Velocity Structure of Self-Similar Spherically Collapsed Halos

Using a generalized self-similar secondary infall model, which accounts for tidal torques acting on the halo, we analyze the velocity profiles of halos in order to gain intuition for N-body simulation results. We analytically calculate the asymptotic behavior of the internal radial and tangential kinetic energy profiles in different radial regimes. We then numerically compute the velocity anisotropy and pseudo-phase-space density profiles and compare them to recent N-body simulations. For cosmological initial conditions, we find both numerically and analytically that the anisotropy profile asymptotes at small radii to a constant set by model parameters. It rises on intermediate scales as the velocity dispersion becomes more radially dominated and then drops off at radii larger than the virial radius where the radial velocity dispersion vanishes in our model. The pseudo-phase-space density is universal on intermediate and large scales. However, its asymptotic slope on small scales depends on the halo mass and on how mass shells are torqued after turnaround. The results largely confirm N-body simulations but show some differences that are likely due to our assumption of a one-dimensional phase space manifold.Comment: 11 pages, 4 figures. Accepted by PR

Scale-Dependent Growth from a Transition in Dark Energy Dynamics

We investigate the observational consequences of the quintessence field rolling to and oscillating near a minimum in its potential, "if" it happens close to the present epoch (z<0.2). We show that in a class of models, the oscillations lead to a rapid growth of the field fluctuations and the gravitational potential on subhorizon scales. The growth in the gravitational potential occurs on timescales << H^(1). This effect is present even when the quintessence parameters are chosen to reproduce an expansion history consistent with observations. For linearized fluctuations, we find that although the gravitational potential power spectrum is enhanced in a scale-dependent manner, the shape of the dark matter/galaxy power spectrum is not significantly affected. We find that the best constraints on such a transition in the quintessence field is provided via the integrated Sachs-Wolfe (ISW) effect in the CMB temperature power spectrum. Going beyond the linearized regime, the quintessence field can fragment into large, localized, long lived excitations (oscillons) with sizes comparable to galaxy clusters; this fragmentation could provide additional observational constraints. Two quoted "signatures" of modified gravity are a scale-dependent growth of the gravitational potential and a difference between the matter power spectrum inferred from measurements of lensing and galaxy clustering. Here, both effects are achieved by a minimally coupled scalar field in general relativity with a canonical kinetic term.Comment: 38 pages, 8 Figures, references added and minor changes in text, matches version published in PR

Dark matter dynamics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 147-150).N-body simulations have revealed a wealth of information about dark matter halos but their results are largely empirical. Here we attempt to shed light on simulation results by using a combination of analytic and numerical methods. First we generalize an analytic model of halo formation, known as Secondary Infall, to include the effects of tidal torque. Given this model we compare its predictions for halo profiles to simulation results and infer that angular momentum plays an important role in setting the structure of dark matter profiles at small radii. Next, we focus on explaining the origin of universality in halos. We find evidence that diffusion -- which can potentially lead to universality -- occurs during halo evolution and is partially sourced by external torques from large scale structure. This is surprising given that the halo is nonlinear and typically thought to be unaffected by neighboring structures. Last, we describe promising ways to analytically describe the evolution of nonlinear halos using a Fokker-Planck formalism.by Phillip Gregory Zukin.Ph.D

The Effects of Varying Cosmological Parameters on Halo Substructure

We investigate how different cosmological parameters, such as those delivered by the WMAP and Planck missions, affect the nature and evolution of dark matter halo substructure. We use a series of flat $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological $N$-body simulations of structure formation, each with a different power spectrum but the same initial white noise field. Our fiducial simulation is based on parameters from the WMAP 7th year cosmology. We then systematically vary the spectral index, $n_s$, matter density, $\Omega_M$, and normalization of the power spectrum, $\sigma_8$, for 7 unique simulations. Across these, we study variations in the subhalo mass function, mass fraction, maximum circular velocity function, spatial distribution, concentration, formation times, accretion times, and peak mass. We eliminate dependence of subhalo properties on host halo mass and average over many hosts to reduce variance. While the "same" subhalos from identical initial overdensity peaks in higher $\sigma_8, n_s$, and $\Omega_m$ simulations accrete earlier and end up less massive and closer to the halo center at $z=0$, the process of continuous subhalo accretion and destruction leads to a steady state distribution of these properties across all subhalos in a given host. This steady state mechanism eliminates cosmological dependence on all properties listed above except subhalo concentration and $V_{max}$, which remain greater for higher $\sigma_8, n_s$ and $\Omega_m$ simulations, and subhalo formation time, which remains earlier. We also find that the numerical technique for computing scale radius and the halo finder used can significantly affect the concentration-mass relationship computed for a simulation.Comment: 15 pages, 15 figures, Accepted to ApJ on March 15, 201

Reconstructing the Peculiar Velocity of the Local Group with Modified Gravity and 2MASS

The peculiar velocity of the Local Group, reconstructed from inhomogeneities in the local density field, differs in direction and magnitude from the velocity inferred from the Cosmic Microwave Background dipole. We investigate whether generalized theories of gravity, which predict a modified growth of perturbations, are able to alleviate this discrepancy. We introduce a general formalism for calculating the real-space peculiar-velocity field for modified gravity and theories with interactions in the dark sector. For different classes of theories - scalar tensor and higher-dimensional gravity - we reconstruct the Local Group peculiar velocity using groups of galaxies identified in the 2MASS Redshift Survey. We show that, for realistic parameters, modifications to General Relativity cannot account for the angular discrepancy between the reconstructed Local Group velocity and the dipole in the Cosmic Microwave Background.Comment: 14 pages, 7 figure

Conditional Knockout of NMDA Receptors in Dopamine Neurons Prevents Nicotine-Conditioned Place Preference

Nicotine from smoking tobacco produces one of the most common forms of addictive behavior and has major societal and health consequences. It is known that nicotine triggers tobacco addiction by activating nicotine acetylcholine receptors (nAChRs) in the midbrain dopaminergic reward system, primarily via the ventral tegmental area. Heterogeneity of cell populations in the region has made it difficult for pharmacology-based analyses to precisely assess the functional significance of glutamatergic inputs to dopamine neurons in nicotine addiction. By generating dopamine neuron-specific NR1 knockout mice using cre/loxP-mediated method, we demonstrate that genetic inactivation of the NMDA receptors in ventral tegmental area dopamine neurons selectively prevents nicotine-conditioned place preference. Interestingly, the mutant mice exhibit normal performances in the conditioned place aversion induced by aversive air puffs. Therefore, this selective effect on addictive drug-induced reinforcement behavior suggests that NMDA receptors in the dopamine neurons are critical for the development of nicotine addiction

Astrophysical Blastwaves

Shock waves are known to significantly alter the medium through with they propagate. Shocks in the interstellar medium have been known to affect star formation. Likewise, shocks in the intergalactic medium are expected to affect galaxy formation and the confinement of intergalactic clouds. Because of their significant influence, it is important to have a sound understanding of the different solutions used to study shocks and see how they differ. In this paper, I will focus on the self similar solution, thoroughly developed by Ostriker and McKee, and the explosion model, developed by Tegmark, Silk, and Evrard

Self-similar spherical collapse with tidal torque

N-body simulations have revealed a wealth of information about dark matter halos; however, their results are largely empirical. Using analytic means, we attempt to shed light on simulation results by generalizing the self-similar secondary infall model to include tidal torque. In this first of two papers, we describe our halo formation model and compare our results to empirical mass profiles inspired by N-body simulations. Each halo is determined by four parameters. One parameter sets the mass scale and the other three define how particles within a mass shell are torqued throughout evolution. We choose torque parameters motivated by tidal torque theory and N-body simulations and analytically calculate the structure of the halo in different radial regimes. We find that angular momentum plays an important role in determining the density profile at small radii. For cosmological initial conditions, the density profile on small scales is set by the time rate of change of the angular momentum of particles as well as the halo mass. On intermediate scales, however, ρ∝r-2, while ρ∝r-3 close to the virial radius