Searching for the missing baryons in clusters

Abstract

Observations of clusters of galaxies suggest that they contain fewer baryons (gas plus stars) than the cosmic baryon fraction. This "missing baryon" puzzle is especially surprising for the most massive clusters, which are expected to be representative of the cosmic matter content of the universe (baryons and dark matter). Here we show that the baryons may not actually be missing from clusters, but rather are extended to larger radii than typically observed. The baryon deficiency is typically observed in the central regions of clusters (∼0.5 the virial radius). However, the observed gas-density profile is significantly shallower than the mass-density profile, implying that the gas is more extended than the mass and that the gas fraction increases with radius. We use the observed density profiles of gas and mass in clusters to extrapolate the measured baryon fraction as a function of radius and as a function of cluster mass. We find that the baryon fraction reaches the cosmic value near the virial radius for all groups and clusters above ∼5 × 10 13 h −1 72 M ⊙ . This suggests that the baryons are not missing, they are simply located in cluster outskirts. Heating processes (such as shock-heating of the intracluster gas, supernovae, and Active Galactic Nuclei feedback) likely contribute to this expanded distribution. Upcoming observations should be able to detect these baryons.cosmology | hot intracluster gas C lusters of galaxies, the largest virialized systems in the universe, are powerful tools in constraining cosmology and tracing the large-scale structure of the universe (1-4, and references therein). The large mass of clusters (∼10 14 to 10 15 h −1 72 M ⊙ ) implies that their contents-dark and baryonic matter-have been accreted from very large regions of ∼10 comoving Mpc, and therefore should be representative of the mean matter content of the universe; on these large scales there are no clear mechanisms to separate dark and baryonic matter (e.g., refs. 5 and 6). The strong gravitational potential of clusters also implies that baryons cannot easily escape from these systems. Therefore, clusters are expected to retain the cosmic baryon fraction, the relative fraction of baryons to total matter on large scales. This basic expectation of a representative baryon fraction in clusters was used in 1993 (6) to suggest that the mass-density of the universe must be low, since the observed baryon fraction in clusters was considerably larger than expected for a critical density universe. Most of the baryons in clusters reside in the X-ray emitting hot intracluster gas, which approximately traces the cluster gravitational potential dominated by dark matter. The rest of the baryons are in the luminous galaxies and in isolated stars that comprise the small amount of faint diffuse intracluster light (ICL).A puzzle has been raised, however, over the last few years: Detailed X-ray observations from Chandra, XMM-Newton, and ROSAT suggest that the cluster baryon fraction (gas plus stars relative to total mass) is considerably low...

Turbulence in the Supermodel: Mass Reconstruction With Nonthermal Pressure for A1835

“…Using the SDSS photometric data for A1835 and A1689, the authors found that the hot regions are associated with a filamentary structure, while the cold regions contact low-density regions outside the clusters. Finally, the increasing f gas profile at r 0.3R reported in Figure 3 confirms the conclusion of Rasheed et al (2010) that the baryons are not missing. They are simply located in the most peripheral regions of the clusters likely for the heating processes (such as shock heating of the gas, supernovae, and AGN feedback) that cause the ICP to expand or hinder its inflow.…”

“…The total baryon fraction (f b = f gas + f stars ) and its evolution with the redshift are used to constrain cosmological parameters since it is believed to be representative of the universe (e.g., White et al 1993;Metzler & Evrard 1994;Ettori et al 2009). Current studies have shown that the cluster baryon fraction f b derived at r 500 is lower than the ratio Ω b /Ω M measured from the cosmic microwave background (CMB) by several experiments (Afshordi et al 2007;Umetsu et al 2009;Vikhlinin et al 2006;Arnaud et al 2007;Sun et al 2009;Komatsu et al 2011) raising the question of where the missing baryons are allocated (Rasheed et al 2010). To address this issue, Landry et al (2012) recently used Chandra X-ray observations to measure the gas mass fraction for a complete sample of massive clusters in the redshift range 0.15-0.30 from the brightest cluster sample (Ebeling et al 1998;Dahle 2006).…”

Are observations of the galaxy cluster A1689 consistent with a neutrino dark matter scenario?

“…It has recently been solved. For a relatively large sample of groups and clusters of galaxies the gas fraction has been measured within r500 and it is concluded that the "missing" baryons are present as X-ray gas located in the outskirts of the clusters (Rasheed et al 2011). Our gas data have allowed an extrapolation which confirms this picture.…”

Quantifying the effect of baryon physics on weak lensing tomography

“…Furthermore, in our model, we made a rather ad hoc assumption that the gas that is removed from the central regions is on the outskirts of the haloes. Although this seems plausible (see, for example Rasheed, Bahcall & Bode 2010) it is not clear how accurately one can observe the distribution of the gas on the outskirts of the haloes. Because of those observational limitations, we expect that an accurate description of the baryonic feedback will have to rely in part on simulations.…”