13,554 research outputs found
Stellar Dynamics around Black Holes in Galactic Nuclei
We classify orbits of stars that are bound to central black holes in galactic
nuclei. The stars move under the combined gravitational influences of the black
hole and the central star cluster. Within the sphere of influence of the black
hole, the orbital periods of the stars are much shorter than the periods of
precession. We average over the orbital motion and end up with a simpler
problem and an extra integral of motion: the product of the black hole mass and
the semimajor axis of the orbit. Thus the black hole enforces some degree of
regularity in its neighborhood. Well within the sphere of influence, (i)
planar, as well as three dimensional, axisymmetric configurations-both of which
could be lopsided-are integrable, (ii) fully three dimensional clusters with no
spatial symmetry whatsover must have semi-regular dynamics with two integrals
of motion. Similar considerations apply to stellar orbits when the black hole
grows adiabatically. We introduce a family of planar, non-axisymmetric
potential perturbations, and study the orbital structure for the harmonic case
in some detail. In the centered potentials there are essentially two main
families of orbits: the familiar loops and lenses, which were discussed in
Sridhar and Touma (1997, MNRAS, 287, L1-L4). We study the effect of
lopsidedness, and identify a family of loop orbits, whose orientation
reinforces the lopsidedness, an encouraging sign for the construction of
self-consistent models of eccentric, discs around black holes, such as in M31
and NGC 4486B.Comment: to appear in MNRAS, 10 pages, latex, 20 POstScript figure
The Lyman Break Galaxies: their Progenitors and Descendants
We study the evolution of Lyman Break Galaxies (LBGs) from z=5 to z=0 by
tracing the merger trees of galaxies in a large-scale hydrodynamic simulation
based on a Lambda cold dark matter model. In particular, we emphasize on the
range of properties of the sample selected by the rest-frame V band luminosity,
in accordance with recent near-IR observations. The predicted rest-frame V band
luminosity function agrees well with the observed one when dust extinction is
taken into account. The stellar content and the star formation histories of
LBGs are also studied. We find that the LBGs intrinsically brighter than
Mv=-21.0 at z=3 have stellar masses of at least 10^9\Msun, with a median of
10^{10}h^{-1}\Msun. The brightest LBGs (Mv<-23) at z=3 merge into
clusters/groups of galaxies at z=0, as suggested from clustering studies of
LBGs. Roughly one half of the galaxies with -23<Mv<-22 at z=3 fall into
groups/clusters, and the other half become typical L* galaxies at z=0 with
stellar mass of ~10^{11}\Msun. Descendants of LBGs at the present epoch have
formed roughly 30% of their stellar mass by z=3, and the half of their current
stellar population is 10 Gyr old, favoring the scenario that LBGs are the
precursors of the present day spheroids. We find that the most luminous LBGs
have experienced a starburst within 500 Myr prior to z=3, but also have formed
stars continuously over a period of 1 Gyr prior to z=3 when all the star
formation in progenitors is coadded. We also study the evolution of the mean
stellar metallicity distribution of galaxies, and find that the entire
distribution shifts to lower metallicity at higher redshift. The observed
sub-solar metallicity of LBGs at z=3 is naturally predicted in our simulation.Comment: 29 pages, including 11 figures, ApJ in press. One reference adde
Neutrinos from Early-Phase, Pulsar-Driven Supernovae
Neutron stars, just after their formation, are surrounded by expanding,
dense, and very hot envelopes which radiate thermal photons. Iron nuclei can be
accelerated in the wind zones of such energetic pulsars to very high energies.
These nuclei photo-disintegrate and their products lose energy efficiently in
collisions with thermal photons and with the matter of the envelope, mainly via
pion production. When the temperature of the radiation inside the envelope of
the supernova drops below K, these pions decay before
losing energy and produce high energy neutrinos. We estimate the flux of muon
neutrinos emitted during such an early phase of the pulsar - supernova envelope
interaction. We find that a 1 km neutrino detector should be able to detect
neutrinos above 1 TeV within about one year after the explosion from a
supernova in our Galaxy. This result holds if these pulsars are able to
efficiently accelerate nuclei to energies eV, as postulated
recently by some authors for models of Galactic acceleration of the extremely
high energy cosmic rays (EHE CRs).Comment: 16 pages, 3 figures, revised version submitted to Ap
The matter content of the jet in M87: evidence for an electron-positron jet
Recent observations have allowed the geometry and kinematics of the M87 jet
to be tightly constrained. We combine these constraints with historical Very
Long Baseline Interferometry (VLBI) results and the theory of synchrotron
self-absorbed radio cores in order to investigate the physical properties of
the jet. Our results strongly suggest the jet to be dominated by an
electron-positron (pair) plasma. Although our conservative constraints cannot
conclusively dismiss an electron-proton plasma, the viability of this solution
is extremely vulnerable to further tightening of VLBI surface brightness
limits. The arguments presented, coupled with future high-resolution
multi-frequency VLBI studies of the jet core, will be able to firmly
distinguish these two possibilities.Comment: 8 pages, 1 ps figure. Revised and accepted for publication in MNRA
X-ray emission from the Ultramassive Black Hole candidate NGC1277: implications and speculation on its origin
We study the X-ray emission from NGC1277, a galaxy in the core of the Perseus
cluster, for which van den Bosch et al. have recently claimed the presence of
an UltraMassive Black Hole (UMBH) of mass 1.7 times 10^10 Msun, unless the IMF
of the stars in the stellar bulge is extremely bottom heavy. The X-rays
originate in a power-law component of luminosity 1.3 times 10^40 erg/s embedded
in a 1 keV thermal minicorona which has a half-light radius of about 360 pc,
typical of many early-type galaxies in rich clusters of galaxies. If Bondi
accretion operated onto the UMBH from the minicorona with a radiative
efficiency of 10 per cent, then the object would appear as a quasar with
luminosity 10^46 erg/s, a factor of almost 10^6 times higher than observed. The
accretion flow must be highly radiatively inefficient, similar to past results
on M87 and NGC3115. The UMBH in NGC1277 is definitely not undergoing any
significant growth at the present epoch. We note that there are 3 UMBH
candidates in the Perseus cluster and that the inferred present mean mass
density in UMBH could be 10^5 Msun/Mpc^3, which is 20 to 30 per cent of the
estimated mean mass density of all black holes. We speculate on the implied
growth of UMBH and their hosts, and discuss the possibiity that extreme AGN
feedback could make all UMBH host galaxies have low stellar masses at redshifts
around 3. Only those which end up at the centres of groups and clusters later
accrete large stellar envelopes and become Brightest Cluster Galaxies. NGC1277
and the other Perseus core UMBH, NGC1270, have not however been able to gather
more stars or gas owing to their rapid orbital motion in the cluster core.Comment: 5 pages, 4 figures, MNRAS in pres
Is it time for integration of surgical skills simulation into the United Kingdom undergraduate medical curriculum? A perspective from King’s College London School of Medicine
PURPOSE: Changes in undergraduate medical curricula, combined with reforms in postgraduate education, have training implications for surgical skills acquisition in a climate of reduced clinical exposure. Confidence and prior experience influences the educational impact of learning. Currently there is no basic surgical skills (BSS) programme integrated into undergraduate curricula in the United Kingdom. We explored the role of a dedicated BSS programme for undergraduates in improving confidence and influencing careers in King's College London School of Medicine, and the programme was evaluated. METHODS: A programme was designed in-line with the established Royal College of Surgeons course. Undergraduates were taught four key skills over four weeks: knot-tying, basic-suturing, tying-at-depth and chest-drain insertion, using low-fidelity bench-top models. A Likert-style questionnaire was designed to determine educational value and influence on career choice. Qualitative data was collected. RESULTS: Only 29% and 42% of students had undertaken previous practice in knot-tying and basic suturing, respectively. 96% agreed that skills exposure prior to starting surgical rotations was essential and felt a dedicated course would augment undergraduate training. There was a significant increase in confidence in the practice and knowledge of all skills taught (p<0.01), with a greater motivation to be actively involved in the surgical firm and theatres. CONCLUSION: A simple, structured BSS programme can increase the confidence and motivation of students. Early surgical skills targeting is valuable for students entering surgical, related allied, and even traditionally non-surgical specialties such as general practice. Such experience can increase the confidence of future junior doctors and trainees. We advocate the introduction of a BSS programme into United Kingdom undergraduate curricula
Monitoring and Performance Analysis of a Large Non-domestic Ground Source Heat Pump Installation
Application of GSHP systems to provide heating and cooling for non-domestic buildings is seen as a viable and effective way of reducing carbon emissions and achieving design renewable energy targets. The application of GSHP systems and their optimal design can be improved through use of reliable system design and simulation models. To assess the validity of design models, availability of high quality field data is critical. Many experimental and monitoring studies have been concerned with domestic scale GSHP installations. In this research work a monitoring system has been implemented to collect high quality dataset from functioning GSHP heating and cooling system in a large educational building at De Montfort University. Operational data have been logged for every minute since the system was commissioned and will provide high quality heat transfer and energy data that will be used for GSHP system model development and validation. Preliminary performance analysis has been carried out and this paper describes the GSHP installation and the monitoring system. Operational data collected over the first year is presented along with a detailed analysis of system performance. The daily average COP of Heat Pumps varied between 3 and 6. The seasonal COP of the system was found to be 4.13. When the ground loop circulating pump was taken into consideration the seasonal COP was found to be 3.41. The reasons for performance variation over different periods are discussed
Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies
We show that the observed upper bound on the line-of-sight velocity
dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a
simple dynamical origin within the LCDM cosmological model, under two main
hypotheses. The first is that most of the stars now in the luminous parts of a
giant elliptical formed at redshift z>6. Subsequently, the stars behaved
dynamically just as an additional component of the dark matter. The second
hypothesis is that the mass distribution characteristic of a newly formed dark
matter halo forgets such details of the initial conditions as the stellar
"collisionless matter" that was added to the dense parts of earlier generations
of halos. We also assume that the stellar velocity dispersion does not evolve
much at z<6, because a massive host halo grows mainly by the addition of
material at large radii well away from the stellar core of the galaxy. These
assumptions lead to a predicted number density of ellipticals as a function of
stellar velocity dispersion that is in promising agreement with the Sloan
Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio
Double-layer shocks in a magnetized quantum plasma
The formation of small but finite amplitude electrostatic shocks in the
propagation of quantum ion-acoustic waves (QIAWs) obliquely to an external
magnetic field is reported in a quantum electron-positron-ion (e-p-i) plasma.
Such shocks are seen to have double-layer (DL) structures composed of the
compressive and accompanying rarefactive slow-wave fronts. Existence of such DL
shocks depends critically on the quantum coupling parameter associated with
the Bohm potential and the positron to electron density ratio . The
profiles may, however, steepen initially and reach a steady state with a number
of solitary waves in front of the shocks. Such novel DL shocks could be a good
candidate for particle acceleration in intense laser-solid density plasma
interaction experiments as well as in compact astrophysical objects, e.g.,
magnetized white dwarfs.Comment: 4 pages, 1 figure (to appear in Physical Review E
Line emission from gamma-ray burst environments
The time and angle dependent line and continuum emission from a dense torus
around a cosmological gamma-ray burst source is simulated, taking into account
photoionization, collisional ionization, recombination, and electron heating
and cooling due to various processes. The importance of the hydrodynamical
interaction between the torus and the expanding blast wave is stressed. Due to
the rapid deceleration of the blast wave as it interacts with the dense torus,
the material in the torus will be illuminated by a drastically different photon
spectrum than observable through a low-column-density line of sight, and will
be heated by the hydrodynamical interaction between the blast wave and the
torus. A model calculation to reproduce the Fe K-alpha line emission observed
in the X-ray afterglow of GRB 970508 is presented. The results indicate that ~
10^{-4} solar masses of iron must be concentrated in a region of less than
10^{-3} pc. The illumination of the torus material due to the hydrodynamic
interaction of the blast wave with the torus is the dominant heating and
ionization mechanism leading to the formation of the iron line. These results
suggest that misaligned GRBs may be detectable as X-ray flashes with pronounced
iron emission line features.Comment: Accepted for publication in ApJ. Updated recombination rate data;
discussion on element abundances added; references update
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