5,788 research outputs found
Mission: Impossible (Escape from the Lyman Limit)
We investigate the intrinsic opacity of high-redshift galaxies to outgoing
ionising photons using high-quality photometry of a sample of 27
spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep
Field. Our measurement is based on maximum-likelihood fitting of model galaxy
spectral energy distributions-including the effects of intrinsic Lyman-limit
absorption and random realizations of intervening Lyman-series and Lyman-limit
absorption-to photometry of galaxies from space- and ground-based broad-band
images. Our method provides several important advantages over the methods used
by previous groups, including most importantly that two-dimensional sky
subtraction of faint-galaxy images is more robust than one-dimensional sky
subtraction of faint-galaxy spectra. We find at the 3sigma statistical
confidence level that on average no more than 4% of the ionising photons escape
galaxies of redshift 1.9<z<3.5. This result is consistent with observations of
low- and moderate-redshift galaxies but is in direct contradiction to a recent
result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies.
Dividing our sample in subsamples according to luminosity, intrinsic
ultraviolet colour, and redshift, we find no evidence for selection effects
that could explain such discrepancy. Even when all systematic effects are
included, the data could not realistically accomodate any escape fraction value
larger than ~15%.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 8 pages, 4 b/w figures, MNRAS styl
Violation of action--reaction and self-forces induced by nonequilibrium fluctuations
We show that the extension of Casimir-like forces to fluctuating fluids
driven out of equilibrium can exhibit two interrelated phenomena forbidden at
equilibrium: self-forces can be induced on single asymmetric objects and the
action--reaction principle between two objects can be violated. These effects
originate in asymmetric restrictions imposed by the objects' boundaries on the
fluid's fluctuations. They are not ruled out by the second law of
thermodynamics since the fluid is in a nonequilibrium state. Considering a
simple reaction--diffusion model for the fluid, we explicitly calculate the
self-force induced on a deformed circle. We also show that the action--reaction
principle does not apply for the internal Casimir forces exerting between a
circle and a plate. Their sum, instead of vanishing, provides the self-force on
the circle-plate assembly.Comment: 4 pages, 1 figure. V2: New title; Abstract partially rewritten;
Largely enhanced introductory and concluding remarks (incl. new Refs.
The Unusual Spectral Energy Distribution of a Galaxy Previously Reported to be at Redshift 6.68
Observations of distant galaxies are important both for understanding how
galaxies form and for probing the physical conditions of the universe at the
earliest epochs. It is, however, extremely difficult to identify galaxies at
redshift z>5, because these galaxies are faint and exhibit few spectral
features. In a previous work, we presented observations that supported the
identification of a galaxy at redshift z = 6.68 in a deep STIS field. Here we
present new ground-based photometry of the galaxy. We find that the galaxy
exhibits moderate detections of flux in the optical B and V images that are
inconsistent with the expected absence of flux at wavelength shortward of the
redshifted Lyman-alpha emission line of a galaxy at redshift z>5. In addition,
the new broad-band imaging data not only show flux measurements of this galaxy
that are incompatible with the previous STIS measurement, but also suggest a
peculiar spectral energy distribution that cannot be fit with any galaxy
spectral template at any redshift. We therefore conclude that the redshift
identification of this galaxy remains undetermined.Comment: 9 pages, 2 figures; To appear in Nature (30 November 2000
Alternative analysis to perturbation theory
We develop an alternative approach to time independent perturbation theory in
non-relativistic quantum mechanics. The method developed has the advantage to
provide in one operation the correction to the energy and to the wave function,
additionally we can analyze the time evolution of the system. To verify our
results, we apply our method to the harmonic oscillator perturbed by a
quadratic potential. An alternative form of the Dyson series, in matrix form
instead of integral form, is also obtained.Comment: 12 pages, no figure
Lost and found: the radial quantum number of Laguerre-Gauss modes
We introduce an operator linked with the radial index in the Laguerre-Gauss
modes of a two-dimensional harmonic oscillator in cylindrical coordinates. We
discuss ladder operators for this variable, and confirm that they obey the
commutation relations of the su(1,1) algebra. Using this fact, we examine how
basic quantum optical concepts can be recast in terms of radial modes.Comment: Some minor typos fixed
On the structure of the sets of mutually unbiased bases for N qubits
For a system of N qubits, spanning a Hilbert space of dimension d=2^N, it is
known that there exists d+1 mutually unbiased bases. Different construction
algorithms exist, and it is remarkable that different methods lead to sets of
bases with different properties as far as separability is concerned. Here we
derive the four sets of nine bases for three qubits, and show how they are
unitarily related. We also briefly discuss the four-qubit case, give the
entanglement structure of sixteen sets of bases,and show some of them, and
their interrelations, as examples. The extension of the method to the general
case of N qubits is outlined.Comment: 16 pages, 10 tables, 1 figur
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