33,668 research outputs found
Search for L5 Earth Trojans with DECam
Most of the major planets in the Solar system support populations of co-orbiting bodies, known as Trojans, at their L4 and L5 Lagrange points. In contrast, Earth has only one known co-orbiting companion. This paper presents the results from a search for Earth Trojans (ETs) using the DECam instrument on the Blanco Telescope at CTIO. This search found no additional Trojans in spite of greater coverage compared to previous surveys of the L5 point. Therefore, the main result of this work is to place the most stringent constraints to date on the population of ETs. These constraints depend on assumptions regarding the underlying population properties, especially the slope of the magnitude distribution (which in turn depends on the size and albedo distributions of the objects). For standard assumptions, we calculate upper limits to a 90 per cent confidence limit on the L5 population of N_(ET) < 1 for magnitude H < 15.5, N_(ET) = 60–85 for H < 19.7, and N_(ET) = 97 for H = 20.4. This latter magnitude limit corresponds to Trojans ∼300 m in size for albedo 0.15. At H = 19.7, these upper limits are consistent with previous L4 ET constraints and significantly improve L5 constraints
Measuring the galaxy power spectrum with future redshift surveys
Precision measurements of the galaxy power spectrum P(k) require a data
analysis pipeline that is both fast enough to be computationally feasible and
accurate enough to take full advantage of high-quality data. We present a
rigorous discussion of different methods of power spectrum estimation, with
emphasis on the traditional Fourier method, the linear (Karhunen-Loeve; KL),
and quadratic data compression schemes, showing in what approximations they
give the same result. To improve speed, we show how many of the advantages of
KL data compression and power spectrum estimation may be achieved with a
computationally faster quadratic method. To improve accuracy, we derive
analytic expressions for handling the integral constraint, since it is crucial
that finite volume effects are accurately corrected for on scales comparable to
the depth of the survey. We also show that for the KL and quadratic techniques,
multiple constraints can be included via simple matrix operations, thereby
rendering the results less sensitive to galactic extinction and mis-estimates
of the radial selection function. We present a data analysis pipeline that we
argue does justice to the increases in both quality and quantity of data that
upcoming redshift surveys will provide. It uses three analysis techniques in
conjunction: a traditional Fourier approach on small scales, a pixelized
quadratic matrix method on large scales and a pixelized KL eigenmode analysis
to probe anisotropic effects such as redshift-space distortions.Comment: Major revisions for clarity. Matches accepted ApJ version. 23 pages,
with 2 figs included. Color figure and links at
http://www.sns.ias.edu/~max/galpower.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/galpower.html (faster from Europe) or
from [email protected]
Gas Absorption in the KH 15D System: Further Evidence for Dust Settling in the Circumbinary Disk
Na I D lines in the spectrum of the young binary KH 15D have been analyzed in
detail. We find an excess absorption component that may be attributed to
foreground interstellar absorption, and to gas possibly associated with the
solids in the circumbinary disk. The derived column density is log N_NaI = 12.5
cm^-2, centered on a radial velocity that is consistent with the systemic
velocity. Subtracting the likely contribution of the ISM leaves log N_NaI ~
12.3 cm^-2. There is no detectable change in the gas column density across the
"knife edge" formed by the opaque grain disk, indicating that the gas and
solids have very different scale heights, with the solids being highly settled.
Our data support a picture of this circumbinary disk as being composed of a
very thin particulate grain layer composed of millimeter-sized or larger
objects that are settled within whatever remaining gas may be present. This
phase of disk evolution has been hypothesized to exist as a prelude to the
formation of planetesimals through gravitational fragmentation, and is expected
to be short-lived if much gas were still present in such a disk. Our analysis
also reveals the presence of excess Na I emission relative to the comparison
spectrum at the radial velocity of the currently visible star that plausibly
arises within the magnetosphere of this still-accreting young star.Comment: Accepted for publication in ApJ, 23 pages, 6 figure
Mass inflation in a D dimensional Reissner-Nordstrom black hole: a hierarchy of particle accelerators ?
We study the geometry inside the event horizon of perturbed D dimensional
Reissner-Nordstrom-(A)dS type black holes showing that, similarly to the four
dimensional case, mass inflation also occurs for D>4. First, using the
homogeneous approximation, we show that an increase of the number of spatial
dimensions contributes to a steeper variation of the metric coefficients with
the areal radius and that the phenomenon is insensitive to the cosmological
constant in leading order. Then, using the code reported in arXiv:0904.2669
[gr-qc] adapted to D dimensions, we perform fully non-linear numerical
simulations. We perturb the black hole with a compact pulse adapting the pulse
amplitude such that the relative variation of the black hole mass is the same
in all dimensions, and determine how the black hole interior evolves under the
perturbation. We qualitatively confirm that the phenomenon is similar to four
dimensions as well as the behaviour observed in the homogeneous approximation.
We speculate about the formation of black holes inside black holes triggered by
mass inflation, and about possible consequences of this scenario.Comment: 8 pages, 6 figure
Weak Lensing Reconstruction and Power Spectrum Estimation: Minimum Variance Methods
Large-scale structure distorts the images of background galaxies, which
allows one to measure directly the projected distribution of dark matter in the
universe and determine its power spectrum. Here we address the question of how
to extract this information from the observations. We derive minimum variance
estimators for projected density reconstruction and its power spectrum and
apply them to simulated data sets, showing that they give a good agreement with
the theoretical minimum variance expectations. The same estimator can also be
applied to the cluster reconstruction, where it remains a useful reconstruction
technique, although it is no longer optimal for every application. The method
can be generalized to include nonlinear cluster reconstruction and photometric
information on redshifts of background galaxies in the analysis. We also
address the question of how to obtain directly the 3-d power spectrum from the
weak lensing data. We derive a minimum variance quadratic estimator, which
maximizes the likelihood function for the 3-d power spectrum and can be
computed either from the measurements directly or from the 2-d power spectrum.
The estimator correctly propagates the errors and provides a full correlation
matrix of the estimates. It can be generalized to the case where redshift
distribution depends on the galaxy photometric properties, which allows one to
measure both the 3-d power spectrum and its time evolution.Comment: revised version, 36 pages, AAS LateX, submitted to Ap
Information content of the non-linear matter power spectrum
We use an ensemble of N-body simulations of the currently favoured
(concordance) cosmological model to measure the amount of information contained
in the non-linear matter power spectrum about the amplitude of the initial
power spectrum. Two surprising results emerge from this study: (i) that there
is very little independent information in the power spectrum in the translinear
regime (k ~ 0.2-0.8 Mpc/h at the present day) over and above the information at
linear scales and (ii) that the cumulative information begins to rise sharply
again with increasing wavenumber in the non-linear regime. In the fully
non-linear regime, the simulations are consistent with no loss of information
during translinear and non-linear evolution. If this is indeed the case then
the results suggest a picture in which translinear collapse is very rapid, and
is followed by a bounce prior to virialization, impelling a wholesale revision
of the HKLM-PD formalism.Comment: 5 pages, 3 figures. Accepted to Monthly Notices of the Royal
Astronomical Society Letters. Revised to match accepted versio
A Topos Perspective on State-Vector Reduction
A preliminary investigation is made of possible applications in quantum
theory of the topos formed by the collection of all -sets, where is a
monoid. Earlier results on topos aspects of quantum theory can be rederived in
this way. However, the formalism also suggests a new way of constructing a
`neo-realist' interpretation of quantum theory in which the truth values of
propositions are determined by the actions of the monoid of strings of finite
projection operators. By these means, a novel topos perspective is gained on
the concept of state-vector reduction
Determination of the Baryon Density from Large Scale Galaxy Redshift Surveys
We estimate the degree to which the baryon density, , can be
determined from the galaxy power spectrum measured from large scale galaxy
redshift surveys, and in particular, the Sloan Digital Sky Survey. A high
baryon density will cause wiggles to appear in the power spectrum, which should
be observable at the current epoch. We assume linear theory on scales and do not include the effects of redshift distortions, evolution,
or biasing. With an optimum estimate of to ,
the uncertainties in are roughly 0.07 and 0.016 in flat
and open () cosmological models, respectively. This result
suggests that it should be possible to test for consistency with big bang
nucleosynthesis estimates of if we live in an open universe.Comment: 23 Pages, 10 Postscript figure
An obstruction based approach to the Kochen-Specker theorem
In [1] it was shown that the Kochen Specker theorem can be written in terms
of the non-existence of global elements of a certain varying set over the
partially ordered set of boolean subalgebras of projection operators on some
Hilbert space. In this paper, we show how obstructions to the construction of
such global elements arise, and how this provides a new way of looking at
proofs of the theorem.Comment: 14 pages, 6 figure
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