|
|
|
Old research pages
I'm keeping these old research pages here for the benefit of anyone
who might arrive here from an external page. They are no longer
updated. Please visit my new web site at
http://sipapu.astro.illinois.edu/~ricker/.
Eventually some content from these pages will migrate to the new site.
Galaxy Cluster Formation and Evolution
Physical questions of interest
|
X-ray surface brightness contours from an image
of the Coma cluster made with ROSAT
(White et al. 1993).
Significant substructure is evident, probably due to recent mergers.
|
Clusters of galaxies are the largest gravitationally bound objects in
the universe. Their formation and evolution are complex, large-scale
events, involving many interacting physical processes, which span a
significant fraction of the age of the universe. In addition to being
intrinsically complex and interesting, their properties permit
us to constrain many quantities of cosmological interest, such as the
ratio of baryonic to dark matter and the rate of expansion of the universe.
(An excellent older review of clusters of galaxies is available
here.)
Clusters consist of three components: galaxies, gas, and dark matter.
The galaxies themselves contribute the least, at most a few percent,
to the total mass (Sarazin 1988).
The remainder
consists of diffuse, hot gas at densities between 10-4 and
10-2 cm-3 and temperatures of
107-108 K (the
intracluster medium, or ICM) and an unseen, presumably collisionless
component which is needed to explain the gravitational stability of clusters
(the dark matter). Through its X-ray emission, the ICM, which contributes
from 10% to 30% of the
total mass needed to bind most clusters, yields the highest-quality
observational data concerning a cluster's dynamical state.
|
Simulated X-ray observation of a simulated merger between
clusters having a mass ratio of 1:3. Contours indicate X-ray
surface brightness, while the colors indicate X-ray temperature,
with blue indicating cooler gas and red indicating hotter gas.
Click on the image to download a 300 DPI JPEG version.
|
Clusters are not static entities. X-ray images of clusters observed with
Einstein showed that more than 22% of clusters contain substructure
which could be interpreted as evidence of a nonvirialized state
(Forman & Jones 1994).
ROSAT images with higher spatial resolution
confirmed this result, showing that many clusters have bimodal
cores, nonconcentric isophotes, and elongated isophotes
(eg. White, Briel, & Henry 1993;
Briel et al. 1991;
Elbaz, Arnaud, & Bohringer 1995).
Indeed, hierarchical models
of large-scale structure formation lead us to expect that clusters
should have formed through mergers of smaller objects. Simulations
show that these mergers
disturb the ICM in ways consistent with the observed structure in
cluster X-ray images (Roettiger, Burns, & Loken 1996;
Ricker 1998), and they
predict that spatially resolved X-ray temperatures should be
especially sensitive to merger shocks. Evidence for the latter
has been found in a few clusters for which X-ray temperature maps
are available from ASCA observations
(Markevitch et al. 1998).
The advent of Chandra and XMM,
with their combination of high sensitivity,
high angular resolution, and high spectral resolution, will
provide much more solid evidence for these shocks.
The Constellation-X satellite will enable us to go
even further: by resolving iron K-shell emission lines in the X-ray
spectrum of the ICM, we will be able to map abundance gradients
and line-of-sight gas velocities, placing even better constraints
on clusters' dynamical histories.
The large-scale motions produced by cluster mergers
can be long-lasting; merger simulations suggest that clusters
can require as much as a Hubble time to recover from a major merger
(Roettiger, Burns, & Loken 1993;
Schindler & Muller 1993;
Pearce, Thomas, & Couchman 1994;
Ricker 1998).
However, the degree to which the ICM is turbulent following a merger
and how this turbulence depends on the parameters of the collision
are still unknown. Turbulent motions have the potential to very
significantly affect both the dynamical state of a cluster and a
number of observed cluster properties. For example,
the constant-density gas cores seen in most clusters may derive
some of their support against gravity from merger-driven turbulent pressure.
Turbulent motions can act to inhibit cooling flows by promoting
mixing with high-entropy gas shocked by mergers.
They may play a role in the stripping of gas from galaxies in
merging clusters. Turbulent motions and merger-driven shocks may
also be responsible for accelerating high-energy cosmic-ray particles
in clusters.
In collaboration with Craig
Sarazin at the University of Virginia and
Don Lamb at the University
of Chicago, I use three-dimensional numerical simulations
to study the dynamical evolution of clusters and to extract physical
information from observations of unrelaxed clusters.
We perform simulations of isolated mergers of idealized clusters
in order to understand the physical processes which are important in
individual mergers.
Numerical technology
To address cluster problems I have developed a hydrodynamical code
using the piecewise-parabolic method (PPM)
(Colella & Woodward 1984)
to solve the 3D Eulerian gas equations and a multigrid
method (Brant 1977)
to solve the Poisson equation for the gravitational
field. The hydrodynamical code, formerly called PPMnD,
is linked via the Poisson solver to a
particle-mesh N-body code (Hockney &
Eastwood 1988) written
by Scott
Dodelson of the Fermilab Theoretical
Astrophysics Group. This allows us to follow the evolution of
gas which is gravitationally coupled to
a collisionless matter component such as dark matter or galaxies.
The combined code, called COSMOS, can use a
nonuniform grid together with isolated or periodic boundary conditions
for both the gas and dark matter.
Please see the COSMOS home page for
more information about COSMOS.
Images and movies
Publications
 |
The Impact of Galaxy Cluster Mergers on Cosmological Parameter Estimation from Surveys of the Sunyaev-Zel'dovich Effect
Wik, D. R., Sarazin, C. L., Ricker, P. M., and Randall, S. W. Ap. J. 680 17 (2008)
|
|
Morphology of Rising Hydrodynamic and Magnetohydrodynamic Bubbles from Numerical Simulations
Robinson, K., Dursi, L. J., Ricker, P. M., et al. Ap. J. 601 621 (2004)
|
|
Simulations of Hot Bubbles in the ICM
Gardini, A., & Ricker, P. M. Mod. Phys. Lett. A 19 2317 (2004)
|
|
The Effect of
Merger Boosts on the Luminosity, Temperature, and
Inferred Mass Functions of Clusters of Galaxies
Randall, S. W., Sarazin, C. L., and Ricker, P. M.
Ap. J. 577 579 (2002)
|
|
Chandra
Observations of Abell 85: Merger of the South Subcluster
Kempner, J. C., Sarazin, C. L., and Ricker, P. M.
Ap. J. 579 236 (2002)
|
|
Off-Axis
Cluster Mergers: Effects of a Strongly Peaked Dark Matter
Profile
Ricker, P. M., and Sarazin, C. L.
Ap. J. 561 621 (2001)
|
|
Off-Axis Cluster Mergers
(HTML,
Postscript)
Ricker, P. M., and Sarazin, C. L.
In Proceedings of the 20th Texas Symposium on Relativistic
Astrophysics, J. Craig Wheeler and Hugo Martel, eds.
(Melville, NY: AIP Press, 2001), 484
|
|
Large-Scale
Simulations of Clusters of Galaxies
(HTML,
Postscript)
Ricker, P. M., Calder, A. C., Dursi, L. J., Fryxell, B., Lamb, D. Q.,
MacNeice, P., Olson, K., Rosner, R., Timmes, F. X., Truran, J. W.,
Tufo, H. M., and Zingale, M.
In Proceedings of the VII International Workshop on
Advanced Computing and Analysis Techniques in Physics Research
(ACAT 2000), P. C. Bhat and M. Kasemann, eds.
(Melville, NY: AIP Press, 2001), 316
|
|
The Detection of Off-Center Cluster Mergers
(Postscript
part 1,
part 2)
Ricker, P. M. and Sarazin, C. L.
BAAS 194 (1999)
|
|
Off-Center
Collisions between Clusters of Galaxies
Ricker, P. M.
Ap. J. 496 670 (1998)
|
References
- Brant, A. Math. Comp. 31 333 (1977)
- Briel, U. G. et al. A&A 246 L10 (1991)
- Colella, P. and Woodward, P. J. Comp. Phys. 54 174 (1984)
- Elbaz, D., Arnaud, M., and Bohringer, H. A&A 293 337
(1995)
- Forman, W. and Jones, C. in Cosmological Aspects of X-Ray Clusters
of Galaxies, ed. W. C. Seitter (Kluwer), 1994
- Hockney, R. W. and Eastwood, J. W. Computer Simulation using
Particles (IOP), 1988
- Markevitch, M. Ap. J. 504 27 (1998)
- Markevitch, M., Forman, W. R., Sarazin, C. L., and Vikhlinin, A.
Ap. J. 503 77 (1998)
- Pearce, F. R., Thomas, P. A., and Couchman, H. M. P. MNRAS
268 953 (1994)
- Ricker, P. M. Ap. J. 496 670 (1998)
- Roettiger, K., Burns, J., and Loken, C. Ap. J. 407 L53
(1993)
- ________ Ap. J. 473 651 (1996)
- Sarazin, C. L. X-Ray Emissions from Clusters of Galaxies
(CUP), 1988
- Schindler, S. and Muller, E. A&A 272 137 (1993)
- White, S. D. M., Briel, U. G., and Henry, J. P. MNRAS 261
L8 (1993)
|
|
|
