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The Observed Concentration-Mass Relation for Galaxy Clusters
astro-ph/0703126
Authors:
Julia
M. Comerford (UC Berkeley), Priyamvada
Natarajan (Yale University)
Comments: 12 pages, 7 figures, submitted to MNRAS
The properties of clusters of galaxies offer key insights into the
assembly
process of structure in the universe. Numerical simulations of cosmic
structure
formation in a hierarchical, dark matter dominated universe suggest
that galaxy
cluster concentrations, which are a measure of a halo's central
density,
decrease gradually with virial mass. However, cluster observations have
yet to
confirm this correlation. The slopes of the run of measured
concentrations with
virial mass are often either steeper or flatter than predicted by
simulations.
In this work, we present the most complete sample of observed cluster
concentrations and masses yet assembled, including new measurements for
10
strong lensing clusters, thereby more than doubling the existing number
of
strong lensing concentration estimates. We fit a power law to the
observed
concentrations as a function of virial mass, and find that the slope is
consistent with the slopes found in simulations, though our
normalization
factor is higher. Observed lensing concentrations appear to be
systematically
larger than X-ray concentrations, a more pronounced effect than found
in
simulations. We also find that at fixed mass, the bulk of observed
cluster
concentrations are distributed log-normally, with the exception of a
few
anomalously high concentration clusters. We examine the physical
processes
likely responsible for the discrepancy between lensing and X-ray
concentrations, and for the anomalously high concentrations in
particular. The
forthcoming Millennium simulation results will offer the most
comprehensive
comparison set to our findings of an observed concentration-mass power
law
relation.
Discovery of a Very Bright, Nearby Gravitational Microlensing
Event astro-ph/0703125
Authors:
B.
Scott Gaudi, Joseph
Patterson, David
S. Spiegel, Thomas
Krajci, R.
Koff, G.
Pojmanski, Subo
Dong, Andrew
Gould, Jose
L. Prieto, Cullen
H. Blake, Peter
W. A. Roming, David
P. Bennett, Joshua
S. Bloom, David
Boyd, Pierre
de Ponthiere, N.
Mirabal, Christopher
W. Morgan, Ronald
R. Remillard, T.
Vanmunster, R.
Mark Wagner, Linda
C. Watson
Comments: Submitted to ApJ, 10 pages, 5 figures. Data available upon
request
We report the serendipitous detection of a very bright, very nearby
microlensing event. In late October 2006, an otherwise unremarkable A0
star at
a distance ~1 kpc (GSC 3656-1328) brightened achromatically by a factor
of
nearly 40 over the span of several days and then decayed in an
apparently
symmetrical way. We present a light curve of the event based on optical
photometry from the Center for Backyard Astrophysics and the All Sky
Automatic
Survey, as well as near-infrared photometry from the Peters Automated
Infrared
Imaging Telescope. This light curve is well-fit by a generic
microlensing
model. We also report optical spectra, and Swift X-ray and UV
observations that
are consistent with the microlensing interpretation. We discuss and
reject
alternative explanations for this variability. The lens star is
probably a
low-mass star or brown dwarf, with a relatively high proper motion of
>20
mas/yr, and may be visible using precise optical/infrared imaging taken
several
years from now. We demonstrate that a modest, all-sky survey telescope
could
detect ~10 such events per year, which would enable searches for very
low-mass
planetary companions to relatively nearby stars.
The XENON10 WIMP Search Experiment at the Gran Sasso
Underground Laboratory astro-ph/0703183 (suggested by Scott)
Authors:
Laura
Baudis (for the XENON Collaboration)
Comments: 7 pages, 6 figures, to be published in the Proceedings of the
Third Symposium on Large TPCs for Low Energy Rare Event Detection, 11 -
12 December 2006, Paris
XENON10 is a new direct dark matter detection experiment using liquid
xenon
as target for weakly interacting, massive particles (WIMPs). A
two-phase
(liquid/gas) time projection chamber with 15 kg fiducial mass has been
installed in a low-background shield at the Gran Sasso Underground
Laboratory
in July 2006. After initial performance tests with various calibration
sources,
the science data run started on August 24, 2006. The detector has been
running
stably since then, and a full analysis of more than 75 live days of
WIMP search
data is now in progress. We present first results on gamma and neutron
calibration runs, as well as a preliminary analysis of a subset of the
WIMP
search data.
A new measure of Delta alpha/alpha at redshift z = 1.84 from
very high resolution spectra of Q1101-264 astro-ph/0703042 (suggested by
Scott)
Authors:
S.A.
Levshakov, P.
Molaro, S.
Lopez, S.
D'Odorico, M.
Centurion, P.
Bonifacio, I.I.
Agafonova, D.
Reimers
Comments: 8 pages, 5 figures, accepted for publication as a research
note in A&A
We probe the evolution of the fine-structure constant, alpha, with
cosmic
time. Accurate positions of the FeII lines 1608, 2382,and 2600 A are
measured
in the z = 1.84 absorption system from a high-resolution (FWHM = 3.8
km/s) and
high signal-to-noise (S/N >= 100) spectrum of the quasar Q1101-264
(z_em =
2.15, V = 16.0), integrated for 15.4 hours. The Single Ion Differential
alpha
Measurement (SIDAM) procedure and the Delta chi^2 method are used to
set
constraints on Delta alpha/alpha. We have found a relative radial
velocity
shift between the 1608 A and 2382,2600 A lines of Delta v = -180 +/- 85
m/s
(both random and systematic errors are included), which, if real, would
correspond to Delta alpha/alpha = (5.4 +/- 2.5) 10^{-6} (1sigma C.L.).
Considering the strong implications of a such variability, additional
observations with comparable accuracy at redshift z ~ 1.8 are required
to
confirm this result.
High-energy neutrinos from astrophysical accelerators of
cosmic ray nuclei
astro-ph/0703001
(suggested by Scott)
Authors:
Luis
A. Anchordoqui, Dan
Hooper, Subir
Sarkar, Andrew
M. Taylor
Comments: 13 pages revtex, 15 ps figures
Report-no: FERMILAB-PUB-06-480-A
Ongoing experimental efforts to detect cosmic sources of high energy
neutrinos are guided by the expectation that astrophysical accelerators
of
cosmic ray protons would also generate neutrinos through interactions
with
ambient matter and/or photons. However there will be a reduction in the
predicted neutrino flux if cosmic ray sources accelerate not only
protons but
also significant number of heavier nuclei, as is indicated by recent
air shower
data. We consider plausible extragalactic sources such as active
galactic
nuclei, gamma-ray bursts and starburst galaxies and demand consistency
with the
observed cosmic ray composition and energy spectrum at Earth after
allowing for
propagation through intergalactic radiation fields. This allows us to
calculate
the expected neutrino fluxes from the sources, normalised to the
observed
cosmic ray spectrum. We find that the likely signals are still within
reach of
next generation neutrino telescopes such as IceCube.
Observation of the GZK Cutoff by the HiRes Experiment
astro-ph/0703099
(suggested by Pasquale and Scott)
Authors:
HiRes
Collaboration
Comments: 4 pages, 4 figures, submitted to PRL
Report-no: RU-PNA-002
The High Resolution Fly's Eye (HiRes) experiment has observed the GZK
cutoff.
HiRes' measurement of the flux of cosmic rays shows a sharp suppression
at an
energy of $6 \times 10^{19}$ eV, exactly the expected cutoff energy. We
observe
the ``Ankle'' of the cosmic ray spectrum as well, at an energy of $4
\times
10^{18}$ eV. We describe the experiment, data collection, analysis, and
estimate the systematic uncertainties. The results are presented and
the
calculation of a $\sim5$ standard deviation observation of the GZK
cutoff is
described.
A robust lower limit on the amplitude of matter fluctuations
in the universe from cluster abundance and weak lensing
astro-ph/0703114
Authors:
Rachel
Mandelbaum, Uros
Seljak
Comments: 10 pages, 5 figures, submitted to JCAP
Cluster abundance measurements are among the most sensitive probes of
the
amplitude of matter fluctuations in the universe, which in turn can
help
constrain other cosmological parameters, like the dark energy equation
of state
or neutrino mass. However, difficulties in calibrating the relation
between the
cluster observable and halo mass, and the lack of completeness
information,
make this technique particularly susceptible to systematic errors. Here
we
argue that a cluster abundance analysis using statistical weak lensing
on the
stacked clusters leads to a robust lower limit on the amplitude of
fluctuations. The method compares the average weak lensing signal
measured
around the whole cluster sample to a theoretical prediction, assuming
that the
clusters occupy the centers of all of the most massive halos above some
minimum
mass threshold. If the amplitude of fluctuations is below a certain
limiting
value, there are too few massive clusters in this model and the
theoretical
prediction falls below the observations. Since any effects that modify
the
model assumptions can only decrease the theoretical prediction, the
limiting
amplitude becomes a robust lower limit. Here, we apply it to a volume
limited
sample of 16,000 group/cluster candidates identified from isolated
luminous red
galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS). We find
$\sigma_8
(\Omega_m/0.25)^{0.5}>0.62$ at the 95% c.l. after taking into
account
observational errors in the lensing analysis. While this is a
relatively weak
constraint, both the scatter in the LRG luminosity-halo mass relation
and the
lensing errors are large; the constraints could improve considerably in
the
future with more sophisticated cluster identification algorithms and
smaller
errors in the lensing analysis. [Abridged]
Forecasting the Bayes factor of a future observation
astro-ph/0703063
Authors:
Roberto
Trotta (Oxford University)
Comments: 7 pages, 2 figures
I present a new procedure to forecast the Bayes factor of a future
observation by computing the Predictive Posterior Odds Distribution
(PPOD).
This can assess the power of future experiments to answer model
selection
questions and the probability of the outcome, and can be helpful in the
context
of experiment design.
As an illustration, I consider a central quantity for our understanding
of
the cosmological concordance model, namely the scalar spectral index of
primordial perturbations, n_S. I show that the Planck satellite has
over 90%
probability of gathering strong evidence against n_S = 1, thus
conclusively
disproving a scale-invariant spectrum. This result is robust with
respect to a
wide range of choices for the prior on n_S.
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