409 research outputs found
Brightest Cluster Galaxies and Core Gas Density in REXCESS Clusters
We investigate the relationship between brightest cluster galaxies (BCGs) and
their host clusters using a sample of nearby galaxy clusters from the
Representative XMM Cluster Structure Survey (REXCESS). The sample was imaged
with the Southern Observatory for Astrophysical Research (SOAR) in R band to
investigate the mass of the old stellar population. Using a metric radius of
12h^-1 kpc, we found that the BCG luminosity depends weakly on overall cluster
mass as L_BCG \propto M_cl^0.18+-0.07, consistent with previous work. We found
that 90% of the BCGs are located within 0.035 r_500 of the peak of the X-ray
emission, including all of the cool core (CC) clusters. We also found an
unexpected correlation between the BCG metric luminosity and the core gas
density for non-cool core (non-CC) clusters, following a power law of n_e
\propto L_BCG^2.7+-0.4 (where n_e is measured at 0.008 r_500). The correlation
is not easily explained by star formation (which is weak in non-CC clusters) or
overall cluster mass (which is not correlated with core gas density). The trend
persists even when the BCG is not located near the peak of the X-ray emission,
so proximity is not necessary. We suggest that, for non-CC clusters, this
correlation implies that the same process that sets the central entropy of the
cluster gas also determines the central stellar density of the BCG, and that
this underlying physical process is likely to be mergers.Comment: 16 pages, 8 figures, accepted Astrophysical Journa
Three-dimensional Magnetohydrodynamic Simulations of Buoyant Bubbles in Galaxy Clusters
We report results of 3D MHD simulations of the dynamics of buoyant bubbles in
magnetized galaxy cluster media. The simulations are three dimensional
extensions of two dimensional calculations reported by Jones & De Young (2005).
Initially spherical bubbles and briefly inflated spherical bubbles all with
radii a few times smaller than the intracluster medium (ICM) scale height were
followed as they rose through several ICM scale heights. Such bubbles quickly
evolve into a toroidal form that, in the absence of magnetic influences, is
stable against fragmentation in our simulations. This ring formation results
from (commonly used) initial conditions that cause ICM material below the
bubbles to drive upwards through the bubble, creating a vortex ring; that is,
hydrostatic bubbles develop into "smoke rings", if they are initially not very
much smaller or very much larger than the ICM scale height. Even modest ICM
magnetic fields with beta = P_gas/P_mag ~ 10^3 can influence the dynamics of
the bubbles, provided the fields are not tangled on scales comparable to or
smaller than the size of the bubbles. Quasi-uniform, horizontal fields with
initial beta ~ 10^2 bifurcated our bubbles before they rose more than about a
scale height of the ICM, and substantially weaker fields produced clear
distortions. On the other hand, tangled magnetic fields with similar, modest
strengths are generally less easily amplified by the bubble motions and are
thus less influential in bubble evolution. Inclusion of a comparably strong,
tangled magnetic field inside the initial bubbles had little effect on our
bubble evolution, since those fields were quickly diminished through expansion
of the bubble and reconnection of the initial field.Comment: 20 pages, 12 figures. Accepted for publication in The Astrophysical
Journa
Cold gas in the Intra Cluster Medium: implications for flow dynamics and powering optical nebulae
We show that the mechanical energy injection rate generated as the
intra-cluster medium (ICM) flows around cold clouds may be sufficient to power
the optical and near infra-red emission of nebulae observed in the central
regions of a sample of seven galaxy clusters. The energy injection rate is
extremely sensitive to the velocity difference between the ICM and cold clouds,
which may help to explain why optical and infra-red luminosity is often larger
than expected in systems containing AGNs. We also find that mass recycling is
likely to be important for the dynamics of the ICM. This effect will be
strongest in the central regions of clusters where there is more than enough
cold gas for its evaporation to contribute significantly to the density of the
hot phase.Comment: 8 pages, 2 figures, accepted for publication in MNRA
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