Engineering topics

Sparre, Martin

Publications and source records attributed to Sparre, Martin.

6 records

DOE OSTI2017

Log-normal Star Formation Histories in Simulated and Observed Galaxies

Gladders et al. have recently suggested that the star formation histories (SFHs) of individual galaxies are characterized by a log-normal function in time, implying a slow decline rather than rapid quenching. We test their conjecture on theoretical SFHs from the cosmological simulation Illustris and on observationally inferred SFHs. While the log-normal form necessarily ignores short-lived features such as starbursts, it fits the overall shape of the majority of SFHs very well. In particular, 85% of the cumulative SFHs are fitted to within a maximum error of 5% of the total stellar mass formed, and 99% to within 10%. The log-normal performs systematically better than the commonly used delayed-τ model, and is superseded only by functions with more than three free parameters. Poor fits are mostly found in galaxies that were rapidly quenched after becoming satellites. We explore the log-normal parameter space of normalization, peak time, and full width at half maximum, and find that the simulated and observed samples occupy similar regions, though Illustris predicts wider, later-forming SFHs on average. The ensemble of log-normal fits correctly reproduces complex metrics such as the evolution of Illustris galaxies across the star formation main sequence, but overpredicts their quenching timescales. SFHs in Illustris are a diverse population not determined by any one physical property of galaxies, but follow a tight relation, where width ∝ (peaktime){sup 3/2}. We show that such a relation can be explained qualitatively (though not quantitatively) by a close connection between the growth of dark matter halos and their galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Particle ejection during mergers of dark matter halos

Dark matter halos are built from accretion and merging. During merging some of the dark matter particles may be ejected with velocities higher than the escape velocity. We use both N-body simulations and single-particle smooth-field simulations to demonstrate that rapid changes to the mean field potential are responsible for such ejection, and in particular that dynamical friction plays no significant role in it. Studying a range of minor mergers, we find that typically between 5–15% of the particles from the smaller of the two merging structures are ejected. We also find that the ejected particles originate essentially from the small halo, and more specifically are particles in the small halo which pass later through the region in which the merging occurs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

The behaviour of shape and velocity anisotropy in dark matter haloes

Dark matter haloes from cosmological N-body simulations typically have triaxial shapes and anisotropic velocity distributions. Recently it has been shown that the velocity anisotropy, β, of cosmological haloes and major merger remnants depend on direction in such a way that β is largest along the major axis and smallest along the minor axis. In this work we use a wide range of non-cosmological N-body simulations to examine halo shapes and direction-dependence of velocity anisotropy profiles. For each of our simulated haloes we define 48 cones pointing in different directions, and from the particles inside each cone we compute velocity anisotropy profiles. We find that elongated haloes can have very distinct velocity anisotropies. We group the behaviour of haloes into three different categories, that range from spherically symmetric profiles to a much more complex behaviour, where significant differences are found for β along the major and minor axes. We encourage future studies of velocity anisotropies in haloes from cosmological simulations to calculate β-profiles in cones, since it reveals information, which is hidden from a spherically averaged profile. Finally, we show that spherically averaged profiles often obey a linear relation between β and the logarithmic density slope in the inner parts of haloes, but this relation is not necessarily obeyed, when properties are calculated in cones.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

A DERIVATION OF (HALF) THE DARK MATTER DISTRIBUTION FUNCTION

All dark matter structures appear to follow a set of universalities, such as phase-space density or velocity anisotropy profiles; however, the origin of these universalities remains a mystery. Any equilibrated dark matter structure can be fully described by two functions, namely the radial and tangential velocity distribution functions (VDFs), and once these two are understood we will understand all the observed universalities. Here, we demonstrate that if we know the radial VDF then we can derive and understand the tangential VDF. This is based on simple dynamical arguments about properties of collisionless systems. We use a range of controlled numerical simulations to demonstrate the accuracy of this result. We therefore boil the question of the dark matter structural properties down to understanding the radial VDF.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

Asymmetric velocity anisotropies in remnants of collisionless mergers

Dark matter haloes in cosmological N-body simulations are affected by processes such as mergers, accretion and the gravitational interaction with baryonic matter. Typically the analysis of dark matter haloes is performed in spherical or elliptical bins and the velocity distributions are often assumed to be constant within those bins. However, the velocity anisotropy, which describes differences between the radial and tangential velocity dispersion, has recently been show to have a strong dependence on direction in the triaxial halos formed in cosmological simulations. In this study we derive properties of particles in cones parallel or perpendicular to the collision axis of merger remnants. We find that the velocity anisotropy has a strong dependence on direction. The finding that the direction-dependence of the velocity anisotropy of a halo depends on the merger history, explains the existence of such trends in cosmological simulations. It also explains why a large diversity is seen in the velocity anisotropy profiles in the outer parts of high-resolution simulations of cosmological haloes.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2010

AN ATTRACTOR FOR DARK MATTER STRUCTURES

Cosmological simulations of dark matter (DM) structures have identified a set of universal profiles, and similar characteristics have been seen in non-cosmological simulations. There has therefore been speculation as to whether these profiles of collisionless systems relate to accretion and merger history, or if there is an attractor for the DM systems. Here, we identify such a one-dimensional attractor in the three-dimensional space spanned by the two radial slopes of the density and velocity dispersion, and the velocity anisotropy. This attractor effectively removes 1 degree of freedom from the Jeans equation. It also allows us to speculate on a new fluid interpretation for the Jeans equation, with an effective polytropic index for the DM particles between 1/2 and 3/4. If this attractor solution holds for other collisionless structures, then it may hold the key to breaking the mass-anisotropy degeneracy, which presently prevents us from uniquely measuring the mass profiles in dwarf galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗