Further evidence for a merger in Abell 2218 from an XMM-Newton observation
MPE Garching, Giessenbachstraße, 85748 Garching, Germany e-mail: gwp@mpe.mpg.de
Received:
4
June
2004
Accepted:
17
December
2004
The galaxy cluster Abell 2218, at 
, is well-known
for the discrepancy between mass estimates derived from X-ray and
strong lensing analyses. With the present XMM-Newton observation, we are
able to trace the gas density and temperature
profiles out to a radius of 
kpc (approximately
the virial radius of the cluster). The overall surface brightness
profile is well fitted over three orders of magnitude with a simple
β-model with a core radius of 
and 
. The
projected temperature profile declines steeply with radius (by 
), and is well described by a polytrope with parameters 
keV and 
. The temperature map
shows a pronounced peak in the central arcminute, where the
temperature rises by a factor of two (from 
to 
keV). The mass profile, calculated assuming hydrostatic equilibrium
and spherical symmetry, is best fitted with a King approximation
to an isothermal sphere, implying a dark matter distribution with a
central core, in contrast with the cusped cores
found in more obviously relaxed clusters. The X-ray
mass is approximately two times less than the strong lensing mass at
kpc, although the agreement between X-ray and weak
lensing mass measurements at larger radius (
kpc) is slightly better. While the X-ray total mass estimates can vary
by 30 per cent depending on the mass model, all measurements are
significantly lower than the corresponding total mass from optical
measurements. Given the X-ray results indicating considerable
disturbance of the intracluster gas, leading to a probable violation
of the assumption of hydrostatic equilibrium, and the observed
substructure in the optical, suggesting a line-of-sight merger, it is
unlikely that the different mass estimates of this cluster can be
reconciled, at least with standard modelling assumptions.
Key words: galaxies: clusters: individual: A2218 / galaxies: intergalactic medium / cosmology: observations / cosmology: dark matter / X-rays: galaxies: clusters
© ESO, 2005