The entire virial radius of the fossil cluster RXJ 1159 + 5531. II. Dark matter and baryon fraction

David A. Buote, Yuanyuan Su, Fabio Gastaldello, Fabrizio Brighenti

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9 Scopus citations


In this second paper on the entire virial region of the relaxed fossil cluster RXJ 1159+5531, we present a hydrostatic analysis of the azimuthally averaged hot intracluster medium (ICM) using the results of Su et al. For a model consisting of ICM, stellar mass from the central galaxy (BCG), and an NFW dark matter (DM) halo, we obtain a good description of the projected radial profiles of ICM emissivity and temperature that yield precise constraints on the total mass profile. The BCG stellar mass component is clearly detected with a K-band stellar mass-to-light ratio, M/LK=0.61±0.11 M/L, consistent with stellar population synthesis models for a Milky Way initial mass function. We obtain a halo concentration, c200 = 8.4 ± 1.0, and virial mass, M200 = (7.9±0.6)×1013M. For its mass, the inferred concentration is larger than most relaxed halos produced in cosmological simulations with Planck parameters, consistent with RXJ 1159+5531 forming earlier than the general halo population. The baryon fraction at r 200, ∫b,200=0.134±0.007, is slightly below the Planck value (0.155) for the universe. However, when we take into account the additional stellar baryons associated with non-central galaxies and the uncertain intracluster light (ICL), ∫b,200 increases by ≈0.015, consistent with the cosmic value and therefore no significant baryon loss from the system. The total mass profile is nearly a power law over a large radial range (∼0.2-10 R e), where the corresponding density slope obeys the - Re scaling relation for massive early-type galaxies. Performing our analysis in the context of MOND still requires a large DM fraction (82.0% ± 2.5% at r = 100 kpc) similar to that obtained using the standard Newtonian approach. The detection of a plausible stellar BCG mass component distinct from the NFW DM halo in the total gravitational potential suggests that ∼1014 M represents the mass scale above which dissipation is unimportant in the formation of the central regions of galaxy clusters.

Original languageEnglish
Article number146
JournalAstrophysical Journal
Issue number2
StatePublished - Aug 1 2016

Bibliographical note

Funding Information:
We thank the anonymous referee for several helpful comments and suggestions, including the suggestion that we make the comparison shown in the right panel of Figure 7. D.A.B. and Y. S. gratefully acknowledge partial support from the National Aeronautics and Space Administration under grants NNX13AF14G and NNX15AM97G issued through the Astrophysics Data Analysis Program. Partial support for this work was also provided by NASA through Chandra Award Nos. GO2- 13159X and GO4-15117X issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. The scientific results reported in this article are based in part on observations made by the Chandra Xray Observatory and by the Suzaku satellite, a collaborative mission between the space agencies of Japan (JAXA) and the USA (NASA). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Publisher Copyright:
© 2016. The American Astronomical Society. All rights reserved..


  • X-rays: galaxies: clusters
  • dark matter
  • galaxies: clusters: individual (RXJ 1159+5531)
  • galaxies: clusters: intracluster medium

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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