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Register a new userNonthermal Pressure in the Outskirts of Abell 2142
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Clumping and turbulence are expected to affect the matter accreted onto the outskirts of galaxy clusters. To determine their impact on the thermodynamic properties of Abell 2142 we perform an analysis of the X-ray temperature data from XMM-Newton via our SuperModel, a state-of-the-art tool for investigating the astrophysics of the intracluster medium already tested on many individual clusters (since Cavaliere et al. 2009). Using the gas density profile corrected for clumpiness derived by Tchernin et al. (2016), we find evidence for the presence of a nonthermal pressure component required to sustain gravity in the cluster outskirts of Abell 2142, that amounts to about 30% of the total pressure at the virial radius. The presence of the nonthermal component implies the gas fraction to be consistent with the universal value at the virial radius and the electron thermal pressure profile to be in good agreement with that inferred from the SZ data. Our results indicate that the presence of gas clumping and of a nonthermal pressure component are both necessary to recover the observed physical properties in the cluster outskirts. Moreover, we stress that an alternative method often exploited in the literature (included Abell 2142) to determine the temperature profile k_BT = P_e/n_e basing on a combination of the Sunyaev-Zeldovich (SZ) pressure P_e and of the X-ray electron density n_e does not allow to highlight the presence of nonthermal pressure support in the cluster outskirts.
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The total mass derived from X-ray emission is biased low in a large number of clusters when compared with the mass estimated via strong and weak lensing. Suzaku and Chandra observations out to the virial radius report in several relaxed clusters steep temperature gradients that on assuming pure thermal hydrostatic equilibrium imply an unphysically decreasing mass profile. Moreover, the gas mass fraction appears to be inconsistent with the cosmic value measured from the CMB. Such findings can be interpreted as an evidence for an additional nonthermal pressure in the outskirts of these clusters. This nonthermal component may be due to turbulence stirred by residual bulk motions of extragalactic gas infalling into the cluster. Here we present a SuperModel analysis of Abell 1835 observed by Chandra out to the virial radius. The SuperModel formalism can include in the equilibrium a nonthermal component whose level and distribution are derived imposing that the gas mass fraction f_{gas} equals the cosmic value at the virial radius. Including such a nonthermal component, we reconstruct from X rays an increasing mass profile consistent with the hydrostatic equilibrium also in the cluster outskirts and in agreement at the virial boundary with the weak lensing value. The increasing f_{gas} profile confirms that the baryons are not missing but located at the cluster outskirts.
We report on a spectral study at radio frequencies of the giant radio halo in A2142 (z=0.0909), which we performed to explore its nature and origin. A2142 is not a major merger and the presence of a giant radio halo is somewhat surprising. We performed deep radio observations with the GMRT at 608 MHz, 322 MHz, and 234 MHz and with the VLA in the 1-2 GHz band. We obtained high-quality images at all frequencies in a wide range of resolutions. The radio halo is well detected at all frequencies and extends out to the most distant cold front in A2142. We studied the spectral index in two regions: the central part of the halo and a second region in the direction of the most distant south-eastern cold front, selected to follow the bright part of the halo and X-ray emission. We complemented our observations with a preliminary LOFAR image at 118 MHz and with the re-analysis of archival VLA data at 1.4 GHz. The two components of the radio halo show different observational properties. The central brightest part has higher surface brightess and a spectrum whose steepness is similar to those of the known radio halos, i.e. $alpha^{rm 1.78~GHz}_{rm 118~MHz}=1.33pm 0.08$. The ridge, which fades into the larger scale emission, is broader in size and has considerably lower surface brightess and a moderately steeper spectrum, i.e. $alpha^{rm 1.78~GHz}_{rm 118~MHz}sim 1.5$. We propose that the brightest part of the radio halo is powered by the central sloshing in A2142, similar to what has been suggested for mini-halos, or by secondary electrons generated by hadronic collisions in the ICM. On the other hand, the steeper ridge may probe particle re-acceleration by turbulence generated either by stirring the gas and magnetic fields on a larger scale or by less energetic mechanisms, such as continuous infall of galaxy groups or an off-axis merger.
We present VLA D-array HI observations of the RSCG42 and FGC1287 galaxy groups, in the outskirts of the Abell 1367 cluster. These groups are projected ~ 1.8 and 2.7 Mpc west from the cluster centre. The Arecibo Galaxy Environment survey provided evidence for HI extending over as much as 200kpc in both groups. Our new, higher resolution observations reveal that the complex HI features detected by Arecibo are in reality two extraordinary long HI tails extending for ~160 and 250 kpc, respectively, i.e., among the longest HI structures ever observed in groups of galaxies. Although in the case of RSCG42 the morphology and dynamics of the HI tail, as well as the optical properties of the group members, support a low-velocity tidal interaction scenario, less clear is the origin of the unique features associated with FGC1287. This galaxy displays an exceptionally long dog leg HI tail and the large distance from the X-ray emitting region of Abell 1367 makes a ram-pressure stripping scenario highly unlikely. At the same time a low-velocity tidal interaction seems unable to explain the extraordinary length of the tail and the lack of any sign of disturbance in the optical properties of FGC1287. An intriguing possibility could be that this galaxy might have recently experienced a high-speed interaction with another member of the Coma-Abell 1367 Great Wall. We searched for the interloper responsible for this feature and, although we find a possible candidate, we show that without additional observations it is impossible to settle this issue. While the mechanism responsible for this extraordinary HI tail remains to be determined, our discovery highlights how little we know about environmental effects in galaxy groups.
We present results obtained with a new XMM-Newton observation of A2142, a famous textbook example of cluster with multiple cold fronts, which has been studied in detail with Chandra but whose large scale properties are presented here for the first time. We report the discovery of a a new cold front, the most distant one ever detected in a galaxy cluster, at about one Mpc from the center to the SE. Residual images, thermodynamics and metal abundance maps are qualitatively in agreement with predictions from numerical simulations of the sloshing phenomenon. However, the scales involved are much larger, similarly to what recently observed in the Perseus cluster. These results show that sloshing is a cluster-wide phenomenon, not confined in the cores, which extends well beyond the cooling region involving a large fraction of the ICM up to almost half of the virial radius. The absence of a cool core and a newly discovered giant radio halo in A2142, in spite of its relaxed X-ray morphology, suggest that large scale sloshing, or the intermediate merger which caused it, may trigger Mpc-scale radio emission and may lead to the disruption of the cluster cool core
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