14,185 research outputs found
Narrow Band X-ray Photometry as a Tool for Studying Galaxy and Cluster Mass Distributions
We explore the utility of narrow band X-ray surface photometry as a tool for
making fully Bayesian, hydrostatic mass measurements of clusters of galaxies,
groups and early-type galaxies. We demonstrate that it is sufficient to measure
the surface photometry with the Chandra X-ray observatory in only three (rest
frame) bands (0.5--0.9 keV, 0.9--2.0 keV and 2.0--7.0 keV) in order to
constrain the temperature, density and abundance of the hot interstellar medium
(ISM). Adopting parametrized models for the mass distribution and radial
entropy profile and assuming spherical symmetry, we show that the constraints
on the mass and thermodynamic properties of the ISM that are obtained by
fitting data from all three bands simultaneously are comparable to those
obtained by fitting similar models to the temperature and density profiles
derived from spatially resolved spectroscopy, as is typically done. We
demonstrate that the constraints can be significantly tightened when exploiting
a recently derived, empirical relationship between the gas fraction and the
entropy profile at large scales, eliminating arbitrary extrapolations at large
radii. This "Scaled Adiabatic Model" (ScAM) is well suited to modest
signal-to-noise data, and we show that accurate, precise measurements of the
global system properties are inferred when employing it to fit data from even
very shallow, snapshot X-ray observations. The well-defined asymptotic
behaviour of the model also makes it ideally suited for use in
Sunyaev-Zeldovich studies of galaxy clusters.Comment: 15 pages, 6 figures. Accepted for Publication in MNRA
Factors determining exchange rates : a simple model and empirical tests
An abstract for this article is not availableEquilibrium (Economics)
Photoionization models for extreme Ly 1216 and HeII 1640 ratios in quasar halos, and PopIII vs AGN diagnostics
We explore mechanisms to produce extremely high Ly-alpha/HeII flux ratios, or
to enhance the observed number of Ly-alpha photons per incident ionizing
photon, in extended AGN-photoionized nebulae at high-redshift. Using
photoionization models, we explore the impact of ionization parameter, gas
metallicity, ionizing spectrum, electron energy distribution, and cloud viewing
angle on the relative fluxes of Ly-alpha, HeII and other lines, and on the
observed number of Ly-alpha photons per incident ionizing photon. We find that
low ionization parameter, a relatively soft or filtered ionizing spectrum, low
gas metallicity, kappa-distributed electron energies, or reflection of Ly-alpha
photons by HI can all result in significantly enhanced Ly-alpha relative to
other lines (>10%), with log Ly-alpha/HeII reaching values up to 4.6. In the
cases of low gas metallicity, reflection by HI, or a hard or filtered ionizing
spectrum, the observed number of Ly-alpha photons per incident ionizing photon
is itself significantly enhanced above the nominal Case B value of 0.66 due to
collisional excitation, reaching values up to 5.3 in our 'extreme case' model.
At low gas metallicity (e.g. 0.1 x Solar), the production of Ly-alpha is
predominantly via collisional excitation rather than recombination. In
addition, we find that collisional excitation of Ly-alpha becomes more
efficient if the ionizing continuum is pre-filtered through an optically thin
screen of gas closer to the AGN. We also show that Ly-alpha / HeII ratios of
the z~3.5 quasars studied by Borisova et al. (2016) are consistent with
AGN-photoionization of gas with moderate to low metallicity and/or low
ionization parameter, without requiring exotic ionization/excitation mechanisms
such as strong line-transfer effects. We also present UV-optical diagnostic
diagrams to distinguish between photoionization by Pop III stars and AGN
photoionization.Comment: Accepted for publication in A&A. 14 pages, 9 figures. Abstract
slightly shortened to meet arxiv character limi
Liquid ethylene-propylene copolymers
Oligomers are prepared by heating solid ethylene-propylene rubber in container that retains solid and permits liquid product to flow out as it is formed. Molecular weight and viscosity of liquids can be predetermined by process temperature. Copolymers have low viscosity for given molecular weight
Emotion and secrecy in Australian asylum-seeker comics: The politics of visual style
This article examines two online comics about Australiaâs policies of detaining asylum seekers, one created by the Australian governmentâs Customs and Border Protection Service (CBPS), and one published by the experimental journalism site The Global Mail. Through an analysis of the way online readers responded to these comics, this article shows how digital comics use visual style to imply particular kinds of relationships between their authors and their audience, while generating audience engagement through abstracted emotions and narrative gaps. These features have political dimensions, as in the CBPSâs comic, which elides crucial details about the governmentâs policies while suggesting (but never directly stating) its disregard for the human rights of asylum seekers, while The Global Mailâs comic uses a hand-drawn visual style to generate reader sympathy for the detainees and opposition to the governmentâs policies. Both comics use visual language to obfuscate key elements about the sources of their messages while also obscuring the voices of the refugees that their images represent.Aaron Humphre
- âŠ