2,049 research outputs found
Galaxy Tracers and Velocity Bias
This paper examines several methods of tracing galaxies in N-body simulations
and their effects on the derived galaxy statistics, especially measurements of
velocity bias. Using two simulations with identical initial conditions, one
following dark matter only and the other following dark matter and baryons,
both collisionless and collisional methods of tracing galaxies are compared to
one another and against a set of idealized criteria. None of the collisionless
methods proves satisfactory, including an elaborate scheme developed here to
circumvent previously known problems. The main problem is that galactic
overdensities are both secularly and impulsively disrupted while orbiting in
cluster potentials. With dissipation, the baryonic tracers have much higher
density contrasts and much smaller cross sections, allowing them to remain
distinct within the cluster potential. The question remains whether the
incomplete physical model introduces systematic biases. Statistical measures
determined from simulations can vary significantly based solely on the galaxy
tracing method utilized. The two point correlation function differs most on
sub-cluster scales with generally good agreement on larger scales. Pairwise
velocity dispersions show less uniformity on all scales addressed here. All
tracing methods show a velocity bias to varying degrees, but the predictions
are not firm: either the tracing method is not robust or the statistical
significance has not been demonstrated. Though theoretical arguments suggest
that a mild velocity bias should exist, simulation results are not yet
conclusive.Comment: ApJ, in press, 23 pages, plain TeX, 8 of 13 figures included, all
PostScript figures (4.8 MB) available via anonymous ftp from
ftp://astro.princeton.edu/summers/tracers . Also available as POPe-616 on
http://astro.princeton.edu/~library/prep.htm
How does interhemispheric communication in visual word recognition work? Deciding between early and late integration accounts of the split fovea theory
Galaxy tracers in N-body simulations
Using the method of smoothed particle hydrodynamics, we have modeled the formation of a compact group of galaxies with sufficient resolution to trace galaxies. Radiative cooling allows the baryons to dissipate their thermal energy and collapse to overdensities characteristic of real galaxies. With their cross section greatly reduced, these galaxy tracers remain distinct during cluster formation while their dark matter halos merge. In addition, the number density, the mass distribution function, and even the morphology of these objects are similar to those of observed galaxies. A viable population of galaxy tracers can be unambiguously defined
From "best practice" to "best fit": a framework for designing and analyzing pluralistic agricultural advisory services worldwide
"The paper develops a framework for the design and analysis of pluralistic agricultural advisory services and reviews research methods from different disciplines that can be used when applying the framework. Agricultural advisory services are defined in the paper as the entire set of organizations that support and facilitate people engaged in agricultural production to solve problems and to obtain information, skills and technologies to improve their livelihoods and well-being... To classify pluralistic agricultural advisory services, the paper distinguishes between organizations from the public, the private and the third sector that can be involved in (a) providing and (b) financing of agricultural advisory services. The framework for analyzing pluralistic agricultural advisory services presented in the paper addresses the need for analytical approaches that help policy-makers to identify those reform options that best fit country-specific frame conditions. Thus, the paper supports a shift from a “one-size-fits-all” to a “best fit” approach in the reform of public services... Based on a review of the literature, the paper presents a variety of quantitative and qualitative methodological approaches derived from different disciplines that can be applied when using the framework in empirical research projects. The disciplines include agricultural and institutional economics, communication theory, adult education, and public administration and management. The paper intends to inform researchers as well as practitioners, policy-makers and development partners who are interested in supporting evidence-based reform of agricultural advisory services. from Authors' AbstractAgricultural extension work, Pro-poor growth, Capacity strengthening,
AEGIS: Chandra Observation of DEEP2 Galaxy Groups and Clusters
We present a 200 ksec Chandra observation of seven spectroscopically
selected, high redshift (0.75 < z < 1.03) galaxy groups and clusters discovered
by the DEEP2 Galaxy Redshift Survey in the Extended Groth Strip (EGS). X-ray
emission at the locations of these systems is consistent with background. The
3-sigma upper limits on the bolometric X-ray luminosities (L_X) of these
systems put a strong constraint on the relation between L_X and the velocity
dispersion of member galaxies sigma_gal at z~1; the DEEP2 systems have lower
luminosity than would be predicted by the local relation. Our result is
consistent with recent findings that at high redshift, optically selected
clusters tend to be X-ray underluminous. A comparison with mock catalogs
indicates that it is unlikely that this effect is entirely caused by a
measurement bias between sigma_gal and the dark matter velocity dispersion.
Physically, the DEEP2 systems may still be in the process of forming and hence
not fully virialized, or they may be deficient in hot gas compared to local
systems. We find only one possibly extended source in this Chandra field, which
happens to lie outside the DEEP2 coverage.Comment: 5 pages, 3 figures. Accepted for publication in AEGIS ApJ Letters
special editio
Resolving the Stellar Populations in a z=4 Lensed Galaxy
We present deep near-infrared Keck/NIRC imaging of a recently-discovered
z=4.04 galaxy (Frye & Broadhurst 1998). This is lensed by the rich foreground
cluster Abell~2390 (z~0.23) into highly-magnified arcs 3-5arcsec in length. Our
H- and K'-band NIRC imaging allows us to map the Balmer+4000Ang break
amplitude. In combination with high-quality archival HST/WFPC2 data, we can
spatially resolve stellar populations along the arcs. The WFPC2 images clearly
reveal several bright knots, which correspond to sites of active star
formation. However, there are considerable portions of the arcs are
significantly redder, consistent with being observed >100Myr after star
formation has ceased. Keck/LRIS long-slit spectroscopy along the arcs reveals
that the Ly-alpha emission is spatially offset by ~1arcsec from the rest-UV
continuum regions. We show that this line emission is most probably powered by
star formation in neighboring HII regions, and that the z=4 system is unlikely
to be an AGN.Comment: Accepted for publication in the Astrophysical Journal. Uses
emulateapj.sty and graphics.sty (included). 34 pages - has 5 tables and 21
encapsulated postscript figures, 4 in colour mail (B&W versions also
provided
A revised Cepheid distance to NGC 4258 and a test of the distance scale
In a previous paper (Maoz et al. 1999), we reported a Hubble Space Telescope
(HST) Cepheid distance to the galaxy NGC 4258 obtained using the calibrations
and methods then standard for the Key Project on the Extragalactic Distance
Scale. Here, we reevaluate the Cepheid distance using the revised Key Project
procedures described in Freedman et al. (2001). These revisions alter the zero
points and slopes of the Cepheid Period-Luminosity (P-L) relations derived at
the Large Magellanic Cloud (LMC), the calibration of the HST WFPC2 camera, and
the treatment of metallicity differences. We also provide herein full
information on the Cepheids described in Maoz et al. 1999. Using the refined
Key Project techniques and calibrations, we determine the distance modulus of
NGC 4258 to be 29.47 +/- 0.09 mag (unique to this determination) +/- 0.15 mag
(systematic uncertainties in Key Project distances), corresponding to a metric
distance of 7.8 +/- 0.3 +/- 0.5 Mpc and 1.2 sigma from the maser distance of
7.2 +/- 0.5 Mpc. We also test the alternative Cepheid P-L relations of Feast
(1999), which yield more discrepant results. Additionally, we place weak limits
upon the distance to the LMC and upon the effect of metallicity in Cepheid
distance determinations.Comment: 26 pages in emulateapj5 format, including 6 figures and 5 tables.
Accepted for publication in the Astrophysical Journa
A Composite Seyfert 2 X-ray Spectrum: Implications for the Origin of the Cosmic X-ray Background
We present a composite 1-10 keV Seyfert 2 X-ray spectrum, derived from ASCA
observations of a distance-limited sample of nearby galaxies. All 29 observed
objects were detected. Above ~3 keV, the composite spectrum is inverted,
confirming that Seyfert 2 galaxies as a class have the spectral properties
necessary to explain the flat shape of the cosmic X-ray background spectrum.
Integrating the composite spectrum over redshift, we find that the total
emission from Seyfert 2 galaxies, combined with the expected contribution from
unabsorbed type 1 objects, provides an excellent match to the spectrum and
intensity of the hard X-ray background. The principal uncertainty in this
procedure is the cosmic evolution of the Seyfert 2 X-ray luminosity function.
Separate composite spectra for objects in our sample with and without polarized
broad optical emission lines are also presented.Comment: 11 pages (AASTeX), including 3 figures. Accepted for publication in
ApJ Letter
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