163 research outputs found
First Assessment of Mountains on Northwestern Ellesmere Island, Nunavut, as Potential Astronomical Observing Sites
Ellesmere Island, at the most northerly tip of Canada, possesses the highest
mountain peaks within 10 degrees of the pole. The highest is 2616 m, with many
summits over 1000 m, high enough to place them above a stable low-elevation
thermal inversion that persists through winter darkness. Our group has studied
four mountains along the northwestern coast which have the additional benefit
of smooth onshore airflow from the ice-locked Arctic Ocean. We deployed small
robotic site testing stations at three sites, the highest of which is over 1600
m and within 8 degrees of the pole. Basic weather and sky clarity data for over
three years beginning in 2006 are presented here, and compared with available
nearby sea-level data and one manned mid-elevation site. Our results point to
coastal mountain sites experiencing good weather: low median wind speed, high
clear-sky fraction and the expectation of excellent seeing. Some practical
aspects of access to these remote locations and operation and maintenance of
equipment there are also discussed.Comment: 21 pages, 2 tables, 15 figures; accepted for publication in PAS
Detecting the orientation of magnetic fields in galaxy clusters
Clusters of galaxies, filled with hot magnetized plasma, are the largest
bound objects in existence and an important touchstone in understanding the
formation of structures in our Universe. In such clusters, thermal conduction
follows field lines, so magnetic fields strongly shape the cluster's thermal
history; that some have not since cooled and collapsed is a mystery. In a
seemingly unrelated puzzle, recent observations of Virgo cluster spiral
galaxies imply ridges of strong, coherent magnetic fields offset from their
centre. Here we demonstrate, using three-dimensional magnetohydrodynamical
simulations, that such ridges are easily explained by galaxies sweeping up
field lines as they orbit inside the cluster. This magnetic drape is then lit
up with cosmic rays from the galaxies' stars, generating coherent polarized
emission at the galaxies' leading edges. This immediately presents a technique
for probing local orientations and characteristic length scales of cluster
magnetic fields. The first application of this technique, mapping the field of
the Virgo cluster, gives a startling result: outside a central region, the
magnetic field is preferentially oriented radially as predicted by the
magnetothermal instability. Our results strongly suggest a mechanism for
maintaining some clusters in a 'non-cooling-core' state.Comment: 48 pages, 21 figures, revised version to match published article in
Nature Physics, high-resolution version available at
http://www.cita.utoronto.ca/~pfrommer/Publications/pfrommer-dursi.pd
Prospects of detecting gamma-ray emission from galaxy clusters: cosmic rays and dark matter annihilations
We study the possibility for detecting gamma-ray emission from galaxy
clusters. We consider 1) leptophilic models of dark matter (DM) annihilation
that include a Sommerfeld enhancement (SFE), 2) different representative
benchmark models of supersymmetric DM, and 3) cosmic ray (CR) induced pion
decay. Among all clusters/groups of a flux-limited X-ray sample, we predict
Virgo, Fornax and M49 to be the brightest DM sources and find a particularly
low CR-induced background for Fornax. For a minimum substructure mass given by
the DM free-streaming scale, cluster halos maximize the substructure boost for
which we find a factor above 1000. Since regions around the virial radius
dominate the annihilation flux of substructures, the resulting surface
brightness profiles are almost flat. This makes it very challenging to detect
this flux with imaging atmospheric Cherenkov telescopes. Assuming cold dark
matter with a substructure mass distribution down to an Earth mass and using
extended Fermi upper limits, we rule out the leptophilic models in their
present form in 28 clusters, and limit the boost from SFE in M49 and Fornax to
be < 5. This corresponds to a limit on SFE in the Milky Way of < 3, which is
too small to account for the increasing positron fraction with energy as seen
by PAMELA and challenges the DM interpretation. Alternatively, if SFE is
realized in Nature, this would imply a limiting substructure mass of M_lim >
10^4 M_sol - a problem for structure formation. Using individual cluster
observations, it will be challenging for Fermi to constrain our selection of DM
benchmark models without SFE. The Fermi upper limits are, however, closing in
on our predictions for the CR flux using an analytic model based on
cosmological hydrodynamical cluster simulations. We limit the CR-to-thermal
pressure in nearby bright galaxy clusters of the Fermi sample to < 10% and in
Norma and Coma to < 3%.Comment: 43 pages, 23 figures, 10 tables. Accepted for publication in Phys.
Rev. D: streamlined paper, added a paragraph about detectability to
introduction, few references added, and few typos correcte
Concept for Predicting Vibrations in Machine Tools Using Machine Learning
Vibrations have a significant influence on quality and costs in metal
cutting processes. Existing methods for predicting vibrations in machine tools enable an informed choice of process settings, however they rely on costly equipment and specialised staff. Therefore, this contribution proposes to reduce the modelling effort required by using machine learning based on data gathered during production. The approach relies on two sub-models, representing the machine structure and machining process respectively. A method is proposed for initialising and updating the models in production
Comparative analysis of the diffuse radio emission in the galaxy clusters A1835, A2029, and Ophiuchus
We recently performed a study of a sample of relaxed, cooling core galaxy
clusters with deep Very Large Array observations at 1.4 GHz. We find that in
the central regions of A1835, A2029, and Ophiuchus the dominant radio galaxy is
surrounded by a diffuse low-brightness radio emission that takes the form of a
mini-halo. Here we present the results of the analysis of the extended diffuse
radio emission in these mini-halos. In order to investigate the morphological
properties of the diffuse radio emission in clusters of galaxies we propose to
fit their azimuthally averaged brightness profile with an exponential,
obtaining the central brightness and the e-folding radius from which the radio
emissivity can be calculated. We investigate the radio properties of the
mini-halos in A1835, A2029, and Ophiuchus in comparison with the radio
properties of a representative sample of mini-halos and halos already known in
the literature. We find that radio halos can have quite different length-scales
but their emissivity is remarkably similar from one halo to the other. In
contrast, mini-halos span a wide range of radio emissivity. Some of them, like
the Perseus mini-halos, are characterized by a radio emissivity which is more
than 100 times greater than that of radio halos. On the other hand, the new
mini-halos in cooling core clusters analyzed in this work, namely A2029,
Ophiuchus, and A1835, have a radio emissivity which is much more typical of
halos in merging clusters rather than similar to that of the other mini-halos
previously known.Comment: 17 pages, 11 figures, A&A in press. For a version with high quality
figures, see http://erg.ca.astro.it/preprints/mini_halo_2
A low-frequency radio halo associated with a cluster of galaxies
Clusters of galaxies are the largest gravitationally bound objects in the
Universe, containing about 10^15 solar masses of hot (10^8 K) gas, galaxies and
dark matter in a typical volume of about 10 Mpc^3. Magnetic fields and
relativistic particles are mixed with the gas as revealed by giant radio
haloes, which arise from diffuse, megaparsec-scale synchrotron radiation at
cluster center. Radio haloes require that the emitting electrons are
accelerated in situ (by turbulence), or are injected (as secondary particles)
by proton collisions into the intergalactic medium. They are found only in a
fraction of massive clusters that have complex dynamics, which suggests a
connection between these mechanisms and cluster mergers. Here we report a radio
halo at low frequencies associated with the merging cluster Abell 521. This
halo has an extremely steep radio spectrum, which implies a high frequency
cut-off; this makes the halo difficult to detect with observations at 1.4 GHz
(the frequency at which all other known radio haloes have been best studied).
The spectrum of the halo is inconsistent with a secondary origin of the
relativistic electrons, but instead supports turbulent acceleration, which
suggests that many radio haloes in the Universe should emit mainly at low
frequencies.Comment: 18 pages, 4 figures, Nature 455, 94
Simulating cosmic rays in clusters of galaxies - II. A unified scheme for radio halos and relics with predictions of the gamma-ray emission
The thermal plasma of galaxy clusters lost most of its information on how
structure formation proceeded as a result of dissipative processes. In
contrast, non-equilibrium distributions of cosmic rays (CR) preserve the
information about their injection and transport processes and provide thus a
unique window of current and past structure formation processes. This
information can be unveiled by observations of non-thermal radiative processes,
including radio synchrotron, hard X-ray, and gamma-ray emission. To explore
this, we use high-resolution simulations of a sample of galaxy clusters
spanning a mass range of about two orders of magnitudes, and follow
self-consistent CR physics on top of the radiative hydrodynamics. We model CR
electrons that are accelerated at cosmological structure formation shocks and
those that are produced in hadronic interactions of CRs with ambient gas
protons. We find that CR protons trace the time integrated non-equilibrium
activities of clusters while shock-accelerated CR electrons probe current
accretion and merging shock waves. The resulting inhomogeneous synchrotron
emission matches the properties of observed radio relics. We propose a unified
model for the generation of radio halos. Giant radio halos are dominated in the
centre by secondary synchrotron emission with a transition to the synchrotron
radiation emitted from shock-accelerated electrons in the cluster periphery.
This model is able to explain the observed correlation of mergers with radio
halos, the larger peripheral variation of the spectral index, and the large
scatter in the scaling relation between cluster mass and synchrotron emission.
Future low-frequency radio telescopes (LOFAR, GMRT, MWA, LWA) are expected to
probe the accretion shocks of clusters. [abridged]Comment: 32 pages, 19 figures, small changes to match the version to be
published by MNRAS, full resolution version available at
http://www.cita.utoronto.ca/~pfrommer/Publications/CRs_non-thermal.pd
Detection of a radio bridge in Abell 3667
We have detected a radio bridge of unpolarized synchrotron emission
connecting the NW relic of the galaxy cluster Abell 3667 to its central
regions. We used data at 2.3 GHz from the S-band Polarization All Sky Survey
(S-PASS) and at 3.3 GHz from a follow up observation, both conducted with the
Parkes Radio Telescope. This emission is further aligned with a diffuse X-ray
tail, and represents the most compelling evidence for an association between
intracluster medium turbulence and diffuse synchrotron emission. This is the
first clear detection of a bridge associated both with an outlying cluster
relic and X-ray diffuse emission. All the indicators point toward the
synchrotron bridge being related to the post-shock turbulent wake trailing the
shock front generated by a major merger in a massive cluster. Although
predicted by simulations, this is the first time such emission is detected with
high significance and clearly associated with the path of a confirmed shock.
Although the origin of the relativistic electrons is still unknown, the
turbulent re-acceleration model provides a natural explanation for the
large-scale emission. The equipartition magnetic field intensity of the bridge
is B_eq = 2.2 +/- 0.3 \mu G. We further detect diffuse emission coincident with
the central regions of the cluster for the first time.Comment: 10 pages, 16 figures, accepted for publication on MNRAS. Replaced
with the version accepted for publication: also the 3.3 GHz image cleaned
from compact sources; 8 more figures; details on data reduction and compact
source cleaning added; new estimate of the magnetic field. Some figures at
resolution lower than the original one
A search for diffuse radio emission in the relaxed, cool-core galaxy clusters A1068, A1413, A1650, A1835, A2029, and Ophiuchus
We analyze sensitive, high-dynamic-range, observations to search for
extended, diffuse, radio emission in relaxed and cool-core galaxy clusters. We
performed deep 1.4 GHz Very Large Array observations, of A1068, A1413, A1650,
A1835, A2029, and complemented our dataset with archival observations of
Ophiuchus. We find that, in the central regions of A1835, A2029, and Ophiuchus,
the dominant radio galaxy is surrounded by diffuse low-brightness radio
emission that takes the form of a mini-halo. We detect no diffuse emission in
A1650, at a surface brightness level of the other mini-halos. We find low
significance indications of diffuse emission in A1068 and A1413, although to be
classified as mini-halos they would require further investigation, possibly
with data of higher signal-to-noise ratio. In the Appendix, we report on the
serendipitous detection of a giant radio galaxy with a total spatial extension
of ~1.6 Mpc.Comment: 13 pages, 10 figures, A&A in press. For a version with high quality
figures, see http://erg.ca.astro.it/preprints/mini_halo_1
Optical properties of MgH2 measured in situ in a novel gas cell for ellipsometry/spectrophotometry
The dielectric properties of alpha-MgH2 are investigated in the photon energy
range between 1 and 6.5 eV. For this purpose, a novel sample configuration and
experimental setup are developed that allow both optical transmission and
ellipsometric measurements of a transparent thin film in equilibrium with
hydrogen. We show that alpha-MgH2 is a transparent, colour neutral insulator
with a band gap of 5.6 +/- 0.1 eV. It has an intrinsic transparency of about
80% over the whole visible spectrum. The dielectric function found in this work
confirms very recent band structure calculations using the GW approximation by
Alford and Chou [J.A. Alford and M.Y. Chou (unpublished)]. As Pd is used as a
cap layer we report also the optical properties of PdHx thin films.Comment: REVTeX4, 15 pages, 12 figures, 5 table
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