3,139 research outputs found
Detailed comparison of Milky Way models based on stellar population synthesis and SDSS star counts at the north Galactic pole
We test the ability of the TRILEGAL and Besancon models to reproduce the CMD
of SDSS data at the north Galactic pole (NGP). We show that a Hess diagram
analysis of colour-magnitude diagrams is much more powerful than luminosity
functions (LFs) in determining the Milky Way structure. We derive a
best-fitting TRILEGAL model to simulate the NGP field in the (g-r, g) CMD of
SDSS filters via Hess diagrams. For the Besancon model, we simulate the LFs and
Hess diagrams in all SDSS filters. We use a chi2 analysis and determine the
median of the relative deviations in the Hess diagrams to quantify the quality
of the fits by the TRILEGAL models and the Besancon model in comparison and
compare this with the Just-Jahreiss model. The input isochrones in the
colour-absolute magnitude diagrams of the thick disc and halo are tested via
the observed fiducial isochrones of globular clusters (GCs). We find that the
default parameter set lacking a thick disc component gives the best
representation of the LF in TRILEGAL. The Hess diagram reveals that a
metal-poor thick disc is needed. In the Hess diagram, the median relative
deviation of the TRILEGAL model and the SDSS data amounts to 25 percent,
whereas for the Just-Jahreiss model the deviation is only 5.6 percent. The
isochrone analysis shows that the representation of the MS of (at least
metal-poor) stellar populations in the SDSS system is reliable. In contrast,
the RGBs fail to match the observed fiducial sequences of GCs. The Besancon
model shows a similar median relative deviation of 26 percent in (g-r, g). In
the u band, the deviations are larger. There are significant offsets between
the isochrone set used in the Besancon model and the observed fiducial
isochrones. In contrast to Hess diagrams, LFs are insensitive to the detailed
structure of the Milky Way components due to the extended spatial distribution
along the line of sight.Comment: 21 pages, 17 figures and 5 tables. Accepted by publication of A&
Analog VLSI neural network integrated circuits
Two analog very large scale integration (VLSI) vector matrix multiplier integrated circuit chips were designed, fabricated, and partially tested. They can perform both vector-matrix and matrix-matrix multiplication operations at high speeds. The 32 by 32 vector-matrix multiplier chip and the 128 by 64 vector-matrix multiplier chip were designed to perform 300 million and 3 billion multiplications per second, respectively. An additional circuit that has been developed is a continuous-time adaptive learning circuit. The performance achieved thus far for this circuit is an adaptivity of 28 dB at 300 KHz and 11 dB at 15 MHz. This circuit has demonstrated greater than two orders of magnitude higher frequency of operation than any previous adaptive learning circuit
Stability domains for time-delay feedback control with latency
We generalize a known analytical method for determining the stability of
periodic orbits controlled by time-delay feedback methods when latencies
associated with the generation and injection of the feedback signal cannot be
ignored. We discuss the case of extended time-delay autosynchronization (ETDAS)
and show that nontrivial qualitative features of the domain of control observed
in experiments can be explained by taking into account the effects of both the
unstable eigenmode and a single stable eigenmode in the Floquet theory.Comment: 9 pages, 6 figures; Submitted to Physical Review
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Modulation of sterol homeostasis by the Cdc42p effectors Cla4p and Ste20p in the yeast Saccharomyces cerevisiae
This article is available open access through the publisher’s website at the link below. Copyright @ 2009 The Authors.The conserved Rho-type GTPase Cdc42p is a key regulator of signal transduction and polarity in eukaryotic cells. In the yeast Saccharomyces cerevisiae, Cdc42p promotes polarized growth through the p21-activated kinases Ste20p and Cla4p. Previously, we demonstrated that Ste20p forms a complex with Erg4p, Cbr1p and Ncp1p, which all catalyze important steps in sterol biosynthesis. CLA4 interacts genetically with ERG4 and NCP1. Furthermore, Erg4p, Ncp1p and Cbr1p play important roles in cell polarization during vegetative growth, mating and filamentation. As Ste20p and Cla4p are involved in these processes it seems likely that sterol biosynthetic enzymes and p21-activated kinases act in related pathways. Here, we demonstrate that the deletion of either STE20 or CLA4 results in increased levels of sterols. In addition, higher concentrations of steryl esters, the storage form of sterols, were observed in cla4Δ cells. CLA4 expression from a multicopy plasmid reduces enzyme activity of Are2p, the major steryl ester synthase, under aerobic conditions. Altogether, our data suggest that Ste20p and Cla4p may function as negative modulators of sterol biosynthesis. Moreover, Cla4p has a negative effect on steryl ester formation. As sterol homeostasis is crucial for cell polarization, Ste20p and Cla4p may regulate cell polarity in part through the modulation of sterol homeostasis.Deutsche Forschungsgemeinschaft and the Austrian FWF
Heating and Turbulence Driving by Galaxy Motions in Galaxy Clusters
Using three-dimensional hydrodynamic simulations, we investigate heating and
turbulence driving in an intracluster medium (ICM) by orbital motions of
galaxies in a galaxy cluster. We consider Ng member galaxies on isothermal and
isotropic orbits through an ICM typical of rich clusters. An introduction of
the galaxies immediately produces gravitational wakes, providing perturbations
that can potentially grow via resonant interaction with the background gas.
When Ng^{1/2}Mg_11 < 100, where Mg_11 is each galaxy mass in units of 10^{11}
Msun, the perturbations are in the linear regime and the resonant excitation of
gravity waves is efficient to generate kinetic energy in the ICM, resulting in
the velocity dispersion sigma_v ~ 2.2 Ng^{1/2}Mg_11 km/s. When Ng^{1/2}Mg_11 >
100, on the other hand, nonlinear fluctuations of the background ICM destroy
galaxy wakes and thus render resonant excitation weak or absent. In this case,
the kinetic energy saturates at the level corresponding to sigma_v ~ 220 km/s.
The angle-averaged velocity power spectra of turbulence driven in our models
have slopes in the range of -3.7 to -4.3. With the nonlinear saturation of
resonant excitation, none of the cooling models considered are able to halt
cooling catastrophe, suggesting that the galaxy motions alone are unlikely to
solve the cooling flow problem.Comment: 12 pages including 3 figures, To appear in ApJ
On the Influence of Minor Mergers on the Radial Abundance Gradient in Disks of Milky Way-like Galaxies
We investigate the influence of stellar migration caused by minor mergers
(mass ratio from 1:70 to 1:8) on the radial distribution of chemical abundances
in the disks of Milky Way-like galaxies during the last four Gyr. A GPU-based
pure N-body tree-code model without hydrodynamics and star formation was used.
We computed a large set of mergers with different initial satellite masses,
positions, and orbital velocities. We find that there is no significant
metallicity change at any radius of the primary galaxy in the case of accretion
of a low-mass satellite of 10 M (mass ratio 1:70) except for the
special case of prograde satellite motion in the disk plane of the host galaxy.
The accretion of a satellite of a mass M (mass
ratio 1:23) results in an appreciable increase of the chemical abundances at
galactocentric distances larger than kpc. The radial abundance
gradient flattens in the range of galactocentric distances from 5 to 15 kpc in
the case of a merger with a satellite with a mass
M. There is no significant change in the abundance gradient slope in
the outer disk (from kpc up to 25 kpc) in any merger while the scatter
in metallicities at a given radius significantly increases for most of the
satellite's initial masses/positions compared to the case of an isolated
galaxy. This argues against attributing the break (flattening) of the abundance
gradient near the optical radius observed in the extended disks of Milky
Way-like galaxies only to merger-induced stellar migration.Comment: 17 pages, 15 figures, accepted for publication in Ap
An Improved Method of Computing Multistate Survivorship Proportions for the Terminal Age Groups
The aging of populations is a phenomenon which has become an important research topic. Demographers, however, have given inadequate attention to the projection of the number of old people and their future age composition. This paper shows that the conventional method for estimating the survivorship proportions of the very old tends to produce misleading results with respect to the size and composition of the aged. Several alternatives are suggested here to overcome these problems. An empirical example is used to point out the problems of the conventional approach and to evaluate the suggested improvements
Transfer of autocollimator calibration for use with scanning gantry profilometers for accurate determination of surface slope and curvature of state of the art x ray mirrors
X ray optics, desired for beamlines at free electron laser and diffraction limited storage ring x ray light sources, must have almost perfect surfaces, capable of delivering light to experiments without significant degradation of brightness and coherence. To accurately characterize such optics at an optical metrology lab, two basic types of surface slope profilometers are used the long trace profilers LTPs and nanometer optical measuring NOM like angular deflectometers, based on electronic autocollimator AC ELCOMAT 3000. The inherent systematic errors of the instrument s optical sensors set the principle limit to their measuring performance. Where autocollimator of a NOM like profiler may be calibrated at a unique dedicated facility, this is for a particular configuration of distance, aperture size, and angular range that does not always match the exact use in a scanning measurement with the profiler. Here we discuss the developed methodology, experimental set up, and numerical methods of transferring the calibration of one reference AC to the scanning AC of the Optical Surface Measuring System OSMS , recently brought to operation at the ALS Xray Optics Laboratory. We show that precision calibration of the OSMS performed in three steps, allows us to provide high confidence and accuracy low spatial frequency metrology and not print into measurements the inherent systematic error of tool in use. With the examples of the OSMS measurements with a state of the art x ray aspherical mirror, available from one of the most advanced vendors of X ray optics, we demonstrate the high efficacy of the developed calibration procedure. The results of our work are important for obtaining high reliability data, needed for sophisticated numerical simulations of beamline performance and optimization of beamline usage of the optics. This work was supported by the U. S. Department of Energy under contract number DE AC02 05CH1123
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