The Physics of Cluster Mergers

Clusters of galaxies generally form by the gravitational merger of smaller clusters and groups. Major cluster mergers are the most energetic events in the Universe since the Big Bang. Some of the basic physical properties of mergers will be discussed, with an emphasis on simple analytic arguments rather than numerical simulations. Semi-analytic estimates of merger rates are reviewed, and a simple treatment of the kinematics of binary mergers is given. Mergers drive shocks into the intracluster medium, and these shocks heat the gas and should also accelerate nonthermal relativistic particles. X-ray observations of shocks can be used to determine the geometry and kinematics of the merger. Many clusters contain cooling flow cores; the hydrodynamical interactions of these cores with the hotter, less dense gas during mergers are discussed. As a result of particle acceleration in shocks, clusters of galaxies should contain very large populations of relativistic electrons and ions. Electrons with Lorentz factors gamma~300 (energies E = gamma m_e c^2 ~ 150 MeV) are expected to be particularly common. Observations and models for the radio, extreme ultraviolet, hard X-ray, and gamma-ray emission from nonthermal particles accelerated in these mergers are described.Comment: 38 pages with 9 embedded Postscript figures. To appear in Merging Processes in Clusters of Galaxies, edited by L. Feretti, I. M. Gioia, and G. Giovannini (Dordrecht: Kluwer), in press (2001

Effects of Photosystem‐II‐Interfering Herbicides Atrazine and Bentazon on the Soybean Transcriptome

Atrazine and bentazon are both photosystem-II (PSII)–inhibiting herbicides that interfere with photosynthetic electron transport, provoking oxidative stress. While atrazine is lethal to soybean [ (L.) Merr.], bentazon does not kill soybean because of the capability of soybeans to metabolize the herbicide. Gene expression profiling was conducted using cDNA microarrays to understand the responses of soybeans to PSII interruption and concomitant stress caused by atrazine and bentazon by monitoring expression at 1, 2, 4, and 8 h after treatment (HAT). The microarray study revealed that 6646 genes were differentially expressed with high statistical significance over the experiment, with 88% of them sharing similar expression pattern between the atrazine and bentazon treatments. Many genes related to xenobiotic detoxification and antioxidation, such as cytochrome P450s, glutathione-S-transferases, superoxide dismutases, catalases, and tocophero cyclases, were induced by the herbicides. The study also discovered plants treated with bentazon started to recover between 4 and 8 HAT as reflected in the decreased amplitude of fold changes of most genes from 4 to 8 HAT. The 12% of the genes that were differentially expressed between atrazine and bentazon were largely related to cell recovery, such as genes related to ribosomal components

Characterisation of the D1 protein in a photosystem II mutant (LF-1) of Scenedesmus obliquus blocked on the oxidising side Evidence supporting non-processing of D1 as the cause of the lesion

AbstractThe D1 polypeptide of the photosystem II reaction centre in a mutant (LF-1) of Scenedesmus obliquus lacking a water-splitting manganese complex is approx. 1.5 kDa larger than that in the wild type but the D2 protein is the same size. The peptide profiles of D1 on partial digestion with papain or endoproteinase Lys-C indicate that the extra segment in the LF-1 protein is located at or near the carboxyl-terminus. The D1 proteins produced by in vitro translation of mRNA from wild type and LF-1 cells have an identical molecular mass to D1 from LF-1 thylakoids whereas D1 from wild-type thylakoids is 1.5 kDa smaller due to C-terminal processing. These results support the hypothesis that in the LF-1 mutant, the D1 protein is incorporated into the PS II reaction centre, but the C-terminal extension is not removed