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528 results on '"Johnstone, R. M."'

112. Chandra imaging of the X-ray core of Abell 1795

Author

Fabian, A. C., primary, Sanders, J. S., additional, Ettori, S., additional, Taylor, G. B., additional, Allen, S. W., additional, Crawford, C. S., additional, Iwasawa, K., additional, and Johnstone, R. M., additional

Published
2001
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137. Detection of optical coronal emission from 10.

Author

Canning, R. E. A., Fabian, A. C., Johnstone, R. M., Sanders, J. S., Crawford, C. S., Hatch, N. A., and Ferland, G. J.

Subjects
OPTICAL detectors, CORONAL holes, GALAXY clusters, ASTRONOMICAL observations, EMISSION-line galaxies, X-rays, LOW temperatures
Abstract

We report a detection erg s) of the optical coronal emission line [Fe ] λ6374 and upper limits of four other coronal lines using high-resolution VIMOS spectra centred on NGC 4696, the brightest cluster galaxy in the Centaurus cluster. Emission from these lines is indicative of gas at temperatures between K, so traces the interstellar gas in NGC 4696. The rate of cooling derived from the upper limits is consistent with the cooling rate from X-ray observations (∼; however, we detect twice the luminosity expected for [Fe ] λ6374 emission, at 10 K, our lowest temperature probe. We suggest this emission is due to the gas being heated rather than cooling out of the intracluster medium. We detect no coronal lines from [Ca ], which are expected from the K gas seen near the centre in X-rays with Chandra. Calcium is, however, likely to be depleted from the gas phase on to dust grains in the central regions of NGC 4696. [ABSTRACT FROM AUTHOR]

Published
2011
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141. Multiwavelength observations of serendipitous Chandra X-ray sources in the field of A 2390.

Author

Crawford, C. S., Gandhi, P., Fabian, A. C., Wilman, R. J., Johnstone, R. M., Barger, A. J., and Cowie, L. L.

Subjects
ARTIFICIAL satellites, GALAXIES
Abstract

We present optical spectra and near-infrared imaging of a sample of 31 serendipitous X-ray sources detected in the field of Chandra observations of the A 2390 cluster of galaxies. The sources have 0.5–7 keV fluxes of (0.6–8)×10[sup -14] erg cm[sup -2] s[sup -1] and lie around the break in the 2–10 keV source counts. They are therefore typical of sources dominating the X-ray Background in that band. 12 of the 15 targets for which we have optical spectra show emission lines at a range of line luminosities, and half of these show broad lines. These active galaxies and quasars have soft X-ray spectra. Including photometric redshifts and published spectra, we have redshifts for 17 of the sources, ranging from z∼0.2 up to z∼3 , with a peak between z=1–2 . 10 of our sources have hard X-ray spectra indicating a spectral slope flatter than that of a typical unabsorbed quasar. Two hard sources that are gravitationally lensed by the foreground cluster are obscured quasars, with intrinsic 2–10 keV luminosities of (0.2–3)×10[sup 45] erg s[sup -1] , and absorbing columns of N[sub H]>10[sup 23] cm[sup -2] . Both of these sources were detected in the mid-infrared by ISOCAM on the Infrared Space Observatory, which when combined with radiative transfer modelling leads to the prediction that the bulk of the reprocessed flux emerges at ∼100 μm. [ABSTRACT FROM AUTHOR]

Published
2002
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149. Na+-gradient-stimulated AIB transport in membrane vesicles from Ehrlich ascites cells

Author

Colombini, M. and Johnstone, R. M.

Abstract

Summary Plasma membrane vesicles derived from Ehrlich ascites cells can accumulate 2-aminoisobutyric acid (AIB) twofold, in the absence of ion gradients or potential differences. In addition, AIB uptake is stimulated specifically by the presence of a Na+ chemical gradient (high Na+ outside). The nature of the counterion (e.g., K+, Li+, Cs+, or ethanolamine) inside the vesicle does not affect the qualitative response although quantitative differences are observed. The level of AIB present in the vesicle decreases as the Na+-gradient is dissipated. Gramicidin, which increases the rate of ion gradient dissipation, inhibits the gradient-stimulated uptake. Valinomycin stimulates AIB uptake when Na+ is present outside the vesicles and K+ is inside, probably by producing a diffusion potential which increases the electrochemical potential difference for Na+. As the Na+-gradient dissipates, AIB accumulation exceeds that predicted from 100% transfer of the energy from the Na+ chemical gradient if a 1:1 relationship between amino acid and Na+ coupling exists. It is possible that a diffusion potential adds to the chemical gradient for Na+ making the electrochemical potential difference for Na+ adequate to energize AIB accumulation. Ouabain inhibits gradient-stimulated AIB uptake without measurable effects on the ion distributions, thus showing a direct action of ouabain on amino acid transport.

Published
1974
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150. Volume enlargement and recovery of Na+-dependent amino acid transport in proteoliposomes derived from Ehrlich ascites cell membranes.

Author

McCormick, J I and Johnstone, R M

Abstract

Na+-dependent amino acid transport can be reconstituted from solubilized Ehrlich cell plasma membranes by addition of asolectin vesicles, gel filtration, and a freeze-thaw cycle. Removal of phosphatidic acid (approximately 10% of the total lipid) by Ba2+ precipitation reduces the efficiency of reconstitution of Na+-dependent amino acid transport by approximately 73% and decreases intravesicular volume of the proteoliposomes by approximately 43%. The loss of transport activity is not due to exclusion of specific proteins during reconstitution. The phosphatidic acid-free liposomes are less permeable and require more time to attain an equilibrium distribution of solute. Transport activity and intravesicular volume can be restored to Ba2+-precipitated asolectin proteoliposomes by addition of egg-phosphatidic acid during reconstitution. The extent of recovery of transport activity is proportional to the change in intravesicular volume and depends on the amount of phosphatidic acid present. Replacement of phosphatidic acid with 20% phosphatidylserine or phosphatidylglycerol leads to increases in intravesicular volume with little or no increase in amino acid transport. Generation of phosphatidic acid in situ by treatment of Ba2+-precipitated proteoliposomes with phospholipase D also restored transport. The observed increase in transport activity (9-fold) is accompanied by a 46% increase in intravesicular volume, presumably caused by vesicle fusion. Phosphatidic acid is also required for successful reconstitution of Na+-dependent amino acid transport from pure phosphatidylcholine:phosphatidylethanolamine (1:1) mixtures with only a small change (approximately 16%) in intravesicular volume. The results provide evidence for both indirect and direct effects of phosphatidic acid on reconstitution of Na+-dependent amino acid transport. The indirect effects occur through enlargement of intravesicular volume, large vesicles showing higher rates of transport. However, there is also evidence to indicate a specific effect of phosphatidic acid on the Na+-dependent amino acid transporter, since other acidic lipids may change intravesicular volume without a commensurate change in transport activity.

Published
1988
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