1,692 research outputs found
Where do "red and dead" early-type void galaxies come from?
Void regions of the Universe offer a special environment for studying
cosmology and galaxy formation, which may expose weaknesses in our
understanding of these phenomena. Although galaxies in voids are observed to be
predominately gas rich, star forming and blue, a sub-population of bright red
void galaxies can also be found, whose star formation was shut down long ago.
Are the same processes that quench star formation in denser regions of the
Universe also at work in voids?
We compare the luminosity function of void galaxies in the 2dF Galaxy
Redshift Survey, to those from a galaxy formation model built on the Millennium
Simulation. We show that a global star formation suppression mechanism in the
form of low luminosity "radio mode" AGN heating is sufficient to reproduce the
observed population of void early-types. Radio mode heating is environment
independent other than its dependence on dark matter halo mass, where, above a
critical mass threshold of approximately M_vir~10^12.5 M_sun, gas cooling onto
the galaxy is suppressed and star formation subsequently fades. In the
Millennium Simulation, the void halo mass function is shifted with respect to
denser environments, but still maintains a high mass tail above this critical
threshold. In such void halos, radio mode heating remains efficient and red
galaxies are found; collectively these galaxies match the observed space
density without any modification to the model. Consequently, galaxies living in
vastly different large-scale environments but hosted by halos of similar mass
are predicted to have similar properties, consistent with observations.Comment: 6 pages, 3 figures, accepted MNRA
From Galaxy-Galaxy Lensing to Cosmological Parameters
Galaxy-galaxy lensing measures the mean excess surface density DS(r) around a
sample of lensing galaxies. We develop a method for combining DS(r) with the
galaxy correlation function xi_gg(r) to constrain Omega_m and sigma_8, going
beyond the linear bias model to reach the level of accuracy demanded by current
and future measurements. We adopt the halo occupation distribution (HOD)
framework, and we test its applicability to this problem by examining the
effects of replacing satellite galaxies in the halos of an SPH simulation with
randomly selected dark matter particles from the same halos. The difference
between dark matter and satellite galaxy radial profiles has a ~10% effect on
DS(r) at r<1 Mpc/h. However, if radial profiles are matched, the remaining
impact of individual subhalos around satellite galaxies and environmental
dependence of the HOD at fixed halo mass is <5% in DS(r) for 0.1<r<15 Mpc/h. We
develop an analytic approximation for DS(r) that incorporates halo exclusion
and scale-dependent halo bias, and we demonstrate its accuracy with tests
against a suite of populated N-body simulations. We use the analytic model to
investigate the dependence of DS(r) and the galaxy-matter correlation function
xi_gm(r) on Omega_m and sigma_8, once HOD parameters for a given cosmological
model are pinned down by matching xi_gg(r). The linear bias prediction is
accurate for r>2 Mpc/h, but it fails at the 30-50% level on smaller scales. The
scaling of DS(r) ~ Omega_m^a(r) sigma_8^b(r) approaches the linear bias
expectation a=b=1 at r>10 Mpc/h, but a(r) and b(r) vary from 0.8 to 1.6 at
smaller r. We calculate a fiducial DS(r) and scaling indices a(r) and b(r) for
two SDSS galaxy samples; galaxy-galaxy lensing measurements for these samples
can be combined with our predictions to constrain Omega_m and sigma_8.Comment: 18 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Topology Synthesis of Structures Using Parameter Relaxation and Geometric Refinement
Typically, structural topology optimization problems undergo relaxation of certain design parameters to allow the existence of intermediate variable optimum topologies. Relaxation permits the use of a variety of gradient-based search techniques and has been shown to guarantee the existence of optimal solutions and eliminate mesh dependencies. This Technical Publication (TP) will demonstrate the application of relaxation to a control point discretization of the design workspace for the structural topology optimization process. The control point parameterization with subdivision has been offered as an alternative to the traditional method of discretized finite element design domain. The principle of relaxation demonstrates the increased utility of the control point parameterization. One of the significant results of the relaxation process offered in this TP is that direct manufacturability of the optimized design will be maintained without the need for designer intervention or translation. In addition, it will be shown that relaxation of certain parameters may extend the range of problems that can be addressed; e.g., in permitting limited out-of-plane motion to be included in a path generation problem
Initial investigations into the damping characteristics of wire rope vibration isolators
Passive dampers composed of coils of multi-strand wire rope are investigated. Analytical results range from those produced by complex NASTRAN models to those of a Coulomb damping model with variable friction force. The latter agrees well with experiment. The Coulomb model is also utilized to generate hysteresis loops. Various other models related to early experimental investigations are described. Significant closed-form static solutions for physical properties of single-and multi-strand wire ropes are developed for certain specific geometries and loading conditions. NASTRAN models concentrate on model generation and mode shapes of 2-strand and 7-strand straight wire ropes with interfacial forces
Cosmic Voids and Galaxy Bias in the Halo Occupation Framework
(Abridged) We investigate the power of void statistics to constrain galaxy
bias and the amplitude of dark matter fluctuations. We use the halo occupation
distribution (HOD) framework to describe the relation between galaxies and dark
matter. After choosing HOD parameters that reproduce the mean space density
n_gal and projected correlation function w_p measured for galaxy samples with
M_r<-19 and M_r<-21 from the Sloan Digital Sky Survey (SDSS), we predict the
void probability function (VPF) and underdensity probability function (UPF) of
these samples by populating the halos of a large, high-resolution N-body
simulation. If we make the conventional assumption that the HOD is independent
of large scale environment at fixed halo mass, then models constrained to match
n_gal and w_p predict nearly identical void statistics, independent of the
scatter between halo mass and central galaxy luminosity or uncertainties in HOD
parameters. Models with sigma_8=0.7 and sigma_8=0.9 also predict very similar
void statistics. However, the VPF and UPF are sensitive to environmental
variations of the HOD in a regime where these variations have little impact on
w_p. For example, doubling the minimum host halo mass in regions with large
scale (5 Mpc/h) density contrast delta<-0.65 has a readily detectable impact on
void probabilities of M_r<-19 galaxies, and a similar change for delta<-0.2
alters the void probabilities of M_r<-21 galaxies at a detectable level. The
VPF and UPF provide complementary information about the onset and magnitude of
density- dependence in the HOD. By detecting or ruling out HOD changes in low
density regions, void statistics can reduce systematic uncertainties in the
cosmological constraints derived from HOD modeling, and, more importantly,
reveal connections between halo formation history and galaxy properties.Comment: emulateapj, 16 pages, 13 figure
Differential effects of inhibitory G protein isoforms on G protein-gated inwardly rectifying K+ currents in adult murine atria.
G protein-gated inwardly rectifying K+ (GIRK) channels are the major inwardly rectifying K+ currents in cardiac atrial myocytes and an important determinant of atrial electrophysiology. Inhibitory G protein α-subunits can both mediate activation via acetylcholine but can also suppress basal currents in the absence of agonist. We studied this phenomenon using whole cell patch clamping in murine atria from mice with global genetic deletion of Gαi2, combined deletion of Gαi1/Gαi3, and littermate controls. We found that mice with deletion of Gαi2 had increased basal and agonist-activated currents, particularly in the right atria while in contrast those with Gαi1/Gαi3 deletion had reduced currents. Mice with global genetic deletion of Gαi2 had decreased action potential duration. Tissue preparations of the left atria studied with a multielectrode array from Gαi2 knockout mice showed a shorter effective refractory period, with no change in conduction velocity, than littermate controls. Transcriptional studies revealed increased expression of GIRK channel subunit genes in Gαi2 knockout mice. Thus different G protein isoforms have differential effects on GIRK channel behavior and paradoxically Gαi2 act to increase basal and agonist-activated GIRK currents. Deletion of Gαi2 is potentially proarrhythmic in the atria.We thank the British Heart Foundation (RG/15/15/31742) and the Intramural
Research Program of the NIH (project Z01ES101643) for funding this research. D.M. was
supported by a grant from la Fédération Française de Cardiologie
George C. Marshall Space Flight Center Research and Technology Report 2014
Many of NASA's missions would not be possible if it were not for the investments made in research advancements and technology development efforts. The technologies developed at Marshall Space Flight Center contribute to NASA's strategic array of missions through technology development and accomplishments. The scientists, researchers, and technologists of Marshall Space Flight Center who are working these enabling technology efforts are facilitating NASA's ability to fulfill the ambitious goals of innovation, exploration, and discovery
Evolutionary Optimization of a Geometrically Refined Truss
Structural optimization is a field of research that has experienced noteworthy growth for many years. Researchers in this area have developed optimization tools to successfully design and model structures, typically minimizing mass while maintaining certain deflection and stress constraints. Numerous optimization studies have been performed to minimize mass, deflection, and stress on a benchmark cantilever truss problem. Predominantly traditional optimization theory is applied to this problem. The cross-sectional area of each member is optimized to minimize the aforementioned objectives. This Technical Publication (TP) presents a structural optimization technique that has been previously applied to compliant mechanism design. This technique demonstrates a method that combines topology optimization, geometric refinement, finite element analysis, and two forms of evolutionary computation: genetic algorithms and differential evolution to successfully optimize a benchmark structural optimization problem. A nontraditional solution to the benchmark problem is presented in this TP, specifically a geometrically refined topological solution. The design process begins with an alternate control mesh formulation, multilevel geometric smoothing operation, and an elastostatic structural analysis. The design process is wrapped in an evolutionary computing optimization toolset
A Stellar Mass Threshold for Quenching of Field Galaxies
We demonstrate that dwarf galaxies (10^7 < M_stellar < 10^9 Msun) with no
active star formation are extremely rare (<0.06%) in the field. Our sample is
based on the NASA-Sloan Atlas which is a re-analysis of the Sloan Digital Sky
Survey Data Release 8. We examine the relative number of quenched versus star
forming dwarf galaxies, defining quenched galaxies as having no Halpha emission
(EW_Halpha < 2 AA) and a strong 4000AA-break. The fraction of quenched dwarf
galaxies decreases rapidly with increasing distance from a massive host,
leveling off for distances beyond 1.5 Mpc. We define galaxies beyond 1.5 Mpc of
a massive host galaxy to be in the field. We demonstrate that there is a
stellar mass threshold of M_stellar < 1.0x10^9 Msun below which quenched
galaxies do not exist in the field. Below this threshold, we find that none of
the 2951 field dwarf galaxies are quenched; all field dwarf galaxies show
evidence for recent star formation. Correcting for volume effects, this
corresponds to a 1-sigma upper limit on the quenched fraction of 0.06%. In more
dense environments, quenched galaxies account for 23% of the dwarf population
over the same stellar mass range. The majority of quenched dwarf galaxies
(often classified as dwarf elliptical galaxies) are within 2 virial radii of a
massive galaxy, and only a few percent of quenched dwarf galaxies exist beyond
4 virial radii. Thus, for galaxies with stellar mass less than 1.0x10^9 Msun,
ending star-formation requires the presence of a more massive neighbor,
providing a stringent constraint on models of star formation feedback.Comment: 9 pages, 6 figures, accepted to Ap
- …