41,241 research outputs found
Group finding in the stellar halo using M-giants in 2MASS: An extended view of the Pisces Overdensity?
A density based hierarchical group-finding algorithm is used to identify
stellar halo structures in a catalog of M-giants from the Two Micron All Sky
Survey (2MASS). The intrinsic brightness of M-giant stars means that this
catalog probes deep into the halo where substructures are expected to be
abundant and easy to detect. Our analysis reveals 16 structures at high
Galactic latitude (greater than 15 degree), of which 10 have been previously
identified. Among the six new structures two could plausibly be due to masks
applied to the data, one is associated with a strong extinction region and one
is probably a part of the Monoceros ring. Another one originates at low
latitudes, suggesting some contamination from disk stars, but also shows
protrusions extending to high latitudes, implying that it could be a real
feature in the stellar halo. The last remaining structure is free from the
defects discussed above and hence is very likely a satellite remnant. Although
the extinction in the direction of the structure is very low, the structure
does match a low temperature feature in the dust maps. While this casts some
doubt on its origin, the low temperature feature could plausibly be due to real
dust in the structure itself. The angular position and distance of this
structure encompass the Pisces overdensity traced by RR Lyraes in Stripe 82 of
the Sloan Digital Sky Survey (SDSS). However, the 2MASS M-giants indicate that
the structure is much more extended than what is visible with the SDSS, with
the point of peak density lying just outside Stripe 82. The morphology of the
structure is more like a cloud than a stream and reminiscent of that seen in
simulations of satellites disrupting along highly eccentric orbits.Comment: Accepted for publication in Ap
How CMB and large-scale structure constrain chameleon interacting dark energy
We explore a chameleon type of interacting dark matter-dark energy scenario
in which a scalar field adiabatically traces the minimum of an effective
potential sourced by the dark matter density. We discuss extensively the effect
of this coupling on cosmological observables, especially the parameter
degeneracies expected to arise between the model parameters and other
cosmological parameters, and then test the model against observations of the
cosmic microwave background (CMB) anisotropies and other cosmological probes.
We find that the chameleon parameters and , which determine
respectively the slope of the scalar field potential and the dark matter-dark
energy coupling strength, can be constrained to and using CMB data alone. The latter parameter in particular is constrained
only by the late Integrated Sachs-Wolfe effect. Adding measurements of the
local Hubble expansion rate tightens the bound on by a factor of
two, although this apparent improvement is arguably an artefact of the tension
between the local measurement and the value inferred from Planck data in
the minimal CDM model. The same argument also precludes chameleon
models from mimicking a dark radiation component, despite a passing similarity
between the two scenarios in that they both delay the epoch of matter-radiation
equality. Based on the derived parameter constraints, we discuss possible
signatures of the model for ongoing and future large-scale structure surveys.Comment: 25 pages, 6 figure
Fuzzy clustering with volume prototypes and adaptive cluster merging
Two extensions to the objective function-based fuzzy
clustering are proposed. First, the (point) prototypes are extended to hypervolumes, whose size can be fixed or can be determined automatically from the data being clustered. It is shown that clustering with hypervolume prototypes can be formulated as the minimization of an objective function. Second, a heuristic cluster merging step is introduced where the similarity among the clusters
is assessed during optimization. Starting with an overestimation of the number of clusters in the data, similar clusters are merged in order to obtain a suitable partitioning. An adaptive threshold for merging is proposed. The extensions proposed are applied to
Gustafson–Kessel and fuzzy c-means algorithms, and the resulting extended algorithm is given. The properties of the new algorithm are illustrated by various examples
Tests of Gravity from Imaging and Spectroscopic Surveys
Tests of gravity on large-scales in the universe can be made using both
imaging and spectroscopic surveys. The former allow for measurements of weak
lensing, galaxy clustering and cross-correlations such as the ISW effect. The
latter probe galaxy dynamics through redshift space distortions. We use a set
of basic observables, namely lensing power spectra, galaxy-lensing and
galaxy-velocity cross-spectra in multiple redshift bins (including their
covariances), to estimate the ability of upcoming surveys to test gravity
theories. We use a two-parameter description of gravity that allows for the
Poisson equation and the ratio of metric potentials to depart from general
relativity. We find that the combination of imaging and spectroscopic
observables is essential in making robust tests of gravity theories. The range
of scales and redshifts best probed by upcoming surveys is discussed. We also
compare our parametrization to others used in the literature, in particular the
gamma parameter modification of the growth factor.Comment: 18 pages, 10 figures, to be submitte
Probing non-standard gravity with the growth index: a background independent analysis
Measurements of the growth index provide a clue as to whether
Einstein's field equations encompass gravity also on large cosmic scales, those
where the expansion of the universe accelerates. We show that the information
encoded in this function can be satisfactorily parameterized using a small set
of coefficients in such a way that the true scaling of the growth
index is recovered to better than in most dark energy and dark gravity
models. We find that the likelihood of current data is maximal for
and , a measurement compatible
with the CDM predictions. Moreover data favor models predicting
slightly less growth of structures than the Planck LambdaCDM scenario. The main
aim of the paper is to provide a prescription for routinely calculating, in an
analytic way, the amplitude of the growth indices in relevant
cosmological scenarios, and to show that these parameters naturally define a
space where predictions of alternative theories of gravity can be compared
against growth data in a manner which is independent from the expansion history
of the cosmological background. As the standard -plane provides a tool
to identify different expansion histories and their relation to various
cosmological models, the -plane can thus be used to locate different
growth rate histories and their relation to alternatives model of
gravity. As a result, we find that the Dvali-Gabadadze-Porrati gravity model is
rejected with a confidence level. By simulating future data sets, such
as those that a Euclid-like mission will provide, we also show how to tell
apart LambdaCDM predictions from those of more extreme possibilities, such as
smooth dark energy models, clustering quintessence or parameterized
post-Friedmann cosmological models.Comment: 29 pages, 21 figure
Distance, Growth Factor, and Dark Energy Constraints from Photometric Baryon Acoustic Oscillation and Weak Lensing Measurements
Baryon acoustic oscillations (BAOs) and weak lensing (WL) are complementary
probes of cosmology. We explore the distance and growth factor measurements
from photometric BAO and WL techniques and investigate the roles of the
distance and growth factor in constraining dark energy. We find for WL that the
growth factor has a great impact on dark energy constraints but is much less
powerful than the distance. Dark energy constraints from WL are concentrated in
considerably fewer distance eigenmodes than those from BAO, with the largest
contributions from modes that are sensitive to the absolute distance. Both
techniques have some well determined distance eigenmodes that are not very
sensitive to the dark energy equation of state parameters w_0 and w_a,
suggesting that they can accommodate additional parameters for dark energy and
for the control of systematic uncertainties. A joint analysis of BAO and WL is
far more powerful than either technique alone, and the resulting constraints on
the distance and growth factor will be useful for distinguishing dark energy
and modified gravity models. The Large Synoptic Survey Telescope (LSST) will
yield both WL and angular BAO over a sample of several billion galaxies. Joint
LSST BAO and WL can yield 0.5% level precision on ten comoving distances evenly
spaced in log(1+z) between redshift 0.3 and 3 with cosmic microwave background
priors from Planck. In addition, since the angular diameter distance, which
directly affects the observables, is linked to the comoving distance solely by
the curvature radius in the Friedmann-Robertson-Walker metric solution, LSST
can achieve a pure metric constraint of 0.017 on the mean curvature parameter
Omega_k of the universe simultaneously with the constraints on the comoving
distances.Comment: 15 pages, 9 figures, details and references added, ApJ accepte
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