12,790 research outputs found
An Extremely Massive Dry Galaxy Merger in a Moderate Redshift Cluster
We have identified perhaps the largest major galaxy merger ever seen. While
analysing Spitzer IRAC images of CL0958+4702, an X-ray selected cluster at
z=0.39, we discovered an unusual plume of stars extending 110 kpc
outward from the bright central galaxy (BCG). Three galaxies 1-1.5 mag fainter
than the BCG lie within 17 kpc (projected) of the BCG and are probably
participating in the merger. The plume is detected in all four IRAC channels
and at optical wavelengths in images from the WIYN telescope; the surface
brightness is remarkably high (24.8 mag arcsec at 50 kpc).
The optical and infrared colors are consistent with those of other BCGs,
suggesting that the plume is composed of old stars and negligible recent star
formation (hence a "dry merger"). The luminosity in the plume is at least
equivalent to a 4L^* galaxy. A diffuse halo extending 110 kpc from the BCG in
one IRAC image suggests the total amount of diffuse light is L_r\sim
1.3x10^{11}h^{-2} L_sun. A Chandra observation shows an X-ray image and
spectrum typical of moderate-mass clusters. We use MMT/Hectospec to measure 905
redshifts in a 1 deg^2 region around the cluster. The velocities of two of the
BCG companions indicate a merger timescale for the companion galaxies of
110 Myr and 0.5-1 Gyr for the plume. We conclude that the BCG and
intracluster light of CL0958 is formed by major mergers at moderate redshifts.
After the major merger is complete, CL0958 will likely become a fossil cluster.Comment: 5 pages, 2 figures, to appear in ApJ
The generation of a Gaussian random process in a position parameter
Analog computer method for approximating stationary Gaussian random process depending only on position paramete
Molecular mechanisms of transcription initiation—structure, function, and evolution of TFE/TFIIE-like factors and open complex formation
Transcription initiation requires that the promoter DNA is melted and the template strand is loaded into the active site of the RNA polymerase (RNAP), forming the open complex (OC). The archaeal initiation factor TFE and its eukaryotic counterpart TFIIE facilitate this process. Recent structural and biophysical studies have revealed the position of TFE/TFIIE within the pre-initiation complex (PIC) and illuminated its role in OC formation. TFE operates via allosteric and direct mechanisms. Firstly, it interacts with the RNAP and induces the opening of the flexible RNAP clamp domain, concomitant with DNA melting and template loading. Secondly, TFE binds physically to single-stranded DNA in the transcription bubble of the OC and increases its stability. The identification of the β-subunit of archaeal TFE enabled us to reconstruct the evolutionary history of TFE/TFIIE-like factors, which is characterised by winged helix (WH) domain expansion in eukaryotes and loss of metal centres including iron-sulfur clusters and Zinc ribbons. OC formation is an important target for the regulation of transcription in all domains of life. We propose that TFE and the bacterial general transcription factor CarD, although structurally and evolutionary unrelated, show interesting parallels in their mechanism to enhance OC formation. We argue that OC formation is used as a way to regulate transcription in all domains of life, and these regulatory mechanisms coevolved with the basal transcription machinery
Binary inspiral, gravitational radiation, and cosmology
Observations of binary inspiral in a single interferometric gravitational
wave detector can be cataloged according to signal-to-noise ratio and
chirp mass . The distribution of events in a catalog composed of
observations with greater than a threshold depends on the
Hubble expansion, deceleration parameter, and cosmological constant, as well as
the distribution of component masses in binary systems and evolutionary
effects. In this paper I find general expressions, valid in any homogeneous and
isotropic cosmological model, for the distribution with and of
cataloged events; I also evaluate these distributions explicitly for relevant
matter-dominated Friedmann-Robertson-Walker models and simple models of the
neutron star mass distribution. In matter dominated Friedmann-Robertson-Walker
cosmological models advanced LIGO detectors will observe binary neutron star
inspiral events with from distances not exceeding approximately
, corresponding to redshifts of (0.26) for
(), at an estimated rate of 1 per week. As the binary system mass
increases so does the distance it can be seen, up to a limit: in a matter
dominated Einstein-deSitter cosmological model with () that limit
is approximately (1.7) for binaries consisting of two
black holes. Cosmological tests based on catalogs of the
kind discussed here depend on the distribution of cataloged events with
and . The distributions found here will play a pivotal role in testing
cosmological models against our own universe and in constructing templates for
the detection of cosmological inspiraling binary neutron stars and black holes.Comment: REVTeX, 38 pages, 9 (encapsulated) postscript figures, uses epsf.st
Rotochemical Heating of Neutron Stars: Rigorous Formalism with Electrostatic Potential Perturbations
The electrostatic potential that keeps approximate charge neutrality in
neutron star matter is self-consistently introduced into the formalism for
rotochemical heating presented in a previous paper by Fernandez and
Reisenegger. Although the new formalism is more rigorous, we show that its
observable consequences are indistinguishable from those of the previous one,
leaving the conclusions of the previous paper unchanged.Comment: 14 pages, including 4 eps figures. Accepted for publication in The
Astrophysical Journa
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