Matthias Steinmetz

The cosmic web and the orientation of angular momenta

We use a 64xa0h−1xa0Mpc dark-matter-only cosmological simulation to examine the large-scale orientation of haloes and substructures with respect to the cosmic web. A web classification scheme based on the velocity shear tensor is used to assign to each halo in the simulation a web type: knot, filament, sheet or void. Using ∼106 haloes that span ∼3 orders of magnitude in mass, the orientation of the halo’s spin and the orbital angular momentum of subhaloes with respect to the eigenvectors of the shear tensor is examined. We find that the orbital angular momentum of subhaloes tends to align with the intermediate eigenvector of the velocity shear tensor for all haloes in knots, filaments and sheets. This result indicates that the kinematics of substructures located deep within the virialized regions of a halo is determined by its infall which in turn is determined by the large-scale velocity shear, a surprising result given the virialized nature of haloes. The non-random nature of subhalo accretion is thus imprinted on the angular momentum measured at z= 0. We also find that the haloes’ spin axis is aligned with the third eigenvector of the velocity shear tensor in filaments and sheets: the halo spin axis points along filaments and lies in the plane of cosmic sheets.

The velocity shear tensor: tracer of halo alignment

The alignment of dark matter (DM) halos and the surrounding large scale structure (LSS) is examined in the context of the cosmic web. Halo spin, shape and the orbital angular momentum of subhaloes is investigated relative to the LSS using the eigen- vectors of the velocity shear tensor evaluated on a grid with a scale of 1 Mpc/h, deep within the non-linear regime. Knots, laments, sheets and voids are associated with biased. We nd that larger mass haloes live in regions where the shear is more isotropic, namely the expansion or collapse is more spherical. A correlation is found between the halos shape and the eigenvectors of the shear tensor, with the longest (shortest) axis of the halos shape being aligned with the slowest (fastest) collapsing eigenvector. This correlation is web independent, suggest- ing that the velocity shear is a fundamental tracer of the halo alignment. A similar result is found for the alignment of halo spin with the cosmic web. It has been shown that high mass haloes exhibit a spin ip with respect to the LSS: we nd the mass at which this spin ip occurs is web dependent and not universal as suggested previously. Although weaker than haloes, subhalo orbits too exhibit an alignment with the LSS, providing a possible insight into the highly correlated co-rotation of the Milky Ways satellite system. The present study suggests that the velocity shear tensor constitutes the natural framework for studying the directional properties of the non-linear LSS and of halos and galaxies.

DWARF GALAXIES AND THE COSMIC WEB

We use a cosmological simulation of the formation of the Local Group of Galaxies to identify a mechanism that enables the removal of baryons from low-mass halos without appealing to feedback or reionization. As the Local Group forms, matter bound to it develops a network of filaments and pancakes. This moving web of gas and dark matter drifts and sweeps a large volume, overtaking many halos in the process. The dark matter content of these halos is unaffected but their gas can be efficiently removed by ram pressure. The loss of gas is especially pronounced in low-mass halos due to their lower binding energy and has a dramatic effect on the star formation history of affected systems. This cosmic web stripping may help to explain the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in ΛCDM and the large diversity of star formation histories and morphologies characteristic of faint galaxies. Although our results are based on a single high-resolution simulation, it is likely that the hydrodynamical interaction of dwarf galaxies with the cosmic web is a crucial ingredient so far missing from galaxy formation models.

The imprint of reionization on the star formation histories of dwarf galaxies

We explore the impact of cosmic reionization on nearby isolated dwarf galaxies using a compilation of SFHs estimated from deep HST data and a cosmological hydrodynamical simulation of the Local Group. The nearby dwarfs show a wide diversity of star formation histories; from ancient systems that have largely completed their star formation

Mergers and the outside-in formation of dwarf spheroidals

sim 10

Cluster kinematics and stellar rotation in NGC 419 with MUSE and adaptive optics

Gyr ago to young dwarfs that have formed the majority of their stars in the past

The Age-Metallicity-Velocity relation in the nearby disk

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The kinematical evolution of the Galactic disk

Gyr to two-component systems characterized by the overlap of comparable numbers of old and young stars. Taken as an ensemble, star formation in nearby dwarfs dips to lower-than-average rates at intermediate times (

METAL-POOR LITHIUM-RICH GIANTS IN THE RAVE SURVEY 1

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LCDM predictions for galaxy halos and visible components

/Gyr