22,610 research outputs found
CDM, Feedback and the Hubble Sequence
We have performed TreeSPH simulations of galaxy formation in a standard LCDM
cosmology, including effects of star formation, energetic stellar feedback
processes and a meta-galactic UV field, and obtain a mix of disk, lenticular
and elliptical galaxies. The disk galaxies are deficient in angular momentum by
only about a factor of two compared to observed disk galaxies. The stellar
disks have approximately exponential surface density profiles, and those of the
bulges range from exponential to r^{1/4}, as observed. The bulge-to-disk ratios
of the disk galaxies are consistent with observations and likewise are their
integrated B-V colours, which have been calculated using stellar population
synthesis techniques. Furthermore, we can match the observed I-band
Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk
galaxies, (M/L_I), is about 0.8. The ellipticals and lenticulars have
approximately r^{1/4} stellar surface density profiles, are dominated by
non-disklike kinematics and flattened due to non-isotropic stellar velocity
distributions, again consistent with observations.Comment: 6 pages, incl. 4 figs. To appear in the proceedings of the
EuroConference "The Evolution of Galaxies: II - Basic Building Blocks", Ile
de La Reunion (France), 16-21 October 2001 (Slightly updated version). A much
more comprehensive paper about this work with links to pictures of some of
the galaxies can be found at http://babbage.sissa.it/abs/astro-ph/020436
Mapping the Asymmetric Thick Disk I. A Search for Triaxiality
A significant asymmetry in the distribution of faint blue stars in the inner
Galaxy, Quadrant 1 (l = 20 to 45 degrees) compared to Quadrant 4 was first
reported by Larsen & Humphreys (1996). Parker et al (2003, 2004) greatly
expanded the survey to determine its spatial extent and shape and the
kinematics of the affected stars. This excess in the star counts was
subsequently confirmed by Juric et al. (2008) using SDSS data. Possible
explanations for the asymmetry include a merger remnant, a triaxial Thick Disk,
and a possible interaction with the bar in the Disk. In this paper we describe
our program of wide field photometry to map the asymmetry to fainter magnitudes
and therefore larger distances. To search for the signature of triaxiality, we
extended our survey to higher Galactic longitudes. We find no evidence for an
excess of faint blue stars at l > 55 degrees including the faintest magnitude
interval. The asymmetry and star count excess in Quadrant 1 is thus not due to
a triaxial Thick Disk.Comment: 36 pages, 8 figures. Accepted by Astronomical Journa
Observations of atmospheric water vapor with the SAGE 2 instrument
The Stratospheric Aerosol and Gas Experiment 2 (SAGE 2) is discussed. The SAGE 2 instrument was a multichannel spectrometer that inferred the vertical distribution of water vapor, aerosols, nitrogen dioxide, and ozone by measuring the extinction of solar radiation at spacecraft sunrise/sunset. At altitudes above 20 km, the SAGE 2 and LIMS (Limb Infrared Monitor of the Stratosphere) data are in close agreement. The discrepancies below this altitude may be attributed to differences in the instruments' field of view and time of data acquisition
Exact String Solutions in Nontrivial Backgrounds
We show how the classical string dynamics in -dimensional gravity
background can be reduced to the dynamics of a massless particle constrained on
a certain surface whenever there exists at least one Killing vector for the
background metric. We obtain a number of sufficient conditions, which ensure
the existence of exact solutions to the equations of motion and constraints.
These results are extended to include the Kalb-Ramond background. The
-brane dynamics is also analyzed and exact solutions are found. Finally, we
illustrate our considerations with several examples in different dimensions.
All this also applies to the tensionless strings.Comment: 22 pages, LaTeX, no figures; V2:Comments and references added;
V3:Discussion on the properties of the obtained solutions extended, a
reference and acknowledgment added; V4:The references renumbered, to appear
in Phys Rev.
Non-linear optomechanical measurement of mechanical motion
Precision measurement of non-linear observables is an important goal in all
facets of quantum optics. This allows measurement-based non-classical state
preparation, which has been applied to great success in various physical
systems, and provides a route for quantum information processing with otherwise
linear interactions. In cavity optomechanics much progress has been made using
linear interactions and measurement, but observation of non-linear mechanical
degrees-of-freedom remains outstanding. Here we report the observation of
displacement-squared thermal motion of a micro-mechanical resonator by
exploiting the intrinsic non-linearity of the radiation pressure interaction.
Using this measurement we generate bimodal mechanical states of motion with
separations and feature sizes well below 100~pm. Future improvements to this
approach will allow the preparation of quantum superposition states, which can
be used to experimentally explore collapse models of the wavefunction and the
potential for mechanical-resonator-based quantum information and metrology
applications.Comment: 8 pages, 4 figures, extensive supplementary material available with
published versio
Time lower bounds for nonadaptive turnstile streaming algorithms
We say a turnstile streaming algorithm is "non-adaptive" if, during updates,
the memory cells written and read depend only on the index being updated and
random coins tossed at the beginning of the stream (and not on the memory
contents of the algorithm). Memory cells read during queries may be decided
upon adaptively. All known turnstile streaming algorithms in the literature are
non-adaptive.
We prove the first non-trivial update time lower bounds for both randomized
and deterministic turnstile streaming algorithms, which hold when the
algorithms are non-adaptive. While there has been abundant success in proving
space lower bounds, there have been no non-trivial update time lower bounds in
the turnstile model. Our lower bounds hold against classically studied problems
such as heavy hitters, point query, entropy estimation, and moment estimation.
In some cases of deterministic algorithms, our lower bounds nearly match known
upper bounds
Faint Fuzzy Star Clusters in NGC1023 as Remnants of Merged Star Cluster Complexes
In the lenticular galaxy NGC1023 a third population of globular clusters
(GCs), called faint fuzzies (FFs), was discovered next to the blue and red GC
populations by Larsen & Brodie. While these FFs have colors comparable to the
red population, the new population is fainter, larger (R_eff > 7 pc) and, most
importantly, shows clear signs of co-rotation with the galactic disk of
NGC1023. We present N-body simulations verifying the hypothesis that these
disk-associated FFs are related to the young massive cluster complexes (CCs)
observed by Bastian et. al in M51, who discovered a mass-radius relation for
these CCs. Our models have an initial configuration based on the observations
from M51 and are placed on various orbits in a galactic potential derived for
NGC1023. All computations end up with a stable object containing 10 to 60% of
the initial CC mass after an integration time of 5 Gyr. A conversion to visual
magnitudes demonstrates that the resulting objects cover exactly the observed
range for FFs. Moreover, the simulated objects show projected half-mass radii
between 3.6 and 13.4 pc, in good agreement with the observed FF sizes. We
conclude that objects like the young massive CCs in M51 are likely progenitors
of the FFs observed in NGC1023.Comment: Accepted for publication in Ap
From the WZWN Model to the Liouville Equation: Exact String Dynamics in Conformally Invariant AdS Background
It has been known for some time that the SL(2,R) WZWN model reduces to
Liouville theory. Here we give a direct and physical derivation of this result
based on the classical string equations of motion and the proper string size.
This allows us to extract precisely the physical effects of the metric and
antisymmetric tensor, respectively, on the {\it exact} string dynamics in the
SL(2,R) background. The general solution to the proper string size is also
found. We show that the antisymmetric tensor (corresponding to conformal
invariance) generally gives rise to repulsion, and it precisely cancels the
dominant attractive term arising from the metric.
Both the sinh-Gordon and the cosh-Gordon sectors of the string dynamics in
non-conformally invariant AdS spacetime reduce here to the Liouville equation
(with different signs of the potential), while the original Liouville sector
reduces to the free wave equation. Only the very large classical string size is
affected by the torsion. Medium and small size string behaviours are unchanged.
We also find illustrative classes of string solutions in the SL(2,R)
background: dynamical closed as well as stationary open spiralling strings, for
which the effect of torsion is somewhat like the effect of rotation in the
metric. Similarly, the string solutions in the 2+1 BH-AdS background with
torsion and angular momentum are fully analyzed.Comment: 24 pages including 4 postscript figures. Enlarged version including a
section on string solutions in 2+1 black hole background. To be published in
Phys. Rev. D., December 199
Hole Spin Coherence in a Ge/Si Heterostructure Nanowire
Relaxation and dephasing of hole spins are measured in a gate-defined Ge/Si
nanowire double quantum dot using a fast pulsed-gate method and dispersive
readout. An inhomogeneous dephasing time
exceeds corresponding measurements in III-V semiconductors by more than an
order of magnitude, as expected for predominately nuclear-spin-free materials.
Dephasing is observed to be exponential in time, indicating the presence of a
broadband noise source, rather than Gaussian, previously seen in systems with
nuclear-spin-dominated dephasing.Comment: 15 pages, 4 figure
Antilocalization of Coulomb Blockade in a Ge-Si Nanowire
The distribution of Coulomb blockade peak heights as a function of magnetic
field is investigated experimentally in a Ge-Si nanowire quantum dot. Strong
spin-orbit coupling in this hole-gas system leads to antilocalization of
Coulomb blockade peaks, consistent with theory. In particular, the peak height
distribution has its maximum away from zero at zero magnetic field, with an
average that decreases with increasing field. Magnetoconductance in the
open-wire regime places a bound on the spin-orbit length ( < 20 nm),
consistent with values extracted in the Coulomb blockade regime ( < 25
nm).Comment: Supplementary Information available at http://bit.ly/19pMpd
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