Cristóbal Sifón, Felipe Menanteau, John P. Hughes, L. Felipe Barrientos, for the ACT collaboration
Dynamical Masses of Galaxy
Clusters Discovered with the
Sunyaev-Zel’dovich Effect
A large spectroscopic follow-up campaign of galaxy clusters, discovered via
the Sunyaev-Zel’dovich Effect (SZE) by the Atacama Cosmology Telescope
collaboration and largely carried out with the Gemini Multi-Object
Spectrograph, has provided the first dynamical mass measurements for
SZE-selected clusters.
Galaxy clusters are the most massive bound objects in the universe. Because of this, their
number density is a sensitive function of the matter content of the universe. Clusters are composed of three main constituents: stars (both in galaxies and outside of them); ionized gas
with typical temperatures of 106-107 Kelvin, known as the intracluster medium (ICM); and socalled “dark matter.” (The presence of dark matter in clusters has been firmly established. The
evidence dates back to the 1930s and the pioneering work of Fritz Zwicky at the California
Institute of Technology, when he applied the Virial Theorem to galaxy velocities in the Coma
cluster.) These three components have been exploited to discover and study clusters, most
typically through X-ray and optical observations.
The power of galaxy clusters as cosmological probes depends largely on our ability to correctly infer their masses. However, we often estimate cluster masses from scaling relations
that connect a given observable to mass itself. We usually obtain these relations through numerical simulations. Additionally, the mass function of galaxy clusters is a very steep one of
both mass and redshift. Therefore, the few massive clusters at high redshift give the strongest
weight to cosmological constraints.
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GeminiFocus
July2013