axisymmetric perturbations and gravitational torques, to overcome the centrifugal barrier. During the course of the collapse, this angular momentum transfer process happens on nearly all spatial scales, and the angular momentum vector of the gas varies with position and time. Collapsing gas also exhibits supersonic turbulent motions which suppress gas fragmentation, and are characterized by density PDF consisting of a lognormal part and a high-density power-law tail
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