Our research revolves around X-ray astronomy and observational cosmology,
studying the physics of galaxy clusters, supermassive black
holes, dark matter and dark energy.
Galaxy clusters and cosmology
Galaxy clusters are the largest objects in the Universe, spanning 10
million light years across and containing as much mass as a million,
billion suns. Our research examines the physics of these remarkable
systems using the best available multi-wavelength data. We also use
the observed distribution and internal properties of clusters, and
their variation with mass and redshift, to probe the natures
of dark matter, the weakly interacting yet dominant matter
component of the Universe, and dark energy, the driving force behind
cosmic acceleration. Measurements of galaxy clusters are highly
complementary to other cosmological probes.
Most of the normal, baryonic matter in the Universe is in gaseous
form. Within galaxy clusters, gravity squeezes this gas, heating it to
10-100 million degrees and causing it to shine brightly at X-ray
wavelengths. As well as revealing, in exquisite detail, the
thermodynamic and metal enrichment histories of galaxy clusters, X-ray
observations made with satellite observatories like
the Chandra X-ray
and Suzaku allow us
to determine their masses, and to separately measure the baryonic and
dark, non-baryonic matter within them.
General Relativity predicts that mass concentrations bend light rays
passing near to them in a phenomenon known as gravitational
lensing. This can both magnify and distort the images of
background galaxies. The effects of lensing can be detected in the
statistical appearance of background objects seen through clusters
(weak lensing). Occasionally, lensing can also lead to large
distortions (strong lensing). Our group is at the forefront
of using deep, wide-field lensing measurements to measure cluster
masses and constrain cosmology.
For further discussion of this work, see e.g.:
Cosmological Parameters from Observations of Galaxy Clusters
Allen, Steven W.; Evrard, August E.; Mantz, Adam B.
Annual Review of Astronomy and Astrophysics, vol. 49, issue 1, pp. 409-470
Weighing the Giants I: Weak Lensing Masses for 51 Massive Galaxy
Clusters - Project Overview, Data Analysis Methods, and Cluster Images
von der Linden, Anja; Allen, Mark T.; Applegate, Douglas E.; Kelly,
Patrick L.; Allen, Steven W.; Ebeling, Harald; Burchat, Patricia R.;
Burke, David L.; Donovan, David; Morris, R. Glenn; Blandford, Roger;
Erben, Thomas; Mantz, Adam
Submitted to MNRAS
Baryons at the Edge of the X-ray-Brightest Galaxy Cluster
Simionescu, Aurora; Allen, Steven W.; Mantz, Adam; Werner, Norbert;
Takei, Yoh; Morris, R. Glenn; Fabian, Andrew C.; Sanders, Jeremy S.;
Nulsen, Paul E. J.; George, Matthew R.; Taylor, Gregory B.
Science, Volume 331, Issue 6024, pp. 1576- (2011).
von der Linden et al., 2012, MNRAS
Black holes, jets and galaxy formation
Classical models for galaxy formation predict that the largest
galaxies should be much bigger and brighter than we observe. A large
and ubiquitous power source must prevent gas from cooling and forming
vast numbers of additional stars. Our team has argued that
supermassive black holes in the centers of galaxies are responsible
for this suppressed star formation. The regions close to these black
holes are commonly observed to pump out huge amounts of energy in
relativistic jets. These jets inflate cavities in the surrounding gas
and drive enormous supergalactic shock waves. Using X-ray and radio
observations, we have shown that this `feedback' occurs with a high
and near-universal efficiency, sufficient to solve the galaxy
formation problem. However, significant questions relating to the
`duty-cycle' of black hole heating, the accretion process, jet
formation and black hole growth remain.
For further discussion of this work, see e.g.:
On the thermodynamic self-similarity of the nearest, most relaxed, giant ellipticals
Werner, N.; Allen, S. W.; Simionescu, A.
accepted for publication in MNRAS.
A wide Chandra view of the core of the Perseus cluster
Fabian, A. C.; Sanders, J. S.; Allen, S. W.; Canning, R. E. A.;
Churazov, E.; Crawford, C. S.; Forman, W.; Gabany, J.;
Hlavacek-Larrondo, J.; Johnstone, R. M.; Russell, H. R.; Reynolds, C.
S.; Salomé, P.; Taylor, G. B.; Young, A. J.
Monthly Notices of the Royal Astronomical Society, Volume 418, Issue
4, pp. 2154-2164.
Feedback under the microscope - I. Thermodynamic structure and AGN-driven shocks in M87
Million, E. T.; Werner, N.; Simionescu, A.; Allen, S. W.; Nulsen, P.
E. J.; Fabian, A. C.; Böhringer, H.; Sanders, J. S.
Monthly Notices of the Royal Astronomical Society, Volume 407, Issue
4, pp. 2046-2062.
Simionescu A., Allen S. W., et al.; NASA; March 2011.
We are involved in the development of both powerful, new X-ray
satellites such as ASTRO-H,
and large, new ground- and space-based optical and near-infrared
telescopes such as the Large Synoptic
and Euclid. Together, the
projects will push back the boundaries of our understanding of the
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