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The hard X-ray tails in neutron star low mass X-ray binaries: BeppoSAX observations and possible theoretical explanation of the GX 17+2 case

Farinelli, R.; Titarchuk, L.; Frontera, F.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 23/02/2007 Português
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We report results of a new spectral analysis of two BeppoSAX observations of the Z source GX 17+2. In one of the two observations the source exhibits a powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have shown that bulk (dynamical) Comptonization of soft photons can be a possible alternative mechanism for producing hard X-ray tails in such systems. We start from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX to better investigate the physical processes at work. We use GX 17+2 as a representative case. Moreover, we suggest that weakening (or disappearance) of the hard X-ray tail can be explained by increasing radiation pressure originated at the surface of the neutron star (NS). As a result the high radiation pressure stops the bulk inflow and consequently this radiation feedback of the NS surface leads to quenching the bulk Comptonization.; Comment: 6 pages, 3 figures, Accepted for publication in ApJ

Pressure induced renormalization of energy scales in the unconventional superconductor FeTe0.6Se0.4

Marty, Karol; Christianson, Andrew D.; Santos, Antonio M. dos; Sipos, Balasz; Matsubayashi, Kazuyuki; Uwatoko, Yoshiya; Fernandez-Baca, Jaime A.; Tulk, Christopher A.; Maier, Thomas A.; Sales, Brian C.; Lumsden, Mark D.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 08/12/2011 Português
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We have carried out a pressure study of the unconventional superconductor FeTe0.6Se0.4 up to 1.5 GPa by neutron scattering, resistivity and magnetic susceptibility measurements. We have extracted the neutron spin resonance energy and the superconducting transition temperature as a function of applied pressure. Both increase with pressure up to a maximum at ~1.3 GPa. This analogous qualitative behavior is evidence for a correlation between these two fundamental parameters of unconventional superconductivity. However, Tc and the resonance energy do not scale linearly and thus a simple relationship between these energies does not exist even in a single sample. The renormalization of the resonance energy relative to the transition temperature is here attributed to an increased hybridization. The present results appear to be consistent with a pressure-induced weakening of the coupling strength associated with the fundamental pairing mechanism.; Comment: 5 pages, 4 figures

Approximately Efficient Cost-Sharing Mechanisms

Roughgarden, Tim; Sundararajan, Mukund
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 29/06/2006 Português
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We make three different types of contributions to cost-sharing: First, we identify several new classes of combinatorial cost functions that admit incentive-compatible mechanisms achieving both a constant-factor approximation of budget-balance and a polylogarithmic approximation of the social cost formulation of efficiency. Second, we prove a new, optimal lower bound on the approximate efficiency of every budget-balanced Moulin mechanism for Steiner tree or SSRoB cost functions. This lower bound exposes a latent approximation hierarchy among different cost-sharing problems. Third, we show that weakening the definition of incentive-compatibility to strategyproofness can permit exponentially more efficient approximately budget-balanced mechanisms, in particular for set cover cost-sharing problems.; Comment: latex source, 22 pages, 1 figure

Localized Excitons and Breaking of Chemical Bonds at III-V (110) Surfaces

Pankratov, Oleg; Scheffler, Matthias
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 20/01/1995 Português
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Electron-hole excitations in the surface bands of GaAs(110) are analyzed using constrained density-functional theory calculations. The results show that Frenkel-type autolocalized excitons are formed. The excitons induce a local surface unrelaxation which results in a strong exciton-exciton attraction and makes complexes of two or three electron-hole pairs more favorable than separate excitons. In such microscopic exciton "droplets" the electron density is mainly concentrated in the dangling orbital of a surface Ga atom whereas the holes are distributed over the bonds of this atom to its As neighbors thus weakening the bonding to the substrate. This finding suggests the microscopic mechanism of a laser-induced emission of neutral Ga atoms from GaAs and GaP (110) surfaces.; Comment: submitted to PRL, 10 pages, 4 figures available upon request from: oleg@theo24.rz-berlin.mpg.de

Controlling friction

Elmer, Franz-Josef
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/11/1997 Português
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Two different controlling methods are proposed to stabilize unstable continuous-sliding states of a dry-friction oscillator. Both methods are based on a delayed-feedback mechanism well-known for stabilizing periodic orbits in deterministic chaos. The feedback variable is the elastic deformation. The control parameter is either the sliding velocity or the normal force. We calculate analytically stability boundaries in the space of control parameter and delay time. Furthermore, we show that our methods are able to turn stick-slip motion into continuous sliding. Controlling friction helps to get a better understanding of friction by measuring, e.g., velocity-weakening friction forces.; Comment: Written in REVTeX, 4 pages, 6 Postscript figures

Eruption of a Kink-Unstable Filament in Active Region NOAA 10696

Williams, D. R.; Toeroek, T.; Demoulin, P.; van Driel-Gesztelyi, L.; Kliem, B.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/07/2005 Português
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We present rapid-cadence Transition Region And Coronal Explorer (TRACE) observations which show evidence of a filament eruption from active region NOAA 10696, accompanied by an X2.5 flare, on 2004 November 10. The eruptive filament, which manifests as a fast coronal mass ejection some minutes later, rises as a kinking structure with an apparently exponential growth of height within TRACE's field of view. We compare the characteristics of this filament eruption with MHD numerical simulations of a kink-unstable magnetic flux rope, finding excellent qualitative agreement. We suggest that, while tether weakening by breakout-like quadrupolar reconnection may be the release mechanism for the previously confined flux rope, the driver of the expansion is most likely the MHD helical kink instability.; Comment: Accepted by ApJ Letters. 4 figures (Fig. 3 in two parts). For MPEG files associated with Figure 1, see: http://www.mssl.ucl.ac.uk/~drw/papers/kink/ktrace.mpg http://www.mssl.ucl.ac.uk/~drw/papers/kink/kmdi.mpg http://www.mssl.ucl.ac.uk/~drw/papers/kink/ksimu.mpg

The life-time of galactic bars: central mass concentrations and gravity torques

Bournaud, F.; Combes, F.; Semelin, B.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/09/2005 Português
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Bars in gas-rich spiral galaxies are short-lived. They drive gas inflows through their gravity torques, and at the same time self-regulate their strength. Their robustness has been subject of debate, since it was thought that only the resulting central mass concentrations (CMCs) were weakening bars, and only relatively rare massive CMCs were able to completely destroy them. Through numerical simulations including gas dynamics, we find that with the gas parameters of normal spiral galaxies, the CMC is not sufficient to fully dissolve the bar. But another overlooked mechanism, the transfer of angular momentum from the infalling gas to the stellar bar, can also strongly weaken the bar. In addition, we show that gravity torques are correctly reproduced in simulations, and conclude that bars are transient features, with life-time of 1-2 Gyr in typical Sb-Sc galaxies, because of the combined effects of CMCs and gravity torques, while most existing works had focussed on the CMC effects alone.; Comment: accepted for publication in MNRAS. The definitive version is available at www.blackwell-synergy.com

Dynamic clustering in active colloidal suspensions with chemical signaling

Theurkauff, I.; Cottin-Bizonne, C.; Palacci, J.; Ybert, C.; Bocquet, L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/02/2012 Português
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In this paper, we explore experimentally the phase behavior of a dense active suspension of self- propelled colloids. In addition to a solid-like and a gas-like phase observed for high and low densities, a novel cluster phase is reported at intermediate densities. This takes the form of a stationary assembly of dense aggregates, with an average size which grows with activity as a linear function of the self-propelling velocity. While different possible scenarii can be considered to account for these observations - such as a generic velocity weakening instability recently put forward -, we show that the experimental results are reproduced by a chemotactic aggregation mechanism, originally introduced to account for bacterial aggregation, and accounting here for diffusiophoretic chemical interaction between colloidal swimmers.; Comment: supplementary video :http:// www-lpmcn.univ-lyon1.fr/~lbocquet/Movie-Theurkauff-SI.avi

Galaxy sizes as a function of environment at intermediate redshift from the ESO Distant Cluster Survey

Kelkar, Kshitija; Aragón-Salamanca, Alfonso; Gray, Meghan E.; Maltby, David; Vulcani, Benedetta; De Lucia, Gabriella; Poggianti, Bianca M.; Zaritsky, Dennis
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/03/2015 Português
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In order to assess whether the environment has a significant effect on galaxy sizes, we compare the mass--size relations of cluster and field galaxies in the $0.4 < z < 0.8$ redshift range from the ESO Distant Cluster Survey (EDisCS) using HST images. We analyse two mass-selected samples, one defined using photometric redshifts ($10.2 \le \log M_\ast/M_{\odot} \le 12.0$), and a smaller more robust subsample using spectroscopic redshifts ($10.6 \le \log M_\ast/M_{\odot} \le 11.8$). We find no significant difference in the size distributions of cluster and field galaxies of a given morphology. Similarly, we find no significant difference in the size distributions of cluster and field galaxies of similar rest-frame $B-V$ colours. We rule out average size differences larger than $10$--$20$\% in both cases. Consistent conclusions are found with the spectroscopic and photometric samples. These results have important consequences for the physical process(es) responsible for the size evolution of galaxies, and in particular the effect of the environment. The remarkable growth in galaxy size observed from $z\sim2.5$ has been reported to depend on the environment at higher redshifts ($z>1$), with early-type/passive galaxies in higher density environments growing earlier. Such dependence disappears at lower redshifts. Therefore...

On the distribution of frequency ratios of kHz QPOs

Boutelier, Martin; Barret, Didier; Lin, Yongfeng; Torok, Gabriel
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 16/09/2009 Português
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The width (W), root mean squared amplitude (Rs) of lower and upper kHz quasi-periodic oscillations (QPOs) from accreting neutron stars vary with frequency. Similarly, the QPO frequency varies with the source count rate (S). Hence, the significance of a QPO, scaling as S x Rs^2/W^(1/2) will also depend on frequency. In addition, the significance also scales up with the square root of the integration time of the Fourier power density spectrum (T). Consequently, depending on the way data are considered, kHz QPOs may be detected only over a limited range of their frequency spans or detected predominantly at some frequencies, leading potentially to biases in the observed distributions of frequencies or frequency ratios. Although subject of much controversy, an observed clustering of QPO frequency ratios around 3/2 in Sco X-1, also seen in other sources, has been previously used as an argument supporting resonance based models of neutron star QPOs. In this paper, we measure how the statistical significance of both kHz QPOs vary with frequency for three prototype neutron star kHz QPO sources, namely 4U1636-536, 4U0614+091 and Sco X-1. As the significance of QPO detection depends on frequency, we show that in sensitivity-limited observations (as in the case of the RXTE/PCA)...

Frustration by competing interactions in the highly-distorted double perovskites La2NaB'O6 (B' = Ru, Os)

Aczel, A. A.; Bugaris, D. E.; Li, L.; Yan, J. -Q.; de la Cruz, C.; Loye, H. -C. zur; Nagler, S. E.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/09/2012 Português
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The usual classical behaviour of S = 3/2, B-site ordered double perovskites generally results in simple, commensurate magnetic ground states. In contrast, heat capacity and neutron powder diffraction measurements for the S = 3/2 systems La2NaB'O6 (B' = Ru, Os) reveal an incommensurate magnetic ground state for La2NaRuO6 and a drastically suppressed ordered moment for La2NaOsO6. This behaviour is attributed to the large monoclinic structural distortions of these double perovskites. The distortions have the effect of weakening the nearest neighbour superexchange interactions, presumably to an energy scale that is comparable to the next nearest neighbour superexchange. The exotic ground states in these materials can then arise from a competition between these two types of antiferromagnetic interactions, providing a novel mechanism for achieving frustration in the double perovskite family.; Comment: 7 pages, 6 figures, submitted to PRB

The dynamical disconnection of sunspots from their magnetic roots

Schuessler, M.; Rempel, M.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/06/2005 Português
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After a dynamically active emergence phase, magnetic flux at the solar surface soon ceases to show strong signs of the subsurface dynamics of its parent magnetic structure. This indicates that some kind of disconnection of the emerged flux from its roots in the deep convection zone should take place. We propose a mechanism for the dynamical disconnection of the surface flux based upon the buoyant upflow of plasma along the field lines. Such flows arise in the upper part of a rising flux loop during the final phases of its buoyant ascent towards the surface. The combination of the pressure buildup by the upflow and the cooling of the upper layers of an emerged flux tube by radiative losses at the surface lead to a progressive weakening of the magnetic field in several Mm depth. When the field strength has become sufficiently low, convective motions and the fluting instability disrupt the flux tube into thin, passively advected flux fragments, thus providing a dynamical disconnection of the emerged part from its roots. We substantiate this scenario by considering the quasi-static evolution of a sunspot model under the effects of radiative cooling, convective energy transport, and pressure buildup by a prescribed inflow at the bottom of the model. For inflow speeds in the range shown by simulations of thin flux tubes...

Quasi-stationary evolution of systems driven by particle evaporation

Velazquez, L.; Cuesta, H. Mosquera; Guzman, F.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 01/03/2005 Português
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We study the quasi-stationary evolution of systems where an energetic confinement is unable to completely retain their constituents. It is performed an extensive numerical study of a gas whose dynamics is driven by binary encounters and its particles are able to escape from the container when their kinetic energies overcome a given cutou Uc .We use a parametric family of differential cross sections in order to modify the effectiveness of this equilibration mechanism. It is verified that when the binary encounters favor an effective exploration of all accessible velocities, the quasi-stationary evolution is reached when the detailed balance is imposed for all those binary collisions which do not provoke particle evaporation. However, the weakening of this effectiveness leads to energy distribution functions which could be very well fitted by using a Michie-King-like profile. We perform a theoretical analysis, in the context of Hamiltonian systems driven by a strong chaotic dynamics and particle evaporation, in order to take into account the effect of the nonhomogeneous character of the confining potential.; Comment: 14 pages, 21 eps figures

The AGN-Disk Dynamics Connection

Sellwood, J. A.; Shen, Juntai
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 27/03/2003 Português
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Any connection between central activity and the large-scale dynamics of disk galaxies requires an efficient mechanism to remove angular momentum from the orbiting material. The only viable means of achieving inflow from kiloparsec scales is through gravitational stresses created by bars and/or mergers. The inflow of gas in bars today appears to stall at a radius of few hundred parsec, however, forming a nuclear ring. Here we suggest that bars in the early Universe may have avoided this problem, and propose that the progenitors of central supermassive black holes (SMBHs) are created by gas that is driven deep into the centers of galaxies by bars in the early stages of disk formation. The coincidence of the QSO epoch with galaxy formation, the short lifetimes of QSOs, and the existence of SMBHs in the centers of most bright galaxies are all naturally accounted for by disk dynamics in this model. The progenitor SMBHs are the seeds for brighter QSO flares during galaxy mergers. We present a new study of bar weakening by central mass concentrations, which shows that bars are less easily destroyed than previously thought. An extremely massive and compact central mass can, however, dissolve the bar, which creates a pseudo-bulge component in the center of the disk.; Comment: 16 pages...

Substantial reduction of Stone-Wales activation barrier in fullerene

Kabir, Mukul; Mukherjee, Swarnakamal; Saha-Dasgupta, Tanusri
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/06/2011 Português
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Stone-Wales transformation is a key mechanism responsible for growth, transformation, and fusion in fullerene, carbon nanotube and other carbon nanostructures. These topological defects also substantially alter the physical and chemical properties of the carbon nanostructures. However, this transformation is thermodynamically limited by very high activation energy (\sim 7 eV in fullerene), which can get reduced due to the presence of hydrogen, extra carbon atom, or due to endohedral metal doping. Using first-principles density functional calculations, we show that the substitutional boron doping substantially reduces the Stone-Wales activation barrier (from \sim 7 eV to 2.54 eV). This reduction is the largest in magnitude among all the mechanisms of barrier reduction reported till date. Analysis of bonding charge density and phonon frequencies suggests that the bond weakening at and around the active Stone-Wales site in B-heterofullerene is responsible for such reduction. The formation of Stone-Wales defect is, therefore, promoted in such heterofullerenes, and is expected to affect their proposed H2 storage properties. Such substitutional doping can also modify the Stone-Wales activation barrier in carbon nanotube and graphene naonostructures...

Melt-Enhanced Rejuvenation of Lithospheric Mantle: Insights from the Colorado Plateau

Roy, Mousumi; Orozco, Rodrigo Osuna; Holtzman, Ben; Gaherty, James
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 12/12/2012 Português
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The stability of the lithospheric mantle beneath the ancient cratonic cores of continents is primarily a function of chemical modification during the process of melt extraction. Processes by which stable continental lithosphere may be destabilized are not well-understood, although destabilization by thickening and removal of negatively-buoyant lithospheric mantle in "delamination" events has been proposed in a number of tectonic settings. In this paper we explore an alternative process for destabilizing continents, namely, thermal and chemical modification during infiltration of metasomatic fluids and melts into the lithospheric column. We consider observations pertinent to the structure and evolution of the Colorado Plateau within the western United States to argue that the physical and chemical state of the margins of the plateau have been variably modified and destabilized by interaction with melts. In the melt-infiltration process explored here, the primary mechanism for weakening and rejuvenating the plate is through thermal effects and the feedback between deformation and melt-infiltration. We speculate on the nature and geometry of a melt-modulated interaction zone between lithosphere and asthenosphere and the seismically-observable consequences of variable melt-infiltration into the margins of regions of thick...

Salt-specific stability and denaturation of a short salt-bridge forming alpha-helix

Dzubiella, Joachim
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/08/2008 Português
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The structure of a single alanine-based Ace-AEAAAKEAAAKA-Nme peptide in explicit aqueous electrolyte solutions (NaCl, KCl, NaI, and KF) at large salt concentrations (3-4 M) is investigated using 1 microsecond molecular dynamics (MD) computer simulations. The peptide displays 71 alpha-helical structure without salt and destabilizes with the addition of NaCl in agreement with experiments of a somewhat longer version. It is mainly stabilized by direct and indirect (i+4)EK salt bridges between the Lys and Glu side chains and a concomitant backbone shielding mechanism. NaI is found to be a stronger denaturant than NaCl, while the potassium salts hardly show influence. Investigation of the molecular structures reveals that consistent with recent experiments Na+ has a much stronger affinity to side chain carboxylates and backbone carbonyls than K+, thereby weakening salt bridges and secondary structure hydrogen bonds. At the same time the large I- has a considerable affinity to the nonpolar alanine in line with recent observations of a large propensity of I- to adsorb to simple hydrophobes, and thereby 'assists' Na+ in its destabilizing action. In the denatured states of the peptide novel long-lived (10-20 ns) 'loop'-configurations are observed in which single Na+ ions and water molecules are hydrogen-bonded to multiple backbone carbonyls. In an attempt to analyze the denaturation behavior within the preferential interaction formalism...

Kinetic simulations of turbulent magnetic-field growth by streaming cosmic rays

Stroman, Thomas; Pohl, Martin; Niemiec, Jacek
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 28/09/2009 Português
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Efficient acceleration of cosmic rays (via the mechanism of diffusive shock acceleration) requires turbulent, amplified magnetic fields in the shock's upstream region. We present results of multidimensional particle-in-cell simulations aimed at observing the magnetic field amplification that is expected to arise from the cosmic-ray current ahead of the shock, and the impact on the properties of the upstream interstellar medium. We find that the initial structure and peak strength of the amplified field is somewhat sensitive to the choice of parameters, but that the field growth saturates in a similar manner in all cases: the back-reaction on the cosmic rays leads to modification of their rest-frame distribution and also a net transfer of momentum to the interstellar medium, substantially weakening their relative drift while also implying the development of a modified shock. The upstream medium becomes turbulent, with significant spatial fluctuations in density and velocity, the latter in particular leading to moderate upstream heating; such fluctuations will also have a strong influence on the shock structure.; Comment: 8 pages, 6 figures, accepted by ApJ

Equilibrium properties of highly asymmetric star-polymer mixtures

Mayer, Christian; Likos, Christos N.; Löwen, Hartmut
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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We employ effective interaction potentials to study the equilibrium structure and phase behavior of highly asymmetric mixtures of star polymers. We consider in particular the influence of the addition of a component with a small number of arms and a small size on a concentrated solution of large stars with a high functionality. By employing liquid integral equation theories we examine the evolution of the correlation functions of the big stars upon addition of the small ones, finding a loss of structure that can be attributed to a weakening of the repulsions between the large stars due to the presence of the small ones. We analyze this phenomenon be means of a generalized depletion mechanism which is supported by computer simulations. By applying thermodynamic perturbation theory we draw the phase diagram of the asymmetric mixture, finding that the addition of small stars melts the crystal formed by the big ones. A systematic comparison between the two- and effective one-component descriptions of the mixture that corroborates the reliability of the generalized depletion picture is also carried out.; Comment: 26 pages, 9 figures, submitted to Phys. Rev. E

Lyman Break Galaxies and the Lyman-alpha Forest

Kollmeier, Juna A.; Weinberg, David H.; Dave', Romeel; Katz, Neal
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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We use hydrodynamic simulations to predict correlations between Lya forest absorption and galaxies at redshift z~3. The probability distribution function (PDF) of Lya flux decrements shifts systematically towards higher values in the vicinity of galaxies, reflecting the overdense environments in which these galaxies reside. The predicted signal remains strong in spectra smoothed over 50-200 km/s, allowing tests with moderate resolution quasar spectra. The strong bias of high redshift galaxies towards high density regions imprints a clear signature on the flux PDF, but the predictions are not sensitive to galaxy baryon mass or star formation rate, and they are similar for galaxies and for dark matter halos. The dependence of the flux PDF on galaxy proximity is sensitive to redshift determination errors, with rms errors of 150-300 km/s substantially weakening the predicted trends. On larger scales, the mean galaxy overdensity in a cube of 5 or 10 Mpc/h (comoving) is strongly correlated with the mean Lya flux decrement on a line of sight through the cube center. The slope of the correlation is ~3 times steeper for galaxies than for dark matter as a result of galaxy bias. The predicted large scale correlation is in qualitative agreement with recently reported observational results. However...