676 research outputs found
Studying the Nature of Dark Energy with Galaxy Clusters
We report on the status of our effort to constrain the nature of dark energy
through the evolution of the cluster mass function. Chandra temperature
profiles for 31 clusters from a local cluster sample are shown. The X-ray
appearance of the proto supermassive binary black hole at the center of the
cluster Abell 400 is described. Preliminary weak lensing results obtained with
Megacam@MMT for a redshift z=0.5 cluster from a distant cluster sample are
given.Comment: 5 pages, to appear in: Aschenbach, B., Burwitz, V., Hasinger, G.,
Leibundgut, B. (eds.), Relativistic Astrophysics and Cosmology - Einstein's
Legacy. ESO Astrophysics Symposia, Springer Verlag, Berlin, German
The 400d Galaxy Cluster Survey weak lensing programme: II: Weak lensing study of seven clusters with MMT/Megacam
Evolution in the mass function of galaxy clusters sensitively traces both the
expansion history of the Universe and cosmological structure formation. Robust
cluster mass determinations are a key ingredient for a reliable measurement of
this evolution, especially at high redshift. Weak gravitational lensing is a
promising tool for, on average, unbiased mass estimates. This weak lensing
project aims at measuring reliable weak lensing masses for a complete X-ray
selected sample of 36 high redshift (0.35<z<0.9) clusters. The goal of this
paper is to demonstrate the robustness of the methodology against commonly
encountered problems, including pure instrumental effects, the presence of
bright (8--9 mag) stars close to the cluster centre, ground based measurements
of high-z (z~0.8) clusters, and the presence of massive unrelated structures
along the line-sight. We select a subsample of seven clusters observed with
MMT/Megacam. Instrumental effects are checked in detail by cross-comparison
with an archival CFHT/MegaCam observation. We derive mass estimates for seven
clusters by modelling the tangential shear with an NFW profile, in two cases
with multiple components to account for projected structures in the
line-of-sight. We firmly detect lensing signals from all seven clusters at more
than and determine their masses, ranging from
to , despite the presence of nearby bright stars. We
retrieve the lensing signal of more than one cluster in the CL 1701+6414 field,
while apparently observing CL 1701+6414 through a massive foreground filament.
We also find a multi-peaked shear signal in CL 1641+4001. Shear structures
measured in the MMT and CFHT images of CL 1701+6414 are highly correlated.Comment: Accepted for publication in Astronomy & Astrophysics; A&A 546, A7
Cosmic Structure Traced by Precision Measurements of the X-Ray Brightest Galaxy Clusters in the Sky
The current status of our efforts to trace cosmic structure with 10^6
galaxies (2MASS), 10^3 galaxy clusters (NORAS II cluster survey), and precision
measurements for 10^2 galaxy clusters (HIFLUGCS) is given. The latter is
illustrated in more detail with results on the gas temperature and metal
abundance structure for 10^0 cluster (A1644) obtained with XMM-Newton.Comment: 4 pages; to be published in the Proceedings of the Conference: The
Emergence of Cosmic Structure, College Park, MD (2002), editors: S.S. Holt
and C. Reynolds; also available at http://www.reiprich.ne
Time-Dependence of the Mass Accretion Rate in Cluster Cooling Flows
We analyze two time-dependent cluster cooling flow models in spherical
symmetry. The first assumes that the intracluster gas resides in a static
external potential, and includes the effects of optically thin radiative
cooling and mass deposition. This corresponds to previous steady-state cooling
flow models calculated by White & Sarazin (1987). Detailed agreement is found
between steady-state models and time-dependent models at fixed times in the
simulations. The mass accretion rate is found either to increase or remain
nearly constant once flows reach a steady state. The time rate of change of the
accretion rate is strongly sensitive to the value of the mass deposition
parameter q, but only mildly sensitive to the ratio beta of gravitational
binding energy to gas temperature. We show that previous scaling arguments
presented by Bertschinger (1988) and White (1988) are valid only for mature
cooling flows with weak mass deposition (q ~< 1). The second set of models
includes the effects of a secularly deepening cluster potential and secondary
infall of gas from the Hubble flow. We find that such heating effects do not
prevent the flows from reaching a steady state within an initial central
cooling time.Comment: 22 pages (AASTeX) with 16 EPS figures; accepted for publication in
The Astrophysical Journa
Convective cores in galactic cooling flows
We use hydrodynamic simulations with adaptive grid refinement to study the
dependence of hot gas flows in X-ray luminous giant elliptical galaxies on the
efficiency of heat supply to the gas. We consider a number of potential heating
mechanisms including Type Ia supernovae and sporadic nuclear activity of a
central supermassive black hole. As a starting point for this research we use
an equilibrium hydrostatic recycling model (Kritsuk 1996). We show that a
compact cooling inflow develops, if the heating is slightly insufficient to
counterbalance radiative cooling of the hot gas in the central few kiloparsecs.
An excessive heating in the centre, instead, drives a convectively unstable
outflow. We model the onset of the instability and a quasi-steady convective
regime in the core of the galaxy in two-dimensions assuming axial symmetry.
Provided the power of net energy supply in the core is not too high, the
convection remains subsonic. The convective pattern is dominated by buoyancy
driven large-scale mushroom-like structures. Unlike in the case of a cooling
inflow, the X-ray surface brightness of an (on average) isentropic convective
core does not display a sharp maximum at the centre. A hybrid model, which
combines a subsonic peripheral cooling inflow with an inner convective core,
appears to be stable. We also discuss observational implications of these
results.Comment: 14 pages, LaTeX requires mn.sty, 12 postscript figures including 3
colour figures, MNRAS accepted; mpeg movies available from
http://www.mpa-garching.mpg.de/Hydro/CoolHyd/coolhyd.htm
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