5,620 research outputs found
Clarifying spherical collapse in coupled dark energy cosmologies
The spherical collapse model is often used to follow the evolution of
overdensities into the nonlinear regime. We describe the correct approach to be
used in coupled dark energy cosmologies, where a fifth force, different from
gravity and mediated by the dark energy scalar field, influences the collapse.
We reformulate the spherical collapse description by deriving it directly from
the set of nonlinear hydrodynamical Navier Stokes equations. By comparing with
the corresponding relativistic equations, we show how the fifth force should be
taken into account within the spherical collapse picture and clarify the
problems arising when an inhomogeneous scalar field is considered within a
spherical collapse picture. We then apply our method to the case of coupled
quintessence, where the fifth force acts among cold dark matter particles, and
to growing neutrino quintessence, where the fifth force acts between neutrinos.
Furthermore, we review this method when applied to standard cosmologies and
apply our analysis to minimally coupled quintessence and check past results for
early dark energy parametrizations.Comment: 16 pages, 13 figures, published in Physical Review D, minor changes
and correction
An imaging K-band survey - I: The catalogue, star and galaxy counts
We present results from a large area (552\,\sqamin) imaging -band survey
to a 5 limit of . We have optical-infrared colours of
almost all the objects in the sample. Star-galaxy discrimination is performed
and the results used to derive the infrared star and galaxy counts. -band
``no-evolution'' galaxy-count models are constructed and compared with the
observed data. In the infrared, there is no counterpart for the large excess of
faint galaxies over the no-evolution model seen in optical counts. However, we
show that the counts can be remarkably insensitive to evolution under
certain reasonable assumptions. Finally, model predictions for -selected
redshift surveys are derived.Comment: MNRAS in press. 21 pages plain TeX; figs plus table 4 available via
anonymous ftp from /pub/kgb/paper1/sissa.uu at ftp.ast.cam.ac.u
Measuring the galaxy power spectrum with multiresolution decomposition -- II. diagonal and off-diagonal power spectra of the LCRS galaxies
The power spectrum estimator based on the discrete wavelet transform (DWT)
for 3-dimensional samples has been studied. The DWT estimator for
multi-dimensional samples provides two types of spectra with respect to
diagonal and off-diagonal modes, which are very flexible to deal with
configuration-related problems in the power spectrum detection. With simulation
samples and mock catalogues of the Las Campanas redshift survey (LCRS), we show
(1) the slice-like geometry of the LCRS doesn't affect the off-diagonal power
spectrum with ``slice-like'' mode; (2) the Poisson sampling with the LCRS
selection function doesn't cause more than 1- error in the DWT power
spectrum; and (3) the powers of peculiar velocity fluctuations, which cause the
redshift distortion, are approximately scale-independent. These results insure
that the uncertainties of the power spectrum measurement are under control. The
scatter of the DWT power spectra of the six strips of the LCRS survey is found
to be rather small. It is less than 1- of the cosmic variance of mock
samples in the wavenumber range h Mpc. To fit the detected
LCRS diagonal DWT power spectrum with CDM models, we find that the best-fitting
redshift distortion parameter is about the same as that obtained from
the Fourier power spectrum. The velocity dispersions for SCDM and
CDM models are also consistent with other detections with
the LCRS. A systematic difference between the best-fitting parameters of
diagonal and off-diagonal power spectra has been significantly measured. This
indicates that the off-diagonal power spectra are capable of providing
information about the power spectrum of galaxy velocity field.Comment: AAS LaTeX file, 41 pages, 10 figures included, accepted for
publication in Ap
Cluster Correlation in Mixed Models
We evaluate the dependence of the cluster correlation length r_c on the mean
intercluster separation D_c, for three models with critical matter density,
vanishing vacuum energy (Lambda = 0) and COBE normalized: a tilted CDM (tCDM)
model (n=0.8) and two blue mixed models with two light massive neutrinos
yielding Omega_h = 0.26 and 0.14 (MDM1 and MDM2, respectively). All models
approach the observational value of sigma_8 (and, henceforth, the observed
cluster abundance) and are consistent with the observed abundance of Damped
Lyman_alpha systems. Mixed models have a motivation in recent results of
neutrino physics; they also agree with the observed value of the ratio
sigma_8/sigma_25, yielding the spectral slope parameter Gamma, and nicely fit
LCRS reconstructed spectra. We use parallel AP3M simulations, performed in a
wide box (side 360/h Mpc) and with high mass and distance resolution, enabling
us to build artificial samples of clusters, whose total number and mass range
allow to cover the same D_c interval inspected through APM and Abell cluster
clustering data. We find that the tCDM model performs substantially better than
n=1 critical density CDM models. Our main finding, however, is that mixed
models provide a surprisingly good fit of cluster clustering data.Comment: 22 pages + 10 Postscript figures. Accepted for publication in Ap
The Angular Power Spectrum of EDSGC Galaxies
We determine the angular power spectrum, C_l, of the Edinburgh/Durham
Southern Galaxy Catalog (EDSGC) and use this statistic to constrain
cosmological parameters. Our methods for determining C_l, and the parameters
that affect it are based on those developed for the analysis of cosmic
microwave background maps. We expect them to be useful for future surveys.
Assuming flat cold dark matter models with a cosmological constant (constrained
by COBE/DMR and local cluster abundances), and a scale--independent bias, b, we
find good fits to the EDSGC angular power spectrum with 1.11 < b < 2.35 and 0.2
< Omega_m < 0.55 at 95% confidence. These results are not significantly
affected by the ``integral constraint'' or extinction by interstellar dust, but
may be by our assumption of Gaussianity.Comment: 11 pages, 9 figures, version to appear in Ap
Simulated Extragalactic Observations with a Cryogenic Imaging Spectrophotometer
In this paper we explore the application of cryogenic imaging
spectrophotometers. Prototypes of this new class of detector, such as
superconducting tunnel junctions (STJs) and transition edge sensors (TESs),
currently deliver low resolution imaging spectrophotometry with high quantum
efficiency (70-100%) and no read noise over a wide bandpass in the visible to
near-infrared. In order to demonstrate their utility and the differences in
observing strategy needed to maximize their scientific return, we present
simulated observations of a deep extragalactic field. Using a simple analytic
technique, we can estimate both the galaxy redshift and spectral type more
accurately than is possible with current broadband techniques. From our
simulated observations and a subsequent discussion of the expected migration
path for this new technology, we illustrate the power and promise of these
devices.Comment: 30 pages, 10 figures, accepted for publication in the Astronomical
Journa
Temperature determination via STJ optical spectroscopy
ESA's Superconducting Tunnel Junction (STJ) optical photon-counting camera
(S-Cam2) incorporates an array of pixels with intrinsic energy sensitivity.
Using the spectral fitting technique common in X-ray astronomy, we fit black
bodies to nine stellar spectra, ranging from cool flare stars to hot white
dwarfs. The measured temperatures are consistent with literature values at the
expected level of accuracy based on the predicted gain stability of the
instrument. Having also demonstrated that systematic effects due to count rate
are likely to be small, we then proceed to apply the temperature determination
method to four cataclysmic variable (CV) binary systems. In three cases we
measure the temperature of the accretion stream, while in the fourth we measure
the temperature of the white dwarf. The results are discussed in the context of
existing CV results. We conclude by outlining the prospects for future versions
of S-Cam.Comment: 9 pages, 9 figures (11 files); uses aa.cls; accepted for publication
in A&
Dynamical Masses in Modified Gravity
Differences in masses inferred from dynamics, such as velocity dispersions or
X-rays, and those inferred from lensing are a generic prediction of modified
gravity theories. Viable models however must include some non-linear mechanism
to restore General Relativity (GR) in dense environments, which is necessary to
pass Solar System constraints on precisely these deviations. In this paper, we
study the dynamics within virialized structures in the context of two modified
gravity models, f(R) gravity and DGP. The non-linear mechanisms to restore GR,
which f(R) and DGP implement in very different ways, have a strong impact on
the dynamics in bound objects; they leave distinctive signatures in the
dynamical mass-lensing mass relation as a function of mass and radius. We
present measurements from N-body simulations of f(R) and DGP, as well as
semi-analytical models which match the simulation results to surprising
accuracy in both cases. The semi-analytical models are useful for making the
connection to observations. Our results confirm that the environment- and
scale-dependence of the modified gravity effects have to be taken into account
when confronting gravity theories with observations of dynamics in galaxies and
clusters.Comment: 18 pages, 16 figures; submitted to PRD; v2: typos corrected,
references added, minor additions (Sec. IID
- …