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Register a new userOn the nature of dark matter in the Coma Cluster
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Recent precise observations of the 2.7 K CMB by the Planck mission toward the Coma cluster are not in agreement with X-ray measurements. To reconcile both types of measuring techniques we suggest that unstable dark matter is the cause of this mismatch. Decaying dark matter, which gravitationally dominates the galaxy cluster, can affect the estimated hot plasma content, which is then missing in the measured SZ effect from exactly the same place in the sky. The model independent lifetime of dark matter decaying entirely to X-rays is estimated to be about 6x10^{24} sec; this lifetime scales down with the fraction of the radiatively decaying dark matter. In addition, it is shown that the potential of such dark matter investigations in space is superior to the largest volume Earth-bound dark matter decay searches. Other clusters might provide additional evidence for or against this suggestion.
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In this paper we study the effect of reacceleration provided by turbulences on electrons produced by dark matter (DM) annihilation in the Coma cluster. We use a simplified phenomenological model to describe the effect of the turbulences, and explore a limited subset of three possible DM models for neutralino particles with different mass and annihilation channel. We find that, for values of the annihilation cross section of the order of the upper limits found with Fermi-LAT measurements in astrophysical objects, and for conservative values of the boosting factor due to DM substructures, the reacceleration due to turbulences can enhance the radio emission produced by DM-originated electrons up to the level of the observed flux of the radio halo in Coma, for moderate reacceleration intensity in relatively short times. Therefore we conclude that, even if it is not possible to distinguish between the fits obtained in this paper because of the scattering present in the radio flux data, the electrons produced by DM annihilation can be possible seed electrons for the reacceleration, as well as secondary electrons of hadronic origin. A possible discriminant between these two classes of models is the flux produced in the gamma ray band, that in the case of DM-originated electrons should be more than two orders of magnitude smaller than the present Fermi-LAT upper limits, whereas in the hadronic case the expected gamma ray flux should be close to the value of present upper limits.
As part of the HST/ACS Coma Cluster Treasury Survey, we have undertaken a Keck/LRIS spectroscopic campaign to determine membership for faint dwarf galaxies. In the process, we discovered a population of Ultra Compact Dwarf galaxies (UCDs) in the core region of the Coma cluster. At the distance of Coma, UCDs are expected to have angular sizes 0.01 < R_e < 0.2 arcsec. With ACS imaging, we can resolve all but the smallest ones with careful fitting. Candidate UCDs were chosen based on magnitude, color, and degree of resolution. We spectroscopically confirm 27 objects as bona fide UCD members of the Coma cluster, a 60% success rate for objects targeted with M_R < -12. We attribute the high success rate in part to the high resolution of HST data and to an apparent large population of UCDs in Coma. We find that the UCDs tend to be strongly clustered around giant galaxies, at least in the core region of the cluster, and have a distribution and colors that are similar to globular clusters. These findings suggest that UCDs are not independent galaxies, but rather have a star cluster origin. This current study provides the dense environment datapoint necessary for understanding the UCD population.
We present a 4 deg^2 weak gravitational lensing survey of subhalos in the very nearby Coma cluster using the Subaru/Suprime-Cam. The large apparent size of cluster subhalos allows us to measure the mass of 32 subhalos detected in a model-independent manner, down to the order of 10^-3 of the virial mass of the cluster. Weak-lensing mass measurements of these shear-selected subhalos enable us to investigate subhalo properties and the correlation between subhalo masses and galaxy luminosities for the first time. The mean distortion profiles stacked over subhalos show a sharply truncated feature which is well-fitted by a Navarro-Frenk-White (NFW) mass model with the truncation radius, as expected due to tidal destruction by the main cluster. We also found that subhalo masses, truncation radii, and mass-to-light ratios decrease toward the cluster center. The subhalo mass function, dn/dln M_sub, in the range of 2 orders of magnitude in mass, is well described by a single power law or a Schechter function. Best-fit power indices of 1.09_-0.32^+0.42 for the former model and 0.99_-0.23^+0.34 for the latter, are in remarkable agreement with slopes of ~0.9-1.0 predicted by the cold dark matter paradigm. The tangential distortion signals in the radial range of 0.02-2Mpc/h from the cluster center show a complex structure which is well described by a composition of three mass components of subhalos, the NFW mass distribution as a smooth component of the main cluster, and a lensing model from a large scale structure behind the cluster. Although the lensing signals are 1 order of magnitude lower than those for clusters at z~0.2, the total signal-to-noise ratio, S/N=13.3, is comparable to, or higher, because the enormous number of background source galaxies compensates for the low lensing efficiency of the low lensing efficiency of the nearby cluster.
(ABRIDGED) We use high resolution (~0.1) F814W ACS images from the HST ACS Treasury survey of the Coma cluster at z~0.02 to study bars in massive disk galaxies (S0s), and in dwarf galaxies in the Coma core. Our study helps constrain the evolution of bars and disks in dense environments and provides a comparison point for studies in lower density environments and at higher redshifts. (1) We characterize the fraction and properties of bars in a sample of 32 bright (M_V <= -18, M_* > 10^9.5 M_sun) S0 galaxies, which dominate the population of massive disk galaxies in the Coma core. Measuring the S0 bar fraction must be handled carefully, as the results depend on the method used: the bar fraction for bright S0s in the Coma core is 50%+/-11%, 65%+/-11%, and 60%+/-11% for three methods of bar detection: strict ellipse fit criteria, relaxed ellipse fit criteria, and visual classification. (2) We compare the S0 bar fraction across different environments (Coma core, A901/902, Virgo). We find that the bar fraction among bright S0 galaxies does not show a statistically significant variation across environments spanning two orders of magnitude in galaxy number density (n~300-10,000 gal/Mpc^3). We speculate that the S0 bar fraction is not significantly enhanced in rich clusters because S0s in rich clusters are less prone to bar instabilities as they are dynamically hot and gas poor due to ram pressure stripping and accelerated star formation. In addition, high-speed encounters in rich clusters may be less effective than slow, strong encounters in inducing bars. (3) We analyze a sample of 333 faint (M_V > -18) dwarf galaxies in the Coma core. Using unsharp-masking, we find only 13 galaxies with bar and/or spiral structure. The paucity of disk structures in Coma dwarfs suggests that either disks are not common in these galaxies, or that any disks present are too hot to develop instabilities.
We use ACS data from the HST Treasury survey of the Coma cluster (z~0.02) to study the properties of barred galaxies in the Coma core, the densest environment in the nearby Universe. This study provides a complementary data point for studies of barred galaxies as a function of redshift and environment. From ~470 cluster members brighter than M_I = -11 mag, we select a sample of 46 disk galaxies (S0--Im) based on visual classification. The sample is dominated by S0s for which we find an optical bar fraction of 47+/-11% through ellipse fitting and visual inspection. Among the bars in the core of the Coma cluster, we do not find any very large (a_bar > 2 kpc) bars. Comparison to other studies reveals that while the optical bar fraction for S0s shows only a modest variation across low-to-intermediate density environments (field to intermediate-density clusters), it can be higher by up to a factor of ~2 in the very high-density environment of the rich Coma cluster core.
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