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Acta Polytechnica Vol. 51 No. 2/2011

XMM Observations of Metal Abundances in Galaxy Clusters

L. Lovisari, S. Schindler, W. Kapferer

Abstract

The hot gas that fills the space between galaxies in clusters is rich in metals. Due to their large potential well, galaxy
clusters accumulate metals over the whole history of the cluster, and retain important information on cluster formation
and evolution.
Wederive detailedmetallicity maps for a sample of 5 clusters, observedwithXMM-Newton, to study the distribution

of metals in the Intra-Cluster Medium (ICM). We show that even in relaxed clusters the distribution of metals shows
many inhomogeneities with several maxima separated by low metallicity regions. We also found a deviation from the
expected temperature-metallicity relation.

Keywords: galaxies, cluster, general – galaxies, abundances – X-ray, galaxies, cluster, intergalactic Medium.

Introduction
Since the first X-ray observations of the 7 keV iron
line feature in the 1970’s we have known that the
intra-cluster medium (ICM) contains not only pri-
mordial elements but also heavy elements. As heavy
elements are only produced in stars which reside
mainly in galaxies, the enriched material must have
been ejected into the ICM by the member galaxies.
Due to the large potential wells of galaxy clusters
they retain all the enriched material. This makes
them excellent laboratories for the study of nucle-
osynthesis and of the chemical enrichment history of
the universe. Because the gas transfer affects the evo-
lution of galaxy and galaxy cluster, it is important to
know when and how the enrichment takes place.

The components in a galaxy cluster interact with
each other in many different ways. A study of the
distribution of the ejected metals can therefore give
us important information on the mechanisms that
transported the enriched gas into the ICM.

Several processes have been proposed for ex-
plaining the observed enrichment in the ICM: ram-
pressure stripping, galactic winds, galaxy-galaxy in-
teractions, AGN outflows, intra-cluster supernovae,
and others. Simulations show an inhomogeneous
distribution of the metals independent of the en-
richment processes [1]. Nevertheless, AGN outflows
and galaxy-galaxy interactions can add metals to the
ICM [2]. Simulations suggest that the metal enrich-
ment of the ICM is primarily due to galactic winds
and ram-pressure stripping. A detailed comparison
between the enrichment due to galactic winds and
due to ram-pressure stripping revealed that these two
processes yield different metal distributions and a dif-
ferent time dependence of the enrichment [3]. In
massive clusters, ram-pressure stripping provides a

much more centrally concentrated distribution than
galactic winds. This is because galactic winds can be
suppressed in the cluster center while ram-pressure
stripping is most efficient there due to the fact that
the ICM density as well as the galaxies velocities are
larger in the cluster center [3].

X-ray spectra are the only measure for the metal-
licity of the ICM. The metallicity is derived mainly
by measuring the equivalent width of the iron line
once the continuum (almost entirely due to thermal
bremsstrahlung) is known. With the first genera-
tion of satellites it was just possible to determine
the radial metallicity profiles [4]. With deep obser-
vations of bright clusters of galaxies by the Chandra
and XMM-Newton satellites it is now possible to ex-
tract the metallicities in certain regions of a galaxy
cluster and to construct X-ray weighted metallicity
maps [5, 6, 7, 8, 9, 10, 11, 12, 13, 14].

Metallicity maps

In order to study the distribution of metals, we pre-
pared adaptively binned abundance maps for a sam-
ple of five clusters (Centaurus, A2029, A496, S159-03
and Hydra A). Appropriately cleaned data sets for
all three XMM-Newton EPIC cameras, with point
sources removed, were used to create the source spec-
tra. A high statistic is required in order to obtain a
metallicity measurement with good accuracy. Thus,
to ensure an acceptable error also in the outskirts of
the clusters we set a minimum required count number
(∼ 5 000 source counts per region in the 0.3–10 keV
band) for proceeding with the spectral fit. In order
to model the emission from a single (or multi) tem-
perature plasma we fit the spectra with an APEC
(+APEC) model multiplied by the Galactic column
density fixed at the Galactic values. The spectral re-

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Acta Polytechnica Vol. 51 No. 2/2011

Fig. 1: left panels: Metallicity maps based on spectra from all three EPIC cameras, right panels: Plot of abundances
against temperature for each bin

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Acta Polytechnica Vol. 51 No. 2/2011

gions for the map were selected following the method
presented in [14], which we can summarize as follows:
a square region centered on the X-ray peak was de-
fined to include the area with high surface brightness.
The region size of the pixels was optimized to be as
small as possible by splitting it into horizontal or ver-
tical segments through its center, while including at
least 5 000 source counts.

The metallicity distribution appears clearly non-
spherical. In Fig. 1 (left panels) we show 3 examples
of the obtained metallicity maps. For Centaurus and
Sérsic 159-03, there is a peak in the center and then
it decreases in the outskirts, while A496 (as A2029
and Hydra A) shows high metallicity clumps both in
the center and in the outskirts. Several clumps are
not significant but most of them deviate significantly
(99 %c.l) from the average profile. We note that,
since Centaurus is a very low redshift (z = 0.011 4),
in these observations we are looking in the very cen-
tral part of the cluster (r < 200 kpc) compared with
the others clusters, and this could explain its differ-
ent shape. On the other hand, Sérsic, for which we
map the metal distribution for more than 350 kpc, we
observe the same shape as Centaurus. Several max-
ima are visible in the metal distribution, which are
not associated with the cluster center. From simula-
tions [15, 16], we know that the maxima are typically
at places where galaxies just have lost a lot of gas due
to ram pressure. Since the gas lost by galaxies is not
mixed immediately with the ICM at the place where
we observe a metal blob, we should also observe a low
temperature due to the fact that the gas in galaxies
is cooler than the ICM. We therefore produced tem-
perature maps of the clusters with the same spatial
resolution obtained for the metal maps, and then we
plotted the abundance of bins against their tempera-
ture, see Fig. 1 (right panels). Since we are searching
for cool high-metallicity clumps due to the ejection
of gas from galaxies and not the cool high-metallicity
bins found in the cool cores, we did not plot the inner
bins. Apart from Centaurus, we see a deviation from
the expected temperature-metallicity relation. Most
of the deviating points corresponds to the locus of
low metallicity and low kT. The deviation could be
due to the iron bias effect that cause an underes-
timation of the metallicity when we attempt to fit
with a single temperature model a plasma that is,
in fact, characterized by a combination of different
temperatures [17, 18]. On the other hand, in the
range of temperature between 3 and 4 keV, the in-
verse iron bias effect, that cause an overestimation
of the metal abundance [19], can play an important
role. The combination of these two effects could ex-
plain the large scatter in the distribution of Fig. 1. It
is also possible that the ejected gas, with T < 1 keV
and metallicity in the range 0.5–1.5 [20, 21], will be

heated up to the temperature of the surrounding gas
(ICM) on a shorter time-scale than the time-scale of
metal mixing. In this case, after a while we should ob-
serve a region of high metallicity (not yet dispersed)
and high temperature (heated up at ICM tempera-
ture). Another possible explanation for the spread
in the distribution could be related to the number of
intracluster supernovae. During ram-pressure strip-
ping events, many stars in fact form in the tail of
stripped gas. The stars evolve and explode as SNe
directly in the ICM and they can enrich the ICM
very efficiently. In this case we should see clumps
of high metallicity (due to SNe explosions) and high
temperature. Obviously, more complex heating and
cooling processes are at work, thus the simple picture
of stripped gas does not hold.

Conclusion
Based on XMM-Newton observations, we have stud-
ied the spatial distribution of metal abundances in
a sample of 5 relaxed clusters. We found that
even for relaxed clusters the distribution of metals
is clearly non-spherical. It appears very inhomoge-
neous, with several maxima separated by low metal-
licity regions. Moreover, we found a deviation from
the temperature-metallicity anti-correlation that can
be partially explained by the iron bias and inverse
iron bias effect. However, a set of simulations that
takes into account thermal conduction should be per-
formed to better understand the physics of the Intra-
Cluster Medium.

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Lorenzo Lovisari
Sabine Schindler
Wolfgang Kapferer
E-mail: Lorenzo.Lovisari@uibk.ac.at
Institute of Astro-and Particle Physics
Technikerstrasse 25, A-6020 Innsbruck, Austria

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