276 research outputs found
Thermal and non-thermal traces of AGN feedback: results from cosmological AMR simulations
We investigate the observable effects of feedback from Active Galactic Nuclei
(AGN) on non-thermal components of the intracluster medium (ICM). We have
modelled feedback from AGN in cosmological simulations with the adaptive mesh
refinement code ENZO, investigating three types of feedback that are sometimes
called quasar, jet and radio mode. Using a small set of galaxy clusters
simulated at high resolution, we model the injection and evolution of Cosmic
Rays, as well as their effects on the thermal plasma. By comparing, both, the
profiles of thermal gas to observed profiles from the ACCEPT sample, and the
secondary gamma-ray emission to the available upper limits from FERMI, we
discuss how the combined analysis of these two observables can constrain the
energetics and mechanisms of feedback models in clusters. Those modes of AGN
feedback that provide a good match to X-ray observations, yield a gamma-ray
luminosity resulting from secondary cosmic rays that is about below the
available upper limits from FERMI. Moreover, we investigate the injection of
turbulent motions into the ICM from AGN, and the detectability of these motions
via the analysis of line broadening of the Fe XXIII line. In the near future,
deeper observations/upper-limits of non-thermal emissions from galaxy clusters
will yield stringent constraints on the energetics and modes of AGN feedback,
even at early cosmic epochs.Comment: 24 pages, 20 figures. MNRAS accepted. A version of the paper with
higher quality figures can be found at this url:
http://www.ira.inaf.it/~vazza/papers/feedback_vazza.pd
On the amplification of magnetic fields in cosmic filaments and galaxy clusters
The amplification of primordial magnetic fields via a small-scale turbulent
dynamo during structure formation might be able to explain the observed
magnetic fields in galaxy clusters. The magnetisation of more tenuous
large-scale structures such as cosmic filaments is more uncertain, as it is
challenging for numerical simulations to achieve the required dynamical range.
In this work, we present magneto-hydrodynamical cosmological simulations on
large uniform grids to study the amplification of primordial seed fields in the
intracluster medium (ICM) and in the warm-hot-intergalactic medium (WHIM). In
the ICM, we confirm that turbulence caused by structure formation can produce a
significant dynamo amplification, even if the amplification is smaller than
what is reported in other papers. In the WHIM inside filaments, we do not
observe significant dynamo amplification, even though we achieve Reynolds
numbers of . The maximal amplification for large
filaments is of the order of for the magnetic energy, corresponding
to a typical field of a few starting from a primordial weak field
of G (comoving). In order to start a small-scale dynamo, we found
that a minimum of resolution elements across the virial radius of
galaxy clusters was necessary. In filaments we could not find a minimum
resolution to set off a dynamo. This stems from the inefficiency of supersonic
motions in the WHIM in triggering solenoidal modes and small-scale twisting of
magnetic field structures. Magnetic fields this small will make it hard to
detect filaments in radio observations.Comment: MNRAS accepted, in press. 18 pages, 18 Figures. New version to match
with the one published in MNRAS. Updated publication list and footnote added
to the title as obituary notic
Convolutional deep denoising autoencoders for radio astronomical images
We apply a Machine Learning technique known as Convolutional Denoising Autoencoder to denoise synthetic images of state-of-the-art radio telescopes, with the goal of detecting the faint, diffused radio sources predicted to characterize the radio cosmic web. In our application, denoising is intended to address both the reduction of random instrumental noise and the minimization of additional spurious artefacts like the sidelobes, resulting from the aperture synthesis technique. The effectiveness and the accuracy of the method are analysed for different kinds of corrupted input images, together with its computational perfoance. Specific attention has been devoted to create realistic mock observations for the training, exploiting the outcomes of cosmological numerical silations, to generate images corresponding to LOFAR HBA 8 h observations at 150 MHz. Our autoencoder can effectively denoise complex images identifying and extracting faint objects at the limits of the instrumental sensitivity. The method can efficiently scale on large data sets, exploiting high-perfoance computing solutions, in a fully automated way (i.e. no human supervision is required after training). It can accurately perfo image segmentation, identifying low brightness outskirts of diffused sources, proving to be a viable solution for detecting challenging extended objects hidden in noisy radio observations
Forecasts for the detection of the magnetised cosmic web from cosmological simulations
The cosmic web contains a large fraction of the total gas mass in the
universe but is difficult to detect at most wavelengths. Synchrotron emission
from shock-accelerated electrons may offer the chance of imaging the cosmic web
at radio wavelengths. In this work we use 3D cosmological ENZO-MHD simulations
(combined with a post-processing renormalisation of the magnetic field to
bracket for missing physical ingredients and resolution effects) to produce
models of the radio emission from the cosmic web. In post-processing we study
the capabilities of 13 large radio surveys to detect this emission. We find
that surveys by LOFAR, SKA1-LOW and MWA have a chance of detecting the cosmic
web, provided that the magnetisation level of the tenuous medium in filaments
is of the order of 1% of the thermal gas energy.Comment: 19 pages, 18 figures. A&A accepted, in press. The public repository
of radio maps for the full volumes studied in this work is available at
http://www.hs.uni-hamburg.de/DE/Ins/Per/Vazza/projects/Public_data.htm
Detecting shocked intergalactic gas with X-ray and radio observations
Detecting the thermal and non-thermal emission from the shocked cosmic gas
surrounding large-scale structures represents a challenge for observations, as
well as a unique window into the physics of the warm-hot intergalactic medium.
In this work, we present synthetic radio and X-ray surveys of large
cosmological simulations in order to assess the chances of jointly detecting
the cosmic web in both frequency ranges. We then propose best observing
strategies tailored for existing (LOFAR, MWA and XMM) or future instruments
(SKA-LOW and SKA-MID, ATHENA and eROSITA). We find that the most promising
targets are the extreme peripheries of galaxy clusters in an early merging
stage, where the merger causes the fast compression of warm-hot gas onto the
virial region. By taking advantage of a detection in the radio band, future
deep X-ray observations will probe this gas in emission, and help us to study
plasma conditions in the dynamic warm-hot intergalactic medium with
unprecedented detail.Comment: 22 pages, 25 Figures. A\&A accepted, in press. Moderate revision
compared to version 1, with a few new figure
The turbulent pressure support in galaxy clusters revisited
Due to their late formation in cosmic history, clusters of galaxies are not
fully in hydrostatic equilibrium and the gravitational pull of their mass at a
given radius is expected not to be entirely balanced by the thermal gas
pressure. Turbulence may supply additional pressure, and recent (X-ray and SZ)
hydrostatic mass reconstructions claim a pressure support of of
the total pressure at . In this work we show that, after carefully
disentangling bulk from small-scale turbulent motions in high-resolution
simulations of galaxy clusters, we can constrain which fraction of the gas
kinetic energy effectively provides pressure support in the cluster's
gravitational potential. While the ubiquitous presence of radial inflows in the
cluster can lead to significant bias in the estimate of the non-thermal
pressure support, we report that only a part of this energy effectively acts as
a source of pressure, providing a support of the order of of the
total pressure at .Comment: 5 pages, 5 pages, accepted, to appear in MNRAS Letter
Properties of cosmological filaments extracted from Eulerian simulations
Using a new parallel algorithm implemented within the VisIt framework, we analysed large cosmological grid simulations to study the properties of baryons in filaments. The procedure allows us to build large catalogues with up to∼3×104 filaments per simulated volume and to investigate the properties of cosmic filaments for very large volumes at high resolution (up to 3003 Mpc3 simulated with 20483 cells). We determined scaling relations for the mass, volume, length and temperature of filaments and compared them to those of galaxy clusters. The longest filaments have a total length of about 200 Mpc with a mass of several 1015 M⊙. We also investigated the effects of different gas physics. Radiative cooling significantly modifies the thermal properties of the warm-hot-intergalactic medium of filaments, mainly by lowering their mean temperature via line cooling. On the other hand, powerful feedback from active galactic nuclei in surrounding haloes can heat up the gas in filaments. The impact of shock-accelerated cosmic rays from diffusive shock acceleration on filaments is small and the ratio between cosmic ray and gas pressure within filaments is of the order of∼10-20 per cen
Efficiency of Turbulent Reacceleration by Solenoidal Turbulence and Its Application to the Origin of Radio Megahalos in Cluster Outskirts
Recent radio observations with the Low Frequency Array (LOFAR) discovered diffuse emission extending beyond the scale of classical radio halos. The presence of such megahalos indicates that the amplification of the magnetic field and acceleration of relativistic particles are working in the cluster outskirts, presumably due to the combination of shocks and turbulence that dissipate energy in these regions. Cosmological magnetohydrodynamical (MHD) simulations of galaxy clusters suggest that solenoidal turbulence has a significant energy budget in the outskirts of galaxy clusters. In this paper, we explore the possibility that this turbulence contributes to the emission observed in megahalos through second-order Fermi acceleration of relativistic particles and magnetic field amplification by the dynamo. We focus on the case of A2255 and find that this scenario can explain the basic properties of the diffuse emission component that is observed under assumptions that are used in previous literature. More specifically, we conduct a numerical follow-up, solving the Fokker-Planck equation by using a snapshot of an MHD simulation and deducing the synchrotron brightness integrated along the lines of sight. We find that a volume-filling emission, ranging between 30% and almost 100% of the projected area, depending on our assumptions on the particle diffusion and transport, can be detected at LOFAR sensitivities. Assuming a magnetic field B ∼ 0.2 μG, as derived from a dynamo model applied to the emitting region, we find that the observed brightness can be matched when ∼1% of the solenoidal turbulent energy flux is channeled into particle acceleration
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