Locating the two black holes in NGC 6240

Mergers play an important role in galaxy evolution and are key to understanding the correlation between central-black hole mass and host-galaxy properties. We used the new technology of adaptive optics at the Keck II telescope to observe NGC 6240, a merger between two disk galaxies. Our high-resolution near-infrared images, combined with radio and x-ray positions, revealed the location and environment of two central supermassive black holes. Each is at the center of a rotating stellar disk, surrounded by a cloud of young star clusters. The brightest of these young clusters lie in the plane of each disk, but surprisingly are seen only on the disks' receding side.     

Assessment of critical loads and their exceedance on European forests using a one-layer steady-state model

Critical loads for N, S and total acidity, and amounts by which they are exceeded by present atmospheric loads, were derived for coniferous and deciduous forests in Europe using the one-layer steady-state model START. Results indicated that present acid loads exceed critical values in approximately 45% of the forested area i.e. 52% of all coniferous forests and 33% of all deciduous forests. The area exceeding critical loads was nearly equal for N (50%) and S (52%). However, the maximum exceedances were much higher for S (up to 12000 mol_c ha^?1 yr^?1 in Czechoslovakia, Poland and Germany) than for N (up to 3500 mol_c ha^?1 yr^?1 in the Netherlands, Belgium and Germany). Furthermore, the critical N loads derived refer to the risk of increased vegetation changes. Higher values, i.e. lower exceedances, were found for N when it was related to an increased risk in forest vitality decrease. The uncertainty in the area exceeding critical loads was estimated to be about ±50% of the given value. This is mainly due to uncertainties in the chemical criteria that have been used. However, despite the uncertainties involved it is clear that large exceedances in critical N and S loads occur in Western and Central Europe. This coincides with the area where a decrease in forest vitality has been reported.     

Calculating critical loads for acidity with the simple mass balance method

The critical load for acid deposition to forest soils, groundwater and surface water can be calculated using a simple mass balance model, taking into account all sources of acidity and alkalinity in the system. The model is very simple to apply, and the major difficulty lies primarily in the estimation of reasonable values for the required input data such as the weathering rate. Methods for its estimation are indicated here. Examples are given for the application of the simple mass balance method (SMB) to Dutch and Swedish forest soils, lakes and shallow groundwater. This work outlines and explains the actual application of the consept as it is being carried out regionally for all European nations.     

Assessment of critical loads and their exceedance on Dutch forests using a multi-layer steady-state model

Critical acid loads for Dutch forests were derived using a multi-layer steady-state model that includes canopy interactions, nutrient cycling, mineral weathering and N transformations. Values were calculated for combinations of 12 tree species and 23 soil types for a 10×10 km grid. Critical acid loads thus derived increased with decreasing soil depth. Nearly 90% of the values varied approximately between 1500 and 4000 mol_c ha^?1 yr^?1 at 10 cm soil depth and between 750 and 2000 mol_c ha^?1 yr^?1 at the bottom of the rootzone. Separate critical loads calculated for N and S at the bottom of the rootzone varied between approximately 300 and 1000 mol_c ha^?1 yr^?1 for N and between 150 and 1250 mol_c ha^?1 yr^?1 for S. Using deposition data of 1990, a median reduction of the deposition by approximately 75% was calculated to achieve the critical loads at the bottom of the rootzone. The overall uncertainty in this value was estimated to be about 10%, although it can be much larger for specific soil types such as clay and peat soils. For N a larger reduction deposition percentage was calculated than for S, especially for coniferous forests with a high present N input.     

Alloantigen recognition is preceded by nonspecific adhesion of cytotoxic T cells and target cells

T-cell receptors bind antigens only when the antigens are exposed on the cell surface. This can be studied best in the interaction of cytolytic T lymphocytes (CTL) with target cells because the recognition and binding event can be separated from the lytic phase. Studies with CTL clones specific for HLA-A2 and HLA-B7 demonstrated that conjugates of CTL's and target cells can be formed in the absence of specific antigen recognition. Furthermore, T-cell receptor and target antigen cannot interact unless there is conjugate formation. This indicates that nonspecific conjugate formation between CTL's and target cells precedes the recognition of specific antigen by the T-cell receptor.     

Recognition of HLA-A2 by cytotoxic T lymphocytes after DNA transfer into human and murine cells

A gene coding for the major histocompatibility antigen HLA-A2 was transferred into human HLA-A2 negative M1 cells and murine L cells. Following transfection, these cells expressed molecules at the cell surface that are biochemically indistinguishable from HLA-A2 antigens on the human cell line JY from which the HLA-A2 gene was isolated. The M1A2 cells were recognized and lysed by a cytolytic T-cell clone specific for HLA-A2. The transfected L cells which express HLA-A2 in association with human beta 2-microglobulin were not lysed by this T-cell clone. The specific cytolysis of M1A2 cells could be inhibited by monoclonal antibodies to HLA-A2, and monoclonal antibodies to T3, T8, and LFA-1 on cytotoxic T lymphocytes. These results suggest that killing by allospecific T cells requires HLA-A2 antigens as well as other species-specific structures on the target cell surface.     

The Hedgehog pathway promotes blood-brain barrier integrity and CNS immune quiescence

The blood-brain barrier (BBB) is composed of tightly bound endothelial cells (ECs) and perivascular astrocytes that regulate central nervous system (CNS) homeostasis. We showed that astrocytes secrete Sonic hedgehog and that BBB ECs express Hedgehog (Hh) receptors, which together promote BBB formation and integrity during embryonic development and adulthood. Using pharmacological inhibition and genetic inactivation of the Hh signaling pathway in ECs, we also demonstrated a critical role of the Hh pathway in promoting the immune quiescence of BBB ECs by decreasing the expression of proinflammatory mediators and the adhesion and migration of leukocytes, in vivo and in vitro. Overall, the Hh pathway provides a barrier-promoting effect and an endogenous anti-inflammatory balance to CNS-directed immune attacks, as occurs in multiple sclerosis.     

Scandium clusters in fullerene cages

The production and spectroscopic characterization of fullerene-encapsulated metal-atom clusters is reported. In particular, both solution and solid-state electron paramagnetic resonance (EPR) spectra of Sc(3)C(82) have been obtained. ScC(82) also gives an EPR spectrum, but Sc2Cn species-the most abundant metallofullerenes in the mass spectrum-are EPR-silent even though Sc(2) is EPR-active in a rare-gas matrix at 4.2 K. The results suggest that the three scandium atoms in Sc(3)C(82) form an equilateral triangle-as was previously suggested for Sc(3) molecules isolated in a cryogenic rare-gas matrix. The spectrum of ScC(82) has features similar to those found earlier for LaC(82) and YC(82), suggesting that it can also be described as a +3 metal cation within a -3 fullerene radical anion. An implication of this work is that production of macroscopic quantities of clustercontaining fullerenes may make possible the fabrication of exotic new structures with regular arrays of metal-atom clusters isolated in fullerene molecules, resulting in a new type of host/guest nanostructured material.     

Growth and hydrolase profiles can be used as characteristics to distinguish Aspergillus niger and other black aspergilli

Wild type Aspergillus niger isolates from different biotopes from all over the world were compared to each other and to the type strains of other black Aspergillus species with respect to growth and extracellular enzyme profiles. The origin of the A. niger isolate did not result in differences in growth profile with respect to monomeric or polymeric carbon sources. Differences were observed in the growth rate of the A. niger isolates, but these were observed on all carbon sources and not specific for a particular carbon source. In contrast, carbon source specific differences were observed between the different species. Aspergillus brasiliensis is the only species able to grow on D-galactose, and A. aculeatus had significantly better growth on Locus Bean gum than the other species. Only small differences were found in the extracellular enzyme profile of the A. niger isolates during growth on wheat bran, while large differences were observed in the profiles of the different black aspergilli. In addition, differences were observed in temperature profiles between the black Aspergillus species, but not between the A. niger isolates, demonstrating no isolate-specific adaptations to the environment.These data indicate that the local environment does not result in stable adaptations of A. niger with respect to growth profile or enzyme production, but that the potential is maintained irrespective of the environmental parameters. It also demonstrates that growth, extracellular protein and temperature profiles can be used for species identification within the group of black aspergilli.