42,635 research outputs found
Internal dynamics of the galaxy cluster Abell 545
Diffuse radio emission in galaxy clusters, and their connection with cluster
mergers, are still debated. We seek to explore the internal dynamics of the
radio halo cluster Abell 545. This cluster is also peculiar for hosting in its
center a very bright, red, diffuse intracluster light due to an old, stellar
population, so bright to be named as "star pile". Our analysis is based on
redshift data for 110 galaxies. We identify 95 cluster members and analyze the
cluster internal dynamics by combining galaxy velocities and positions. We also
use both photometric and X-ray data. We estimate the cluster redshift,
z=0.1580, a velocity dispersion of 1200 km/s, and ICM temperature kT_X~8 keV.
Our optical and X-ray analyses detect substructures. Optical data reveal three
main galaxy clumps (center, NNW, and NE), and possibly a fourth clump at South.
There is not a dominant galaxy and the four brightest galaxies avoid the
cluster core (>~0.4h distant from the cluster center) and are >~1500 km/s far
from the mean cluster velocity. The analysis of the X-ray surface brightness
distribution provides us evidence of a disturbed dynamical phase. Located in
the star pile region there is the brightest galaxies of the cluster core (CBCG)
and a very compact elliptical galaxy. We show that the star pile has a similar
redshift to that of the CBCG. Both the star pile and the CBCG are at rest in
the cluster rest frame. The emerging picture of Abell 545 is that of a massive,
M(R<1.6 h_70^-1 Mpc)=1.1-1.8x10^15 h_70^-1 Msun, very complex cluster with
merging occurring along two directions. A545 gives another proof in the favor
of the connection between cluster merger and extended, diffuse radio emission.
The star pile, likely due to the process of a brightest galaxy forming in the
cluster core. A545 represents a textbook cluster where to study the
simultaneous formation of a galaxy system and its brightest galaxy.Comment: 16 pages, 11 figures and 2 tables. Accepted in A&
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
Modified Baryonic Dynamics: two-component cosmological simulations with light sterile neutrinos
In this article we continue to test cosmological models centred on Modified
Newtonian Dynamics (MOND) with light sterile neutrinos, which could in
principle be a way to solve the fine-tuning problems of the standard model on
galaxy scales while preserving successful predictions on larger scales. Due to
previous failures of the simple MOND cosmological model, here we test a
speculative model where the modified gravitational field is produced only by
the baryons and the sterile neutrinos produce a purely Newtonian field (hence
Modified Baryonic Dynamics). We use two component cosmological simulations to
separate the baryonic N-body particles from the sterile neutrino ones. The
premise is to attenuate the over-production of massive galaxy cluster halos
which were prevalent in the original MOND plus light sterile neutrinos
scenario. Theoretical issues with such a formulation notwithstanding, the
Modified Baryonic Dynamics model fails to produce the correct amplitude for the
galaxy cluster mass function for any reasonable value of the primordial power
spectrum normalisation.Comment: 11 pages, 2 figures. Submitted to JCA
The unrelaxed dynamical structure of the galaxy cluster Abell 85
For the first time, we explore the dynamics of the central region of a galaxy
cluster within ~kpc from its center by combining optical
and X-ray spectroscopy. We use (1) the caustic technique that identifies the
cluster substructures and their galaxy members with optical spectroscopic data,
and (2) the X-ray redshift fitting procedure that estimates the redshift
distribution of the intracluster medium (ICM). We use the spatial and redshift
distributions of the galaxies and of the X-ray emitting gas to associate the
optical substructures to the X-ray regions. When we apply this approach to
Abell 85 (A85), a complex dynamical structure of A85 emerges from our analysis:
a galaxy group, with redshift is passing through the
cluster center along the line of sight dragging part of the ICM present in the
cluster core; two additional groups, at redshift and
, are going through the cluster in opposite directions,
almost perpendicularly to the line of sight, and have substantially perturbed
the dynamics of the ICM. An additional group in the outskirts of A85, at
redshift , is associated to a secondary peak of the X-ray
emission, at redshift . Although our analysis and
results on A85 need to be confirmed by high-resolution spectroscopy, they
demonstrate how our new approach can be a powerful tool to constrain the
formation history of galaxy clusters by unveiling their central and surrounding
structures.Comment: 12 pages, 11 figures, accepted by Ap
Internal dynamics of the galaxy cluster Abell 959
The connection of cluster mergers with the presence of extended, diffuse
radio sources in galaxy clusters is still being debated. In this paper we aim
to obtain new insights into the internal dynamics of Abell 959, showing
evidence of a diffuse radio source, analyzing velocities and positions of
member galaxies. Our analysis is based on redshift data for 107 galaxies in the
cluster field acquired at the Telescopio Nazionale Galileo. We also use
photometric data from the Sloan Digital Sky Survey (Data Release 6). We combine
galaxy velocities and positions to select 81 galaxies recognized as cluster
members and determine global dynamical properties. We analyze the cluster
searching for substructures by using several statistical methods. We also study
the 2D galaxy distribution in the field of the cluster. We compare our results
with those from X-ray and gravitational lensing analyses. We estimate a cluster
redshift of z=0.2883 +/- 0.0004. We detect an NE high velocity group at 5' from
the cluster center with a relative line--of--sight (LOS) velocity of ~ +1900
km/s with respect to the main system. We also detect a central, dense structure
elongated along the SE--NW direction likely connected with the two dominant
galaxies and their surrounding cores. This elongated central structure is
probably the trace of an old cluster merger. The LOS velocity dispersion of
galaxies is very high (1025 (-75/+104) km/s). The virial mass is M(<R=1.48
Mpc)= 1.15 (-0.19/+0.25) Msun. Our results suggest that this cluster is forming
along two main directions of mass accretion and show the typical
characteristics of radio clusters; i.e., it is very massive and shows a young
dynamical state. However, deeper radio observations are needed to clarify the
nature of the diffuse radio emission in Abell 959.Comment: 12 pages, 13 figures, 3 tables. Accepted for publication on Astronomy
and Astrophysic
Lopsidedness of cluster galaxies in modified gravity
We point out an interesting theoretical prediction for elliptical galaxies
residing inside galaxy clusters in the framework of modified Newtonian dynamics
(MOND), that could be used to test this paradigm. Apart from the central
brightest cluster galaxy, other galaxies close enough to the centre experience
a strong gravitational influence from the other galaxies of the cluster. This
influence manifests itself only as tides in standard Newtonian gravity, meaning
that the systematic acceleration of the centre of mass of the galaxy has no
consequence. However, in the context of MOND, a consequence of the breaking of
the strong equivalence principle is that the systematic acceleration changes
the own self-gravity of the galaxy. We show here that, in this framework,
initially axisymmetric elliptical galaxies become lopsided along the external
field's direction, and that the centroid of the galaxy, defined by the outer
density contours, is shifted by a few hundreds parsecs with respect to the
densest point.Comment: accepted for publication in JCA
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