220 research outputs found
Beyond the Thin Lens Approximation
We obtain analytic formulae for the null geodesics of
Friedmann-Lema\^{\i}tre-Robertson-Walker spacetimes with scalar perturbations
in the longitudinal gauge. From these we provide a rigorous derivation of the
cosmological lens equation, and obtain an expression for the magnification of a
bundle of light rays without restriction to static or thin lens scenarios. We
show how the usual magnification matrix naturally emerges in the appropriate
limits.Comment: 37 pages plus 3 appended figures, plain TeX. Submitted to Ap
Using the Sunyaev-Zel'dovich Effect to Probe the Gas in Clusters
The thermal Sunyaev-Zel'dovich effect is an important probe of clusters of
galaxies, and has the attractive property of being proportional to the thermal
energy content of the intracluster medium. With the assistance of X-ray data,
the effect can be used to measure the number of hot electrons in clusters, and
thus measure cluster baryon contents. Cluster absolute distances and other
structural parameters can also be measured by combining thermal
Sunyaev-Zel'dovich, X-ray, and other data. This review presents an introduction
to the effect, shows some representative results, and sketches imminent
developments.Comment: Review for Carnegie Observatories Centennial Symposium 3. 16 pages of
LaTeX with 5 embedded Postscript figures figure
Cluster-Induced Fluctuations in the Microwave Background Radiation
The research proposed was to detect, map and interpret the Sunyaev-Zel dovich (SZ) effects in two samples of distant clusters of galaxies with the OVRO 40-m telescope: an optically selected sample of 26 clusters at the North Ecliptic Pole, and an X-ray selected sample of clusters based on the Einstein Medium Sensitivity Survey, to make small maps of the strongest cluster SZ effects using the OVRO 40-m telescope, to combine the SZ and X-ray data for well-detected clusters to determine the value of the Hubble constant and set limits to the value of the deceleration parameter, and to study the properties of cluster atmospheres using the SZ effect
Entrepreneurship In Multinational Corporations: The Initiative Process In Foreign Subsidiaries
This is an empirical study of initiatives in the subsidiaries of multinational corporations. An initiative is the subsidiary-driven creation of a value-adding activity. While previous research has focused on the world product mandates earned as a result of initiatives, or the corporate systems that facilitate subsidiary initiatives, this study is concerned with the internal processes that actively drive subsidiary initiatives. This issue is of vital importance to subsidiary managers who are looking for ways to enhance their value-added role in the corporation: it also has substantial implications for corporate strategy and for theoretical models of multinational management and entrepreneurship.;Two research questions drove this research: What forms do subsidiary initiatives take? and What is the initiative process? The research was undertaken using an inductive approach, building knowledge from an iterative combination of empirical evidence and theory. A total of 39 initiatives from six Canadian subsidiaries of U.S. multinational corporations formed the final sample. Data was gathered through over 100 personal interviews, but also from two questionnaires and archival and secondary sources.;There were two key findings. First, four distinct types of subsidiary initiatives were identified, labelled reconfiguration , local market , competitive bid and mandate extension . These four types were empirically validated, and conceptualized in terms of the market opportunities they tapped into. Second, the initiative process was analyzed in depth, and found to consist of four phases representing the increasing viability of the initiative. The process was found to be strongly influenced by the organizational context of the subsidiary, as predicted by Burgelman\u27s model of corporate venturing. The key contribution, however, is the observation that the internally-defined subsidiary context is a more critical driver of initiative than the externally-defined corporate context.;The implication for the management of multinational subsidiaries is that the assignment of subsidiary roles has limitations, because it assumes the opportunity set of the subsidiary can be prejudged and better understood from the centre than the periphery. Subsidiaries should have sufficient slack that opportunities can be identified and developed; and they should attempt to build an internal management context to foster the entrepreneurial spirit. Theoretical and managerial implications are developed around these core insights
Galaxy gas ejection in radio galaxies: the case of 3C 35
We report results from XMM-Newton and Chandra observations of the nearby (z =
0.067) giant radio galaxy 3C 35. We find evidence for an X-ray emitting gas
belt, orthogonal to and lying between the lobes of 3C 35, which we interpret as
fossil-group gas driven outwards by the expanding radio lobes. We also detect
weak emission from a second, more extended group-type environment, as well as
inverse-Compton X-ray emission from the radio lobes. The morphological
structure of the radio lobes and gas belt point to co-evolution. Furthermore,
the radio source is powerful enough to eject galaxy-scale gas out to distances
of 100kpc, and the ages of the two features are comparable (tsynch~140Myr,
tbelt~80 Myr). The destruction of 3C 35's atmosphere may offer clues as to how
fossil systems are regulated: radio galaxies need to be of power comparable to
3C 35 to displace and regulate fossil-group gas. We discuss the implications of
the gas belt in 3C 35 in terms of AGN fuelling and feedback.Comment: 18 pages, accepted to MNRA
Null Geodesics in Perturbed Spacetimes
We present a generalization and refinement of the Sachs-Wolfe technique which
unifies many of the approaches taken to date and clarifies both the physical
and the mathematical character of the method. We illustrate the formalism with
a calculation of the behavior of light passing a moving lens on a Minkowski
background.Comment: 24 page
3-D Simulations of Protostellar Jets in Stratified Ambient Media
We present fully three-dimensional hydrodynamical simulations of radiative
cooling jets propagating into stratified isothermal ambient media with
power-law density and pressure distributions. The parameters used are mainly
suitable for protostellar jets but results applicable to extragalactic jets are
also presented. Comparisons are made with previous simulations of jets through
homogeneous media. We find that for radiative cooling jets propagating into
regions where the ambient medium has an increasing density (and pressure)
gradient, the ambient gas tends to compress the cold, low-pressure cocoon of
shocked material that surrounds the beam and destroy the bow shock-like
structure at the head. The compressing medium collimates the jet and promotes
the development of Kelvin-Helmholtz instabilities which cause beam focusing,
wiggling and the formation of internal traveling shocks,
, via pinching along the beam. This remarkably resembles the structure of
some observed systems (e.g. Haro 6-5B northern and HH 24G jets). These effects
are larger for jets with smaller density ratio between jet and environment
(tested for =1, 3, and 10) and larger Mach number
(tested for 12 and 24, where is the jet velocity and the
ambient sound speed). In an ambient medium of decreasing density (and
pressure), the beam is poorly collimated and relaxes, becoming faint. This
could explain ''invisible'' jet sections, like the gap between the parent
source and collimated beam (e.g., in HH30 jet). Although, on average, jets
propagating into an increasing (decreasing) density environment are decelerated
(accelerated) by the increasing (decreasing) ram pressure of the ambient
medium, we find that their propagation velocities have an oscillating pattern.Comment: 33 pp, LaTeX file, 13 figures upon request. To appear in the
Astrophys. J., vol 471, nov. 10t
The gas distribution in the high-redshift cluster MS 1054-0321
We investigate the gas mass distribution in the high redshift cluster MS
1054-0321 using Chandra X-ray and OCRA SZ effect data. We use a superposition
of offset -type models to describe the composite structure of MS
1054-0321. We find gas mass fractions f_{gas}^\rm{X\mbox{-}ray} =
0.087_{-0.001}^{+0.005} and f_{gas}^\rm{SZ} = 0.094_{-0.001}^{+0.003} for
the (main) eastern component of MS 1054-0321 using X-ray or SZ data, but
f_{gas}^\rm{X\mbox{-}ray} = 0.030 _{-0.014}^{+0.010} for the western
component. The gas mass fraction for the eastern component is in agreement with
some results reported in the literature, but inconsistent with the cosmic
baryon fraction. The low gas mass fraction for the western component is likely
to be a consequence of gas stripping during the ongoing merger. The gas mass
fraction of the integrated system is : we suggest that
the missing baryons from the western component are present as hot diffuse gas
which is poorly represented in existing X-ray images. The missing gas could
appear in sensitive SZ maps.Comment: 12 pages, 4 figures, and 5 table
Determining Tangential Peculiar Velocities of Clusters of Galaxies using Gravitational Lensing
We propose two new methods for measuring tangential peculiar velocities of
rich clusters of galaxies. Our first method is based on weak gravitational
lensing and takes advantage of the differing images of background galaxies
caused by moving and stationary gravitational potentials. Our second method is
based on measuring relative frequency shifts between multiple images of a
single strongly lensed background galaxy. We illustrate this method using the
example of galaxy cluster CL 0024+1654.Comment: LateX, 4 pages, 2 figures, accepted for publication in Part 1 of The
Astrophysical Journa
Interaction between the intergalactic medium and central radio source in the NGC 4261 group of galaxies
Using observations from the Chandra and XMM-Newton X-ray observatories, we
examine the interaction between the intra-group medium and central radio source
in the nearby NGC 4261 galaxy group. We confirm the presence of cavities
associated with the radio lobes and estimate their enthalpy to be ~2.4x10^58
erg. The mechanical power output of the jets is >=10^43 erg/s, at least a
factor of 60 greater than the cooling luminosity in the region the lobes
inhabit. We identify rims of compressed gas enclosing the lobes, but find no
statistically significant temperature difference between them and their
surroundings, suggesting that the lobe expansion velocity is approximately
sonic (Mach<=1.05). The apparent pressure of the radio lobes, based on the
synchrotron minimum energy density argument, is a factor of 5 lower than that
of the intra-group medium. Pressure balance could be achieved if entrainment of
thermal gas provided additional non-radiating particles in the lobe plasma, but
the energy required to heat these particles would be ~20 per cent. of the
mechanical energy output of the radio source. NGC 4261 has a relatively compact
cool core, which should probably be categorised as a galactic corona. The
corona is capable of fuelling the active nucleus for considerably longer than
the inferred source lifetime, but can be only inefficiently heated by the AGN
or conduction. The expansion of the radio lobes has affected the structure of
the gas in the galaxy, compressing and moving the material of the corona
without causing significant shock heating, and expelling gas from the immediate
neighbourhood of the jets. We discuss the possible implications of this
environment for the duration of the AGN outburst, and consider mechanisms which
might lead to the cessation of nuclear activity.Comment: Accepted for publication in MNRAS, 17 pages, 6 figure
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