611 research outputs found
The Stellar Mass Evolution of Galaxies in the NICMOS Ultra Deep Field
We measure the build-up of the stellar mass of galaxies from z=6 to z=1.
Using 15 band multicolor imaging data in the NICMOS Ultra Deep Field we derive
photometric redshifts and masses for 796 galaxies down to H(AB)=26.5. The
derived evolution of the global stellar mass density of galaxies is consistent
with previous star formation rate density measurements over the observed range
of redshifts. Beyond the observed range, maintaining consistency between the
global stellar mass and the observed star formation rate suggests the epoch of
galaxy formation was z=16.Comment: 18 pages, 3 figures, submitted to AJ. Data at:
http://orca.phys.uvic.ca/~gwyn/MMM/nicmos.htm
A Testable Solution of the Cosmological Constant and Coincidence Problems
We present a new solution to the cosmological constant (CC) and coincidence
problems in which the observed value of the CC, , is linked to other
observable properties of the universe. This is achieved by promoting the CC
from a parameter which must to specified, to a field which can take many
possible values. The observed value of Lambda ~ 1/(9.3 Gyrs)^2\Lambda$-values
and does not rely on anthropic selection effects. Our model includes no
unnatural small parameters and does not require the introduction of new
dynamical scalar fields or modifications to general relativity, and it can be
tested by astronomical observations in the near future.Comment: 31 pages, 4 figures; v2: version accepted by Phys. Rev.
Direct Detection Rates of Dark Matter Coupled to Dark Energy
We investigate the effect of a coupling between dark matter and dark energy
on the rates for the direct detection of dark matter. The magnitude of the
effect depends on the strength of this new interaction relative to
gravity. The resulting isothermal velocity distribution for dark matter in
galaxy halos is still Maxwell-Boltzmann (M-B), but the characteristic velocity
and the escape velocity are increased by . We adopt a
phenomenological approach and consider values of near unity. For such
values we find that: (i) The (time averaged) event rate increases for light
WIMPs, while it is somewhat reduced for WIMP masses larger than 100 GeV. (ii)
The time dependence of the rate arising from the modulation amplitude is
decreased compared to the standard M-B velocity distribution. (iii) The average
and maximum WIMP energy increase proportionally to , which, for
sufficiently massive WIMPs, allows the possibility of designing experiments
measuring rays following nuclear de-excitation.Comment: 16 pages, 7 figure
A Method for Individual Source Brightness Estimation in Single- and Multi-band Data
We present a method of reliably extracting the flux of individual sources
from sky maps in the presence of noise and a source population in which number
counts are a steeply falling function of flux. The method is an extension of a
standard Bayesian procedure in the millimeter/submillimeter literature. As in
the standard method, the prior applied to source flux measurements is derived
from an estimate of the source counts as a function of flux, dN/dS. The key
feature of the new method is that it enables reliable extraction of properties
of individual sources, which previous methods in the literature do not. We
first present the method for extracting individual source fluxes from data in a
single observing band, then we extend the method to multiple bands, including
prior information about the spectral behavior of the source population(s). The
multi-band estimation technique is particularly relevant for classifying
individual sources into populations according to their spectral behavior. We
find that proper treatment of the correlated prior information between
observing bands is key to avoiding significant biases in estimations of
multi-band fluxes and spectral behavior, biases which lead to significant
numbers of misclassified sources. We test the single- and multi-band versions
of the method using simulated observations with observing parameters similar to
that of the South Pole Telescope data used in Vieira, et al. (2010).Comment: 11 emulateapj pages, 3 figures, revised to match published versio
Galaxy Satellites and the Weak Equivalence Principle
Numerical simulations of the effect of a long-range scalar interaction (LRSI)
acting only on nonbaryonic dark matter, with strength comparable to gravity,
show patterns of disruption of satellites that can agree with what is seen in
the Milky Way. This includes the symmetric Sagittarius stellar stream. The
exception presented here to the Kesden and Kamionkowski demonstration that an
LRSI tends to produce distinctly asymmetric streams follows if the LRSI is
strong enough to separate the stars from the dark matter before tidal
disruption of the stellar component, and if stars dominate the mass in the
luminous part of the satellite. It requires that the Sgr galaxy now contains
little dark matter, which may be consistent with the Sgr stellar velocity
dispersion, for in the simulation the dispersion at pericenter exceeds virial.
We present other examples of simulations in which a strong LRSI produces
satellites with large mass-to-light ratio, as in Draco, or free streams of
stars, which might be compared to "orphan" streams.Comment: 14 pages, accepted for publication in PR
Core Collapse via Coarse Dynamic Renormalization
In the context of the recently developed "equation-free" approach to
computer-assisted analysis of complex systems, we extract the self-similar
solution describing core collapse of a stellar system from numerical
experiments. The technique allows us to side-step the core "bounce" that occurs
in direct N-body simulations due to the small-N correlations that develop in
the late stages of collapse, and hence to follow the evolution well into the
self-similar regime.Comment: 5 pages, 3 figure
The Power of General Relativity
We study the cosmological and weak-field properties of theories of gravity
derived by extending general relativity by means of a Lagrangian proportional
to . This scale-free extension reduces to general relativity when
. In order to constrain generalisations of general relativity of
this power class we analyse the behaviour of the perfect-fluid Friedmann
universes and isolate the physically relevant models of zero curvature. A
stable matter-dominated period of evolution requires or . The stable attractors of the evolution are found. By considering the
synthesis of light elements (helium-4, deuterium and lithium-7) we obtain the
bound We evaluate the effect on the power spectrum of
clustering via the shift in the epoch of matter-radiation equality. The horizon
size at matter--radiation equality will be shifted by for a value of
We study the stable extensions of the Schwarzschild
solution in these theories and calculate the timelike and null geodesics. No
significant bounds arise from null geodesic effects but the perihelion
precession observations lead to the strong bound assuming that Mercury follows a timelike geodesic. The combination of
these observational constraints leads to the overall bound on theories of this type.Comment: 26 pages and 5 figures. Published versio
Computational capacity of the universe
Merely by existing, all physical systems register information. And by
evolving dynamically in time, they transform and process that information. The
laws of physics determine the amount of information that a physical system can
register (number of bits) and the number of elementary logic operations that a
system can perform (number of ops). The universe is a physical system. This
paper quantifies the amount of information that the universe can register and
the number of elementary operations that it can have performed over its
history. The universe can have performed no more than ops on
bits.Comment: 17 pages, TeX. submitted to Natur
Ly-alpha Emission-Line Galaxies at z = 3.1 in the Extended Chandra Deep Field South
We describe the results of an extremely deep, 0.28 deg^2 survey for z = 3.1
Ly-alpha emission-line galaxies in the Extended Chandra Deep Field South. By
using a narrow-band 5000 Anstrom filter and complementary broadband photometry
from the MUSYC survey, we identify a statistically complete sample of 162
galaxies with monochromatic fluxes brighter than 1.5 x 10^-17 ergs cm^-2 s^-1
and observers frame equivalent widths greater than 80 Angstroms. We show that
the equivalent width distribution of these objects follows an exponential with
a rest-frame scale length of w_0 = 76 +/- 10 Angstroms. In addition, we show
that in the emission line, the luminosity function of Ly-alpha galaxies has a
faint-end power-law slope of alpha = -1.49 +/- 0.4, a bright-end cutoff of log
L^* = 42.64 +/- 0.2, and a space density above our detection thresholds of 1.46
+/- 0.12 x 10^-3 h70^3 galaxies Mpc^-3. Finally, by comparing the emission-line
and continuum properties of the LAEs, we show that the star-formation rates
derived from Ly-alpha are ~3 times lower than those inferred from the
rest-frame UV continuum. We use this offset to deduce the existence of a small
amount of internal extinction within the host galaxies. This extinction,
coupled with the lack of extremely-high equivalent width emitters, argues that
these galaxies are not primordial Pop III objects, though they are young and
relatively chemically unevolved.Comment: 45 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Forces between electric charges in motion: Rutherford scattering, circular Keplerian orbits, action-at-a-distance and Newton's third law in relativistic classical electrodynamics
Standard formulae of classical electromagnetism for the forces between
electric charges in motion derived from retarded potentials are compared with
those obtained from a recently developed relativistic classical electrodynamic
theory with an instantaneous inter-charge force. Problems discussed include
small angle Rutherford scattering, Jackson's recent `torque paradox' and
circular Keplerian orbits. Results consistent with special relativity are
obtained only with an instantaneous interaction. The impossiblity of stable
circular motion with retarded fields in either classical electromagnetism or
Newtonian gravitation is demonstrated.Comment: 26 pages, 5 figures. QED and special relativity forbid retarded
electromagnetic forces. See also physics/0501130. V2 has typos corrected,
minor text modifications and updated references. V3 has further typos removed
and added text and reference
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