Astronomical seeing and ground-layer turbulence in the Canadian High Arctic

We report results of a two-year campaign of measurements, during arctic winter darkness, of optical turbulence in the atmospheric boundary-layer above the Polar Environment Atmospheric Laboratory in northern Ellesmere Island (latitude +80 deg N). The data reveal that the ground-layer turbulence in the Arctic is often quite weak, even at the comparatively-low 610 m altitude of this site. The median and 25th percentile ground-layer seeing, at a height of 20 m, are found to be 0.57 and 0.25 arcsec, respectively. When combined with a free-atmosphere component of 0.30 arcsec, the median and 25th percentile total seeing for this height is 0.68 and 0.42 arcsec respectively. The median total seeing from a height of 7 m is estimated to be 0.81 arcsec. These values are comparable to those found at the best high-altitude astronomical sites

Impact of tangled magnetic fields on AGN-blown bubbles

There is growing consensus that feedback from AGN is the main mechanism responsible for stopping cooling flows in clusters of galaxies. AGN are known to inflate buoyant bubbles that supply mechanical power to the intracluster gas (ICM). High Reynolds number hydrodynamical simulations show that such bubbles get entirely disrupted within 100 Myr, as they rise in cluster atmospheres, which is contrary to observations. This artificial mixing has consequences for models trying to quantify the amount of heating and star formation in cool core clusters of galaxies. It has been suggested that magnetic fields can stabilize bubbles against disruption. We perform MHD simulations of fossil bubbles in the presence of tangled magnetic fields using the high order PENCIL code. We focus on the physically-motivated case where thermal pressure dominates over magnetic pressure and consider randomly oriented fields with and without maximum helicity and a case where large scale external fields drape the bubble.We find that helicity has some stabilizing effect. However, unless the coherence length of magnetic fields exceeds the bubble size, the bubbles are quickly shredded. As observations of Hydra A suggest that lengthscale of magnetic fields may be smaller then typical bubble size, this may suggest that other mechanisms, such as viscosity, may be responsible for stabilizing the bubbles. However, since Faraday rotation observations of radio lobes do not constrain large scale ICM fields well if they are aligned with the bubble surface, the draping case may be a viable alternative solution to the problem. A generic feature found in our simulations is the formation of magnetic wakes where fields are ordered and amplified. We suggest that this effect could prevent evaporation by thermal conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures, color printing recommended

Impact of Sodium Layer variations on the performance of the E-ELT MCAO module

Multi-Conjugate Adaptive Optics systems based on sodium Laser Guide Stars may exploit Natural Guide Stars to solve intrinsic limitations of artificial beacons (tip-tilt indetermination and anisoplanatism) and to mitigate the impact of the sodium layer structure and variability. The sodium layer may also have transverse structures leading to differential effects among Laser Guide Stars. Starting from the analysis of the input perturbations related to the Sodium Layer variability, modeled directly on measured sodium layer profiles, we analyze, through a simplified end-to-end simulation code, the impact of the low/medium orders induced on global performance of the European Extremely Large Telescope Multi-Conjugate Adaptive Optics module MAORY.Comment: 7 pages, 5 figures, SPIE conference Proceedin

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

An unlikely radio halo in the low X-ray luminosity galaxy cluster RXC J1514.9-1523

We report the discovery of a giant radio halo in the galaxy cluster RXC J1514.9-1523 at z=0.22 with a relatively low X-ray luminosity, $L_{X \, [0.1-2.4 \rm \, kev]} \sim 7 \times 10^{44}$ erg s$^{-1}$. This faint, diffuse radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz. The source is barely detected at 1.4 GHz in a NVSS pointing that we have reanalyzed. The integrated radio spectrum of the halo is quite steep, with a slope \alpha = 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are common in more X-ray luminous cluster mergers, there is a less than 10% probability to detect a halo in systems with L_X \ltsim 8 \times 10^{44} erg s$^{-1}$. The detection of a new giant halo in this borderline luminosity regime can be particularly useful for discriminating between the competing theories for the origin of ultrarelativistic electrons in clusters. Furthermore, if our steep radio spectral index is confirmed by future deeper radio observations, this cluster would provide another example of the recently discovered population of ultra-steep spectrum radio halos, predicted by the model in which the cluster cosmic ray electrons are produced by turbulent reacceleration.Comment: 4 pages, 2 figures - Accepted for publication on A&A Research Note

Radio Galaxy NGC 1265 unveils the Accretion Shock onto the Perseus Galaxy Cluster

We present a consistent 3D model for the head-tail radio galaxy NGC 1265 that explains the complex radio morphology and spectrum by a past passage of the galaxy and radio bubble through a shock wave. Using analytical solutions to the full Riemann problem and hydrodynamical simulations, we study how this passage transformed the plasma bubble into a toroidal vortex ring. Adiabatic compression of the aged electron population causes it to be energized and to emit low-surface brightness and steep-spectrum radio emission. The large infall velocity of NGC 1265 and the low Faraday rotation measure values and variance of the jet strongly argue that this transformation was due to the accretion shock onto Perseus situated roughly at R_200. Estimating the volume change of the radio bubble enables inferring a shock Mach number of M = 4.2_{-1.2}^{+0.8}, a density jump of 3.4_{-0.4}^{+0.2}, a temperature jump of 6.3_{-2.7}^{+2.5}, and a pressure jump of 21.5 +/- 10.5 while allowing for uncertainties in the equation of state of the radio plasma and volume of the torus. Extrapolating X-ray profiles, we obtain upper limits on the gas temperature and density in the infalling warm-hot intergalactic medium of kT < 0.4 keV and n < 5e-5 / cm^3. The orientation of the ellipsoidally shaped radio torus in combination with the direction of the galaxy's head and tail in the plane of the sky is impossible to reconcile with projection effects. Instead, this argues for post-shock shear flows that have been caused by curvature in the shock surface with a characteristic radius of 850 kpc. The energy density of the shear flow corresponds to a turbulent-to-thermal energy density of 14%. The shock-injected vorticity might be important in generating and amplifying magnetic fields in galaxy clusters. Future LOFAR observations of head-tail galaxies can be complementary probes of accretion shocks onto galaxy clusters.Comment: 14 pages, 4 figures, ApJ, in print; v3: typos corrected to match the published version; v2: improved presentation, added 2D numerical simulations and exact solution to the 1D Riemann problem of a shock overrunning a spherical bubble that gets transformed into a vortex rin

Author Correction: Enhanced tenacity of mycobacterial aerosols from necrotic neutrophils

The original version of this Article contained errors within the affiliations section. Affiliation 4 was incorrectly given as ‘Leibniz Research Alliance INFECTIONS’21, Leipzig, Germany’. The correct affiliation is listed below: Leibniz Research Alliance INFECTIONS’21, Borstel, 23845, Germany Also, Affiliation 5 was incorrectly given as ‘German Center for Infection Research, TTU-TB, Borstel, 23845, Germany’. The correct affiliation is listed below: German Center for Infection Research (DZIF), Partner Site Hamburg-Lübeck-Borstel, Germany. Finally, the original HTML version of this Article omitted an affiliation for G. Gabriel. The correct affiliations for G. Gabriel are listed below: Heinrich Pette Institute, Leibniz Institute for Experimental Virology, Hamburg, 20251, Germany. Leibniz Research Alliance INFECTIONS’21, Borstel, 23845, Germany. German Center for Infection Research (DZIF), Partner Site Hamburg-Lübeck-Borstel, Germany. These errors have now been corrected in the PDF and HTML versions of the Article

Enhanced tenacity of mycobacterial aerosols from necrotic neutrophils

The tuberculosis agent Mycobacterium tuberculosis is primarily transmitted through air, but little is known about the tenacity of mycobacterium-containing aerosols derived from either suspensions or infected neutrophils. Analysis of mycobacterial aerosol particles generated from bacterial suspensions revealed an average aerodynamic diameter and mass density that may allow distant airborne transmission. The volume and mass of mycobacterial aerosol particles increased with elevated relative humidity. To more closely mimic aerosol formation that occurs in active TB patients, aerosols from mycobacterium-infected neutrophils were analysed. Mycobacterium-infected intact neutrophils showed a smaller particle size distribution and lower viability than free mycobacteria. In contrast, mycobacterium-infected necrotic neutrophils, predominant in M. tuberculosis infection, revealed particle sizes and viability rates similar to those found for free mycobacteria, but in addition, larger aggregates of viable mycobacteria were observed. Therefore, mycobacteria are shielded from environmental stresses in multibacillary aggregates generated from necrotic neutrophils, which allows improved tenacity but emphasizes short distance transmission between close contacts

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

A radio minihalo in the extreme cool-core galaxy cluster RXCJ1504.1-0248

Aims. We report the discovery of a radio minihalo in RXCJ1504.1-0248, a massive galaxy cluster that has an extremely luminous cool core. To date, only 9 radio minihalos are known, thus the discovery of a new one, in one of the most luminous cool-core clusters, provides important information on this peculiar class of sources and sheds light on their origin. Methods. The diffuse radio source is detected using GMRT at 327 MHz and confirmed by pointed VLA data at 1.46 GHz. The minihalo has a radius of $\sim$140 kpc. A Chandra gas temperature map shows that the minihalo emission fills the cluster cool core and has some morphological similarities to it, as has been previously observed for other minihalos. Results. The Chandra data reveal two subtle cold fronts in the cool core, likely created by sloshing of the core gas, as observed in most cool-core clusters. Following previous work, we speculate that the origin of the minihalo is related to sloshing. Sloshing may result in particle acceleration by generating turbulence and/or amplifying the magnetic field in the cool core, leading to the formation of a minihalo.Comment: 4 pages, 1 table, 3 color figures. Accepted for publication in A&A Letter