7,520 research outputs found
Radio AGN in 13,240 galaxy clusters from the Sloan Digital Sky Survey
We correlate the positions of 13,240 Brightest Cluster Galaxies (BCGs) with
0.1 <= z <= 0.3 from the maxBCG catalog with radio sources from the FIRST
survey to study the sizes and distributions of radio AGN in galaxy clusters. We
find that 19.7% of our BCGs are associated with FIRST sources, and this
fraction depends on the stellar mass of the BCG, and to a lesser extent on the
richness of the parent cluster (in the sense of increasing radio loudness with
increasing mass). The intrinsic size of the radio emission associated with the
BCGs peaks at 55 kpc, with a tail extending to 200 kpc. The radio power of the
extended sources places them on the divide between FR I and FR II type sources,
while sources compact in the radio tend to be somewhat less radio-luminous. We
also detect an excess of radio sources associated with the cluster, instead of
with the BCG itself, extending out to ~1.4 Mpc.Comment: 14 pages, 6 figures, accepted for publication in ApJ
Intergalactic Helium Absorption in Cold Dark Matter Models
Observations from the HUT and the HST have recently detected HeII absorption
along the lines of sight to two high redshift quasars. We use cosmological
simulations with gas dynamics to investigate HeII absorption in the cold dark
matter (CDM) theory of structure formation. We consider two Omega=1 CDM models
with different normalizations and one Omega_0=0.4 CDM model, all incorporating
the photoionizing UV background spectrum computed by Haardt & Madau (1996). The
simulated gas distribution, combined with the H&M spectral shape, accounts for
the relative observed values of taubar_HI and taubar_HeII, the effective mean
optical depths for HI and HeII absorption. If the background intensity is as
high as H&M predict, then matching the absolute values of taubar_HI and
taubar_HeII requires a baryon abundance larger (by factors between 1.5 and 3
for the various CDM models) than our assumed value of Omega_b h^2=0.0125. The
simulations reproduce the evolution of taubar_heII over the observed redshift
range, 2.2 < z < 3.3, if the HeII photoionization rate remains roughly
constant. HeII absorption in the CDM simulations is produced by a diffuse,
fluctuating, intergalactic medium, which also gives rise to the HI ly-alpha
forest. Much of the HeII opacity arises in underdense regions where the HI
optical depth is very low. We compute statistical properties of the HeII and HI
absorption that can be used to test the CDM models and distinguish them from an
alternative scenario in which the HeII absorption is caused by discrete,
compact clouds. The CDM scenario predicts that a substantial amount of baryonic
material resides in underdense regions at high redshift. HeII absorption is the
only sensitive probe of such extremely diffuse, intergalactic gas, so it can
provide a vital test of this fundamental prediction.Comment: Accepted for publication in ApJ, 36 pages, LaTeX (aaspp4), 12
figures. Changes include addition of more information on statistical
uncertainties and on the adopted UV background. Also available at
http://www-astronomy.mps.ohio-state.edu/~racc
Characterization of Lyman Alpha Spectra and Predictions of Structure Formation Models: A Flux Statistics Approach
In gravitational instability models, \lya absorption arises from a continuous
fluctuating medium, so that spectra provide a non-linear one-dimensional
``map'' of the underlying density field. We characterise this continuous
absorption using statistical measures applied to the distribution of absorbed
flux. We describe two simple members of a family of statistics which we apply
to simulated spectra in order to show their sensitivity as probes of
cosmological parameters (H, , the initial power spectrum of
matter fluctuations) and the physical state of the IGM. We make use of SPH
simulation results to test the flux statistics, as well as presenting a
preliminary application to Keck HIRES data.Comment: Contribution to proceedings of the 18th Texas Symposium on
Relativistic Astrophysics (eds A. Olinto, J. Frieman and D. Schramm, World
Scientific),Chicago, December 1996, 3 pages, LaTeX (sprocl), 2 figures. Also
available at http://www-astronomy.mps.ohio-state.edu/~racc
Comparing the penetrative stab performance of the UK HOSDB P1/B and Stanley Tools 1992 trimming blades on certified body armour specimens
This body of research assessed the penetrative stab performance of the United Kingdom Home
Office Scientific Development Branch’s (HOSDB) P1/B certified blades against the readily accessible
Stanley Tools 1992 trimming blade. All tests were performed against the HOSDB KR1-E1 impact
energy of 24 Joules on HOSDB KR1 certified body armour specimens. Results demonstrated that the
Stanley 1992 trimming blade posed a 30% greater stab threat in comparison to the HOSDB P1/B
blade.
This paper demonstrates the threat imposed by readily available utility blades and a requirement to
enhance existing body stab resistant body armour test procedures testing procedures
Using Perturbative Least Action to Recover Cosmological Initial Conditions
We introduce a new method for generating initial conditions consistent with
highly nonlinear observations of density and velocity fields. Using a variant
of the Least Action method, called Perturbative Least Action (PLA), we show
that it is possible to generate several different sets of initial conditions,
each of which will satisfy a set of highly nonlinear observational constraints
at the present day. We then discuss a code written to test and apply this
method and present the results of several simulations.Comment: 24 pages, 6 postscript figures. Accepted for publication in
Astrophysical Journa
Structural Calculations for a Medical Clinic in the Dominican Republic with Mission TwentyFive35
Located in the northern region of the Dominican Republic, Rural Resilience is a clinic situated on a vocational campus in the rural town of Via Tapia. In collaboration with the Amoveo Group, Mission TwentyFive35 initiated the design and construction of this vocational campus to address the community’s limited access to healthcare and education, clean water, food, and the lack of vocational job training. Planned to be completed in several phases, this campus aims as a means to solve these shortcomings. This project provides the structural calculations and drawings for the clinic within Mission TwentyFive35’s campus. In coordination with the non profit humanitarian organization Journeyman International, an architecture student was paired with a construction management student and an architectural engineering student to design the building, which will be reviewed by an in-country engineer before construction may begin
Measuring the complexity of social associations using mixture models
This is final version. Available on open access from Springer via the DOI in this record.We propose a method for examining and measuring the complexity of animal social networks that are characterized using association indices. The method focusses on the diversity of types of dyadic relationship within the social network. Binomial mixture models cluster dyadic relationships into relationship types, and variation in the preponderance and strength of these relationship types can be used to estimate association complexity using Shannon’s information index. We use simulated data to test the method, and find that models chosen using integrated complete likelihood give estimates of complexity that closely reflect the true complexity of social systems, but these estimates can be downwardly biased by low intensity sampling and upwardly biased by extreme overdispersion within components. We also illustrate the use of the method on two real data sets. The method could be extended for use on interaction rate data using Poisson mixture models, or on multidimensional relationship data using multivariate mixture models
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