3,711 research outputs found
A Slow Merger History of Field Galaxies Since z~1
Using deep infrared observations conducted with the CISCO imager on the
Subaru Telescope, we investigate the field-corrected pair fraction and the
implied merger rate of galaxies in redshift survey fields with Hubble Space
Telescope imaging. In the redshift interval, 0.5 < z < 1.5, the fraction of
infrared-selected pairs increases only modestly with redshift to 7% +- 6% at
z~1. This is nearly a factor of three less than the fraction, 22% +- 8%,
determined using the same technique on HST optical images and as measured in a
previous similar study. Tests support the hypothesis that optical pair
fractions at z~1 are inflated by bright star-forming regions that are unlikely
to be representative of the underlying mass distribution. By determining
stellar masses for the companions, we estimate the mass accretion rate
associated with merging galaxies. At z~1, we estimate this to be 2x10^{9 +-
0.2} solar masses per galaxy per Gyr. Although uncertainties remain, our
results suggest that the growth of galaxies via the accretion of pre-existing
fragments remains as significant a phenomenon in the redshift range studied as
that estimated from ongoing star formation in independent surveys.Comment: 5 pages, accepted for publication in ApJ Letter
An atom fiber for guiding cold neutral atoms
We present an omnidirectional matter wave guide on an atom chip. The
rotational symmetry of the guide is maintained by a combination of two current
carrying wires and a bias field pointing perpendicular to the chip surface. We
demonstrate guiding of thermal atoms around more than two complete turns along
a spiral shaped 25mm long curved path (curve radii down to 200m) at
various atom--surface distances (35-450m). An extension of the scheme for
the guiding of Bose-Einstein condensates is outlined
A Proper Motion Survey for White Dwarfs with the Wide Field Planetary Camera 2
We have performed a search for halo white dwarfs as high proper motion
objects in a second epoch WFPC2 image of the Groth-Westphal strip. We identify
24 high proper motion objects with mu > 0.014 ''/yr. Five of these high proper
motion objects are identified as strong white dwarf candidates on the basis of
their position in a reduced proper motion diagram. We create a model of the
Milky Way thin disk, thick disk and stellar halo and find that this sample of
white dwarfs is clearly an excess above the < 2 detections expected from these
known stellar populations. The origin of the excess signal is less clear.
Possibly, the excess cannot be explained without invoking a fourth galactic
component: a white dwarf dark halo. We present a statistical separation of our
sample into the four components and estimate the corresponding local white
dwarf densities using only the directly observable variables, V, V-I, and mu.
For all Galactic models explored, our sample separates into about 3 disk white
dwarfs and 2 halo white dwarfs. However, the further subdivision into the thin
and thick disk and the stellar and dark halo, and the subsequent calculation of
the local densities are sensitive to the input parameters of our model for each
Galactic component. Using the lowest mean mass model for the dark halo we find
a 7% white dwarf halo and six times the canonical value for the thin disk white
dwarf density (at marginal statistical significance), but possible systematic
errors due to uncertainty in the model parameters likely dominate these
statistical error bars. The white dwarf halo can be reduced to around 1.5% of
the halo dark matter by changing the initial mass function slightly. The local
thin disk white dwarf density in our solution can be made consistent with the
canonical value by assuming a larger thin disk scaleheight of 500 pc.Comment: revised version, accepted by ApJ, results unchanged, discussion
expande
Trapping and manipulating neutral atoms with electrostatic fields
We report on experiments with cold thermal Li atoms confined in combined
magnetic and electric potentials. A novel type of three-dimensional trap was
formed by modulating a magnetic guide using electrostatic fields. We observed
atoms trapped in a string of up to six individual such traps, a controlled
transport of an atomic cloud over a distance of 400m, and a dynamic
splitting of a single trap into a double well potential. Applications for
quantum information processing are discussed.Comment: 4 pages, 4 figure
A nonlinear detection algorithm for periodic signals in gravitational wave detectors
We present an algorithm for the detection of periodic sources of
gravitational waves with interferometric detectors that is based on a special
symmetry of the problem: the contributions to the phase modulation of the
signal from the earth rotation are exactly equal and opposite at any two
instants of time separated by half a sidereal day; the corresponding is true
for the contributions from the earth orbital motion for half a sidereal year,
assuming a circular orbit. The addition of phases through multiplications of
the shifted time series gives a demodulated signal; specific attention is given
to the reduction of noise mixing resulting from these multiplications. We
discuss the statistics of this algorithm for all-sky searches (which include a
parameterization of the source spin-down), in particular its optimal
sensitivity as a function of required computational power. Two specific
examples of all-sky searches (broad-band and narrow-band) are explored
numerically, and their performances are compared with the stack-slide technique
(P. R. Brady, T. Creighton, Phys. Rev. D, 61, 082001).Comment: 9 pages, 3 figures, to appear in Phys. Rev.
Evolution of the Near-Infrared Tully-Fisher Relation: Constraints on the Relationship Between the Stellar and Total Masses of Disk Galaxies since z=1
Using a combination of Keck spectroscopy and near-infrared imaging, we
investigate the K-band and stellar mass Tully-Fisher relation for 101 disk
galaxies at 0.2 < z < 1.2, with the goal of placing the first observational
constraints on the assembly history of halo and stellar mass. Our main result
is a lack of evolution in either the K-band or stellar mass Tully-Fisher
relation from z = 0 - 1.2. Furthermore, although our sample is not
statistically complete, we consider it suitable for an initial investigation of
how the fraction of total mass that has condensed into stars is distributed
with both redshift and total halo mass. We calculate stellar masses from
optical and near-infrared photometry and total masses from maximum rotational
velocities and disk scale lengths, utilizing a range of model relationships
derived analytically and from simulations. We find that the stellar/total mass
distribution and stellar-mass Tully-Fisher relation for z > 0.7 disks is
similar to that at lower redshift, suggesting that baryonic mass is accreted by
disks along with dark matter at z < 1, and that disk galaxy formation at z < 1
is hierarchical in nature. We briefly discuss the evolutionary trends expected
in conventional structure formation models and the implications of extending
such a study to much larger samples.Comment: ApJ, in press, 9 page
Provenance-Centered Dataset of Drug-Drug Interactions
Over the years several studies have demonstrated the ability to identify
potential drug-drug interactions via data mining from the literature (MEDLINE),
electronic health records, public databases (Drugbank), etc. While each one of
these approaches is properly statistically validated, they do not take into
consideration the overlap between them as one of their decision making
variables. In this paper we present LInked Drug-Drug Interactions (LIDDI), a
public nanopublication-based RDF dataset with trusty URIs that encompasses some
of the most cited prediction methods and sources to provide researchers a
resource for leveraging the work of others into their prediction methods. As
one of the main issues to overcome the usage of external resources is their
mappings between drug names and identifiers used, we also provide the set of
mappings we curated to be able to compare the multiple sources we aggregate in
our dataset.Comment: In Proceedings of the 14th International Semantic Web Conference
(ISWC) 201
Making Code Voting Secure against Insider Threats using Unconditionally Secure MIX Schemes and Human PSMT Protocols
Code voting was introduced by Chaum as a solution for using a possibly
infected-by-malware device to cast a vote in an electronic voting application.
Chaum's work on code voting assumed voting codes are physically delivered to
voters using the mail system, implicitly requiring to trust the mail system.
This is not necessarily a valid assumption to make - especially if the mail
system cannot be trusted. When conspiring with the recipient of the cast
ballots, privacy is broken.
It is clear to the public that when it comes to privacy, computers and
"secure" communication over the Internet cannot fully be trusted. This
emphasizes the importance of using: (1) Unconditional security for secure
network communication. (2) Reduce reliance on untrusted computers.
In this paper we explore how to remove the mail system trust assumption in
code voting. We use PSMT protocols (SCN 2012) where with the help of visual
aids, humans can carry out addition correctly with a 99\% degree of
accuracy. We introduce an unconditionally secure MIX based on the combinatorics
of set systems.
Given that end users of our proposed voting scheme construction are humans we
\emph{cannot use} classical Secure Multi Party Computation protocols.
Our solutions are for both single and multi-seat elections achieving:
\begin{enumerate}[i)]
\item An anonymous and perfectly secure communication network secure against
a -bounded passive adversary used to deliver voting,
\item The end step of the protocol can be handled by a human to evade the
threat of malware. \end{enumerate} We do not focus on active adversaries
Fluid Interpretation of Cardassian Expansion
A fluid interpretation of Cardassian expansion is developed. Here, the
Friedmann equation takes the form where contains
only matter and radiation (no vacuum). The function g(\rhom) returns to the
usual 8\pi\rhom/(3 m_{pl}^2) during the early history of the universe, but
takes a different form that drives an accelerated expansion after a redshift . One possible interpretation of this function (and of the right hand
side of Einstein's equations) is that it describes a fluid with total energy
density \rho_{tot} = {3 m_{pl}^2 \over 8 \pi} g(\rhom) = \rhom + \rho_K
containing not only matter density (mass times number density) but also
interaction terms . These interaction terms give rise to an effective
negative pressure which drives cosmological acceleration. These interactions
may be due to interacting dark matter, e.g. with a fifth force between
particles . Such interactions may be intrinsically four
dimensional or may result from higher dimensional physics. A fully relativistic
fluid model is developed here, with conservation of energy, momentum, and
particle number. A modified Poisson's equation is derived. A study of
fluctuations in the early universe is presented, although a fully relativistic
treatment of the perturbations including gauge choice is as yet incomplete.Comment: 25 pages, 1 figure. Replaced with published version. Title changed in
journa
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