227 research outputs found
Chaotic cold accretion on to black holes
No full textBondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when t_cool/t_ff ≲ 10. The clouds decouple from the hot gas, `raining' on to the centre. Subsonic turbulence of just over 100 km s^-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (t_turb/t_cool ≲ 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in many systems, such as hot galactic haloes, groups and clusters. In this mode, the black hole can quickly react to the state of the entire host galaxy, leading to efficient self-regulated AGN feedback and the symbiotic Magorrian relation. Chaotic accretion can generate high-velocity clouds, likely leading to strong variations in the AGN luminosity, and the deflection or mass-loading of jets. During phases of overheating, the hot mode becomes the single channel of accretion, though strongly suppressed by turbulence. High-resolution data could determine the current mode of accretion: assuming quiescent feedback, the cold mode results in a quasi-flat-temperature core as opposed to the cuspy profile of the hot mode
Cause and Effect of Feedback: Multiphase Gas in Cluster Cores Heated by AGN Jets
No full textMultiwavelength data indicate that the X-ray-emitting plasma in the cores of galaxy clusters is not cooling catastrophically. To a large extent, cooling is offset by heating due to active galactic nuclei (AGNs) via jets. The cool-core clusters, with cooler/denser plasmas, show multiphase gas and signs of some cooling in their cores. These observations suggest that the cool core is locally thermally unstable while maintaining global thermal equilibrium. Using high-resolution, three-dimensional simulations we study the formation of multiphase gas in cluster cores heated by collimated bipolar AGN jets. Our key conclusion is that spatially extended multiphase filaments form only when the instantaneous ratio of the thermal instability and free-fall timescales (t(TI)/t(ff)) falls below a critical threshold of approximate to 10. When this happens, dense cold gas decouples from the hot intracluster medium (ICM) phase and generates inhomogeneous and spatially extended Ha filaments. These cold gas clumps and filaments ``rain'' down onto the central regions of the core, forming a cold rotating torus and in part feeding the supermassive black hole. Consequently, the self-regulated feedback enhances AGN heating and the core returns to a higher entropy level with t(TI)/t(ff) > 10. Eventually, the core reaches quasi-stable global thermal equilibrium, and cold filaments condense out of the hot ICM whenever t(TI)/t(ff) less than or similar to 10. This occurs despite the fact that the energy from AGN jets is supplied to the core in a highly anisotropic fashion. The effective spatial redistribution of heat is enabled in part by the turbulent motions in the wake of freely falling cold filaments. Increased AGN activity can locally reverse the cold gas flow, launching cold filamentary gas away from the cluster center. Our criterion for the condensation of spatially extended cold gas is in agreement with observations and previous idealized simulations
Peculiar substrate specificity of δ1-pyrroline-5-carboxylate reductase in the obligately fermentative bacterium Zymomonas mobilis
No full textBackground: The enzyme that catalyzes the last step in proline synthesis, δ1-pyrroline-5-carboxylate reductase, showed in most cases a distinct preference in vitro for NADPH as the electron donor. Methods and results: A Zymomonas mobilis gene coding for a δ1-pyrroline-5-carboxylate reductase was cloned and heterologously expressed, and the recombinant protein was purified and characterized. The enzyme showed higher affinity to, and higher catalytic rate with NADH, with a specific activity of about 600 nkat (mg protein)−1. The molecular basis of this feature was investigated by analysis of the dinucleotide binding domain in silico. Conclusions: We postulate that the main determinants of coenzyme preference for P5C reductases are the length and the sequence of the motif A, whereas the overall sequence identity is insufficient to predict it a priori. Results are discussed in view of the obligately fermentative metabolism of this bacterium. Graphic abstract: [Figure not available: see fulltext.
Functional Characterization of Saccharomyces cerevisiae P5C Reductase, the Enzyme at the Converging Point of Proline and Arginine Metabolism
No full textThe enzyme that, in Saccharomyces cerevisiae, catalyzes the last step in both proline synthesis and arginine catabolism, δ1-pyrroline-5-carboxylate (P5C) reductase, was purified to near homogeneity and characterized thoroughly. Retention patterns upon gel permeation chromatography were consistent with a homodecameric composition of the holomer. High lability of the purified preparations and stabilization by reducing compounds suggested susceptibility to reactive-oxygen-species-mediated damage. Both NADH and NADPH were used as the electron donor, the latter resulting in a 3-fold higher Vmax. However, a higher affinity toward NADH was evident, and the NADPH-dependent activity was inhibited by NAD+, proline, arginine, and a variety of anions. With proline and arginine, the inhibition was of the competitive type with respect to the specific substrate, and of the uncompetitive- or mixed-type with respect to NADPH, respectively. The results suggest that, contrary to the enzyme from higher plants, yeast P5C reductase may preferentially use NADH in vivo. An in silico analysis was also performed to investigate the structural basis of such enzyme features. Superposition of the protein model with the experimental structure of P5C reductase from Medicago truncatula allowed us to hypothesize on the possible allosteric sites for arginine and anion binding, and the cysteine pairs that may be involved in disulfide formation
Crystal structure of Medicago truncatula histidinol-phosphate aminotransferase (HISN6) in apo form
No full textRaw dataset for the crystal structure of Medicago truncatula histidinol-phosphate aminotransferase (HISN6) in apo form</p
Structure of E. coli Class 2 L-asparaginase EcAIII, mutant M200W (crystal M200W#1)
No full textStructure of E. coli Class 2 L-asparaginase EcAIII, mutant M200W (crystal M200W#1)</p
Determination and haemolytic activity of saponins in hairy root culture of Platycodon grandiforum A.DC
No full textS u m m a r y The sum of saponins in the hairy root lines (6, 17) of Platycodon grandiforum A.DC. was compared. Hairy root line 6 showed a higher total saponin content (6.92%) than the line 17 (6.01%). According to the Chinese Pharmacopoeia standards the content of saponins in Platycodi Radix should be not less than 2%. Our results seem to indicate that the hairy root culture of Platycodon grandiforum A.DC. is a good source of saponins. The Haemolytic 104 N. Urbańska, J. Nartowska, A. Skorupska, D. Ruszkowski, J. Giebułtowicz, O. Olszowska Index of the hairy root line 6 was 1600. Digitonin was used as a reference. Moreover, the haemolytic activity of TLC subfractions of saponins varied
Shallow dark matter cusps in galaxy clusters
No full textWe study the evolution of the stellar and dark matter components in a galaxy cluster of 10 15 M ⊙ from z = 3 to the present epoch using the high‐resolution collisionless simulations of Ruszkowski & Springel. At z = 3 the dominant progenitor haloes were populated with spherical model galaxies with and without accounting for adiabatic contraction. We apply a weighting scheme which allows us to change the relative amount of dark and stellar material assigned to each simulation particle in order to produce luminous properties which agree better with abundance‐matching arguments and observed bulge sizes at z = 3. This permits the study of the effect of initial compactness on the evolution of the mass–size relation. We find that for more compact initial stellar distributions the size of the final brightest cluster galaxy grows with mass according to r ∝ M 2 , whereas for more extended initial distributions, r ∝ M . Our results show that collisionless mergers in a cosmological context can reduce the strength of inner dark matter cusps with changes in logarithmic slope of 0.3–0.5 at fixed radius. Shallow cusps such as those found recently in several strong lensing clusters thus do not necessarily conflict with cold dark matter, but may rather reflect on the initial structure of the progenitor galaxies, which was shaped at high redshift by their formation process.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/92100/1/j.1365-2966.2012.21262.x.pd
Mechanical ventilation simulation for cross-training step-down nurses to the surgical ICU
No full textPurpose of Project: The purpose of this project was to modify the current cross-training program to include simulation-based education to increase step-down nurses’ knowledge and confidence in providing care for patients requiring invasive mechanical ventilation while increasing learning satisfaction with simulation in cross-training to the surgical ICU.
Methodology: This quality improvement project used a one-group pre- and post-intervention design with a convenience sample of step-down nurses at a hospital in New York City. Participants evaluated their level of self-confidence, knowledge, and learner satisfaction before and after the use of simulated scenarios. Two months after the educational intervention, the participants were asked to retake the post-knowledge survey.
Results: Overall, there was a significant increase in the total participant confidence scores preintervention (M = 27.9) and post-intervention (M = 32.0; p = .003). There was a significant increase in the total number of correctly answered questions prior to the simulation intervention (M = 7.1) compared with the scores two-months post intervention (M = 10.3; p = .000). Following the intervention there was a significant increase in pre-intervention satisfaction scores (M = 20.2) and post-intervention confidence scores; (M =24.4; p = .000).
Implications for Practice: Simulation provides the ability to engage within complex scenarios without the pressure of potentially harming a patient, allowing for submersion into learning that is otherwise excludedDNPIncludes bibliographical reference
- …
