Collaborative Housing in Scandinavia and Western Europe

Special Studies Experience, Summer 2011 -- Oslo, Norway; Stockholm, Sweden; Utrecht, Rotterdam and Delft, Netherlands; Rennes and Angers, France; London, Englandhttp://deepblue.lib.umich.edu/bitstream/2027.42/110143/1/Poster_Danovich.pd

Four phases of angular-momentum buildup in high-z galaxies: from cosmic-web streams through an extended ring to disc and bulge

We study the angular-momentum (AM) buildup in high-$z$ massive galaxies using high-resolution cosmological simulations. The AM originates in co-planar streams of cold gas and merging galaxies tracing cosmic-web filaments, and it undergoes four phases of evolution. (I) Outside the halo virial radius ($R_{\rm v}\!\sim\!100\,{\rm kpc}$), the elongated streams gain AM by tidal torques with a specific AM (sAM) $\sim\!1.7$ times the dark-matter (DM) spin due to the gas' higher quadrupole moment. This AM is expressed as stream impact parameters, from $\sim\!0.3R_{\rm v}$ to counter rotation. (II) In the outer halo, while the incoming DM mixes with the existing halo of lower sAM to a spin $\lambda_{\rm dm}\!\sim\!0.04$, the cold streams transport the AM to the inner halo such that their spin in the halo is $\sim\!3\lambda_{\rm dm}$. (III) Near pericenter, the streams dissipate into an irregular rotating ring extending to $\sim\!0.3R_{\rm v}$ and tilted relative to the inner disc. Torques exerted partly by the disc make the ring gas lose AM, spiral in, and settle into the disc within one orbit. The ring is observable with 30\% probability as a damped Lyman-$\alpha$ absorber. (IV) Within the disc, $<\!0.1R_{\rm v}$, torques associated with violent disc instability drive AM out and baryons into a central bulge, while outflows remove low-spin gas, introducing certain sensitivity to feedback strength. Despite the different AM histories of gas and DM, the disc spin is comparable to the DM-halo spin. Counter rotation can strongly affect disc evolution.Comment: Resubmitted to MNRAS after responding to referee's comments. (27 pages, 20 figures

Auger recombination of dark excitons in WS2{\rm WS_2} and WSe2{\rm WSe_2} monolayers

We propose a novel phonon assisted Auger process unique to the electronic band structure of monolayer transition metal dichalcogenides (TMDCs), which dominates the radiative recombination of ground state excitons in Tungsten based TMDCs. Using experimental and DFT computed values for the exciton energies, spin-orbit splittings, optical matrix element, and the Auger matrix elements, we find that the Auger process begins to dominate at carrier densities as low as $10^{9-10}~{\rm cm^{-2}}$, thus providing a plausible explanation for the low quantum efficiencies reported for these materials.Comment: 5 pages, 2 figure

Coplanar streams, pancakes and angular-momentum exchange in high-z disc galaxies

We study the feeding of massive galaxies at high redshift through streams from the cosmic web using the Mare Nostrum hydrocosmological simulation. Our statistical sample consists of 350 dark matter haloes of ≃1012 M⊙ at z= 2.5. We find that ∼70 per cent of the influx into the virial radius Rv is in narrow streams covering 10 per cent of the virial shell. On average 64 per cent of the stream influx is in one stream, and 95 per cent is in three dominant streams. The streams that feed a massive halo tend to lie in a plane that extends from half to a few Rv, hereafter ‘the stream plane' (SP). The streams are typically embedded in a thin sheet of low-entropy gas, a Zel'dovich pancake, which carries ∼20 per cent of the influx into Rv. The filaments-in-a-plane configuration about the massive haloes at the nodes of the cosmic web differs from the large-scale structure of the web where the filaments mark the intersections of slanted sheets. The SP is only weakly aligned with the angular momentum (AM) near Rv, consistent with the fact that typically 80 per cent of the AM is carried by one dominant stream. The galactic disc plane shows a weak tendency to be perpendicular to the large-scale SP, consistent with tidal-torque theory. Most interesting, the direction of the disc AM is only weakly correlated with the AM direction at Rv. This indicates a significant AM exchange at the interphase between streams and disc in the greater environment of the disc inside an ‘AM sphere' of radius ∼0.3Rv. The required large torques are expected based on the perturbed morphology and kinematics within this interaction sphere. This AM exchange may or may not require a major modification of the standard disc modelling based on AM conservation, depending on the extent to which the amplitude of the disc AM is affected, which is yet to be studie

INFLUENCE OF VARIOUS ANOMALIES IN THE VEHICLE AND TRACK PARAMETERS ON THE POSSIBILITY OF DERAILMENT

A detailed non linear model of the four-axle real vehicle, considering both vertical and lateral vibrations has been proposed. Inertial and elastic-viscous properties of the track have been taken into account. Comparison of the theoretical investigations and on-track tests has confirmed the rather good authenticity of the model. The calculations for anomalies either in track or in vehicle parameters as well as for their combinations have been performed

Co-planar streams, pancakes, and angular-momentum exchange in high-z disc galaxies

We study the feeding of massive galaxies at high redshift through streams from the cosmic web using the Mare Nostrum hydro-cosmological simulation. Our statistical sample consists of 350 dark-matter haloes of ~10^12 Msun at z = 2.5. We find that ~70% of the influx into the virial radius Rv is in narrow streams covering 10% of the virial shell. On average 64% of the stream influx is in one stream, and 95% is in three dominant streams. The streams that feed a massive halo tend to lie in a plane that extends from half to a few Rv, hereafter "the stream plane" (SP). The streams are typically embedded in a thin sheet of low-entropy gas, a Zel'dovich pancake, which carries ~20% of the influx into Rv. The filaments-in-a-plane configuration about the massive haloes at the nodes of the cosmic web differs from the large- scale structure of the web where the filaments mark the intersections of slanted sheets. The stream plane is only weakly aligned with the angular momentum (AM) near Rv, consistent with the fact that typically 80% of the AM is carried by one dominant stream. The galactic disc plane shows a weak tendency to be perpendicular to the large-scale SP, consistent with tidal-torque theory. Most interesting, the direction of the disc AM is only weakly correlated with the AM direction at Rv. This indicates a significant AM exchange at the interphase between streams and disc in the greater environment of the disc inside an "AM sphere" of radius ~0.3Rv . The required large torques are expected based on the perturbed morphology and kinematics within this interaction sphere. This AM exchange may or may not require a major modification of the standard disc modeling based on AM conservation, depending on the extent to which the amplitude of the disc AM is affected, which is yet to be studied.Comment: There are minor revisions to v

Four phases of angular-momentum buildup in high-z galaxies: from cosmic-web streams through an extended ring to disc and bulge

We study the angular-momentum (AM) buildup in high-z massive galaxies using high-resolution cosmological simulations. The AM originates in co-planar streams of cold gas and merging galaxies tracing cosmic-web filaments, and it undergoes four phases of evolution. (I) Outside the halo virial radius (Rv∼100 kpc), the elongated streams gain AM by tidal torques with a specific AM (sAM) ∼1.7times the dark matter (DM) spin due to the gas' higher quadrupole moment. This AM is expressed as stream impact parameters, from ∼0.3Rv to counter rotation. (II) In the outer halo, while the incoming DM mixes with the existing halo of lower sAM to a spin λdm∼0.04, the cold streams transport the AM to the inner halo such that their spin in the halo is ∼3λdm. (III) Near pericentre, the streams dissipate into an irregular rotating ring extending to ∼0.3Rv and tilted relative to the inner disc. Torques exerted partly by the disc make the ring gas lose AM, spiral in, and settle into the disc within one orbit. The ring is observable with 30 per cent probability as a damped Lyman α absorber. (IV) Within the disc, <0.1Rv, torques associated with violent disc instability drive AM out and baryons into a central bulge, while outflows remove low-spin gas, introducing certain sensitivity to feedback strength. Despite the different AM histories of gas and DM, the disc spin is comparable to the DM-halo spin. Counter rotation can strongly affect disc evolutio

Fast relaxation of photo-excited carriers in 2-D transition metal dichalcogenides

We predict a fast relaxation of photo-excited carriers in monolayer transition metal dichalcogenides, which is mediated by the emission of longitudinal optical (LO) and homopolar (HP) phonons. By evaluating Born effective charges for MoS2, MoSe2, WS2, and WSe2, we find that, due to the polar coupling of electrons with LO phonons, and the HP phonons lattice deformation potential, the cooling times for hot electrons and holes from excitation energies of several hundred meV are at ps-scale