1,340 research outputs found
Cosmic ray diffusion fronts in the Virgo cluster
The pair of large radio lobes in the Virgo cluster, each about 23 kpc in
radius, have curiously sharp outer edges where the radio-synchrotron continuum
flux declines abruptly. However, just adjacent to this sharp transition, the
radio flux increases. This radio limb-brightening is observed over at least
half of the perimeter of both lobes. We describe slowly propagating steady
state diffusion fronts that explain these counterintuitive features. Because of
the natural buoyancy of radio lobes, the magnetic field is largely tangent to
the lobe boundary, an alignment that polarizes the radio emission and
dramatically reduces the diffusion coefficient of relativistic electrons. As
cosmic ray electrons diffuse slowly into the cluster gas, the local magnetic
field and gas density are reduced as gas flows back toward the radio lobe.
Radio emission peaks can occur because the synchrotron emissivity increases
with magnetic field and then decreases with the density of non-thermal
electrons. A detailed comparison of steady diffusion fronts with quantitative
radio observations may reveal information about the spatial variation of
magnetic fields and the diffusion coefficient of relativistic electrons. On
larger scales, some reduction of the gas density inside the Virgo lobes due to
cosmic ray pressure must occur and may be measurable. Such X-ray observations
could reveal important information about the presence of otherwise unobservable
non-thermal components such as relativistic electrons of low energy or proton
cosmic rays.Comment: 11 pages, 5 figures, Accepted by Ap
Polarization Effects on the process with Large Extra Dimensions
We study large extra dimension effects on the polarizations of the pair
and electron beam at the process. It is shown that the
measurements of the cross section for transversely polarized pair with the
right-handed electron beam remarkably enhance the possibilities to see the low
scale quantum gravity effects. Higher Linear Collider bounds on the string
scale in this model can be obtained by using the left-handed electron beam.Comment: Final version to appear in Phys.Lett.B. More references are adde
Spitzer Observations of Passive and Star Forming Early-type Galaxies: an Infrared Color-Color Sequence
We describe the infrared properties of a large sample of early type galaxies,
comparing data from the Spitzer archive with Ks-band emission from 2MASS. While
most representations of this data result in correlations with large scatter, we
find a remarkably tight relation among colors formed by ratios of luminosities
in Spitzer-MIPS (24, 70 and 160 um) bands and the Ks-band. Remarkably, this
correlation among E and S0 galaxies follows that of nearby normal galaxies of
all morphological types. In particular, the tight infrared color-color
correlation for S0 galaxies alone follows that of the entire Hubble sequence of
normal galaxies, roughly in order of galaxy type from ellipticals to spirals to
irregulars. The specific star formation rate of S0 galaxies estimated from the
24um luminosity increases with decreasing Ks-band luminosity (or stellar mass)
from essentially zero, as with most massive ellipticals, to rates typical of
irregular galaxies. Moreover, the luminosities of the many infrared-luminous S0
galaxies can significantly exceed those of the most luminous (presumably
post-merger) E galaxies. Star formation rates in the most infrared-luminous S0
galaxies approach 1-10 solar masses per year. Consistently with this picture we
find that while most early-type galaxies populate an infrared red sequence,
about 24% of the objects (mostly S0s) are in an infrared blue cloud together
with late type galaxies. For those early-type galaxies also observed at radio
frequencies we find that the far-infrared luminosities correlate with the mass
of neutral and molecular hydrogen, but the scatter is large. This scatter
suggests that the star formation may be intermittent or that similar S0
galaxies with cold gaseous disks of nearly equal mass can have varying radial
column density distributions that alter the local and global SF rates.Comment: 14 Pages, 13 figures, Accepted by Ap
On the exclusion of intra-cluster plasma from AGN-blown bubbles
Simple arguments suggest that magnetic fields should be aligned tangentially
to the surface of an AGN-blown bubble. If this is the case, charged particles
from the fully ionised intra-cluster medium (ICM) will be prevented,
ordinarily, from crossing the boundary by the Lorentz force. However, recent
observations indicate that thermal material may occupy up to 50% of the volume
of some bubbles. Given the effect of the Lorentz force, the thermal content
must then be attributed to one, or a combination, of the following processes:
i) the entrainment of thermal gas into the AGN outflow that inflated the
bubble; ii) rapid diffusion across the magnetic field lines at the ICM/bubble
interface; iii) magnetic reconnection events which transfer thermal material
across the ICM/bubble boundary. Unless the AGN outflow behaves as a magnetic
tower jet, entrainment may be significant and could explain the observed
thermal content of bubbles. Alternatively, the cross-field diffusion
coefficient required for the ICM to fill a typical bubble is roughly 10^16 cm^2
s^-1, which is anomalously high compared to predictions from turbulent
diffusion models. Finally, the mass transfer rate due to magnetic reconnection
is uncertain, but significant for plausible reconnection rates. We conclude
that entrainment into the outflow and mass transfer due to magnetic
reconnection events are probably the most significant sources of thermal
content in AGN-blown bubbles.Comment: Accepted for publication in MNRAS, 8 pages, 1 figur
Cellular mRNA Activates Transcription Elongation by Displacing 7SK RNA
The positive transcription elongation factor P-TEFb is a pivotal regulator of gene expression in higher cells. Originally identified in Drosophila, attention was drawn to human P-TEFb by the discovery of its role as an essential cofactor for HIV-1 transcription. It is recruited to HIV transcription complexes by the viral transactivator Tat, and to cellular transcription complexes by a plethora of transcription factors. P-TEFb activity is negatively regulated by sequestration in a complex with the HEXIM proteins and 7SK RNA. The mechanism of P-TEFb release from the inhibitory complex is not known. We report that P-TEFb-dependent transcription from the HIV promoter can be stimulated by the mRNA encoding HIC, the human I-mfa domain-containing protein. The 3′-untranslated region of HIC mRNA is necessary and sufficient for this action. It forms complexes with P-TEFb and displaces 7SK RNA from the inhibitory complex in cells and cell extracts. A 314-nucleotide sequence near the 3′ end of HIC mRNA has full activity and contains a predicted structure resembling the 3′-terminal hairpin of 7SK that is critical for P-TEFb binding. This represents the first example of a cellular mRNA that can regulate transcription via P-TEFb. Our findings offer a rationale for 7SK being an RNA transcriptional regulator and suggest a practical means for enhancing gene expression
Imaging diagnosis: magnetic resonance imaging of diffuse leptomeningeal oligodendrogliomatosis in a dog with "dural tail sign"
A case of diffuse leptomeningeal oligodendrogliomatosis affecting the brain and spinal cord of a dog is presented. A 7.5-year old, male neutered Staffordshire bull terrier presented for evaluation of a chronic history of tetraparesis and seizures, with a multifocal neuroanatomical localization was determined. Extra-axial intradural lesions with an atypical presentation of a dural tail sign were seen on MRI. Histologically, the lesions were consistent with leptomeningeal oligodendrogliomatosis. To the authors’ knowledge, a dural tail sign has not previously been reported as an MRI characteristic of diffuse leptomeningeal oligodendrogliomatosis in dogs
Galactic Coronae in the Intracluster Environment: Semi-confined Stellar-feedback-driven Outflows
Recently X-ray observations have shown the common presence of compact
galactic coronae around intermediate-mass spheroid galaxies embedded in the
intracluster/intragroup medium (ICM). We conduct 2-D hydrodynamic simulations
to study the quasi-steady-state properties of such coronae as the natural
products of the ongoing distributed stellar feedback semi-confined by the
thermal and ram pressures of the ICM. We find that the temperature of a
simulated corona depends primarily on the specific energy of the feedback,
consistent with the lack of the correlation between the observed hot gas
temperature and K-band luminosity of galaxies. The simulated coronae typically
represent subsonic outflows, chiefly because of the semi-confinement. As a
result, the hot gas density increases with the ICM thermal pressure. The ram
pressure, on the other hand, chiefly affects the size and lopsidedness of the
coronae. The density increase could lead to the compression of cool gas clouds,
if present, and hence the formation of stars. The increase also enhances
radiative cooling of the hot gas, which may fuel central supermassive black
holes, explaining the higher frequency of active galactic nuclei observed in
clusters than in the field. The radiation enhancement is consistent with a
substantially higher surface brightness of the X-ray emission detected from
coronae in cluster environment. The total X-ray luminosity of a corona,
however, depends on the relative importance of the surrounding thermal and ram
pressures. These environment dependences should at least partly explain the
large dispersion in the observed diffuse X-ray luminosities of spheroids with
similar stellar properties. Furthermore, we show that an outflow powered by the
distributed feedback can naturally produce a positive radial gradient in the
hot gas entropy, mimicking a cooling flow.Comment: accepted by MNRAS, comments are welcom
All Curled Up: A Numerical Investigation of Shock-Bubble Interactions and the Role of Vortices in Heating Galaxy Clusters
Jets from active galactic nuclei in the centers of galaxy clusters inflate
cavities of low density relativistic plasma and drive shock and sound waves
into the intracluster medium. When these waves overrun previously inflated
cavities, they form a differentially rotating vortex through the
Richtmyer-Meshkov instability. The dissipation of energy captured in the vortex
can contribute to the feedback of energy into the atmospheres of cool core
clusters. Using a series of hydrodynamic simulations we investigate the
efficiency of this process: we calculate the kinetic energy in the vortex by
decomposing the velocity field into its irrotational and solenoidal parts.
Compared to the two-dimensional case, the 3-dimensional Richtmyer-Meshkov
instability is about a factor of 2 more efficient. The energy in the vortex
field for weak shocks is E_vortex ~ rho_ICM v_shock^2 V_bubble (with dependence
on the geometry, density contrast, and shock width). For strong shocks, the
vortex becomes dynamically unstable, quickly dissipating its energy via a
turbulent cascade. We derive a number of diagnostics for observations and
laboratory experiments of shock-bubble interactions, like the shock-vortex
standoff distance, which can be used to derive lower limits on the Mach number.
The differential rotation of the vortex field leads to viscous dissipation,
which is sufficiently efficient to react to cluster cooling and to dissipate
the vortex energy within the cooling radius of the cluster for a reasonable
range of vortex parameters. For sufficiently large filling factors (of order a
few percent or larger), this process could thus contribute significantly to AGN
feedback in galaxy clusters.Comment: 16 pages, 24 figure
Foldy-Wouthuysen transformation for relativistic particles in external fields
A method of Foldy-Wouthuysen transformation for relativistic spin-1/2
particles in external fields is proposed. It permits determination of the
Hamilton operator in the Foldy-Wouthuysen representation with any accuracy.
Interactions between a particle having an anomalous magnetic moment and
nonstationary electromagnetic and electroweak fields are investigated.Comment: 22 page
Spin configuration of top quark pair production with large extra dimensions at photon-photon colliders
Top quark pair production at photon-photon colliders is studied in low scale
quantum gravity scenario. From the dependence of the cross sections on the spin
configuration of the top quark and anti-quark, we introduce a new observable,
top spin asymmetry. It is shown that there exists a special top spin basis
where with the polarized parent electron beams the top spin asymmetry vanishes
in the standard model but retains substantial values with the large extra
dimension effects. We also present lower bounds of the quantum gravity scale
from total cross sections with various combinations of the laser,
electron beam, and top quark pair polarizations. The measurements of the top
spin state with unpolarized initial beams are
shown to be most effective, enhancing by about 5% the bounds with respect
to totally unpolarized case.Comment: 18 pages, 4 figures, ReVTe
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