Showing papers by "Joel R. Primack published in 2005"

The Shape of Dark Matter Halos: Dependence on Mass, Redshift, Radius, and Formation

Abstract: Using six high resolution dissipationless simulations with a varying box size in a flat LCDM universe, we study the mass and redshift dependence of dark matter halo shapes for M_vir = 9.0e11 - 2.0e14, over the redshift range z=0-3, and for two values of sigma_8=0.75 and 0.9. Remarkably, we find that the redshift, mass, and sigma_8 dependence of the mean smallest-to-largest axis ratio of halos is well described by the simple power-law relation = (0.54 +- 0.02)(M_vir/M_*)^(-0.050 +- 0.003), where s is measured at 0.3 R_vir and the z and sigma_8 dependences are governed by the characteristic nonlinear mass, M_*=M_*(z,sigma_8). We find that the scatter about the mean s is well described by a Gaussian with sigma ~ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the halos through ~100 snapshots in time. We determine the formation scalefactor a_c as defined by Wechsler et al. (2002) and find that it can be related to the halo shape at z = 0 and its evolution over time.

Observational Gamma-ray Cosmology

Abstract: We discuss how measurements of the absorption of gamma‐rays from GeV to TeV energies via pair production on the extragalactic background light (EBL) can probe important issues in galaxy formation. Semi‐analytic models (SAMs) of galaxy formation, based on the flat LCDM hierarchical structure formation scenario, are used to make predictions of the EBL from 0.1 to 1000 microns. SAMs incorporate simplified physical treatments of the key processes of galaxy formation — including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production. We will summarize SAM successes and failures in accounting for observations at low and high redshift. New ground‐ and space‐based gamma ray telescopes will help to determine the EBL, and also help to explain its origin by constraining some of the most uncertain features of galaxy formation theory, including the stellar initial mass function, the history of star formation, and the reprocessing of light by dust. On a separate topic concerning gamma ray cosmology, we discuss a new theoretical insight into the distribution of dark matter at the center of the Milky Way, and its implications concerning the high energy gamma rays observed from the Galactic center.

Dark energy and dark matter haloes

Abstract: We investigate the effect of dark energy on the density profiles of dark matter haloes with a suite of cosmological N-body simulations and use our results to test analytic models. We consider constant equation of state models, and allow both w −1 and w −1 quintessence models seem to exacerbate the central density problem relative to the standard w =− 1 model. Fo re xample, models with w �− 0.5 seem disfavoured by the data, which can be matched only by allowing extremely low normalizations, σ 8 0.6. Meanwhile w< −1 models help to reduce the apparent discrepancy. We confirm that the halo mass function of Jenkins et al. provides an excellent approximation to the abundance of haloes in our simulations and extend its region of validity to include models with w< −1. Ke yw ords: methods: N-body simulations ‐ cosmology: theory ‐ dark matter ‐ large-scale structure of Universe.

The effects of feedback in simulations of disk-galaxy major mergers

Abstract: Using hydrodynamic simulations of disc-galaxy major mergers, we investigate the star formation history and remnant properties when various parametrizations of a simple stellar feedback model are implemented The simulations include radiative cooling, a density-dependent star formation recipe and a model for feedback from massive stars The feedback model stores supernova feedback energy within individual gas particles and dissipates this energy on a time-scale specified by two free parameters; tau_fb, which sets the dissipative time-scale, and n, which sets the effective equation of state in star-forming regions Using a self-consistent disc galaxy, modelled after a local Sbc spiral, in both isolated and major-merger simulations, we investigate parametrizations of the feedback model that are selected with respect to the quiescent disc stability These models produce a range of star formation histories that are consistent with the star formation relation found by Kennicutt All major mergers produce a population of new stars that is highly centrally concentrated, demonstrating a distinct break in the r1/4 surface density profile, consistent with previous findings The half-mass radius and one-dimensional velocity dispersion are affected by the feedback model used Finally, we compare our results to those of previous simulations of star formation in disc-galaxy major mergers, addressing the effects of star formation normalization, the version of smoothed particle hydrodynamics (SPH) employed and assumptions about the interstellar medium

Feedback in simulations of disc-galaxy major mergers

Abstract: Using hydrodynamic simulations of disc-galaxy major mergers, we investigate the star formation history and remnant properties when various parametrizations of a simple stellar feedback model are implemented. The simulations include radiative cooling, a density-dependent star formation recipe and a model for feedback from massive stars. The feedback model stores supernova feedback energy within individual gas particles and dissipates this energy on a time-scale specified by two free parameters; tau_fb, which sets the dissipative time-scale, and n, which sets the effective equation of state in star-forming regions. Using a self-consistent disc galaxy, modelled after a local Sbc spiral, in both isolated and major-merger simulations, we investigate parametrizations of the feedback model that are selected with respect to the quiescent disc stability. These models produce a range of star formation histories that are consistent with the star formation relation found by Kennicutt. All major mergers produce a population of new stars that is highly centrally concentrated, demonstrating a distinct break in the r1/4 surface density profile, consistent with previous findings. The half-mass radius and one-dimensional velocity dispersion are affected by the feedback model used. Finally, we compare our results to those of previous simulations of star formation in disc-galaxy major mergers, addressing the effects of star formation normalization, the version of smoothed particle hydrodynamics (SPH) employed and assumptions about the interstellar medium.

Observational Gamma-ray Cosmology

Abstract: We discuss how measurements of the absorption of gamma-rays from GeV to TeV energies via pair production on the extragalactic background light (EBL) can probe important issues in galaxy formation. Semi-analytic models (SAMs) of galaxy formation, based on the flat LCDM hierarchical structure formation scenario, are used to make predictions of the EBL from 0.1 to 1000 microns. SAMs incorporate simplified physical treatments of the key processes of galaxy formation -- including gravitational collapse and merging of dark matter halos, gas cooling and dissipation, star formation, supernova feedback and metal production. We will summarize SAM successes and failures in accounting for observations at low and high redshift. New ground- and space-based gamma ray telescopes will help to determine the EBL, and also help to explain its origin by constraining some of the most uncertain features of galaxy formation theory, including the stellar initial mass function, the history of star formation, and the reprocessing of light by dust. On a separate topic concerning gamma ray cosmology, we discuss a new theoretical insight into the distribution of dark matter at the center of the Milky Way, and its implications concerning the high energy gamma rays observed from the Galactic center.

Dark-Matter Haloes in Elliptical Galaxies: Lost and Found

Abstract: There is strong evidence that the mass in the Universe is dominated by dark matter, which exerts gravitational attraction but whose exact nature is unknown. In particular, all galaxies are believed to be embedded in massive haloes of dark matter. This view has recently been challenged by surprisingly low random stellar velocities in the outskirts of ordinary elliptical galaxies, which were interpreted as indicating a lack of dark matter (Mendez et al. 2001; Romanowsky et al. 2003). Here we show that the low velocities are in fact compatible with galaxy formation in dark-matter haloes. Using numerical simulations of disc-galaxy mergers, we find that the stellar orbits in the outer regions of the resulting ellipticals are very elongated. These stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories. The elongated orbits, combined with the steeply falling density profile of the observed tracers, explain the observed low velocities even in the presence of large amounts of dark matter. Projection effects when viewing a triaxial elliptical can lead to even lower observed velocities along certain lines of sight.

Simulations of Dust in Interacting Galaxies

Abstract: A new Monte‐Carlo radiative‐transfer code, Sunrise, is used to study the effects of dust in N‐body/hydrodynamic simulations of interacting galaxies. Dust has a profound effect on the appearance of the simulated galaxies. At peak luminosities, ∼ 90% of the bolometric luminosity is absorbed, and the dust obscuration scales with luminosity in such a way that the brightness at UV/visual wavelengths remains roughly constant. A general relationship between the fraction of energy absorbed and the ratio of bolometric luminosity to baryonic mass is found.Comparing to observations, the simulations are found to follow a relation similar to the observed IRX‐β relation found by Meurer et al. when similar luminosity objects are considered. The highest‐luminosity simulated galaxies depart from this relation and occupy the region where local (U)LIRGs are found. This agreement is contingent on the presence of Milky‐Way‐like dust, while SMC‐like dust results in far too red a UV continuum slope to match observations.The sim...