102 research outputs found
First evidence of a shock in a radio mini-halo cluster: implications from new LOFAR data
No full textDiffuse non-thermal radio emission in galaxy clusters provides direct information on the relativistic particles and magnetic fields in the ICM. With the advent of the LOFAR array, new properties of the steep diffuse emission in galaxy cluster have been revealed in the low radio frequencies. One of these recent discoveries is the presence of ultra-steep spectrum radio emission on Mpc scale outside the mini-halo. The origin of the mini-halos in cool-core cluster is still debated. New LOFAR observations of the cool-core galaxy cluster RBS797 were requested with the aim to investigate signs of steep large scale emission extending beyond the mini-halo. In order to perform an accurate investigation of this scenario, we carried out a multi-wavelength study to reveal the presence of surface brightness discontinuities in the ICM. The low resolution LOFAR image produced in this thesis work does not show Mpc diffuse emission. Comparing the low resolution 140MHz LOFAR image with the 1.4 GHz VLA image, we found a more extended diffuse emission at the high frequency. Spectral index analysis of this mini-halo was performed and a flattening of the spectral index toward the external region was found. The X-ray analysis was carried out by means of detailed morphological and spectroscopic study of archival Chandra observation. We found an outer discontinuity, which presents thermodynamic properties consistent with those typically found in shock fronts. The presence of a possible “cocoon shock” propagating in the ICM, which compresses and heats the plasma, can re-accelerate relativistic particles to higher energy, and hence, can explain a larger extension of the 1.4 GHz emission with respect to the 140 MHz LOFAR emission, and the flattening of the spectral index increasing with the distance from the center. Therefore, cocoon shock in this cool-core cluster can provide new evidence of the link between the AGN feedback and the mini-halo origin
Magnetic fields and relativistic electrons fill entire galaxy cluster
No full textThe hot plasma within merging galaxy clusters is predicted to be filled with shocks and turbulence that may convert part of their kinetic energy into relativistic electrons and magnetic fields generating synchrotron radiation. Analyzing Low Frequency Array (LOFAR) observations of the galaxy cluster Abell 2255, we show evidence of radio synchrotron emission distributed over very large scales of at least 5 megaparsec. The pervasive radio emission witnesses that shocks and turbulence efficiently transfer kinetic energy into relativistic particles and magnetic fields in a region that extends up to the cluster outskirts. The strength of the emission requires a magnetic field energy density at least 100 times higher than expected from a simple compression of primordial fields, presumably implying that dynamo operates efficiently also in the cluster periphery. It also suggests that nonthermal components may contribute substantially to the pressure of the intracluster medium in the cluster periphery
LOFAR detection of extended emission around a mini halo in the galaxy cluster Abell 1413
No full textContext. The relation between giant radio halos and mini halos in galaxy clusters is not understood. The former are usually associated with merging clusters, while the latter are found in relaxed systems. In recent years, the advent of low-frequency radio observations has challenged this dichotomy by finding intermediate objects with a hybrid radio morphology.
Aims. We aim to investigate the presence of diffuse radio emission in the cluster Abell 1413 and determine its dynamical status to explore the relation between mini halos and giant radio halos.
Methods. We used LOFAR observations centred at 144 MHz to study the diffuse radio emission. To investigate the dynamical state of the system, we used newly analysed XMM-Newton archival data. Abell 1413 shows features that are typically present in both relaxed (e.g., peaked X-ray surface brightness distribution and some large-scale inhomogeneities) and disturbed (e.g., flatter temperature and metallicity profiles) clusters.
Results. This suggests that Abell 1413 is neither disturbed nor fully relaxed, and we argue that it is an intermediate-phase cluster. At 144 MHz, we discover a wider diffuse component surrounding the previously known mini halo at the cluster center. By fitting the radio surface-brightness profile with a double-exponential model, we can disentangle the two components. We find an inner mini halo with an e-folding radius, re, 1 = 28 ± 5 kpc, and an extended component with re, 2 = 290 ± 60 kpc. We also evaluated the point-to-point correlation between the radio and X-ray surface brightness, finding a sublinear relation for the outer emission and a superlinear relation for the mini halo. The mini halo and the diffuse emission extend over different scales and show different features, confirming the double nature of the radio emission and suggesting that the mechanisms responsible for the re-acceleration of the radio-emitting particle might be different
Correlating Galactic Magnetic Fields with Regions of Dense Star Formation using LOFAR and CALIFA
No full textabstract: I test the hypothesis that galactic magnetic fields originate from regions of dense
star formation (Dahlem et al. 2006) by comparing maps of 120-240 MHz synchrotron emission and hydrogen alpha (Hα) emission of the tidally-interacting, edge-on, barred spiral galaxy UGC 9665. Synchrotron emission traces magnetic field strength to a rough first order, while Hα emission traces recent massive star formation. UGC 9665 was selected because it was included in the LOw Frequency ARray (LOFAR) TwoMetre Sky Survey (LoTSS; Shimwell et al. (2017)) as well as the Calar Alto Legacy Integral Field Area Survey (CALIFA; Sanchez et al. (2012)). I generated vertical intensity profiles at several distances along the disk from the galactic center for synchrotron emission and Hα in order to measure how the intensity of each falls off with distance from the midplane. In addition to correlating the vertical profiles to see if there is a relationship between star formation and magnetic field strength, I fit the LOFAR vertical profiles to characteristic Gaussian and exponential functions given by Dumke et al. (1995). Fitting these equations have been shown to be good indicators of the main mode of cosmic ray transport, whether it is advection (exponential fit) or diffusion (Gaussian fit) (Heesen et al. 2016). Cosmic rays originate from supernova,
and core collapse supernovae occur in star forming regions, which also produce
advective winds, so I test the correlation between star-forming regions and advective regions as predicted by the Heesen et al. (2016) method. Similar studies should be conducted on different galaxies in the future in order to further test these hypotheses and how well LOFAR and CALIFA complement each other, which will be made possible by the full release of the LOFAR Two-Metre Sky Survey (LoTSS) (Shimwell et al. 2017)
Gentle reenergization of electrons in merging galaxy clusters
No full textCosmic ray electrons reenergized in merging galaxy clusters: A phenomenon observed with low-frequency radio telescopes.</jats:p
Future Science Prospects for AMI
No full textAuthors:- Keith Grainge, Paul Alexander, Richard Battye, Mark Birkinshaw, Andrew Blain, Malcolm Bremer, Sarah Bridle, Michael Brown, Richard Davis, Clive Dickinson, Alastair Edge, George Efstathiou, Robert Fender, Martin Hardcastle, Jennifer Hatchell, Michael Hobson, Matthew Jarvis, Benjamin Maughan, Ian McHardy, Matthew Middleton, Anthony Lasenby, Richard Saunders, Giorgio Savini, Anna Scaife, Graham Smith, Mark Thompson, Glenn White, Kris Zarb-Adami, James Allison, Jane Buckle, Alberto Castro-Tirado, Maria Chernyakova, Roger Deane, Farhan Feroz, Ricardo Genova Santos, David Green, Diana Hannikainen, Ian Heywood, Natasha Hurley-Walker, Ruediger Kneissl, Karri Koljonen, Shrinivas Kulkarni, Sera Markoff, Carrie MacTavish, Michael McCollough, Simone Migliari, Jon M. Miller, James Miller-Jones, Malak Olamaie, Zsolt Paragi, Timothy Pearson, Guy Pooley, Katja Pottschmidt, Rafael Rebolo, John Richer, Julia Riley, Jerome Rodriguez, Carmen Rodriguez-Gonzalvez, Anthony Rushton, Petri Savolainen, Paul Scott, Timothy Shimwell, Marco Tavani, John Tomsick, Valeriu Tudose, Kurt van der Heyden, Alexander van der Horst, Angelo Varlotta, Elizabeth Waldram, Joern Wilms, Andrzej Zdziarski, Jonathan Zwart, Yvette Perrott, Clare Rumsey, Michel SchammelThe Arcminute Microkelvin Imager (AMI) is a telescope specifically designed for high sensitivity measurements of low-surface-brightness features at cm-wavelength and has unique, important capabilities. It consists of two interferometer arrays operating over 13.5-18 GHz that image structures on scales of 0.5-10 arcmin with very low systematics. The Small Array (AMI-SA; ten 3.7-m antennas) couples very well to Sunyaev-Zel'dovich features from galaxy clusters and to many Galactic features. The Large Array (AMI-LA; eight 13-m antennas) has a collecting area ten times that of the AMI-SA and longer baselines, crucially allowing the removal of the effects of confusing radio point sources from regions of low surface-brightness, extended emission. Moreover AMI provides fast, deep object surveying and allows monitoring of large numbers of objects. In this White Paper we review the new science - both Galactic and extragalactic - already achieved with AMI and outline the prospects for much more.https://arxiv.org/abs/1208.196
Walk on the Low Side: LOFAR explores the low-frequency radio emission of GASP jellyfish galaxies
No full textJellyfish galaxies, characterized by long filaments of stripped interstellar
medium extending from their disks, are the prime laboratories to study the
outcomes of ram pressure stripping. At radio wavelengths, they often show
unilateral emission extending beyond the stellar disk, and an excess of radio
luminosity with respect to that expected from their current star formation
rate. We present new 144 MHz images provided by the LOFAR Two-metre Sky Survey
for a sample of six galaxies from the GASP survey. These galaxies are
characterized by a high global luminosity at 144 MHz ( W
Hz), in excess compared to their ongoing star formation rate. The
comparison of radio and H images smoothed with a Gaussian beam
corresponding to 10 kpc reveals a sub-linear spatial correlation between
the two emissions with an average slope . In their stellar disk we
measure , which is close to the radio-to-star formation linear
relation. We speculate that, as a consequence of the ram pressure, in these
jellyfish galaxies the cosmic rays transport is more efficient than in normal
galaxies. Radio tails typically have higher radio-to-H ratios than the
disks, thus we suggest that the radio emission is boosted by the electrons
stripped from the disks. In all galaxies, the star formation rate has decreased
by a factor within the last yr. The observed radio emission
is consistent with the past star formation, so we propose that this recent
decline may be the cause of their radio luminosity-to-star formation rate
excess.Comment: 22 pages, 7 figures. Accepted for publication on ApJ on 24/08/202
Multiphysics analysis with CAD-based parametric breeding blanket creation for rapid design iteration
No full textInternational audienceInternational Atomic Energy Agency Nuclear FusionPAPER • THE FOLLOWING ARTICLE ISOPEN ACCESSMultiphysics analysis with CAD-based parametric breeding blanket creation for rapid design iterationJonathan Shimwell1, Rémi Delaporte-Mathurin2, Jean-Charles Jaboulay3, Julien Aubert3, Chris Richardson4, Chris Bowman5, Andrew Davis1, Andrew Lahiff1, James Bernardi6, Sikander Yasin7,8Show full author listPublished 8 March 2019 • © EURATOM 2019Nuclear Fusion, Volume 59, Number 4Citation Jonathan Shimwell et al 2019 Nucl. Fusion 59 046019DOI 10.1088/1741-4326/ab0016DownloadArticle PDFFiguresReferencesDownload PDF2222 Total downloads1111 total citations on Dimensions.Article has an altmetric score of 9Turn on MathJaxShare this articleShare this content via emailShare on Facebook (opens new window)Share on Twitter (opens new window)Share on Mendeley (opens new window)Article informationAbstractBreeding blankets are designed to ensure tritium self-sufficiency in deuterium–tritium fusion power plants. In addition to this, breeder blankets play a vital role in shielding key components of the reactor, and provide the main source of heat which will ultimately be used to generate electricity. Blanket design is critical to the success of fusion reactors and integral to the design process. Neutronic simulations of breeder blankets are regularly performed to ascertain the performance of a particular design. An iterative process of design improvements and parametric studies are required to optimize the design and meet performance targets. Within the EU DEMO program the breeding blanket design cycle is repeated for each new baseline design. One of the key steps is to create three-dimensional models suitable primarily for use in neutronics, but could be used in other computer-aided design (CAD)-based physics and engineering analyses. This article presents a novel blanket design tool which automates the process of producing heterogeneous 3D CAD-based geometries of the helium-cooled pebble bed, water-cooled lithium lead, helium-cooled lithium lead and dual-coolant lithium lead blanket types. The paper shows a method of integrating neutronics, thermal analysis and mechanical analysis with parametric CAD to facilitate the design process. The blanket design tool described in this paper provides parametric geometry for use in neutronics and engineering simulations. This paper explains the methodology of the design tool and demonstrates use of the design tool by generating all four EU blanket designs using the EU DEMO baseline. Neutronics and heat transfer simulations using the models have been carried out. The approach described has the potential to considerably speed up the design cycle and greatly facilitate the integration of multiphysics studies
AMI observations of Lynds dark nebulae: further evidence for anomalous cm-wave emission
No full textObservations at 14.2 to 17.9 GHz made with the Arcminute Microkelvin Imager (AMI) Small Array towards 14 Lynds dark nebulae with a resolution of ≈2 arcmin are reported. These sources are selected from the Submillimetre Common-User Bolometre Array (SCUBA) observations of Visser, Richer & Chandler as small angular diameter clouds well matched to the synthesized beam of the AMI Small Array. Comparison of the AMI observations with radio observations at lower frequencies with matched uv-plane coverage is made, in order to search for any anomalous excess emission which can be attributed to spinning dust. Possible emission from spinning dust is identified as a source within a 2-arcmin radius of the SCUBA position of the Lynds dark nebula, exhibiting an excess with respect to lower frequency radio emission. We find five sources which show a possible spinning dust component in their spectra. These sources have rising spectral indices in the frequency range 14.2-17.9 GHz with α17.914.2 = -0.7 ± 0.7 to -2.9 ± 0.4, where S ∝ ν-α. Of these five one has already been reported, L1111, we report one new definite detection, L675 (16σ), and three new probable detections (L944, L1103 and L1246). The relative certainty of these detections is assessed on the basis of three criteria: the extent of the emission, the coincidence of the emission with the SCUBA position and the likelihood of alternative explanations for the excess. Extended microwave emission makes the likelihood of the anomalous emission arising as a consequence of a radio counterpart to a protostar or a protoplanetary disc unlikely. We use a 2-arcmin radius in order to be consistent with the IRAS identifications of dark nebulae, and our third criterion is used in the case of L1103 where a high flux density at 850 μm relative to the far-infrared data suggests a more complicated emission spectrum. © 2009 RAS
Surface brightness discontinuities in radio halos. Insights from the MeerKAT Galaxy Cluster Legacy Survey
No full textDynamical motions in the ICM can imprint distinctive features on X-ray images
that map the thermal emission from galaxy clusters, such as sharp surface
brightness discontinuities due to shocks and cold fronts. The gas dynamics
during cluster mergers may also drive large-scale turbulence in the ICM, which
in turn generates extended synchrontron sources known as radio halos. Surface
brightness edges have been found numerous times in the thermal gas of clusters
based on X-ray observations. In contrast, edges in radio halos have only been
observed in a handful of cases. Our goal is to search for new radio surface
brightness discontinuities in the ICM. We inspected the images of the Bullet
Cluster and the other 25 radio halos reported in the MeerKAT Galaxy Cluster
Legacy Survey. To aid the identification of surface brightness discontinuities,
we applied a gradient-filtering edge-detection method to the radio images. We
find that the adopted filtering technique is helpful in identifying surface
brightness edges in radio images, allowing us to identify at least one gradient
in half of the radio halos studied. For the Bullet Cluster, we find excellent
agreement between the locations of the 4 radio discontinuities detected and the
X-ray edges. This similarity informs us that there is substantial interplay
between thermal and nonthermal components in galaxy clusters. This interplay is
likely due to the frozen-in ICM magnetic field, which mediates the advection of
cosmic rays while being dragged by thermal gas flows. We conclude that radio
halos are shaped by dynamical motions in the ICM and that they often display
surface brightness discontinuities, which appear to be co-located with edges in
the thermal gas emission. Our results demonstrate that new and future
generations of radio telescopes will provide a complementary approach to X-rays
to efficiently detect shocks and cold fronts in the ICM.Comment: v2: Matches accepted version in A&A (minor changes, added two new
figures) corrected by language editor. Abstract abridged to meet arXiv
requirement
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