334 research outputs found
The Teaching Postdoc: What Four Early Career Scholars Learned from an Honors College
Authors examine the impact of honors education on four early career scholars who each spent two years in a postdoctoral position (2017–2023), highlighting how initial in-depth exposure to honors curricula and culture positively influences professional identity and teaching practice. Fellowships in a Teaching, Research, and Mentoring (TRM) Program offer nascent scholars from a range of disciplines the opportunity to work with students, extend curricular offerings, and launch career trajectories in a variety of ways. Accounts of these experiences should encourage honors program administrators to look to the ranks of early career scholars as desirable instructors
Cosmological Implications of Lyman-Break Galaxy Clustering
We review our analysis of the clustering properties of ``Lyman-break''
galaxies (LBGs) at redshift z~3, previously discussed in Wechsler et al (1998).
We examine the likelihood of spikes found by Steidel et al (1998) in the
redshift distribution of LBGs, within a suite of models for the evolution of
structure in the Universe. Using high-resolution dissipationless N-body
simulations, we analyze deep pencil-beam surveys from these models in the same
way that they are actually observed, identifying LBGs with the most massive
dark matter halos. We find that all the models (with SCDM as a marginal
exception) have a substantial probability of producing spikes similar to those
observed, because the massive halos are much more clumped than the underlying
matter -- i.e., they are biased. Therefore, the likelihood of such a spike is
not a good discriminator among these models. The LBG correlation functions are
less steep than galaxies today (gamma~1.4), but show similar or slightly longer
correlation lengths. We have extened this analysis and include a preliminary
comparison to the new data presented in Adelberger et al (1998). We also
discuss work in progress, in which we use semi-analytic models to identify
Lyman-break galaxies within dark-matter halos.Comment: 4 pages, 2 figures, Latex, uses aipproc.sty; to appear in the
proceedings of the 9th Annual October Maryland Astrophysics Conference,
"After the Dark Ages: When the Galaxies Were Young (the Universe at 2<z<5)
Clusters in Various Cosmological Models: Abundance and Evolution
The combination of measurements of the local abundance of rich clusters of
galaxies and its evolution to higher redshift offers the possibility of a
direct measurement of with little contribution from other
cosmological parameters. We investigate the significance of recent claims that
this evolution indicates that must be small. The most recent cluster
velocity dispersion function from a compilation including the ESO Northern
Abell Cluster Survey (ENACS) results in a significantly higher normalization
for models, corresponding to for , compared
to the Eke, Cole, & Frenk result of . Using the ENACS
data for a calibration results in strong evolution in the abundance of
clusters, and we find that the velocity dispersion function is consistent with
. The results are dependent upon the choice and analysis of
low-redshift and high-redshift data, so at present, the data is not good enough
to determine unambiguously.Comment: 4 pages Latex using sprocl.sty, 1 figure. To appear in Proceedings of
12th Potsdam Cosmology Workshop, "Large-Scale Structure: Tracks and Traces"
Sept. 15-19, 199
CDM-Variant Cosmological Models - I: Simulations and Preliminary Comparisons
We present two matched sets of five simulations each, covering five presently
favored simple modifications to the standard cold dark matter (CDM) scenario.
One simulation suite, with a linear box size of 75 Mpc/h, is designed for high
resolution and good statistics on the group/poor cluster scale, and the other,
with a box size of 300 Mpc/h, is designed for good rich cluster statistics. All
runs had 57 million cold particles, and models with massive neutrinos had an
additional 113 million hot particles. We consider separately models with
massive neutrinos, tilt, curvature, and a nonzero cosmological constant in
addition to the standard CDM model. We find that our tilted
Omega+Omega_Lambda=1 (TLCDM) model produces too much small-scale power by a
factor of ~3, and our open Lambda=0 (OCDM) model also exceeds observed
small-scale power by a factor of 2. In addition, we take advantage of the large
dynamic range in detectable halo masses our simulations allow to check the
shape of the Press-Schechter approximation. We find good fits at cluster masses
for delta_c=1.27--1.35 for a Gaussian filter and delta_c=1.57--1.73 for a
tophat filter. However, Press-Schechter overpredicts the number density of
halos compared to the simulations in the high resolution suite by a weakly
cosmology-dependent factor of 1.5--2 at galaxy and group masses, which cannot
be fixed by adjusting delta_c within reasonable bounds. An appendix generalizes
the spherical collapse model to any isotropic cosmology.Comment: 18 pages Latex using Monthly Notices style, with 13 inlined EPS
figures. This version matches the one accepted by MNRAS. The appendix has
been removed and may now be found instead at
http://fozzie.gsfc.nasa.gov/thesis/appendixC.ps.g
Evaluating Semi-Analytic Halo Merging Histories
We evaluate the accuracy of semi-analytic merger-trees by comparing them with
the merging histories of dark-matter halos in N-body simulations, focusing on
the joint distribution of the number of progenitors and their masses. We first
confirm that the halo mass function as predicted directly by the
Press-Schechter (PS) model deviates from the simulations by up to 50% depending
on the mass scale and redshift, while the means of the projected distributions
of progenitor number and mass for a halo of a given mass are more accurately
predicted by the Extended PS model. We then use the full merger trees to study
the joint distribution as a function of redshift and parent-halo mass. We find
that while the deviation of the mean quantities due to the inaccuracy of the
Extended PS model partly propagates into the higher moments of the
distribution, the merger-tree procedure does not introduce a significant
additional source of error. In particular, certain properties of the merging
history such as the mass ratio of the progenitors and the total accretion rate
are reproduced quite accurately for galaxy sized halos (\sim 10^{12}\msun),
and less so for larger masses. We conclude that although there could be deviations in the absolute numbers and masses of progenitors and in the
higher order moment of these distributions, the relative properties of
progenitors for a given halo are reproduced fairly well by the merger trees.
They can thus provide a useful framework for modelling galaxy formation once
the above-mentioned limitations are taken into account.Comment: 10 pages including 9 figures, submitted to MNRA
Probing Galaxy Formation with TeV Gamma Ray Absorption
We present here the extragalactic background light (EBL) predicted by
semi-analytic models of galaxy formation, and show how measurements of the
absorption of gamma rays of TeV energies via pair production on the EBL
can probe cosmology and the formation of galaxies. Semi-analytic models permit
a physical treatment of the key processes of galaxy formation -- including
gravitational collapse and merging of dark matter halos, gas cooling and
dissipation, star formation, supernova feedback and metal production -- and
have been shown to reproduce key observations at low and high redshift. Using
this approach, we investigate the consequences of variations in input
assumptions such as the stellar initial mass function and the underlying
cosmology. We conclude that observational studies of the absorption of TeV gamma rays will help to constrain the star formation
history of the universe, and the nature and extent of the extinction of
starlight due to dust and reradiation of the absorbed energy at infrared
wavelengths.Comment: 17 pages, 8 figures, presented at the VERITAS Workshop on TeV
Astrophysics of Extragalactic Sources, eds. M. Catanese and T. Weekes, to be
published in Astroparticle Physic
Semi-Analytic Modelling of Galaxy Formation: The Local Universe
Using semi-analytic models of galaxy formation, we investigate galaxy
properties such as the Tully-Fisher relation, the B and K-band luminosity
functions, cold gas contents, sizes, metallicities, and colours, and compare
our results with observations of local galaxies. We investigate several
different recipes for star formation and supernova feedback, including choices
that are similar to the treatment in Kauffmann, White & Guiderdoni (1993) and
Cole et al. (1994) as well as some new recipes. We obtain good agreement with
all of the key local observations mentioned above. In particular, in our best
models, we simultaneously produce good agreement with both the observed B and
K-band luminosity functions and the I-band Tully-Fisher relation. Improved
cooling and supernova feedback modelling, inclusion of dust extinction, and an
improved Press-Schechter model all contribute to this success. We present
results for several variants of the CDM family of cosmologies, and find that
models with values of --0.5 give the best agreement with
observations.Comment: 26 pages, LaTeX, MNRAS format, 23 inlined postscript figures.
Accepted for publication in MNRAS. Revised version contains substantial
changes including improved models. High resolution figures, original version,
and summary of changes may be found at
http://www.fiz.huji.ac.il/~rachels/papers/sp.htm
Isolation of a potently neutralizing and protective human monoclonal antibody targeting yellow fever virus
Yellow fever virus (YFV) causes sporadic outbreaks of infection in South America and sub-Saharan Africa. While live-attenuated yellow fever virus vaccines based on three substrains of 17D are considered some of the most effective vaccines in use, problems with production and distribution have created large populations of unvaccinated, vulnerable individuals in areas of endemicity. To date, specific antiviral therapeutics have not been licensed for human use against YFV or any other related flavivirus. Recent advances in monoclonal antibody (mAb) technology have allowed the identification of numerous candidate therapeutics targeting highly pathogenic viruses, including many flaviviruses. Here, we sought to identify a highly neutralizing antibody targeting the YFV envelope (E) protein as a therapeutic candidate. We used human B cell hybridoma technology to isolate mAbs from circulating memory B cells from human YFV vaccine recipients. These antibodies bound to recombinant YFV E protein and recognized at least five major antigenic sites on E. Two mAbs (designated YFV-136 and YFV-121) recognized a shared antigenic site and neutralized the YFV-17D vaccine strai
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