arXiv subjects

D. Lee

Publications and source records attributed to D. Lee.

51 records · Page 3Linked to original sources

arXiv2012

MOONS: a Multi-Object Optical and Near-infrared Spectrograph for the VLT

MOONS is a new conceptual design for a Multi-Object Optical and Near-infrared Spectrograph for the Very Large Telescope (VLT), selected by ESO for a Phase A study. The baseline design consists of 1000 fibers deployable over a field of view of 500 square arcmin, the largest patrol field offered by the Nasmyth focus at the VLT. The total wavelength coverage is 0.8um-1.8um and two resolution modes: medium resolution and high resolution. In the medium resolution mode (R=4,000-6,000) the entire wavelength range 0.8um-1.8um is observed simultaneously, while the high resolution mode covers simultaneously three selected spectral regions: one around the CaII triplet (at R=8,000) to measure radial velocities, and two regions at R=20,000 one in the J-band and one in the H-band, for detailed measurements of chemical abundances. The grasp of the 8.2m Very Large Telescope (VLT) combined with the large multiplex and wavelength coverage of MOONS - extending into the near-IR - will provide the observational power necessary to study galaxy formation and evolution over the entire history of the Universe, from our Milky Way, through the redshift desert and up to the epoch of re-ionization at z>8-9. At the same time, the high spectral resolution mode will allow astronomers to study chemical abundances of stars in our Galaxy, in particular in the highly obscured regions of the Bulge, and provide the necessary follow-up of the Gaia mission. Such characteristics and versatility make MOONS the long-awaited workhorse near-IR MOS for the VLT, which will perfectly complement optical spectroscopy performed by FLAMES and VIMOS.

astro-ph.IM↗

arXiv2012

Multi-Object Spectroscopy with the European ELT: Scientific synergies between EAGLE & EVE

The EAGLE and EVE Phase A studies for instruments for the European Extremely Large Telescope (E-ELT) originated from related top-level scientific questions, but employed different (yet complementary) methods to deliver the required observations. We re-examine the motivations for a multi-object spectrograph (MOS) on the E-ELT and present a unified set of requirements for a versatile instrument. Such a MOS would exploit the excellent spatial resolution in the near-infrared envisaged for EAGLE, combined with aspects of the spectral coverage and large multiplex of EVE. We briefly discuss the top-level systems which could satisfy these requirements in a single instrument at one of the Nasmyth foci of the E-ELT.

astro-ph.IM↗

arXiv2011

Sloshing of the Magnetized Cool Gas in the Cores of Galaxy Clusters

X-ray observations of many clusters of galaxies reveal the presence of edges in surface brightness and temperature, known as "cold fronts". In relaxed clusters with cool cores, these edges have been interpreted as evidence for the "sloshing" of the core gas in the cluster's gravitational potential. The smoothness of these edges has been interpreted as evidence for the stabilizing effect of magnetic fields "draped" around the front surfaces. To check this hypothesis, we perform high-resolution magnetohydrodynamics simulations of magnetized gas sloshing in galaxy clusters initiated by encounters with subclusters. We go beyond previous works on the simulation of cold fronts in a magnetized intracluster medium by simulating their formation in realistic, idealized mergers with high resolution (Δx ~ 2 kpc). Our simulations sample a parameter space of plausible initial magnetic field strengths and field configurations. In the simulations, we observe strong velocity shears associated with the cold fronts amplifying the magnetic field along the cold front surfaces, increasing the magnetic field strength in these layers by up to an order of magnitude, and boosting the magnetic pressure up to near-equipartition with thermal pressure in some cases. In these layers, the magnetic field becomes strong enough to stabilize the cold fronts against Kelvin-Helmholtz instabilities, resulting in sharp, smooth fronts as those seen in observations of real clusters. These magnetic fields also result in strong suppression of mixing of high and low-entropy gas in the cluster, seen in our simulations of mergers in the absence of a magnetic field. As a result, the heating of the core due to sloshing is very modest and is unable to stave off a cooling catastrophe.

astro-ph.CO↗

arXiv2011

The excitation of 5-min oscillations in the solar corona

We aim to study excitation of the observed 5-min oscillations in the solar corona by localized pulses that are launched in the photosphere. We solve the full set of nonlinear one-dimensional Euler equations numerically for the velocity pulse propagating in the solar atmosphere that is determined by the realistic temperature profile. Numerical simulations show that an initial velocity pulse quickly steepens into a leading shock, while the nonlinear wake in the chromosphere leads to the formation of consecutive pulses. The time interval between arrivals of two neighboring pulses to a detection point in the corona is approximately 5 min. Therefore, the consecutive pulses may result in the 5-min oscillations that are observed in the solar corona. The 5-min oscillations observed in the solar corona can be explained in terms of consecutive shocks that result from impulsive triggers launched within the solar photosphere by granulation and/or reconnection.

astro-ph.SR↗

arXiv2010

Cosmological MHD simulations of cluster formation with anisotropic thermal conduction

(abridged) The ICM has been suggested to be buoyantly unstable in the presence of magnetic field and anisotropic thermal conduction. We perform first cosmological simulations of galaxy cluster formation that simultaneously include magnetic fields, radiative cooling and anisotropic thermal conduction. In isolated and idealized cluster models, the magnetothermal instability (MTI) tends to reorient the magnetic fields radially. Using cosmological simulations of the Santa Barbara cluster we detect radial bias in the velocity and magnetic fields. Such radial bias is consistent with either the inhomogeneous radial gas flows due to substructures or residual MTI-driven field rearangements that are expected even in the presence of turbulence. Although disentangling the two scenarios is challenging, we do not detect excess bias in the runs that include anisotropic thermal conduction. The anisotropy effect is potentially detectable via radio polarization measurements with LOFAR and SKA and future X-ray spectroscopic studies with the IXO. We demonstrate that radiative cooling boosts the amplification of the magnetic field by about two orders of magnitude beyond what is expected in the non-radiative cases. At z=0 the field is amplified by a factor of about 10^6 compared to the uniform magnetic field evolved due to the universal expansion alone. Interestingly, the runs that include both radiative cooling and anisotropic thermal conduction exhibit stronger magnetic field amplification than purely radiative runs at the off-center locations. In these runs, shallow temperature gradients away from the cluster center make the ICM neutrally buoyant. The ICM is more easily mixed in these regions and the winding up of the frozen-in magnetic field is more efficient resulting in stronger magnetic field amplification.

astro-ph.CO↗

arXiv2010

Local Probing of Mesoscopic Physics of Ferroelectric Domain Walls

Domain wall dynamics in ferroic materials underpins functionality of data storage and information technology devices. Using localized electric field of a scanning probe microscopy tip, we experimentally demonstrate a surprisingly rich range of polarization reversal behaviors in the vicinity of the initially flat 180°ferroelectric domain wall. The nucleation bias is found to increase by an order of magnitude from a 2D nucleus at the wall to 3D nucleus in the bulk. The wall is thus significantly ferroelectrically softer than the bulk. The wall profoundly affects switching on length scales of the order of micrometers. The mechanism of correlated switching is analyzed using analytical theory and phase-field modeling. The long-range effect is ascribed to wall bending under the influence of a tip bias well below the bulk nucleation field and placed many micrometers away from the wall. These studies provide an experimental link between the macroscopic and mesoscopic physics of domain walls and atomistic models of nucleation.

cond-mat.mtrl-sci↗

arXiv2010

Epitaxial growth of multiferroic Pb(Zr0.57Ti0.43)O3-Pb(Fe2/3W1/3)O3 solid-solution thin films and their magnetoelectric effects

We report on epitaxial growth of single-phase [Pb(Zr0.57Ti0.43)O3]0.8[Pb(Fe2/3W1/3)O3]0.2 (PZT-PFW) solid-solution thin films using pulsed laser deposition. X-ray diffraction measurements reveal that the films have a tetragonal structure. The films exhibit ferroelectric properties and weak ferromagnetic responses at room temperature. Magnetoelectric effects were investigated; the nonlinear magnetoelectric coefficient was measured and found to be comparable to those of multiferroic hexagonal manganites, but at least two orders of magnitude smaller than that for polycrystalline PZT-PFW films.

cond-mat.mtrl-sci↗

arXiv2009

Oxygen vacancy induced re-entrant spin glass behavior in multiferroic ErMnO3 thin films

Epitaxial thin films of hexagonal ErMnO3 fabricated on Pt(111)/Al2O3(0001) and YSZ(111) substrates exhibited both ferroelectric character and magnetic ordering at low temperatures. As the temperature was reduced, the ErMnO3 films first showed antiferromagnetism. At lower temperatures, the films deposited at lower oxygen partial pressures exhibited spin glass behavior. This re-entrant spin glass behavior was attributed to competition between an antiferromagnetic interaction in the hexagonal geometry and a ferromagnetic interaction caused by a change in Mn valence induced by excess electrons from the oxygen vacancies.

cond-mat.str-el↗

arXiv2007

Electronic structures of hexagonal RMnO3 (R = Gd, Tb, Dy, and Ho) thin films

We investigated the electronic structure of multiferroic hexagonal RMnO3 (R = Gd, Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. Using artificially stabilized hexagonal RMnO3, we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system. We observed two optical transitions located near 1.7 eV and 2.3 eV, in addition to the predominant absorption above 5 eV. With the help of first-principles calculations, we attribute the low-lying optical absorption peaks to inter-site transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the Mn 3d3z2-r2 state. As the ionic radius of the rare earth ion increased, the lowest peak showed a systematic increase in its peak position. We explained this systematic change in terms of a flattening of the MnO5 triangular bipyramid.

cond-mat.str-el↗

arXiv2006

Multiferroic properties of epitaxially stabilized hexagonal DyMnO3 thin films

We fabricated epitaxial thin films of hexagonal DyMnO3, which otherwise form in a bulk perovskite structure, via deposition on Pt(111)//Al2O3 (0001) and YSZ(111) substrates: each of which has in-plane hexagonal symmetry. The polarization hysteresis loop demonstrated the existence of ferroelectricity in our hexagonal DyMnO3 films at least below 70 K. The observed 2.2 uC/cm^2 remnant polarization at 25 K corresponded to a polarization enhancement by a factor of 10 compared to that of the bulk orthorhombic DyMnO3. Interestingly, this system showed an antiferroelectric-like feature in its hysteresis loop. Our hexagonal DyMnO3 films showed an antiferromagnetic Neel temperature around 60 K and a spin reorientation transition around 40 K. We also found a clear hysteresis in the temperature dependence of the magnetization, which was measured after zero-field-cooling and field-cooling. This hysteresis may well have been of spin glass origin, which was likely to arise from the geometric frustration of antiferromagnetically-coupled Mn spins with an edge-sharing triangular lattice.

cond-mat.mtrl-sci↗

arXiv2006

Deep spectroscopy of 9C J1503+4528: a very young CSS radio source at z=0.521

9C J1503+4528 is a very young CSS radio galaxy, with an age of order 10^4 years. This source is therefore an ideal laboratory for the study ofthe intrinsic host galaxy/IGM properties, interactions between the radio source and surrounding ISM, links between star formation and AGN activity and the radio source triggering mechanism. Here we present the results of a spectroscopic analysis of this source, considering each of these aspects of radio source physics. We find that shock ionization by the young radio source is important in the central regions of the galaxy on scales similar to that of the radio source itself, whilst evidence for an AGN ionization cone is observed at greater distances. Line and continuum features require the presence of a young stellar population, the best-fit model for which implies an age of 5x10^6 years, significantly older than the radio source. Most interestingly, the relative sizes of radio source and extended emission line region suggest that both AGN and radio source are triggered at approximately the same time. If both the triggering of the radio source activity and the formation of the young stellar population had the same underlying cause, this source provides a sequence for the events surrounding the triggering process. We propose that the AGN activity in 9C J1503+4528 was causedby a relatively minor interaction, and that a super-massive black hole powering the radio jets must have been in place before the AGN was triggered.

astro-ph↗

arXiv2005

The triton and three-nucleon force in nuclear lattice simulations

We study the triton and three-nucleon force at lowest chiral order in pionless effective field theory both in the Hamiltonian and Euclidean nuclear lattice formalism. In the case of the Euclidean lattice formalism, we derive the exact few-body worldline amplitudes corresponding to the standard many-body lattice action. This will be useful for setting low-energy coefficients in future nuclear lattice simulations. We work in the Wigner SU(4)-symmetric limit where the S-wave scattering lengths {1}S{0} and {3}S{1} are equal. By comparing with continuum results, we demonstrate for the first time that the nuclear lattice formalism can be used to study few-body nucleon systems.

nucl-th↗

arXiv2005

Deep spectroscopy of a young radio source at z=0.521

9C J1503+4528 is a very young CSS radio galaxy, with an age of order 10^4 years. This source is an ideal laboratory for the study of the intrinsic host galaxy/IGM properties, radio source interactions, evidence for young stellar populations and the radio source triggering mechanism. Here we present the results of a spectroscopic analysis of this source, considering each of these aspects of radio source physics.

astro-ph↗

arXiv2005

Optical studies of carrier and phonon dynamics in Ga_{1-x}Mn_{x}As

We present a time-resolved optical study of the dynamics of carriers and phonons in Ga_{1-x}Mn_{x}As layers for a series of Mn and hole concentrations. While band filling is the dominant effect in transient optical absorption in low-temperature-grown (LT) GaAs, band gap renormalization effects become important with increasing Mn concentration in Ga_{1-x}Mn_{x}As, as inferred from the sign of the absorption change. We also report direct observation on lattice vibrations in Ga1-xMnxAs layers via reflective electro-optic sampling technique. The data show increasingly fast dephasing of LO phonon oscillations for samples with increasing Mn and hole concentration, which can be understood in term of phonon scattering by the holes.

cond-mat.mtrl-sci↗

arXiv2001

The early-type galaxy population in Abell 2218

(Abridged) We present morphological, photometric, kinematic and line-strength data of 48 early-type members of the rich cluster Abell 2218 at z=0.18. This sample is both larger and spans a wider galaxy luminosity range, down to M*_B+1, than previous studies. We compare A2218 with similar samples drawn from local clusters to identify evolutionary changes between the samples which have occured over the last 3Gyrs. The overall picture is one of little or no evolution in nearly all galaxy parameters. Zeropoint offsets in the Faber--Jackson, Mg_b-sigma and Fundamental Plane relations are all consistent with passively evolving stellar populations. The slopes of these relations have not changed significantly. We do however find a significant spread in the estimated luminosity-weighted ages of the stellar populations in the galaxies, based on line diagnostic diagrams. This age spread is seen in both the disky early-type galaxies (S0) and also the ellipticals. We observe both ellipticals with a strong contribution from a young stellar population and lenticulars dominated by old stellar populations. On average, we find no evidence for systematic differences between the populations of ellipticals and lenticulars. This points to a common formation epoch for the bulk of the stars in most of the early-type galaxies in A2218. This result can be reconciled with the claims of rapid morphological evolution in distant clusters if the suggested transformation from spirals to lenticulars does not involve significant new star formation.

astro-ph↗