Issue |
A&A
Volume 431, Number 2, February IV 2005
|
|
---|---|---|
Page(s) | 405 - 413 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20041925 | |
Published online | 04 February 2005 |
Simulating the soft X-ray excess in clusters of galaxies
1
Dipartimento di Astronomia dell'Universitá di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: clm@bao.ac.cn
2
Department of Astronomy, Beijing Normal University, Beijing 100875, PR China
3
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
4
INFN - National Institute for Nuclear Physics, Trieste, Italy
5
INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
6
Dipartimento di Fisica Generale “Amedeo Avogadro”, Universitá degli Studi di Torino, Torino, Italy
7
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Strasse 1, Garching bei München, Germany
8
Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
9
INAF, Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
10
Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, 35122 Padova, Italy
Received:
30
August
2004
Accepted:
6
October
2004
The detection of an excess of soft X-ray or Extreme Ultraviolet
(EUV) radiation, above the thermal contribution from the hot
intracluster medium (ICM), has been a controversial subject ever since
the initial discovery of this phenomenon. We use a large-scale
hydrodynamical simulation of a concordance ΛCDM model, to
investigate the possible thermal origin of such an excess in a set of
20 simulated clusters having temperatures in the range 1-7 keV.
Simulated clusters are analysed by mimicking the observational
procedure applied to ROSAT-PSPC data, which for the first time showed
evidence for the soft X-ray excess: we compare the low-energy
(e.g. [ 0.2-1] keV) part of the spectrum of each cluster with that
predicted for a plasma having temperature and metallicity as computed
after weighting by the emissivity in a harder band (e.g., [ 1-2] keV). For
distances to the cluster center we detect a
significant excess in most of the simulated clusters, whose relative
amount changes from cluster to cluster and, for the same
cluster, by changing the projection direction. In about 30 per cent of
the cases the soft X-ray flux is measured to be at least 50 per cent
larger than predicted by the one-temperature plasma model. We find
that this excess is generated in most cases within the
virialized regions of the cluster. It mainly comes from low-entropy and
high-density gas associated with merging sub-halos, rather than from
diffuse warm gas. Only in a few cases does the excess arise from
fore/background groups observed in projection, while no evidence is
found for a significant contribution from gas lying within
large-scale filaments. We compute the distribution of the relative
soft excess, as a function of the distance to the cluster center , and
compare it with the observational result by Bonamente et al. ([CITE])
for the Coma cluster. As in the observations, we find that the
relative excess increases with the distance from the cluster center,
with no significant excess detected for
. However,
an excess as large as that reported for the Coma cluster at scales
is found to be rather unusual in
our set of simulated clusters.
Key words: cosmology: diffuse radiation / galaxies: clusters: general / X-rays: galaxies: clusters / methods: numerical
© ESO, 2005
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