Deep Learning as a Parton Shower

We make the connection between certain deep learning architectures and the renormalisation group explicit in the context of QCD by using a deep learning network to construct a toy parton shower model. The model aims to describe proton-proton collisions at the Large Hadron Collider. A convolutional autoencoder learns a set of kernels that efficiently encode the behaviour of fully showered QCD collision events. The network is structured recursively so as to ensure self-similarity, and the number of trained network parameters is low. Randomness is introduced via a novel custom masking layer, which also preserves existing parton splittings by using layer-skipping connections. By applying a shower merging procedure, the network can be evaluated on unshowered events produced by a matrix element calculation. The trained network behaves as a parton shower that qualitatively reproduces jet-based observables.Comment: 26 pages, 13 figure

Chandra and XMM-Newton Observations of the Double Cluster Abell 1758

Abell 1758 was classified as a single rich cluster of galaxies by Abell, but a ROSAT observation showed that this system consists of two distinct clusters (A1758N and A1758S) separated by approximately 8\arcmin (a projected separation of 2 Mpc in the rest frame of the clusters). Only a few galaxy redshifts have been published for these two clusters, but the redshift of the Fe lines in the Chandra and XMM-Newton spectra shows that the recessional velocities of A1758N and A1758S are within 2,100 km s$^{-1}$. Thus, these two clusters most likely form a gravitationally bound system, but our imaging and spectroscopic analyses of the X-ray data do not reveal any sign of interaction between the two clusters. The Chandra and XMM-Newton observations show that A1758N and A1758S are both undergoing major mergers. A1758N is in the late stages of a large impact parameter merger between two 7 keV clusters. The two remnant cores have a projected separation of 800 kpc. Based on the measured pressure jumps preceding the two cores, they are receding from one another at less than 1,600 km s$^{-1}$. The two cores are surrounded by hotter gas ($\mathrm{kT}=9$--12 keV) that was probably shock heated during the early stages of the merger. The gas entropy in the two remnant cores is comparable with the central entropy observed in dynamically relaxed clusters, indicating that the merger-induced shocks stalled as they tried to penetrate the high pressure cores of the two merging systems.Each core also has a wake of low entropy gas indicating that this gas was ram pressure stripped without being strongly shocked (abridged). (A copy of the paper with higher resolution images is available at http://asc.harvard.edu/~lpd/a1758.ps).Comment: paper plus 13 figure

Intracluster medium of the merging cluster Abell 3395

We present a detailed imaging and spectral analysis of the merging environment of the bimodal cluster A3395 using X-ray and radio observations. X-ray images of the cluster show five main constituents of diffuse emission : A3395 NE, A3395 SW, A3395 NW, A3395 W, and a filament connecting NE to W. X-ray surface-brightness profiles of the cluster did not show any shock fronts in the cluster. Temperature and entropy maps show high temperature and high entropy regions in the W, the NW, the filament and between the NE and SW subclusters. The NE, SW and W components have X-ray bolometric luminosities similar to those of rich clusters of galaxies but have relatively higher temperatures. Similarly, the NW component has X-ray bolometric luminosity similar to that of isolated groups but with much higher temperature. It is, therefore, possible that all the components of the cluster have been heated by the ongoing mergers. The NE subcluster is the most massive and luminous constituent and other subclusters are found to be gravitationally bound to it. The W component is most probably either a clump of gas stripped off the SW due to ram pressure or a separate subcluster that has merged or is merging with the SW. No X-ray cavities are seen associated with the Wide Angle Tailed (WAT) radio source near the centre of the SW subcluster. Minimum energy pressure in the radio emission-peaks of the WAT galaxy is comparable with the external thermal pressure. The radio spectrum of the WAT suggests a spectral age of ~10Myr

A Puzzling Merger in A3266: the Hydrodynamic Picture from XMM-Newton

Using the mosaic of nine XMM-Newton observations, we study the hydrodynamic state of the merging cluster of galaxies Abell 3266. The high quality of the spectroscopic data and large field of view of XMM-Netwon allow us to determine the thermodynamic conditions of the intracluster medium on scales of order of 50 kpc. A high quality entropy map reveals the presence of an extended region of low entropy gas, running from the primary cluster core toward the northeast along the nominal merger axis. The mass of the low entropy gas amounts to approximately 2e13 solar masses, which is comparable to the baryonic mass of the core of a rich cluster. We test the possibility that the origin of the observed low entropy gas is either related to the disruption a preexisting cooling core in Abell 3266 or to the stripping of gas from an infalling subcluster companion. We find that both the radial pressure and entropy profiles as well as the iron abundance of Abell 3266 do not resemble those in other known cooling core clusters (Abell 478). Thus we conclude that the low entropy region is subcluster gas in the process of being stripped off from its dark matter halo. In this scenario the subcluster would be falling onto the core of A3266 from the foreground. This would also help interpret the observed high velocity dispersion of the galaxies in the cluster center, provided that the mass of the subcluster is at most a tenth of the mass of the main cluster.Comment: 6 pages, ApJ sub

The late merging phase of a galaxy cluster : XMM EPIC Observations of A3266

We present a mosaic of five XMM-Newton observations of the nearby ($z=0.0594$) merging galaxy cluster Abell 3266. We use the spectro-imaging capabilities of \xmm to build precise (projected) temperature, entropy, pressure and Fe abundance maps. The temperature map exhibits a curved, large-scale hot region, associated with elevated entropy levels, very similar to that foreseen in numerical simulations. The pressure distribution is disturbed in the central region but is remarkably regular on large scales. The Fe abundance map indicates that metals are inhomogeneously distributed across the cluster. Using simple physical calculations and comparison with numerical simulations, we discuss in detail merging scenarios that can reconcile the observed gas density, temperature and entropy structure, and the galaxy density distribution

The merging galaxy cluster A520 --- a broken-up cool core, a dark subcluster, and an X-ray channel

We present results from a deep Chandra X-ray observation of a merging galaxy cluster A520. A high-resolution gas temperature map, after the subtraction of the cluster-scale emission, reveals a long trail of dense, cool clumps --- apparently the fragments of a cool core that has been completely stripped from the infalling subcluster by ram pressure. In this scenario, we can assume that the clumps are still connected by the magnetic field lines. The observed temperature variations imply that thermal conductivity is suppressed by a factor >100 across the presumed direction of the magnetic field (as found in other clusters), and is also suppressed -along- the field lines by a factor of several. Two massive clumps in the periphery of A520, visible in the weak lensing mass map and the X-ray image, have apparently been completely stripped of gas during the merger, but then re-accreted the surrounding high-entropy gas upon exit from the cluster. An X-ray hydrostatic mass estimate for one of the clumps (that has simple geometry) agrees with the lensing mass. Its current gas mass to total mass ratio is very low, 1.5-3%, which makes it a "dark subcluster". We also found a curious low X-ray brightness channel (likely a low-density sheet in projection) going across the cluster along the direction of an apparent secondary merger. The channel may be caused by plasma depletion in a region of an amplified magnetic field (with plasma $\beta\sim 10-20$). The shock in A520 will be studied in a separate paper.Comment: Accepted for publication in ApJ. 13 pages, 7 figures. (Author affiliation updated (v2), updated with final revisions prior to publication (v3).

Cold fronts in galaxy clusters

Cold fronts have been observed in a large number of galaxy clusters. Understanding their nature and origin is of primary importance for the investigation of the internal dynamics of clusters. To gain insight on the nature of these features, we carry out a statistical investigation of their occurrence in a sample of galaxy clusters observed with XMM-Newton and we correlate their presence with different cluster properties. We have selected a sample of 45 clusters starting from the B55 flux limited sample by Edge et al. (1990) and performed a systematic search of cold fronts. We find that a large fraction of clusters host at least one cold front. Cold fronts are easily detected in all systems that are manifestly undergoing a merger event in the plane of the sky while the presence of such features in the remaining clusters is related to the presence of a steep entropy gradient, in agreement with theoretical expectations. Assuming that cold fronts in cool core clusters are triggered by minor merger events, we estimate a minimum of 1/3 merging events per halo per Gyr.Comment: Accepted for publication in Astronomy & Astrophysics. Version with full resolution figures available at: http://www.iasf-milano.inaf.it/~simona/pub/coldfronts/ghizzardi.pd