Retrograde Accretion and Merging Supermassive Black Holes

We investigate whether a circumbinary gas disc can coalesce a supermassive black hole binary system in the centre of a galaxy. This is known to be problematic for a prograde disc. We show that in contrast, interaction with a retrograde circumbinary disc is considerably more effective in shrinking the binary because there are no orbital resonances. The binary directly absorbs negative angular momentum from the circumbinary disc by capturing gas into a disc around the secondary black hole, or discs around both holes if the binary mass ratio is close to unity. In many cases the binary orbit becomes eccentric, shortening the pericentre distance as the eccentricity grows. In all cases the binary coalesces once it has absorbed the angular momentum of a gas mass comparable to that of the secondary black hole. Importantly, this conclusion is unaffected even if the gas inflow rate through the disc is formally super--Eddington for either hole. The coalescence timescale is therefore always $\sim M_2/\dot M$, where $M_2$ is the secondary black hole mass and $\dot M$ the inflow rate through the circumbinary disc.Comment: 8 pages, 4 figures. Accepted for publication in MNRAS. Movies of the simulations can be found at: http://www.astro.le.ac.uk/users/cjn12/RetroBinaryMovies.htm

Folylpolyglutamate synthesis in a methionine auxotroph of Neurospora crassa

Folylpolyglutamate synthesis in a auxotroph of Neurospora crass

Glycine stimulates polyglutamyl folate synthesis in N. crassa

Glycine stimulates polyglutamyl folate synthesi

Seasonality of the red blood cell stress response in rainbow trout (Oncorhynchus mykiss)

The β-adrenergic stress response in red blood cells (RBCs) of rainbow trout shows seasonal changes in expression. We have explored the mechanisms underpinning this response by following over a period of 27 months changes in β-adrenergic receptor (β-AR) binding characteristics, β-adrenergically stimulated RBC Na+/H+ exchanger (βNHE) activity, together with β-AR and βNHE mRNA levels and plasma steroid hormone and lactate levels. These parameters were measured at approximately monthly intervals in a single population of fish held under semi-natural conditions. Membrane-bound, high-affinity β-ARs were present in RBCs at all sampling times, varying from 668 ± 112 to 2654 ± 882 receptors cell-1 (mean ± SEM; n=8). βNHE activity, however, was reduced by 57 and 34% in December 1999 and February 2001, respectively, compared to an otherwise sustained influx that averaged 110.4 ± 2.3 mmol l-1 RBCs h-1 (n = 119). Only one reduction coincided with a spawning period but both were preceded by transient increases in circulating testosterone. βNHE activity measured under standard conditions was not correlated with the number or affinity of β-ARs nor with water temperature, but both β-AR numbers and βNHE activity were positively related to their respective mRNA levels (P = 0.005 and 0.038, respectively). Pharmaceutical intervention in the transduction cascade linking the β-AR and βNHE failed to indicate any failure of the transduction elements in RBCs displaying low βNHE activity. Similarly, we failed to demonstrate any link between seasonal cortisol fluctuations and seasonally reduced βNHE activity. However, the βNHE activity of age-separated RBC fractions showed that younger RBCs had a significantly higher βNHE response than older RBCs, consistent with the seasonal reductions in βNHE being linked to turnover of red cells and erythropoiesis. Testosterone is known to induce erythropoiesis and we conclude that seasonal reductions in βNHE are not caused by changes in β-AR numbers, but may be linked to testosterone-induced erythropoiesis

Novel Candidate Genes Identified in the Brain during Nociception in Common Carp (Cyprinus carpio) and Rainbow Trout (Oncorhynchus mykiss)

Recent studies have demonstrated that teleost fish possess nociceptors that detect potentially painful stimuli and that the physiological properties of these fibres are markedly similar to those found in mammals. This finding led to suggestions of possible pain perception in fish, contrary to the view that the sensory response in these animals is limited to the spinal cord and hindbrain and as such is reflexive. Therefore, the aim of this study was to determine if the brain is active at the molecular level by using a microarray analysis of gene expression in the forebrain, midbrain and hindbrain of two fish species. A comparison between the two species at different time points showed that many genes were differentially regulated in response to a noxious stimulus compared with controls. A number of genes that are involved in mammalian nociception, such as brain-derived neurotrophic factor (BDNF) and the cannabinoid CB1 receptor were regulated in the fish brain after a nociceptive event. Novel candidates that showed significant regulation in both species were also identified. In particular, the Van Gogh-like 2 gene, was regulated in both carp and trout and should be pursued to establish its precise role in nociception

Behavioural Analysis of a Nociceptive Event in Fish: Comparisons Between Three Species Demonstrate Specific Responses

Nociception is the sensory mechanism by which potentially harmful stimuli are detected in animals and humans. The behavioural responses to noxious stimulation have been studied in two fish species thus far. However, since species-specific differences are seen in mammals, more species need to be examined to determine whether nociceptive responses are generic in fish. The present study investigated the behavioural and respiratory response to an acute noxious or potentially painful stimulus in common carp (Cyprinus carpio), zebrafish (Danio rerio) and rainbow trout (Oncorhynchus mykiss). Behavioural parameters such as frequency of swimming, use of cover and any anomalous behaviour were measured along with ventilation rate before and after noxious stimulation in the carp and zebrafish. Although no differences in behavioural or ventilation rate response were observed between noxiously stimulated carp and control fish, anomalous behaviours of rocking from side to side and rubbing of lips against the tank walls were observed in two of the five fish. In contrast, zebrafish displayed a significant reduction in frequency of swimming and an increase in ventilation rate, which was similar to the results obtained from rainbow trout. Zebrafish did not display any anomalous behaviour. These dissimilarities in response to potentially painful stimulation demonstrate that there are species-specific behavioural and physiological responses to a nociceptive event in fish

Investigation of Van Gogh-like 2 mRNA Regulation and Localisation in Response to Nociception in the Brain of Adult Common Carp (Cyprinus carpio)

The Van Gogh-like 2 (vangl2) gene is typically associated with planar cell polarity pathways, which is essential for correct orientation of epithelial cells during development. The encoded protein of this gene is a transmembrane protein and is highly conserved through evolution. Van Gogh-like 2 was selected for further study on the basis of consistent regulation after a nociceptive stimulus in adult common carp and rainbow trout in a microarray study. An in situ hybridisation was conducted in the brain of mature common carp (Cyprinus carpio), 1.5 and 3 h after a nociceptive stimulus comprising of an acetic acid injection to the lips of the fish and compared with a saline-injected control. The vangl2 gene was expressed in all brain regions, and particularly intensely in neurons of the telencephalon and in ependymal cells. In the cerebellum, a greater number (P = 0.018) of Purkinje cells expressed vangl2 after nociception (n=7) compared with controls (n = 5). This regulation opens the possibility that vangl2 is involved in nociceptive processing in the adult fish brain and may be a novel target for central nociception in vertebrates

Convergence of SPH simulations of self-gravitating accretion discs: Sensitivity to the implementation of radiative cooling

Recent simulations of self-gravitating accretion discs, carried out using a three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate, have been interpreted as implying that three-dimensional global discs fragment much more easily than would be expected from a two-dimensional local model. Subsequently, global and local two-dimensional models have been shown to display similar fragmentation properties, leaving it unclear whether the three-dimensional results reflect a physical effect or a numerical problem associated with the treatment of cooling or artificial viscosity in SPH. Here, we study how fragmentation of self-gravitating disc flows in SPH depends upon the implementation of cooling. We run disc simulations that compare a simple cooling scheme, in which each particle loses energy based upon its internal energy per unit mass, with a method in which the cooling is derived from a smoothed internal energy density field. For the simple per particle cooling scheme, we find a significant increase in the minimum cooling time scale for fragmentation with increasing resolution, matching previous results. Switching to smoothed cooling, however, results in lower critical cooling time scales, and tentative evidence for convergence at the highest spatial resolution tested. We conclude that precision studies of fragmentation using SPH require careful consideration of how cooling (and, probably, artificial viscosity) is implemented, and that the apparent non-convergence of the fragmentation boundary seen in prior simulations is likely a numerical effect. In real discs, where cooling is physically smoothed by radiative transfer effects, the fragmentation boundary is probably displaced from the two-dimensional value by a factor that is only of the order of unity.Comment: 9 pages, 11 figures, MNRAS in pres

Dust size distributions in coagulation/fragmentation equilibrium: Numerical solutions and analytical fits

Context. Grains in circumstellar disks are believed to grow by mutual collisions and subsequent sticking due to surface forces. Results of many fields of research involving circumstellar disks, such as radiative transfer calculations, disk chemistry, magneto-hydrodynamic simulations largely depend on the unknown grain size distribution. Aims. As detailed calculations of grain growth and fragmentation are both numerically challenging and computationally expensive, we aim to find simple recipes and analytical solutions for the grain size distribution in circumstellar disks for a scenario in which grain growth is limited by fragmentation and radial drift can be neglected. Methods. We generalize previous analytical work on self-similar steady-state grain distributions. Numerical simulations are carried out to identify under which conditions the grain size distributions can be understood in terms of a combination of power-law distributions. A physically motivated fitting formula for grain size distributions is derived using our analytical predictions and numerical simulations. Results. We find good agreement between analytical results and numerical solutions of the Smoluchowski equation for simple shapes of the kernel function. The results for more complicated and realistic cases can be fitted with a physically motivated "black box" recipe presented in this paper. Our results show that the shape of the dust distribution is mostly dominated by the gas surface density (not the dust-to-gas ratio), the turbulence strength and the temperature and does not obey an MRN type distribution.Comment: 16 pages, 9 figures, accepted for publication in A&