The dynamical state of massive galaxy clusters

Abstract

We study the mass distribution of a sample of 24 X-ray bright Abell clusters through weak gravitational lensing. This method is independent of the dynamical state of the galaxy cluster. Hence, by comparing dynamical and lensing mass estimators, we can access the dynamical state of these clusters. We have found that clusters with ICM temperatures above 8 keV show strong deviations from the relaxation, as well as the presence of prominent sub-structures. For the remaining clusters (the majority of the sample) we have found agreement among the several mass estimators, which indicates that most of the clusters are in or close to a state of dynamical equilibrium.

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arXiv:astro-ph/0604205v1 10 Apr 2006
To appear in “Latin American Regional IAU Meeting (2005)”RevMexAA(SC)
THE DYNAMICAL STATE OF MASSIVE GALAXY CLUSTERS
E. S. Cypriano1,2L. Sodr´ e Jr.3J.-P. Kneib4and L. E. Campusano5
RESUMEN
Se eval´ ua la distribuci´ on de masa para 24 c´ umulos de Abell brillantes en rayos-X usando el efecto debil de los
lentes gravitacionales. Este m´ etodo es independiente del estado din´ amico del c´ umulo, y por lo tanto la com-
paracion con masas “dinamicas” posibilita una determinaci´ on del estado din´ amico de los c´ umulos. Los c´ umulos
con temperaturas ICM mayores que 8 keV presentan desviaciones con respecto a los cumulos dinamicamente
relajados, como tambi´ en presentan sub-estructuras significativas. Los distintos indicadores de masa dan valores
consistentes entre si para los dem´ as (la mayor´ ıa de los c´ umulos de la muestra), lo cual es indicativo de estados
cercanos al equilibrio din´ amico.
ABSTRACT
We study the mass distribution of a sample of 24 X-ray bright Abell clusters through weak gravitational lensing.
This method is independent of the dynamical state of the galaxy cluster. Hence, by comparing dynamical and
lensing mass estimators, we can access the dynamical state of these clusters. We have found that clusters
with ICM temperatures above 8 keV show strong deviations from the relaxation, as well as the presence of
prominent sub-structures. For the remaining clusters (the majority of the sample) we have found agreement
among the several mass estimators, which indicates that most of the clusters are in or close to a state of
dynamical equilibrium.
Key Words: GALAXY CLUSTERS — GRAVITATIONAL LENSING
1. INTRODUCTION
There are several ways to measure the masses of
galaxy clusters but the most widely used methods
are the dynamical ones, for instance the study of
the line-of-sight velocity distribution of the member
galaxies and the X-ray measurements of the temper-
ature and distribution of the intra-cluster gas. They
relay on the assumption that galaxies and/or the
intra-cluster gas are reliable tracers of the potential
well or that all cluster components are in a state dy-
namical equilibrium. Gravitational lensing methods,
on the other hand, are completely independent of the
cluster dynamical state.
The goal of this study is to take advantage of
both kind of methods to identify whether a cluster
is in dynamical equilibrium or not by comparing in-
1SOAR
(esc@star.ucl.ac.uk).
2Laborat´ orio Nacional de Astrof´ ısica, CP 21, 37500-000,
Itajub´ a, MG, Brazil.
3Departamento de Astronomia, Instituto de Astronomia,
Geof´ ısica e Cincias Atmosf´ ericas, Universidade de S˜ ao Paulo,
Rua do Mat˜ ao 1226, Cidade Universit´ aria, 05508-090 S˜ ao
Paulo, Brazil (laerte@astro.iag.usp.br).
4Laboratoire d’Astrophysique de Marseille, Traverse du
Siphon, B.P.8 13376 Marseille Cedex 12, France (jean-
paul.kneib@oamp.fr).
5Universidad de Chile, Departamento de Astronom´ ıa,
Casilla 36-D, Santiago, Chile (luis@das.uchile.cl).
Telescope, Casilla603,La Serena,Chile
dicators of the dynamical mass: the velocity disper-
sion of the galaxies (σv) and the X-ray measured gas
temperature (TX) with weak-lensing equivalent in-
dicators. Disagreement between dynamical and non-
dynamical mass indicators can be interpreted as an
indication that the assumption of dynamical equilib-
rium is not valid.
2. RESULTS
In Cypriano et al. (2004) we have fitted the weak-
shear data with singular isothermal profiles (spheri-
cal and elliptical) from a sample of 24 Abell clusters
with X-ray luminosities higher than 5×1044h−2
s−1. Here we will present the results of the sam-
ple members with independent measurements of ei-
ther TX or σv. The mass related parameter that
came out from these fits is an equivalent of the line
of sight velocity dispersion (σSIS or σSIE, respec-
tively for spherical and elliptical profiles). Assum-
ing energy equipartition between cluster galaxies and
gas, we obtain TSIS(or TSIE) through the relation:
σ2= kT/µmH, where µ = 0.61 is the mean molec-
ular weight of the gas, mH is the hydrogen mass,
and k is the Boltzmann constant. In Figure 1 we
compare the actually measured dynamical mass in-
dicators with their weak-lensing counterparts.
It can be noted in Figure 1 that for most clus-
ters the weak-lensing results agree with the dynam-
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2 CYPRIANO ET AL.
Fig. 1. Comparison between the actually measured ICM temperatures (left panel) and velocity dispersions (right panel)
with those inferred by the fitting of isothermal profiles (spheric–SIS and elliptical–SIE) to the shear data. The squares
correspond to the spherical model and triangles to the elliptical. The error bars of the latter were suppressed for clarity.
The solid line is defined by TSIS,SIE = TX or σSIS,SIE = σv. The dotted and long dashed lines show the best-fit
obtained with the SIS and SIE models, respectively, when the origin is kept constant. The short dashed line indicates
TX = 8 keV. Clusters with higher temperatures show signals of dynamic activity.
ical data within 1.5 σ. However, this is not true for
the entire sample. For clusters with TX < 8 keV
(or equivalently σv < 1122 km s−1) the ratio be-
tween the dynamical estimator and its weak-lensing
equivalent is consistent with the expected value of 1.
Nonetheless, for clusters with TX> 8 keV strong dis-
agreements can be found. Among these cases three
clusters stands out: A2744, A1451 and A2163 (all
labeled in the plots). These clusters show tempera-
tures and velocity dispersions significantly different
from the lensing estimations, suggesting that they
should be dynamically active. Actually, detailed in-
dividual analysis of these clusters provide support
for this conclusion.
The study of the A2163 temperature map made
by Markevitch & Vikhlinin (2001) with Chandra
data shows at least two shocked regions and other
evidences that the central region of this cluster is
in a state of violent motion.
Valtchanov et al. (2002) describes A1451 as being
in the final stage of establishing equilibrium after
a merger event, whereas its high X-ray temperature
(13.4 keV) would be probably due to a shock oc-
curred recently. A2744 seems to be an exception,
since, contrarily to A1451 and A2163, which have
both dynamical indicators in excess compared to the
lensing estimates, this cluster has a temperature sig-
nificantly lower than that inferred from weak-lensing,
but a higher velocity dispersion.
The case of A2744 can be understood if,
as suggested bythe
In the same way,
dynamical analysisof
Girardi & Mezzeti (2001),
tures along the line-of-sight.
artificially increased upwards, since it has been
measured over two superimposed galaxy velocity
distributions. Gravitational lensing is sensitive to
the mass projected on the plane of the sky, therefore
its result is a weighted sum of the mass of both
structures. Finally, TX, which is obtained through
X-ray spectroscopy, will be biased in favour of just
one of the components, the brightest in X-rays,
giving a mass estimation lower than that obtained
with the other two methods.
In the hierarchical scenario, where most massive
structures are formed through the merger and ac-
cretion of less massive ones, massive galaxy clusters
should be forming at the present epoch. The results
presented here reinforces this idea by detecting dy-
namical activity among such systems, but not in the
less massive ones. One of the many consequences of
this result is that massive clusters can be used as
cosmological tools only if extra care is taken. Other-
wise the presence of non-relaxed structures can bias
the results.
there are two struc-
In this case σv is
REFERENCES
Cypriano, E. S., Sodr´ e Jr., L., Kneib, J.-P. & Campu-
sano, L. E. 2004, ApJ, 613, 95
Girardi, M & Mezzeti, M. 2001, ApJ, 548, 79
Markevitch, M. & Vikhlinin, A. 2001, ApJ, 563, 95
Valtchanov, I., Murphy, T., Pierre, M., Hunstead, R. &
L´ emonon, L. 2002, A&A, 392, 795