Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session G8: Plasma Astrophysics of Clusters of Galaxies |
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Sponsoring Units: DAP GPAP Chair: Steven Spangler, University of Iowa Room: Governor's Square 17 |
Sunday, May 3, 2009 8:30AM - 8:42AM |
G8.00001: GeV Gamma-ray Observations of Galaxy Clusters with the Fermi/LAT Keith Bechtol The Fermi Gamma Ray Space Telescope has steadily explored the gamma-ray sky since its launch on June 11, 2008. The Fermi Large Area Telescope (LAT) provides all-sky coverage in the energy range from 100 MeV to 300 GeV offering a new perspective of the high energy Universe. Recent radio observations of galaxy clusters motivate a search for populations of energetic particles confined by intercluster magnetic fields. Galaxy clusters could be the site of distinctive particle acceleration processes including acceleration along large scale structure formation shocks. Studies of how cosmic rays and magnetic fields interact with the intercluster medium will provide a better understanding of the pressure and energy budget of galaxy clusters. The previous gamma-ray space telescope, EGRET, placed upper limits on the non-thermal emission. We now present early results on galaxy clusters using data from the Fermi/LAT. [Preview Abstract] |
Sunday, May 3, 2009 8:42AM - 8:54AM |
G8.00002: MHD Turbulence, Dynamo, and the Origin of Magnetic Fields in Galaxy Clusters Hui Li, Hao Xu We present self-consistent cosmological magnetohydrodynamic (MHD) simulations that simultaneously follow the formation of a galaxy cluster and the magnetic field ejection by an active galactic nuclei (AGN). We find that the magnetic fields ejected by the AGN, though initially distributed in relatively small volumes, can be transported throughout the cluster and be further amplified by the intra-cluster medium (ICM) turbulence during the cluster formation process. The ICM turbulence is generated and sustained by the frequent mergers of smaller halos. A cluster-wide dynamo process is shown to exist in the ICM and amplify the magnetic field energy and flux. The total magnetic energy in the cluster can reach $\sim$ $10^{61}$ ergs while micro Gauss ($\mu$G) fields can distribute over $\sim$ Mpc scales throughout the whole cluster. This finding shows that magnetic fields from AGNs, being further amplified by the ICM turbulence through small-scale dynamo processes, can be the origin of cluster-wide magnetic fields. [Preview Abstract] |
Sunday, May 3, 2009 8:54AM - 9:06AM |
G8.00003: Are ICM Magnetic Fields Generated from Scratch by Cosmic Rays? Mikhail Medvedev, Olga Zakutnyaya The origin of the micro-Gauss magnetic fields in galaxy clusters is one of the outstanding problem of modern cosmology. The intra-cluster medium (ICM) plasma is not static, as is seen in cosmological simulations and deduced from observational data. The motions are turbulent and supersonic with a number of merger and accretion shocks. We propose here that cosmic rays (CR) accelerated by the shocks are a natural and inevitable source of magnetic fields which are produced due to the CR streaming motion via a Weibel-type plasma instability. We develop a self-similar model of a CR foreshock and demonstrate that, in contrast to the conventional lore, the generated magnetic fields (i) are large-scale (of order the shock curvature radius, $\sim$~tens of kpc or more) hence they are effectively decoupled from dissipation and are long-lived on the Hubble time and (ii) are strong enough, of order of a fraction of the CR pressure, to meet observational constraints. Unlike other shock-related models of the field generation (e.g., via the Bell instability or the Richtmeyer-Meshkov vorticity instability), our model does not require preexisting seed fields; the fields are generated in the ICM at essentially a few cluster light-crossing times. [Preview Abstract] |
Sunday, May 3, 2009 9:06AM - 9:18AM |
G8.00004: Anisotropic thermal conduction in cosmological cluster formation simulations Mateusz Ruszkowski, Ian Parrish, Marcus Brueggen We investigate the role of the magnetothermal instability (MTI) in the cosmological cluster formation simulations. Our simulations self-consistently incorporate the effects of the field amplification by the structure formation (i.e., gravitational collapse and shearing) and by anisotropic thermal conduction, as well as the effects of violent sloshing motions (e.g., due to mergers) that tend to slow down the field growth. We quantify the effects of these processes on the temperature and density profiles, the strength and topology of the magnetic fields as well as the effective thermal conduction in the intarcluster medium. [Preview Abstract] |
Sunday, May 3, 2009 9:18AM - 9:30AM |
G8.00005: Limits on Gas Evaporation from Galaxy Clusters Olga Zakutnyaya, Mikhail Medvedev Resent observations of a number of galaxy clusters using the Sunyaev-Zel'dovich effect indicate that about 1/3 of baryonic mass is missing from the hot intra-cluster medium (ICM), which is significantly larger than the fraction of stars and cool gas, which account for only about 10\%. Here we address the question whether the remaining $22\pm10\%$ can be accounted for by thermal evaporation of gas from clusters. We have found that evaporation can occur only from the cluster ``surface'', $r\sim r_{\rm vir}$, and not from it's interior. We evaluated particle diffusion through the magnetized ICM for several scenarios of ICM turbulence and found that diffusivity is suppressed by at least a factor of 100 or more, compared to the Spitzer value. Thus, only particles from radii $r\ga0.9r_{\rm vir}$ can evaporate. Diffusion of particles from inside the cluster, $r\la0.9r_{\rm vir}$, takes longer than the Hubble time. This lowers the cluster-averaged fraction of the evaporated hot gas to few percent or less. However, if the missing hot component {\it is indeed} due to evaporation, this strongly constrains the magnetic field structure in the cluster envelope, namely either (i) the gas is completely unmagnetized ($B\le10^{-21}$~gauss) in the cluster halo or (ii) the magnetic fields in the ICM are rather homogeneous and non-turbulent. [Preview Abstract] |
Sunday, May 3, 2009 9:30AM - 9:42AM |
G8.00006: Measuring Sunyaev-Zel'dovich Scaling Relations with APEX-SZ Amy Bender Accurately measuring the masses of galaxy clusters is critical to the precise constraint of cosmological parameters using cluster surveys. Detecting clusters with the Sunyaev-Zel'dovich effect (SZE) is extremely promising and the observed flux is theoretically shown to be an excellent proxy for total cluster mass. We present 150 GHz observations of the SZE taken using the APEX-SZ camera; a 330 element TES bolometer array mounted on the APEX telescope in northern Chile. We combine SZE and X-ray analysis for several clusters and compare relationships between the observable SZE flux and total cluster mass with expected values from theory and simulations. Our sample contains both relaxed and merging clusters over a wide redshift range allowing us to probe the dependence of our correlations on the dynamical and evolutionary state of the clusters. [Preview Abstract] |
Sunday, May 3, 2009 9:42AM - 9:54AM |
G8.00007: Supermassive Black Holes and Spiral Structure in Disk Galaxies Daniel Kennefick, Marc Seigar, Julia Kennefick, Claud Lacy Our collaboration has recently identified an interesting relation between supermassive black hole (SMBH) mass and the pitch angle of spiral arms in disk galaxies whose SMBH mass has been measured with some precision. I discuss this relation and the uses to which it might be put in permitting estimates of SMBH mass for distant normal galaxies for which only imaging data is available, as well as its possible significance for our understanding of galactic structure. [Preview Abstract] |
Sunday, May 3, 2009 9:54AM - 10:06AM |
G8.00008: Galactic archaeology in action space Robyn Sanderson Working in action space offers an instructive alternative view of the process of hierarchical assembly in galaxies, but performing the necessary canonical transformation formally requires both complete phase space information of a stellar population and knowledge of the correct galactic potential, neither of which is generally available. I use the approximate-action method pioneered by MacMillan and Binney (2008) to examine the remnant of a late-time merger in M31, which was modeled by Fardal et al. (2007). [Preview Abstract] |
Sunday, May 3, 2009 10:06AM - 10:18AM |
G8.00009: Galaxy Mergers with Adaptive Mesh Refinement: Star Formation and Hot Gas Outflow Ji-hoon Kim, John Wise, Tom Abel In hierarchical structure formation, galaxy mergers are frequent and known to affect galaxy properties dramatically. Because of the non-linear coupling between pc and Mpc scales, high-resolution simulations are indispensable to comprehend galactic interactions. To this end, we present the first adaptive mesh refinement (AMR) simulation of two merging, low mass, initially gas-rich galaxies (${2.0 \times10^{10} M_{\odot}}$ each), including star formation and feedback. With galaxies resolved by $\sim$$2 \times 10^7$ total computational elements, we achieved unprecedented resolution of the multiphase interstellar medium, finding that a widespread starburst occurs in the merging galaxies via shock-induced star formation. Using the high dynamic range of AMR we also follow the interplay between the galaxies and the embedding medium depicting how galactic outflows and a hot metal-rich halo form. These results demonstrate that AMR provides a powerful tool in understanding interacting galaxies. [Preview Abstract] |
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