90 research outputs found
The inverse-Compton ghost HDF 130 and the giant radio galaxy 6C 0905+3955: matching an analytic model for double radio source evolution
We present new GMRT observations of HDF 130, an inverse-Compton (IC) ghost of
a giant radio source that is no longer being powered by jets. We compare the
properties of HDF 130 with the new and important constraint of the upper limit
of the radio flux density at 240 MHz to an analytic model. We learn what values
of physical parameters in the model for the dynamics and evolution of the radio
luminosity and X-ray luminosity (due to IC scattering of the cosmic microwave
background (CMB)) of a Fanaroff-Riley II (FR II) source are able to describe a
source with features (lobe length, axial ratio, X-ray luminosity, photon index
and upper limit of radio luminosity) similar to the observations. HDF 130 is
found to agree with the interpretation that it is an IC ghost of a powerful
double-lobed radio source, and we are observing it at least a few Myr after jet
activity (which lasted 5--100 Myr) has ceased. The minimum Lorentz factor of
injected particles into the lobes from the hotspot is preferred to be
for the model to describe the observed quantities well,
assuming that the magnetic energy density, electron energy density, and lobe
pressure at time of injection into the lobe are linked by constant factors
according to a minimum energy argument, so that the minimum Lorentz factor is
constrained by the lobe pressure. We also apply the model to match the features
of 6C 0905+3955, a classical double FR II galaxy thought to have a low-energy
cutoff of in the hotspot due to a lack of hotspot
inverse-Compton X-ray emission. The models suggest that the low-energy cutoff
in the hotspots of 6C 0905+3955 is , just slightly above
the particles required for X-ray emission.Comment: 9 pages, 3 figure
The Lifetime of FRIIs in Groups and Clusters: Implications for Radio-Mode Feedback
We determine the maximum lifetime t_max of 52 FRII radio sources found in 26
central group galaxies from cross correlation of the Berlind SDSS group catalog
with the VLA FIRST survey. Mock catalogs of FRII sources were produced to match
the selection criteria of FIRST and the redshift distribution of our parent
sample, while an analytical model was used to calculate source sizes and
luminosities. The maximum lifetime of FRII sources was then determined via a
comparison of the observed and model projected length distributions. We
estimate the average FRII lifetime is 1.5x10^7 years and the duty cycle is
~8x10^8 years. Degeneracies between t_max and the model parameters: jet power
distribution, axial ratio, energy injection index, and ambient density
introduce at most a factor of two uncertainty in our lifetime estimate. In
addition, we calculate the radio active galactic nuclei (AGN) fraction in
central group galaxies as a function of several group and host galaxy
properties. The lifetime of radio sources recorded here is consistent with the
quasar lifetime, even though these FRIIs have substantially sub-Eddington
accretion. These results suggest a fiducial time frame for energy injection
from AGN in feedback models. If the morphology of a given extended radio source
is set by large-scale environment, while the lifetime is determined by the
details of the accretion physics, this FRII lifetime is relevant for all
extended radio sources.Comment: 18 pages, 7 figures. Accepted for publication in ApJ. High resolution
paper available at http://www.astronomy.ohio-state.edu/~bird/BMK07.pd
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Inducing Cross-Clade Neutralizing Antibodies against HIV-1 by Immunofocusing
Background: Although vaccines are important in preventing viral infections by inducing neutralizing antibodies (nAbs), HIV-1 has proven to be a difficult target and escapes humoral immunity through various mechanisms. We sought to test whether HIV-1 Env mimics may serve as immunogens. Methodology/Principal Findings: Using random peptide phage display libraries, we identified the epitopes recognized by polyclonal antibodies of a rhesus monkey that had developed high-titer, broadly reactive nAbs after infection with a simian-human immunodeficiency virus (SHIV) encoding env of a recently transmitted HIV-1 clade C (HIV-C). Phage peptide inserts were analyzed for conformational and linear homology using computational analysis; some peptides mimicked various domains of the original HIV-C Env, such as conformational V3 loop epitopes and the conserved linear region of the gp120 C-terminus. Next, we devised a novel prime/boost strategy to test the immunogenicity of such phage-displayed peptides and primed mice only once with HIV-C gp160 DNA followed by boosting with mixtures of recombinant phages. Conclusions/Significance: This strategy, which was designed to focus the immune system on a few Env epitopes (immunofocusing), not only induced HIV-C gp160 binding antibodies and cross-clade nAbs, but also linked a conserved HIV Env region for the first time to the induction of nAbs: the C-terminus of gp120. The identification of conserved antigen mimics may lead to novel immunogens capable of inducing broadly reactive nAbs
Cosmological Constraints from the ROSAT Deep Cluster Survey
The ROSAT Deep Cluster Survey (RDCS) has provided a new large deep sample of
X-ray selected galaxy clusters. Observables such as the flux number counts
n(S), the redshift distribution n(z) and the X-ray luminosity function (XLF)
over a large redshift baseline (z\lesssim 0.8) are used here in order to
constrain cosmological models. Our analysis is based on the Press-Schechter
approach, whose reliability is tested against N-body simulations. Following a
phenomenological approach, no assumption is made a priori on the relation
between cluster masses and observed X-ray luminosities. As a first step, we use
the local XLF from RDCS, along with the high-luminosity extension provided by
the XLF from the BCS, in order to constrain the amplitude of the power
spectrum, \sigma_8, and the shape of the local luminosity-temperature relation.
We obtain \sigma_8=0.58 +/- 0.06 for Omega_0=1 for open models at 90%
confidence level, almost independent of the L-T shape. The density parameter
\Omega_0 and the evolution of the L-T relation are constrained by the RDCS XLF
at z>0 and the EMSS XLF at z=0.33, and by the RDCS n(S) and n(z) distributions.
By modelling the evolution for the amplitude of the L-T relation as (1+z)^A, an
\Omega_0=1 model can be accommodated for the evolution of the XLF with 1<A<3 at
90% confidence level, while \Omega_0=0.4^{+0.3}_{-0.2} and \Omega_0<0.6 are
implied by a non--evolving L-T for open and flat models, respectively.Comment: 12 pages, 9 colour figures, LateX, uses apj.sty, ApJ, in press, May
20 issu
The Evolution of X-ray Clusters and the Entropy of the Intra Cluster Medium
The thermodynamics of the diffuse, X-ray emitting gas in clusters of galaxies
is determined by gravitational processes associated with shock heating,
adiabatic compression, and non-gravitational processes such as heating by SNe,
stellar winds, activity in the central galactic nucleus, and radiative cooling.
The effect of gravitational processes on the thermodynamics of the Intra
Cluster Medium (ICM) can be expressed in terms of the ICM entropy S ~
ln(T/\rho^{2/3}). We use a generalized spherical model to compute the X-ray
properties of groups and clusters for a range of initial entropy levels in the
ICM and for a range of mass scales, cosmic epochs and background cosmologies.
We find that the statistical properties of the X-ray clusters strongly depend
on the value of the initial excess entropy. Assuming a constant, uniform value
for the excess entropy, the present-day X-ray data are well fitted for the
following range of values K_* = kT/\mu m_p \rho^{2/3} = (0.4\pm 0.1) \times
10^{34} erg cm^2 g^{-5/3} for clusters with average temperatures kT>2 keV; K_*
= (0.2\pm 0.1) \times 10^{34} erg cm^2 g^{-5/3} for groups and clusters with
average temperatures kT<2 keV. These values correspond to different excess
energy per particle of kT \geq 0.1 (K_*/0.4\times 10^{34}) keV. The dependence
of K_* on the mass scale can be well reproduced by an epoch dependent external
entropy: the relation K_* = 0.8(1+z)^{-1}\times 10^{34} erg cm^2 g^{-5/3} fits
the data over the whole temperature range. Observations of both local and
distant clusters can be used to trace the distribution and the evolution of the
entropy in the cosmic baryons, and ultimately to unveil the typical epoch and
the source of the heating processes.Comment: 53 pages, LateX, 19 figures, ApJ in press, relevant comments and
references adde
Data quality in the human and environmental health sciences: Using statistical confidence scoring to improve QSAR/QSPR modeling
A greater number of toxicity data are becoming publicly available allowing for in silico modeling. However, questions often arise as how to incorporate data quality and how to deal with contradicting data if more than a single datum point is available for the same compound. In this study, two well-known and studied QSAR/QSPR models for skin permeability and aquatic toxicology have been investigated in the context of statistical data quality. In particular, the potential benefits of the incorporation of the statistical Confidence Scoring (CS) approach within modelling and validation. As a result, robust QSAR/QSPR models for the skin permeability coefficient and the toxicity of nonpolar narcotics to Aliivibrio fischeri assay were created. CSweighted linear regression for training and CS-weighted root mean square error (RMSE) for validation were statistically superior compared to standard linear regression and standard RMSE. Strategies are proposed as to how to interpret data with high and low CS, as well as how to deal with large datasets containing multiple entries
Tracing cosmic evolution with clusters of galaxies
The most successful cosmological models to date envision structure formation
as a hierarchical process in which gravity is constantly drawing lumps of
matter together to form increasingly larger structures. Clusters of galaxies
currently sit atop this hierarchy as the largest objects that have had time to
collapse under the influence of their own gravity. Thus, their appearance on
the cosmic scene is also relatively recent. Two features of clusters make them
uniquely useful tracers of cosmic evolution. First, clusters are the biggest
things whose masses we can reliably measure because they are the largest
objects to have undergone gravitational relaxation and entered into virial
equilibrium. Mass measurements of nearby clusters can therefore be used to
determine the amount of structure in the universe on scales of 10^14 to 10^15
solar masses, and comparisons of the present-day cluster mass distribution with
the mass distribution at earlier times can be used to measure the rate of
structure formation, placing important constraints on cosmological models.
Second, clusters are essentially ``closed boxes'' that retain all their gaseous
matter, despite the enormous energy input associated with supernovae and active
galactic nuclei, because the gravitational potential wells of clusters are so
deep. The baryonic component of clusters therefore contains a wealth of
information about the processes associated with galaxy formation, including the
efficiency with which baryons are converted into stars and the effects of the
resulting feedback processes on galaxy formation. This article reviews our
theoretical understanding of both the dark-matter component and the baryonic
component of clusters. (Abridged)Comment: 54 pages, 15 figures, Rev. Mod. Phys. (in press
The duty cycle of local radio galaxies
We use a volume- and flux-limited sample of local ()
radio galaxies with optical counterparts to address the question of how long a
typical galaxy spends in radio-active and quiescent states. The length of the
active phase has a strong dependence on the stellar mass of the host galaxy.
Radio sources in the most massive hosts are also retriggered more frequently.
The time spent in the active phase has the same dependence on stellar mass as
does the gas cooling rate, suggesting the onset of the quiescent phase is due
to fuel depletion. We find radio and emission line AGN activity to be
independent, consistent with these corresponding to different accretion states.Comment: accepted for publication in MNRAS; 15 pages, 14 figure
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