Phase separating binary fluids under oscillatory shear

We apply lattice Boltzmann methods to study the segregation of binary fluid mixtures under oscillatory shear flow in two dimensions. The algorithm allows to simulate systems whose dynamics is described by the Navier-Stokes and the convection-diffusion equations. The interplay between several time scales produces a rich and complex phenomenology. We investigate the effects of different oscillation frequencies and viscosities on the morphology of the phase separating domains. We find that at high frequencies the evolution is almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low viscosity) and diffusive (high viscosity) regimes, respectively. When the period of the applied shear flow becomes of the same order of the relaxation time $T_R$ of the shear velocity profile, anisotropic effects are clearly observable. In correspondence with non-linear patterns for the velocity profiles, we find configurations where lamellar order close to the walls coexists with isotropic domains in the middle of the system. For particular values of frequency and viscosity it can also happen that the convective effects induced by the oscillations cause an interruption or a slowing of the segregation process, as found in some experiments. Finally, at very low frequencies, the morphology of domains is characterized by lamellar order everywhere in the system resembling what happens in the case with steady shear.Comment: 1 table and 12 figures in .gif forma

The Vimos VLT Deep Survey: Compact structures in the CDFS

We have used the Vimos VLT Deep Survey in combination with other spectroscopic, photometric and X-ray surveys from literature to detect several galaxy structures in the Chandra Deep Field South (CDFS). Both a friend-of-friend based algorithm applied to the spectroscopic redshift catalog and an adaptative kernel galaxy density and colour maps correlated with photometric redshift estimates have been used. We mainly detect a chain-like structure at z=0.66 and two massive groups at z=0.735 and 1.098 showing signs of ongoing collapse. We also detect two galaxy walls at z=0.66 and at z=0.735 (extremely compact in redshift space). The first one contains the chain-like structure and the last one contains in its center one of the two massive groups. Finally, other galaxy structures that are probably loose low mass groups are detected. We compare the group galaxy population with simulations in order to assess the richness of these structures and we study their galaxy morphological contents. The higher redshift structures appear to probably have lower velocity dispersion than the nearby ones. The number of moderatly massive structures we detect is consistent with what is expected for an LCDM model, but a larger sample is required to put significant cosmological constraints.Comment: 15 pages, 17 figures, accepted in A&

Galaxy luminosity function per morphological type up to z=1.2

We have computed the evolution of the rest-frame B-band luminosity function (LF) for bulge and disk-dominated galaxies since z=1.2. We use a sample of 605 spectroscopic redshifts with I_{AB}<24 in the Chandra Deep Field South from the VIMOS-VLT Deep Survey, 3555 galaxies with photometric redshifts from the COMBO-17 multi-color data, coupled with multi-color HST/ACS images from the Great Observatories Origin Deep Survey. We split the sample in bulge- and disk-dominated populations on the basis of asymmetry and concentration parameters measured in the rest-frame B-band. We find that at z=0.4-0.8, the LF slope is significantly steeper for the disk-dominated population (\alpha=-1.19 \pm 0.07) compared to the bulge-dominated population (\alpha=-0.53 \pm 0.13). The LF of the bulge-dominated population is composed of two distinct populations separated in rest-frame color: 68% of red (B-I)_{AB}>0.9 and bright galaxies showing a strongly decreasing LF slope \alpha=+0.55 \pm 0.21, and 32% of blue (B-I)_{AB}<0.9 and more compact galaxies which populate the LF faint-end. We observe that red bulge-dominated galaxies are already well in place at z~1, but the volume density of this population is increasing by a factor 2.7 between z~1 and z~0.6. It may be related to the building-up of massive elliptical galaxies in the hierarchical scenario. In addition, we observe that the blue bulge-dominated population is dimming by 0.7 magnitude between z~1 and z~0.6. Galaxies in this faint and more compact population could possibly be the progenitors of the local dwarf spheroidal galaxies.Comment: 9 pages, 4 figures, 2 tables, accepted for publication in Astronomy and Astrophysic

UV Morphology and Star Formation in the Tidal Tails of NGC 4038/39

We present GALEX FUV (1530 A) and NUV (2310 A) observations of the archetypal merging system NGC 4038/39, ``The Antennae". Both tails are relatively bright in the UV, especially in the vicinity of the Tidal Dwarf Galaxy candidates at the end of the southern tail. The UV light generally falls within the optically delineated tails, although the UV light is considerably more structured, with a remarkably similar morphology to the tidal HI. The UV colors suggest that there has been continuing star formation within the tidal tails, even outside the previously studied Tidal Dwarf regions. Within the inner disk regions, there are interesting UV features which appear to be related to the extended soft X-ray loops and halo recently discovered by CHANDRA.Comment: Uses apjl.cls, emulateapj.sty. 4pgs, 2 figures, 1 table. This paper will be published as part of the Galaxy Evolution Explorer (GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS/ after November 22, 2004. Version with full resolution figures available at http://www.nrao.edu/~jhibbard/n4038/GALEX

Assimilating Seizure Dynamics

Observability of a dynamical system requires an understanding of its state—the collective values of its variables. However, existing techniques are too limited to measure all but a small fraction of the physical variables and parameters of neuronal networks. We constructed models of the biophysical properties of neuronal membrane, synaptic, and microenvironment dynamics, and incorporated them into a model-based predictor-controller framework from modern control theory. We demonstrate that it is now possible to meaningfully estimate the dynamics of small neuronal networks using as few as a single measured variable. Specifically, we assimilate noisy membrane potential measurements from individual hippocampal neurons to reconstruct the dynamics of networks of these cells, their extracellular microenvironment, and the activities of different neuronal types during seizures. We use reconstruction to account for unmeasured parts of the neuronal system, relating micro-domain metabolic processes to cellular excitability, and validate the reconstruction of cellular dynamical interactions against actual measurements. Data assimilation, the fusing of measurement with computational models, has significant potential to improve the way we observe and understand brain dynamics

Modeling CICR in rat ventricular myocytes: voltage clamp studies

<p>Abstract</p> <p>Background</p> <p>The past thirty-five years have seen an intense search for the molecular mechanisms underlying calcium-induced calcium-release (CICR) in cardiac myocytes, with voltage clamp (VC) studies being the leading tool employed. Several VC protocols including lowering of extracellular calcium to affect <it>Ca</it>²⁺loading of the sarcoplasmic reticulum (SR), and administration of blockers caffeine and thapsigargin have been utilized to probe the phenomena surrounding SR <it>Ca</it>²⁺release. Here, we develop a deterministic mathematical model of a rat ventricular myocyte under VC conditions, to better understand mechanisms underlying the response of an isolated cell to calcium perturbation. Motivation for the study was to pinpoint key control variables influencing CICR and examine the role of CICR in the context of a physiological control system regulating cytosolic <it>Ca</it>²⁺concentration ([<it>Ca</it>²⁺]<it>_myo</it>).</p> <p>Methods</p> <p>The cell model consists of an electrical-equivalent model for the cell membrane and a fluid-compartment model describing the flux of ionic species between the extracellular and several intracellular compartments (cell cytosol, SR and the dyadic coupling unit (DCU), in which resides the mechanistic basis of CICR). The DCU is described as a controller-actuator mechanism, internally stabilized by negative feedback control of the unit's two diametrically-opposed <it>Ca</it>²⁺channels (trigger-channel and release-channel). It releases <it>Ca</it>²⁺flux into the cyto-plasm and is in turn enclosed within a negative feedback loop involving the SERCA pump, regulating[<it>Ca</it>²⁺]<it>_myo</it>.</p> <p>Results</p> <p>Our model reproduces measured VC data published by several laboratories, and generates graded <it>Ca</it>²⁺release at high <it>Ca</it>²⁺gain in a homeostatically-controlled environment where [<it>Ca</it>²⁺]<it>_myo</it>is precisely regulated. We elucidate the importance of the DCU elements in this process, particularly the role of the ryanodine receptor in controlling SR <it>Ca</it>²⁺release, its activation by trigger <it>Ca</it>²⁺, and its refractory characteristics mediated by the luminal SR <it>Ca</it>²⁺sensor. Proper functioning of the DCU, sodium-calcium exchangers and SERCA pump are important in achieving negative feedback control and hence <it>Ca</it>²⁺homeostasis.</p> <p>Conclusions</p> <p>We examine the role of the above <it>Ca</it>²⁺regulating mechanisms in handling various types of induced disturbances in <it>Ca</it>²⁺levels by quantifying cellular <it>Ca</it>²⁺balance. Our model provides biophysically-based explanations of phenomena associated with CICR generating useful and testable hypotheses.</p