1,985 research outputs found
Observed Faraday Effects in Damped Lyman-Alpha Absorbers and Lyman Limit Systems: The Magnetised Environment of Galactic Building Blocks at Redshift=2
Protogalactic environments are typically identified using quasar absorption
lines, and these galactic building blocks can manifest as Damped Lyman-Alpha
Absorbers (DLAs) and Lyman Limit Systems (LLSs). We use radio observations of
Faraday effects to test whether DLAs and LLSs host a magnetised medium, by
combining DLA and LLS detections throughout the literature with 1.4 GHz
polarization data from the NRAO VLA Sky Survey (NVSS). We obtain a control, a
DLA, and a LLS sample consisting of 114, 19, and 27 lines-of-sight respectively
- all of which are polarized at to ensure Rician bias is
negligible. Using a Bayesian framework, we are unable to detect either coherent
or random magnetic fields in DLAs: the regular coherent magnetic fields within
the DLAs must be G, and the lack of depolarization is consistent
with the weakly magnetised gas in DLAs being non-turbulent and quiescent.
However, we find mild suggestive evidence that LLSs have coherent magnetic
fields: after controlling for the redshift-distribution of our data, we find a
71.5% probability that LLSs have a higher RM than a control sample. We also
find strong evidence that LLSs host random magnetic fields, with a 95.5%
probability that LLS lines-of-sight have lower polarized fractions than a
control sample. The regular coherent magnetic fields within the LLSs must be
G, and the magnetised gas must be highly turbulent with a typical
scale on the order of -20 pc, which is similar to that of the Milky
Way. This is consistent with the standard dynamo pedagogy, whereby magnetic
fields in protogalaxies increase in coherence and strength as a function of
cosmic time. Our results are consistent with a hierarchical galaxy formation
scenario, with the DLAs, LLSs, and strong magnesium II (MgII) systems exploring
three different stages of magnetic field evolution in galaxies.Comment: Submitted to Ap
The stripping of a galaxy group diving into the massive cluster A2142
Structure formation in the current Universe operates through the accretion of
group-scale systems onto massive clusters. The detection and study of such
accreting systems is crucial to understand the build-up of the most massive
virialized structures we see today. We report the discovery with XMM-Newton of
an irregular X-ray substructure in the outskirts of the massive galaxy cluster
Abell 2142. The tip of the X-ray emission coincides with a concentration of
galaxies. The bulk of the X-ray emission of this substructure appears to be
lagging behind the galaxies and extends over a projected scale of at least 800
kpc. The temperature of the gas in this region is 1.4 keV, which is a factor of
~4 lower than the surrounding medium and is typical of the virialized plasma of
a galaxy group with a mass of a few 10^13M_sun. For this reason, we interpret
this structure as a galaxy group in the process of being accreted onto the main
dark-matter halo. The X-ray structure trailing behind the group is due to gas
stripped from its original dark-matter halo as it moves through the
intracluster medium (ICM). This is the longest X-ray trail reported to date.
For an infall velocity of ~1,200 km s-1 we estimate that the stripped gas has
been surviving in the presence of the hot ICM for at least 600 Myr, which
exceeds the Spitzer conduction timescale in the medium by a factor of >~400.
Such a strong suppression of conductivity is likely related to a tangled
magnetic field with small coherence length and to plasma microinstabilities.
The long survival time of the low-entropy intragroup medium suggests that the
infalling material can eventually settle within the core of the main cluster.Comment: 11 pages, 7 figures, accepted for publication in A&
High resolution Ge/Li/ spectrometer reduces rate-dependent distortions at high counting rates
Modified spectrometer system with a low-noise preamplifier reduces rate-dependent distortions at high counting rates, 25,000 counts per second. Pole-zero cancellation minimizes pulse undershoots due to multiple time constants, baseline restoration improves resolution and prevents spectral shifts
Deep Chandra observations of the stripped galaxy group falling into Abell 2142
In the local Universe, the growth of massive galaxy clusters mainly operates
through the continuous accretion of group-scale systems. The infalling group in
Abell 2142 is the poster child of such an accreting group, and as such, it is
an ideal target to study the astrophysical processes induced by structure
formation. We present the results of a deep (200 ks) observation of this
structure with Chandra, which highlights the complexity of this system in
exquisite detail. In the core of the group, the spatial resolution of Chandra
reveals the presence of a leading edge and a complex AGN-induced activity. The
morphology of the stripped gas tail appears straight in the innermost 250 kpc,
suggesting that magnetic draping efficiently shields the gas from its
surroundings. However, beyond kpc from the core, the tail flares and
the morphology becomes strongly irregular, which could be explained by a
breaking of the drape, e.g. because of turbulent motions. The power spectrum of
surface-brightness fluctuations is relatively flat (),
which indicates that thermal conduction is strongly inhibited even beyond the
region where magnetic draping is effective. The amplitude of density
fluctuations in the tail is consistent with a mild level of turbulence with a
Mach number . Overall, our results show that the processes
leading to the thermalization and mixing of the infalling gas are slow and
relatively inefficient.Comment: Accepted for publication in A&
The effective potential, critical point scaling and the renormalization group
The desirability of evaluating the effective potential in field theories near
a phase transition has been recognized in a number of different areas. We show
that recent Monte Carlo simulations for the probability distribution for the
order parameter in an equilibrium Ising system, when combined with low-order
renormalization group results for an ordinary system, can be used to
extract the effective potential. All scaling features are included in the
process.Comment: REVTEX file, 22 pages, three figures, submitted to Phys. Rev.
Deep Chandra observations of the stripped galaxy group falling into Abell 2142
In the local Universe, the growth of massive galaxy clusters mainly operates through the continuous accretion of group-scale systems. The infalling group in Abell 2142 is the poster child of such an accreting group, and as such, it is an ideal target to study the astrophysical processes induced by structure formation. We present the results of a deep (200 ks) observation of this structure with Chandra that highlights the complexity of this system in exquisite detail. In the core of the group, the spatial resolution of Chandra reveals a leading edge and complex AGN-induced activity. The morphology of the stripped gas tail appears straight in the innermost 250 kpc, suggesting that magnetic draping efficiently shields the gas from its surroundings. However, beyond ~ 300 kpc from the core, the tail flares and the morphology becomes strongly irregular, which could be explained by a breaking of the drape, for example, caused by turbulent motions. The power spectrum of surface-brightness fluctuations is relatively flat (P2D ∝ k⁻²∙³ which indicates that thermal conduction is strongly inhibited even beyond the region where magnetic draping is effective. The amplitude of density fluctuations in the tail is consistent with a mild level of turbulence with a Mach number M3D ~ 0:1 -0:25. Overall, our results show that the processes leading to the thermalization and mixing of the infalling gas are slow and relatively inefficient
Casimir versus Helmholtz forces: Exact results
Recently, attention has turned to the issue of the ensemble dependence of
fluctuation induced forces. As a noteworthy example, in systems the
statistical mechanics underlying such forces can be shown to differ in the
constant magnetic canonical ensemble (CE) from those in the
widely-studied constant grand canonical ensemble (GCE). Here, the
counterpart of the Casimir force in the GCE is the \textit{Helmholtz} force in
the CE. Given the difference between the two ensembles for finite systems, it
is reasonable to anticipate that these forces will have, in general, different
behavior for the same geometry and boundary conditions. Here we present some
exact results for both the Casimir and the Helmholtz force in the case of the
one-dimensional Ising model subject to periodic and antiperiodic boundary
conditions and compare their behavior. We note that the Ising model has
recently being solved in Phys.Rev. E {\bf 106} L042103(2022), using a
combinatorial approach, for the case of fixed value of its order parameter.
Here we derive exact result for the partition function of the one-dimensional
Ising model of spins and fixed value using the transfer matrix method
(TMM); earlier results obtained via the TMM were limited to and even.
As a byproduct, we derive several specific integral representations of the
hypergeometric function of Gauss. Using those results, we rigorously derive
that the free energies of the CE and grand GCE are related to each other via
Legendre transformation in the thermodynamic limit, and establish the leading
finite-size corrections for the canonical case, which turn out to be much more
pronounced than the corresponding ones in the case of the GCE.Comment: 33 pages, 7 figures. The derivations in Appendix C are simplifie
Theory of monolayers with boundaries: Exact results and Perturbative analysis
Domains and bubbles in tilted phases of Langmuir monolayers contain a class
of textures knows as boojums. The boundaries of such domains and bubbles may
display either cusp-like features or indentations. We derive analytic
expressions for the textures within domains and surrounding bubbles, and for
the shapes of the boundaries of these regions. The derivation is perturbative
in the deviation of the bounding curve from a circle. This method is not
expected to be accurate when the boundary suffers large distortions, but it
does provide important clues with regard to the influence of various energetic
terms on the order-parameter texture and the shape of the domain or bubble
bounding curve. We also look into the effects of thermal fluctuations, which
include a sample-size-dependent effective line tension.Comment: replaced with published version, 21 pages, 16 figures include
Winning quick and dirty: the greedy random walk
As a strategy to complete games quickly, we investigate one-dimensional
random walks where the step length increases deterministically upon each return
to the origin. When the step length after the kth return equals k, the
displacement of the walk x grows linearly in time. Asymptotically, the
probability distribution of displacements is a purely exponentially decaying
function of |x|/t. The probability E(t,L) for the walk to escape a bounded
domain of size L at time t decays algebraically in the long time limit, E(t,L)
~ L/t^2. Consequently, the mean escape time ~ L ln L, while ~
L^{2n-1} for n>1. Corresponding results are derived when the step length after
the kth return scales as k^alpha$ for alpha>0.Comment: 7 pages, 6 figures, 2-column revtext4 forma
The two-component giant radio halo in the galaxy cluster Abell 2142
We report on a spectral study at radio frequencies of the giant radio halo in
A2142 (z=0.0909), which we performed to explore its nature and origin. A2142 is
not a major merger and the presence of a giant radio halo is somewhat
surprising. We performed deep radio observations with the GMRT at 608 MHz, 322
MHz, and 234 MHz and with the VLA in the 1-2 GHz band. We obtained high-quality
images at all frequencies in a wide range of resolutions. The radio halo is
well detected at all frequencies and extends out to the most distant cold front
in A2142. We studied the spectral index in two regions: the central part of the
halo and a second region in the direction of the most distant south-eastern
cold front, selected to follow the bright part of the halo and X-ray emission.
We complemented our observations with a preliminary LOFAR image at 118 MHz and
with the re-analysis of archival VLA data at 1.4 GHz. The two components of the
radio halo show different observational properties. The central brightest part
has higher surface brightess and a spectrum whose steepness is similar to those
of the known radio halos, i.e. . The ridge, which fades into the larger scale emission, is broader in
size and has considerably lower surface brightess and a moderately steeper
spectrum, i.e. . We propose that
the brightest part of the radio halo is powered by the central sloshing in
A2142, similar to what has been suggested for mini-halos, or by secondary
electrons generated by hadronic collisions in the ICM. On the other hand, the
steeper ridge may probe particle re-acceleration by turbulence generated either
by stirring the gas and magnetic fields on a larger scale or by less energetic
mechanisms, such as continuous infall of galaxy groups or an off-axis merger.Comment: 18 pages, 10 figures, 4 tables - A&A, accepte
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