2,714 research outputs found
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
HST Imaging of the Host Galaxies of High Redshift Radio-Loud Quasars
We present rest-frame UV and Ly-alpha images of spatially-resolved structures
around five high-redshift radio-loud quasars obtained with the WFPC2 camera on
the Hubble Space Telescope. We find that all five quasars are extended and this
"fuzz" contains ~5-40% of the total continuum flux and 15-65% of the Ly-alpha
flux within a radius of about 1.5 arcsec. The rest-frame UV luminosities of the
hosts are log lambda P_lambda = 11.9 to 12.5 solar luminosities (assuming no
internal dust extinction), comparable to the luminous radio galaxies at similar
redshifts and a factor 10 higher than both radio-quiet field galaxies at z~2-3
and the most UV-luminous low redshift starburst galaxies. The Ly-alpha
luminosities of the hosts are (in the log) approximately 44.3-44.9 erg/s which
are also similar to the those of luminous high redshift radio galaxies and
considerably larger than the Ly-alpha luminosities of high redshift field
galaxies. To generate the Ly-alpha luminosities of the hosts would require
roughly a few percent of the total observed ionizing luminosity of the quasar.
We find good alignment between the extended Ly-alpha and the radio sources,
strong evidence for jet-cloud interactions in two cases, again resembling radio
galaxies, and what is possibly the most luminous radio-UV synchrotron jet in
one of the hosts at z=2.110.Comment: 36 pages (latex, aas macros), 3 figures (3 gif and 10 postscript
files), accepted for publication in the the Astrophysical Journal Supplement
Serie
GRB990712: First Indication of Polarization Variability in a Gamma-ray Burst Afterglow
We report the detection of significant polarization in the optical afterglow
of GRB990712 on three instances 0.44, 0.70 and 1.45 days after the gamma-ray
burst, with (P, theta) being (2.9% +- 0.4%, 121.1 degr +- 3.5 degr), (1.2% +-
0.4%, 116.2 degr +- 10.1 degr) and (2.2% +- 0.7%, 139.2 degr +- 10.4 degr)
respectively. The polarization is intrinsic to the afterglow. The degree of
polarization is not constant, and smallest at the second measurement. The
polarization angle does not vary significantly during these observations. We
find that none of the existing models predict such polarization variations at
constant polarization angle, and suggest ways in which these models might be
modified to accommodate the observed behavior of this afterglow.Comment: 10 pages including 6 figures, accepted by ApJ. Uses aastex 5.
Generalized Massive Gravity and Galilean Conformal Algebra in two dimensions
Galilean conformal algebra (GCA) in two dimensions arises as contraction of
two copies of the centrally extended Virasoro algebra ( with ). The central charges of
GCA can be expressed in term of Virasoro central charges. For finite and
non-zero GCA central charges, the Virasoro central charges must behave as
asymmetric form . We propose that, the bulk
description for 2d GCA with asymmetric central charges is given by general
massive gravity (GMG) in three dimensions. It can be seen that, if the
gravitational Chern-Simons coupling behaves as of order
O() or (), the central charges
of GMG have the above dependence. So, in non-relativistic scaling
limit , we calculated GCA parameters and finite
entropy in term of gravity parameters mass and angular momentum of GMG.Comment: 9 page
Uniqueness in MHD in divergence form: right nullvectors and well-posedness
Magnetohydrodynamics in divergence form describes a hyperbolic system of
covariant and constraint-free equations. It comprises a linear combination of
an algebraic constraint and Faraday's equations. Here, we study the problem of
well-posedness, and identify a preferred linear combination in this divergence
formulation. The limit of weak magnetic fields shows the slow magnetosonic and
Alfven waves to bifurcate from the contact discontinuity (entropy waves), while
the fast magnetosonic wave is a regular perturbation of the hydrodynamical
sound speed. These results are further reported as a starting point for
characteristic based shock capturing schemes for simulations with
ultra-relativistic shocks in magnetized relativistic fluids.Comment: To appear in J Math Phy
Estimating the detectable rate of capture of stellar mass black holes by massive central black holes in normal galaxies
The capture and subsequent inspiral of stellar mass black holes on eccentric
orbits by central massive black holes, is one of the more interesting likely
sources of gravitational radiation detectable by LISA. We estimate the rate of
observable events and the associated uncertainties. A moderately favourable
mass function could provide many detectable bursts each year, and a detection
of at least one burst per year is very likely given our current understanding
of the populations in cores of normal spiral galaxies.Comment: 3 pages 2-column revtex Latex macro. No figures. Classical and
Quantum Gravity, accepte
Ejection of Matter and Energy from NGC 4258
It has been claimed that the megamaser observations of the nucleus of NGC
4258 show that a massive black hole is present in its center (Miyoshi et al.
1995, Greenhill et al. 1995). We show that the evidence of ejection of gas,
radio plasma, and X-ray emitting QSOs from this nucleus all show that the
ejection is coming from the center in a curving flow within a cone with angle
~40 degrees, centered at P.A. 100 degrees. This is close to the direction in
which the velocities from the megamaser have been measured, so that the
evidence taken as a whole suggests that the masering gas also is being ejected
in the same direction at velocities +/- 900 km/sec and not rotating about a
massive black hole. Thus it does not provide evidence for a black hole in the
center.
Subject headings: galaxies: nuclei: individual (NGC 4258) -- black holes --
masersComment: 7 pages, 1 Figure, LaTex using epsf.tex, submitted to Ap.J.Letter
Electron-positron outflow from black holes
Gamma-ray bursts (GRBs) appear as the brightest transient phenomena in the
Universe. The nature of the central engine in GRBs is a missing link in the
theory of fireballs to their stellar mass progenitors. Here it is shown that
rotating black holes produce electron-positron outflow when brought into
contact with a strong magnetic field. The outflow is produced by a coupling of
the spin of the black hole to the orbit of the particles. For a nearly extreme
Kerr black hole, particle outflow from an initial state of electrostatic
equilibrium has a normalized isotropic emission of erg/s, where B is the
external magnetic field strength, B_c=4.4 x 10^{13}G, and M is the mass of the
black hole. This initial outflow has a half-opening angle
. A connection with fireballs in -ray bursts is
given.Comment: 10 pages LaTe
Observations of HI Absorbing Gas in Compact Radio Sources at Cosmological Redshifts
We present an overview of the occurrence and properties of atomic gas
associated with compact radio sources at redshifts up to z=0.85. Searches for
HI 21cm absorption were made with the Westerbork Synthesis Radio Telescope at
UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57
sources with usable spectra (33%). We have found a large range in line depths,
from tau=0.16 to tau<=0.001. There is a substantial variety of line profiles,
including Gaussians of less than 10km/s, to more typically 150km/s, as well as
irregular and multi-peaked absorption profiles, sometimes spanning several
hundred km/s. Assuming uniform coverage of the entire radio source, we obtain
column depths of atomic gas between 1e19 and 3.3e21(Tsp/100K)(1/f)cm^(-2).
There is evidence for significant gas motions, but in contrast to earlier
results at low redshift, there are many sources in which the HI velocity is
substantially negative (up to v=-1420km/s) with respect to the optical
redshift, suggesting that in these sources the atomic gas, rather than falling
into the centre, may be be flowing out, interacting with the jets, or rotating
around the nucleus.Comment: 10 pages, accepted for publication in A&
High confinement, high yield Si3N4 waveguides for nonlinear optical application
In this paper we present a novel fabrication technique for silicon nitride
(Si3N4) waveguides with a thickness of up to 900 nm, which are suitable for
nonlinear optical applications. The fabrication method is based on etching
trenches in thermally oxidized silicon and filling the trenches with Si3N4.
Using this technique no stress-induced cracks in the Si3N4 layer were observed
resulting in a high yield of devices on the wafer. The propagation losses of
the obtained waveguides were measured to be as low as 0.4 dB/cm at a wavelength
of around 1550 nm.Comment: 10 pages, 4 figure
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