2,272 research outputs found
A large H survey of star formation in relaxed and merging galaxy cluster environments at
We present the first results from the largest H survey of star formation and AGN activity in galaxy clusters. Using 9 different narrow band filters, we select H emitters within clusters and their larger scale environment over a total volume of Mpc. The sample includes both relaxed and merging clusters, covering the redshift range and spanning from to . We find that the H luminosity function (LF) for merging clusters has a higher characteristic density compared to relaxed clusters. drops from cluster core to cluster outskirts for both merging and relaxed clusters, with the merging cluster values dex higher at each projected radius. The characteristic luminosity drops over the Mpc distance from the cluster centre for merging clusters and increases for relaxed objects. Among disturbed objects, clusters hosting large-scale shock waves (traced by radio relics) are overdense in H emitters compared to those with turbulence in their intra-cluster medium (traced by radio haloes). We speculate that the increase in star formation activity in disturbed, young, massive galaxy clusters can be triggered by interactions between gas-rich galaxies, shocks and/or the intra-cluster medium, as well as accretion of filaments and galaxy groups. Our results indicate that disturbed clusters represent vastly different environments for galaxy evolution compared to relaxed clusters or average field environments
The spectacular cluster chain Abell 781 as observed with LOFAR, GMRT, and XMM-Newton
Context: A number of merging galaxy clusters show the presence of large-scale radio emission associated with the intra-cluster medium (ICM). These synchrotron sources are generally classified as radio haloes and radio relics. Aims. Whilst it is commonly accepted that mergers play a crucial role in the formation of radio haloes and relics, not all the merging clusters show the presence of giant diffuse radio sources and this provides important information concerning current models. The Abell 781 complex is a spectacular system composed of an apparent chain of clusters on the sky. Its main component is undergoing a merger and hosts peripheral emission that is classified as a candidate radio relic and a disputed radio halo. Methods. We used new LOw Frequency ARay (LOFAR) observations at 143 MHz and archival Giant Metrewave Radio Telescope (GMRT) observations at 325 and 610 MHz to study radio emission from non-thermal components in the ICM of Abell 781. Complementary information came from XMM-Newton data, which allowed us to investigate the connection with the thermal emission and its complex morphology. Results. The origin of the peripheral emission is still uncertain. We speculate that it is related to the interaction between a head tail radio galaxy and shock. However, the current data allow us only to set an upper limit of M < 1.4 on the Mach number of this putative shock. Instead, we successfully characterise the surface brightness and temperature jumps of a shock and two cold fronts in the main cluster component of Abell 781. Their positions suggest that the merger is involving three substructures. We do not find any evidence for a radio halo either at the centre of this system or in the other clusters of the chain. We place an upper limit to the diffuse radio emission in the main cluster of Abell 781 that is a factor of 2 below the current radio power-mass relation for giant radio haloes
A SINFONI view of flies in the Spiderweb: a galaxy cluster in the making
The environment of the high-z radio galaxy PKS 1138-262 at z~2.2 is a prime
example of a forming galaxy cluster. We use deep SINFONI data to perform a
detailed study of the kinematics of the galaxies within 60 kpc of the radio
core and we link this to the kinematics of the protocluster on the megaparsec
scale. Identification of optical emission lines shows that 11 galaxies are at
the redshift of the protocluster. The density of line emitters is more than an
order of magnitude higher in the core of the protocluster than the larger scale
environment. This implies a matter overdensity in the core of delta_m~70 which
is similar to the outskirts of local galaxy clusters. The velocity distribution
of the confirmed satellite galaxies shows a broad, double-peaked velocity
structure with sigma=1360+/-206 km/s. A similar broad, double-peaked
distribution was found in a previous study targeting the large scale
protocluster structure, indicating that a common process is acting on both
small and large scales. Including all spectroscopically confirmed protocluster
galaxies, a velocity dispersion of 1013+/-87 km/s is found. We show that the
protocluster has likely decoupled from the Hubble flow and is a dynamically
evolved structure. Comparison to the Millenium simulation indicates that the
protocluster velocity distribution is consistent with that of the most massive
haloes at z~2, but we rule out that the protocluster is a fully virialized
structure based on dynamical arguments and its X-ray luminosity. Comparison to
merging haloes in the Millennium simulation shows that the structure as
observed in and around the Spiderweb galaxy is best interpreted as being the
result of a merger between two massive haloes. We propose that this merger can
result in an increase in star formation and AGN activity in the protocluster
core and is possibly an important stage in the evolution of massive cD
galaxies.Comment: 12 pages, 7 figures. Accepted for publication in MNRA
Infalling groups and galaxy transformations in the cluster A2142
We study galaxy populations and search for possible merging substructures in
the rich galaxy cluster A2142. Normal mixture modelling revealed in A2142
several infalling galaxy groups and subclusters. The projected phase space
diagram was used to analyse the dynamics of the cluster and study the
distribution of various galaxy populations in the cluster and subclusters. The
cluster, supercluster, BCGs, and one infalling subcluster are aligned. Their
orientation is correlated with the alignment of the radio and X-ray haloes of
the cluster. Galaxies in the centre of the main cluster at the clustercentric
distances have older stellar populations (with the median age
of ~Gyrs) than galaxies at larger clustercentric distances.
Star-forming and recently quenched galaxies are located mostly in the infall
region at the clustercentric distances ,
where the median age of stellar populations of galaxies is about ~Gyrs.
Galaxies in A2142 have higher stellar masses, lower star formation rates, and
redder colours than galaxies in other rich groups. The total mass in infalling
groups and subclusters is ,
approximately half of the mass of the cluster, sufficient for the mass growth
of the cluster from redshift (half-mass epoch) to the present. The
cluster A2142 may have formed as a result of past and present mergers and
infallen groups, predominantly along the supercluster axis. Mergers cause
complex radio and X-ray structure of the cluster and affect the properties of
galaxies in the cluster, especially in the infall region. Explaining the
differences between galaxy populations, mass, and richness of A2142, and other
groups and clusters may lead to better insight about the formation and
evolution of rich galaxy clusters.Comment: 16 pages, 13 figures, A&A, in pres
Starburst radio galaxies: general properties, evolutionary histories and triggering
In this paper we discuss the results of a programme of spectral synthesis
modelling of a sample of starburst radio galaxies in the context of scenarios
for the triggering of the activity and the evolution of the host galaxies. The
starburst radio galaxies -- comprising ~15 - 25% of all powerful extragalactic
radio sources -- frequently show disturbed morphologies at optical wavelengths,
and unusual radio structures, although their stellar masses are typical of
radio galaxies as a class. In terms of the characteristic ages of their young
stellar populations (YSP), the objects can be divided into two groups: those
with YSP ages t_ysp < 0.1 Gyr, in which the radio source has been triggered
quasi-simultaneously with the main starburst episode, and those with older YSP
in which the radio source has been triggered or re-triggered a significant
period after the starburst episode. Combining the information on the YSP with
that on the optical morphologies of the host galaxies, we deduce that the
majority of the starburst radio galaxies have been triggered in galaxy mergers
in which at least one of the galaxies is gas rich. However, the triggering (or
re-triggering) of the radio jets can occur immediately before, around, or a
significant period after the final coalescence of the merging nuclei,
reflecting the complex gas infall histories of the merger events. Overall, our
results provide further evidence that powerful radio jet activity can be
triggered via a variety of mechanisms, including different evolutionary stages
of major galaxy mergers; clearly radio-loud AGN activity is not solely
associated with a particular stage of a unique type of gas accretion event.Comment: 16 pages, 3 Figures, accepted for publication in MNRA
Radio observations of the double-relic galaxy cluster Abell 1240
We present LOFAR 120 − 168 MHz images of the merging galaxy cluster Abell 1240 that hosts double radio relics. In combination with the GMRT 595 − 629 MHz and VLA 2 − 4 GHz data, we characterised the spectral and polarimetric properties of the radio emission. The spectral indices for the relics steepen from their outer edges towards the cluster centre and the electric field vectors are approximately perpendicular to the major axes of the relics. The results are consistent with the picture that these relics trace large-scale shocks propagating outwards during the merger. Assuming diffusive shock acceleration (DSA), we obtain shock Mach numbers of M = 2.4 and 2.3 for the northern and southern shocks, respectively. For M ≲ 3 shocks, a pre-existing population of mildly relativistic electrons is required to explain the brightness of the relics due to the high (> 10 per cent) particle acceleration efficiency required. However, for M ≳ 4 shocks the required efficiency is ≳ 1% and ≳ 0.5%, respectively, which is low enough for shock acceleration directly from the thermal pool. We used the fractional polarization to constrain the viewing angle to ≥ 53 ± 3° and ≥ 39 ± 5° for the northern and southern shocks, respectively. We found no evidence for diffuse emission in the cluster central region. If the halo spans the entire region between the relics (∼1.8 Mpc) our upper limit on the power is P1.4 GHz = (1.4 ± 0.6) × 1023 W Hz−1 which is approximately equal to the anticipated flux from a cluster of this mass. However, if the halo is smaller than this, our constraints on the power imply that the halo is underluminous
The Red Sequence of High-Redshift Clusters: a Comparison with Cosmological Galaxy Formation Models
We compare the results from a semi-analytic model of galaxy formation with
spectro-photometric observations of distant galaxy clusters observed in the
range 0.8< z< 1.3. We investigate the properties of their red sequence (RS)
galaxies and compare them with those of the field at the same redshift. In our
model we find that i) a well-defined, narrow RS is obtained already by z= 1.2;
this is found to be more populated than the field RS, analogously to what
observed and predicted at z=0; ii) the predicted U-V rest-frame colors and
scatter of the cluster RS at z=1.2 have average values of 1 and 0.15
respectively, with a cluster-to-cluster variance of 0.2 and 0.06, respectively.
The scatter of the RS of cluster galaxies is around 5 times smaller than the
corresponding field value; iii) when the RS galaxies are considered, the mass
growth histories of field and cluster galaxies at z=1.2 are similar, with 90 %
of the stellar mass of RS galaxies at z=1.2 already formed at cosmic times
t=2.5 Gyr, and 50 % at t=1 Gyr; v) the predicted distribution of stellar ages
of RS galaxies at z=1.2 peaks at 3.7 Gyr for both cluster and field
populations; however, for the latter the distribution is significantly skewed
toward lower ages. When compared with observations, the above findings show an
overall consistency, although the average value 0.07 of the observed cluster RS
scatter (U-V colors) at z=1.2 is smaller than the corresponding model central
value. We discuss the physical origin and the significance of the above results
in the framework of cosmological galaxy formation.Comment: 14 pages, accepted for publication in ApJ. Updated one referenc
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
The dynamical status of ZwCl 2341.1+0000: a very elongated galaxy structure with a complex radio emission
We study the dynamical status of the galaxy system ZwCl 2341.1+0000, a
filamentary multi-Mpc galaxy structure associated with a complex diffuse radio
emission. Our analysis is mainly based on new spectroscopic data for 128
galaxies acquired at the TNG telescope. We also use optical data available in
the SDSS and X-ray data from the Chandra archive. We select 101 cluster member
galaxies and compute the cluster redshift ~0.2693 and the global LOS
velocity dispersion ~1000 km/s. Our optical analysis agrees with the presence
of at least three, likely four or more, optical subclusters causing the SSE-NNW
elongation of the galaxy distribution and a significant velocity gradient in
the S-N direction. In particular, we detect an important low-velocity subclump
in the southern region, roughly coincident with the brightest peak of the
diffuse radio emission but with a clear offset between the optical and radio
peaks. We also detect one (or two) optical subcluster(s) at north, in
correspondence with the second brightest radio emission, and another one in the
central cluster region, where a third diffuse radio source has been recently
detected. A more refined analysis involving the study of the 2D galaxy
distribution suggests an even more complex structure. As for the X-ray
analysis, we confirm the SSE-NNW elongation of the intracluster medium and
detect four significant peaks. The X-ray emission is strongly asymmetric and
offsetted with respect to the galaxy distribution, thus suggesting a merger
caught in the phase of post-core-core passage. Our findings support two
possible hypotheses for the nature of the diffuse radio emission of ZwCl
2341.1+0000: a 2 relics+halo scenario or diffuse emission associated with the
infall and merging of several galaxy groups during the first phase of the
cluster formation.Comment: 14 pages, 14 figures, 4 tables; MNRAS in pres
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