We use numerical simulations with hydrodynamics to demonstrate that a class of cold fronts in galaxy clusters can be attributed to oscillations of the dark matter distribution. The oscillations are initiated by the off-axis passage of a low-mass substructure. From the simulations, we derive three observable morphological features indicative of oscillations: 1) The existence of compressed isophotes; 2) The regions of compression must be alternate (opposite and staggered) and lie on an axis passing through the center of the cluster; 3) The gradient of each compression region must pass through the center of the cluster. Four of six clusters reported in the literature to have cold fronts have morphologies consistent with the presence of oscillations. The clusters with oscillations are A496, A1795, A2142, and RX J1720.1+2638. Galaxy clusters A2256 and A3667 are not consistent so the cold fronts are interpreted as group remnants. The oscillations may be able to provide sufficient energy to solve the cooling-flow problem and, importantly, provide it over an extended duration.

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Filamentary gas spanning the region between the galaxy clusters Abell 3391 and Abell 3395 has been detected using ASCA and ROSAT archived data. The gas has a minimum Ñux of 1.3 ] 10~12 ergs cm~2 s~1 (0.8È10 keV). Within this Ðlament resides a galaxy group for which a Ñux of (2.0^0.3) ] 10~13 ergs cm~2 s~1 (0.8È10 keV) is determined. An analysis using ray-tracing determines light scattered into the Ðlamentary region contributes 13% of the count rate. The structure in which the Ðlamentary gas resides is postulated to be a Ðlament aligned nearly lengthwise with the line of sight. IdentiÐcation of this structure as a Ðlament is based on the angular and redshift distribution of surrounding galaxies and clusters. The distribution is compatible with the structure being a quasi-linear structure tilted to the line of sight such that the ratio of the depth to the tangential distance is between 6 : 1 and 18 : 1. The Ðlamentary gas contains on the order of 1013 of gas, which is close to 2% of the total mass of the system. M _ Subject headings : galaxies : clusters : individual (A3391, A3395) È large-scale structure of universe È X-rays : di †use background È X-rays : galaxies : clusters

Sir2, a member of the sirtuin family of protein acylases, deacetylates lysine residues within many proteins and is associated with lifespan extension in a variety of model organisms. Recent studies have questioned the positive effects of Sir2 on lifespan in Drosophila. Several studies have shown that increased expression of the Drosophila Sir2 homolog (dSir2) extends life span while other studies have reported no effect on life span or suggested that increased dSir2 expression was cytotoxic. To attempt to reconcile the differences in these observed effects of dSir2 on Drosophila life span, we hypothesized that a critical level of dSir2 may be necessary to mediate life span extension. Using approaches that allow us to titrate dSir2 expression, we describe here a strong dose-dependent effect of dSir2 on life span. Using the two transgenic dSir2 lines that were reported not to extend life span, we are able to show significant life span extension when dSir2 expression is induced between 2 and 5-fold. However, higher levels decrease life span and can induce cellular toxicity, manifested by increased expression of the JNK-signaling molecule Puc phosphatase and induction of dnaJ-H. Our results help to resolve the apparently conflicting reports by demonstrating that the effects of increased dSir2 expression on life span in Drosophila are dependent upon dSir2 dosage.

We have analyzed spatially resolved spectra of the A754 cluster of galaxies obtained with ASCA. Through earlier observations with HEAO-1, Einstein, and ROSAT as well as optical studies, A754 has been established as the prototype system for a merger in progress. The combination of spectral and spatial resolution over a broad energy band provided by ASCA has set unprecedented constraints on the hydrodynamical effects of a cluster merger. We find significant gas temperature variations over the cluster face, indicating shock heating of the atmosphere during the merger. The hottest region, > 12 keV (90% confidence), is located in the region of the Northwest Galaxy clump though the entire region along the cluster axis appears to be hotter than the mean cluster temperature (∼9 keV). The cool, ≤5 keV, gas originally found with the HEAO1-A2 experiment, resides in the exterior of the cluster atmosphere and in plume of gas we identify with a stripped cool atmosphere of the infalling subcluster. We have also attempted to reconstruct an iron abundance map of this merging system. Though poorly constrained, no significant deviations of abundance from the mean value are apparent in the individual regions.A754 is the only cluster so far which shows the significant temperature pattern expected in a subcluster merger, in both the ROSAT (Henry & Briel 1995) and ASCA data, providing the first possibility to compare it with theoretical predictions. The cluster does not feature a hot peak accompanied by two hot lobes perpendicular to the cluster axis, predicted by hydrodynamic simulations of a head-on merger. The observed temperature and surface brightness maps suggest that the two colliding subunits have missed each other by about 1 Mpc, and are now moving perpendicular to the cluster axis in the image plane (as, e.g., in the simulations by Evrard et al. 1996).