37 research outputs found
Filaments of the radio cosmic web: opportunities and challenges for SKA
The detection of the diffuse gas component of the cosmic web remains a
formidable challenge. In this work we study synchrotron emission from the
cosmic web with simulated SKA1 observations, which can represent an fundamental
probe of the warm-hot intergalactic medium. We investigate radio emission
originated by relativistic electrons accelerated by shocks surrounding cosmic
filaments, assuming diffusive shock acceleration and as a function of the
(unknown) large-scale magnetic fields. The detection of the brightest parts of
large () filaments of the cosmic web should be within reach of the
SKA1-LOW, if the magnetic field is at the level of a percent
equipartition with the thermal gas, corresponding to for the
most massive filaments in simulations. In the course of a 2-years survey with
SKA1-LOW, this will enable a first detection of the "tip of the iceberg" of the
radio cosmic web, and allow for the use of the SKA as a powerful tool to study
the origin of cosmic magnetism in large-scale structures. On the other hand,
the SKA1-MID and SKA1-SUR seem less suited for this science case at low
redshift (), owing to the missing short baselines and the
consequent lack of signal from the large-scale brightness fluctuations
associated with the filaments. In this case only very long exposures ( hr) may enable the detection of filament for field of view in
the SKA1-SUR PAF Band1.Comment: 10 pages, 4 figures, to appear in Proceedings of 'Advancing
Astrophysics with the SKA (AASKA14) - Cosmic Magnetism' Chapter
MC: Dynamical Analysis of the Merging Galaxy Cluster MACS J1149.5+2223
We present an analysis of the merging cluster MACS J1149.5+2223 using
archival imaging from Subaru/Suprime-Cam and multi-object spectroscopy from
Keck/DEIMOS and Gemini/GMOS. We employ two and three dimensional substructure
tests and determine that MACS J1149.5+2223 is composed of two separate mergers
between three subclusters occurring 1 Gyr apart. The primary merger gives
rise to elongated X-ray morphology and a radio relic in the southeast. The
brightest cluster galaxy is a member of the northern subcluster of the primary
merger. This subcluster is very massive
(16.7 M).
The southern subcluster is also very massive
(10.8 M),
yet it lacks an associated X-ray surface brightness peak, and it has been
unidentified previously despite the detailed study of this \emph{Frontier
Field} cluster. A secondary merger is occurring in the north along the line of
sight with a third, less massive, subcluster
(1.20 M).
We perform a Monte Carlo dynamical analysis on the main merger and estimate a
collision speed at pericenter of 2770 km
s. We show the merger to be returning from apocenter with core
passage occurring 1.16 Gyr before the observed
state. We identify the line of sight merging subcluster in a strong lensing
analysis in the literature and show that it is likely bound to MACS J1149
despite having reached an extreme collision velocity of 4000 km
s.Comment: 17 pages, 12 figure
Mega-parsec scale magnetic fields in low density regions in the SKA era: filaments connecting galaxy clusters and groups
The presence of magnetic fields in galaxy clusters has been well established
in recent years, and their importance for the understanding of the physical
processes at work in the Intra Cluster Medium has been recognized. Halo and
relic sources have been detected in several tens clusters. A strong correlation
is present between the halo and relic radio power and the X-ray luminosity.
Since cluster X-Ray luminosity and mass are related, the correlation between
the radio power and X-ray luminosity could derive from a physical dependence of
the radio power on the cluster mass, therefore the cluster mass could be a
crucial parameter in the formation of these sources. The goal of this project
is to investigate the existence of non-thermal structures beyond the Mpc scale,
and associated with lower density regions with respect to clusters of galaxies:
galaxy filaments connecting rich clusters. We present a piece of evidence of
diffuse radio emission in intergalactic filaments. Moreover, we present and
discuss the detection of radio emission in galaxy groups and in faint X-Ray
clusters, to analyze non-thermal properties in low density regions with
physical conditions similar to galaxy filaments. We discuss how SKA1
observations will allow the investigation of this topic and the study of the
presence of diffuse radio sources in low density regions. This will be a
fundamental step to understand the origin and properties of cosmological
magnetic fields.Comment: 9 pages, 5 figures - to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)10
Combining survey data with single-dish observations
Sky surveys represent a fundamental data source for astronomy. Radio surveys, as well as pointed observations, performed with interferometers might be affected by the so-called zero-spacing
problem when observing extended sources at relatively high frequencies. This problem cause
the most extended features in the source to be filtered out due to the lack of antenna-pairs with
small separations. Here we show that interferometric survey data can be successfully combined
with pointed observations performed with a single-dish telescope to produce images with high
resolution and correct total power information.Sky surveys represent a fundamental data source for astronomy. Radio surveys, as well as pointed
observations, performed with interferometers might be affected by the so-called zero-spacing
problem when observing extended sources at relatively high frequencies. This problem cause
the most extended features in the source to be filtered out due to the lack of antenna-pairs with
small separations. Here we show that interferometric survey data can be successfully combined
with pointed observations performed with a single-dish telescope to produce images with high
resolution and correct total power information
Mpc-scale diffuse radio emission in two massive cool-core clusters of galaxies
Radio haloes are diffuse synchrotron sources on scales of ~1 Mpc that are found in merging clusters of galaxies, and are believed to be powered by electrons re-accelerated by mergerdriven turbulence. We present measurements of extended radio emission on similarly large scales in two clusters of galaxies hosting cool cores: Abell 2390 and Abell 2261. The analysis is based on interferometric imaging with the Karl G. Jansky Very Large Array, Very Large Array and Giant Metrewave Radio Telescope.We present detailed radio images of the targets, subtract the compact emission components and measure the spectral indices for the diffuse components. The radio emission in A2390 extends beyond a known sloshing-like brightness discontinuity, and has a very steep in-band spectral slope at 1.5 GHz that is similar to some known ultrasteep spectrum radio haloes. The diffuse signal in A2261 is more extended than in A2390 but has lower luminosity. X-ray morphological indicators, derived from XMM-Newton X-ray data, place these clusters in the category of relaxed or regular systems, although some asymmetric features that can indicate past minor mergers are seen in the X-ray brightness images. If these two Mpc-scale radio sources are categorized as giant radio haloes, they question the common assumption of radio haloes occurring exclusively in clusters undergoing violent merging activity, in addition to commonly used criteria for distinguishing between radio haloes and minihaloes
Progress with the LOFAR Imaging Pipeline
One of the science drivers of the new Low Frequency Array (LOFAR) is
large-area surveys of the low-frequency radio sky. Realizing this goal requires
automated processing of the interferometric data, such that fully calibrated
images are produced by the system during survey operations. The LOFAR Imaging
Pipeline is the tool intended for this purpose, and is now undergoing
significant commissioning work. The pipeline is now functional as an automated
processing chain. Here we present several recent LOFAR images that have been
produced during the still ongoing commissioning period. These early LOFAR
images are representative of some of the science goals of the commissioning
team members.Comment: 11 pages, 6 figures. Accepted for publication in proceedings of
"ISKAF2010 Science Meeting", PoS(ISKAF2010)05
Magnetic field evolution in cosmic filaments with LOFAR data
Measuring the magnetic field in cosmic filaments reveals how the Universe is
magnetised and the process that magnetised it. Using the Rotation Measures (RM)
at 144-MHz from the LoTSS DR2 data, we analyse the rms of the RM extragalactic
component as a function of redshift to investigate the evolution with redshift
of the magnetic field in filaments. From previous results, we find that the
extragalactic term of the RM rms at 144-MHz is dominated by the contribution
from filaments (more than 90 percent). Including an error term to account for
the minor contribution local to the sources, we fit the data with a model of
the physical filament magnetic field, evolving as
and with a density drawn from cosmological simulations of five magnetogenesis
scenarios. We find that the best-fit slope is in the range with uncertainty of --0.5, which is consistent with
no evolution. The comoving field decreases with redshift with a slope of
. The mean field strength at is in
the range --84~nG. For a typical filament gas overdensity of
the filament field strength at is in the range
--26~nG. A primordial stochastic magnetic field model with
initial comoving field of --0.11~nG is favoured. The
primordial uniform field model is rejected
Discovering the most elusive radio relic in the sky: Diffuse Shock Acceleration caught in the act?
The origin of radio relics is usually explained via diffusive shock
acceleration (DSA) or re-acceleration of electrons at/from merger shocks in
galaxy clusters. The case of acceleration is challenged by the low predicted
efficiency of low-Mach number merger shocks, unable to explain the power
observed in most radio relics. In this Letter we present the discovery of a new
giant radio relic around the galaxy cluster Abell 2249 () using
LOFAR. It is special since it has the lowest surface brightness of all known
radio relics. We study its radio and X-ray properties combinig LOFAR data with
uGMRT, JVLA and XMM. This object has a total power of W Hz and integrated spectral index . We infer for this radio relic a lower bound on the magnetisation of
G, a shock Mach number of , and a low
acceleration efficiency consistent with DSA. This result suggests that a
missing population of relics may become visible thanks to the unprecedented
sensitivity of the new generation of radio telescopes.Comment: Letter, 5 pages, 4 figures, accepted for publication on MNRAS Letter
Spectro-polarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening
Observations of radio relics at very high frequency (>10 GHz) can help to
understand how particles age and are (re-)accelerated in galaxy cluster
outskirts and how magnetic fields are amplified in these environments. In this
work, we present new single-dish 18.6 GHz Sardinia Radio Telescope and 14.25
GHz Effelsberg observations of the well known northern radio relic of CIZA
J2242.8+5301. We detected the relic which shows a length of 1.8 Mpc and a
flux density equal to and at 14.25 GHz and 18.6 GHz respectively. The
resulting best-fit model of the relic spectrum from 145 MHz to 18.6 GHz is a
power-law spectrum with spectral index : no evidence of
steepening has been found in the new data presented in this work. For the first
time, polarisation properties have been derived at 18.6 GHz, revealing an
averaged polarisation fraction of and a magnetic field aligned with
the 'filaments' or 'sheets' of the relic.Comment: 10 pages, 8 figure