Abstract. Intergalactig dust in galaxy clusters, recently detected in the central Coma cluster by ISO far infra-red observations, may present substantial opacity to extreme ultraviolet (EUV) and soft X-ray photons. Low-energy X-ray photons in clusters of galaxies are produced by a hot intra-cluster medium (ICM) and oftentimes, in addition, by a yet unidentified soft excess source. EUV and soft X-ray radiation from the central regions of many galaxy clusters is often depleted with respect to the predictions from the hot ICM alone, while at large radii soft excess emission is detected above the hot ICM radiation. A scenario is here proposed whereby in the centers of some galaxy clusters intergalactic dust is responsible for intrinsic EUV and soft X-ray extinction, or for the lack or lesser soft excess emission with respect to outer cluster regions

M Bonamente

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Mem. S.A.It. Vol. 74, 181
c© SAIt 2003 Memorie della
Extreme ultra-violet and soft X-ray extinction
by dust in clusters of galaxies
M. Bonamente
University of Alabama in Huntsville, Huntsville, AL, U.S.A.
e-mail: bonamem@email.uah.edu
Abstract. Intergalactig dust in galaxy clusters, recently detected in the central
Coma cluster by ISO far infra-red observations, may present substantial opacity
to extreme ultraviolet (EUV) and soft X-ray photons. Low-energy X-ray photons
in clusters of galaxies are produced by a hot intra-cluster medium (ICM) and of-
tentimes, in addition, by a yet unidentified soft excess source. EUV and soft X-ray
radiation from the central regions of many galaxy clusters is often depleted with
respect to the predictions from the hot ICM alone, while at large radii soft excess
emission is detected above the hot ICM radiation. A scenario is here proposed
whereby in the centers of some galaxy clusters intergalactic dust is responsible
for intrinsic EUV and soft X-ray extinction, or for the lack or lesser soft excess
emission with respect to outer cluster regions.
Key words. EUV and soft X-ray emission – dust extinction – galaxy clusters
1. Extreme ultraviolet and soft
X-rays from galaxy clusters and
the cluster soft excess
phenomenon
Clusters of galaxies are strong emitters of
X-rays, which originate from a very diffuse
(n ∼ 10−3 cm−3) and hot (T∼ 108 K)
phase of the intra-cluster medium (ICM)
with overall mass budgets of ∼ 1013 − 1014
Msun. The hot ICM emission is mainly free-
free bremsstrahlung, as nearly all species
are fully ionized at such temperatures, and
it is more suitably detected above ∼ 1 keV.
The extreme ultraviolet (EUV) and soft
X-ray bands around 0.1-0.4 keV offers a
Send offprint requests to: M. Bonamente
unique window to investigate the presence
of other emitting/absorbing phases of the
ICM.
Recently, pointed observations with
the Extreme Ultraviolet Explorer (EUVE),
ROSAT and BeppoSAX missions detected
excess EUV and soft X-ray radiation, above
the contribution from the hot phase of the
ICM, from several galaxy clusters (Sérsic
159-03, Bonamente et al. 2001a; Coma
and Virgo, Lieu et al. 1996a,b; Bonamente
et al. 2001b; A1795, Mittaz et al. 1998;
A2199, Lieu et al. 1999; Kaastra et al.
1999; A3571 and A3558, Bonamente et al.
2001c), indicating that the cluster soft ex-
cess phenomenon may be a common fea-
ture of clusters. In Fig. 1 it is shown the
soft excess emission of A2199 from a deep
182 Bonamente: Dust extinction in clusters
EUVE observation, plotted as fractional
excess above the hot ICM predictions. It is
clear that the central regions do not show
the excess, in reality excess absorption is
found. The excess emission is confined to
large distances from the cluster’s center
(Lieu et al. 2000), and similar distribu-
tion of the soft excess is also found in other
sources like Virgo (Fig. 2), where the cen-
ter has now only a modest excess, and sig-
nificantly larger fractional excess is found
at larger distances from the cluster’s cen-
ter (e.g., Bonamente et al. 2001b).
The excess EUV and soft X-ray radia-
tion is explained as either a second ther-
mal phase of the ICM at lower tempera-
tures (a ‘warm’ phase at T∼ 106 K), or
as a non-thermal Inverse-Compton emis-
sion due to scattering of microwave back-
ground photons with a population of rel-
ativistic electrons (e.g., Lieu et al. 1999;
Bonamente et al. 2001a). These findings on
EUV and soft X-ray emission from galaxy
clusters call for an interpretation of the lack
or lesser soft excess emission near the clus-
ter center found in many sources (as, e.g.,
in Virgo or A2199, see Fig. 1; similar results
are also displayed by other sources). As the
hot ICM distribution has its density peak
near the cluster’s center, the radiative cool-
ing time may oftentimes become shorter
than the cluster’s age, and there significant
cooling of the hot phase may be expected
(e.g., Fabian 1988). Presence of ‘cold’ gas
(T ≤ 104 K) may then be responsible for
the central absorption of EUV and soft X-
ray radiation, although definitive proof of
its presence has not yet been found, despite
a more than decade-long search (Fabian
et al. 2001). In this paper it is investi-
gated the alternative possibility of EUV
and soft X-ray extinction by intra-cluster
dust, whose presence is revealed by its far
infra-red thermal emission.
2. Infra-red emission from
intra-cluster dust
Intergalactic dust at T ∼ 20 K (Dwek et
al. 1990) has the peak of its black-body
Fig. 1. EUV (∼ 65-190 eV) fractional excess
emission of A2199 from a EUVE/DS obser-
vation. Fractional excess is defined as η =
(p−q)/q, where p is detected EUV flux, and q is
predicted EUV flux from the hot ICM. Excess
is found at large distances from the center.
Fig. 2. Soft X-ray (∼ 0.2-0.4 keV) fractional
excess emission of the Virgo cluster form a
ROSAT/PSPC observation.
emission at ∼ 150 µm. Extended far infra-
red emission from the central Coma clus-
ter was detected by ISOPHOT at 120 and
185 µm (Stickel et al. 1998), implying that
∼ 108 − 109 Msun of intergalactic dust
is present within a ≤ 0.4 Mpc diameter.
Earlier, presence of intracluster dust was
Bonamente: Dust extinction in clusters 183
surmised by the extinction of ultraviolet
objects behind Coma (Boyle et al. 1998;
Dwek et al. 1990), although IRAS had
failed to detect any FIR emission. Similarly,
upper limits on the dust mass near the cen-
ter of other galaxy clusters are found to
be typically ≤ 108 Msun by optical, infra-
red and sub-mm observations (Maoz 1995;
Annis & Jewitt 1993), and IR emission as-
sociated with central galaxies in clusters
was reported for a significant fraction of
the observed clusters (Cox et al. 1995;
Bregman et al. 1990).
3. EUV and soft X-ray extinction by
dust grains
The details of scattering and absorption
properties of dust grains strongly depend
on their size and chemical composition
(e.g., Spitzer 1978). Mie theory calcula-
tions of EUV and soft X-ray opacities of C,
Fe and Si grains (Cruise 1993) show that
the scattering cross sections of ∼ 1−0.1µm
grains can be as large as ∼ 2 times their ge-
ometrical cross section. Considering a to-
tal mass of M=109 Msun of dust grains
with an average radius of a=0.2 µm and a
grain density of ρ=1 g cm−3, distributed
over a volume of diameter 0.4 Mpc and
with a scattering cross section σ = πa2,
it is readily found that a EUV/soft X-
ray photon has a mean free path of ∼
0.1 Mpc when crossing such volume of
space. In addition, absorption will also con-
tribute significantly to the extinction, es-
pecially for grains whose composition in-
cludes elements (like C) with strong ab-
sorption properties of EUV and soft X-ray
radiation. Dust grains may be ejected in
the intergalactic space by galactic winds or
other mass loss process with an approxi-
mate power law distribution of sizes (a−k),
the size a ranging from 1 µm to a few
Å (e.g., Dwek et al. 1990). Survival of dust
grains in the harsh intergalactic environ-
ment is controlled by the efficiency of sput-
tering by the hot gas. For gas temperatures
above 107 K, Draine & Salpeter (1979a,b)
showed that sputtering is a function of gas
density n and grain size a,
τsputt ∼ 10
6a(µm)
n(cm−3)
.
For a ∼ 0.2 µm and n ∼ 10−3 − 10−4
cm−3, τsputt ∼ 108 − 109 years, typically
shorter than the cluster’s age. This implies
that the dust must be periodically or con-
tinuously injected in the cluster environ-
ment, although presently the mechanism of
dust formation itself is not completely un-
derstood. Clearly dust particles with very
small sizes are very rapidly distroyed by the
interaction with the hot gas, and are not ex-
pected to survive for long periods of time.
4. Discussion and conclusions
Combined observations of galaxy clusters
in the far infra-red and EUV/soft X-ray
bands indicate that dust masses of the or-
der of 108−109 Msun in the central few 100
kpc may result in substantial extinction of
the low energy X-ray emission. This phe-
nomenon may on one hand explain deficit
EUV and soft X-ray fluxes with respect to
the hot ICM predictions observed in some
cases, and on the other explain the increas-
ing trend of the cluster soft excess of other
galaxy clusters. In fact, excess EUV and
soft X-ray radiation is seldom present in the
centers of clusters, and the excess compo-
nent is more often visible at large distances
(≥ 200 kpc) from the center. Evidence
for soft X-ray dust absorption was found
by Arnaud & Mushotzky (1998) through
analysis of BBXRT X-ray data, where the
complex spectrum of the Perseus cluster
seemed to be best explained with a dust
absorption model.
Acknowledgements. The author is pleased to
thank M. Busso and M. Marengo for insight-
ful discussions on this topic, and Prof. P.
Maffei for introducing and motivating him to
Astronomy.
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Extreme ultra-violet and soft X-ray extinction by dust in clusters of galaxies

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Extreme ultra-violet and soft X-ray extinction by dust in clusters of galaxies

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