Dissertation

Characterizing the Substructure of Dark Matter Halos

Jiang, Fangzhou

2016

Overview

Hierarchical structure formation in the standard Λ+cold dark matter (CDM) model produces gravitationally bound clumpy halos with abundant substructure. These subhalos are the remnants of dark matter halos that have been accreted by their host halo over cosmic time, and have survived tidal destruction. Understanding halo substructure is extremely important, as subhalos are believed to host satellite galaxies, boost the dark matter annihilation signal, cause tidal heating of fragile structures such as stellar streams and disks, and are potentially responsible for interesting phenomena in gravitational lensing. Most importantly, the demographics of subhalos contain information of the Universe, thus providing a stringent testbed for the cosmological model. This thesis provides a comprehensive study of dark matter subhalos, using a combination of cosmological N-body simulations and semi-analytic modeling. We start with developing a new, semi-analytic model describing halo assembly and subhalo evolution. The model combines Monte-Carlo techniques of generating halo merging histories and simple analytical descriptions for the evolution of subhalos, thus offering extremely fast computation, the agility to experiment with different cosmologies, and the control of specific physical processes. The model accurately predicts the distributions of subhalo mass and structural parameters in cosmological simulations, and outperforms simulations in terms of mass resolution and statistical power. Taking advantage of the speed and agility of the model, we present universal fitting formulae for subhalo mass and maximum circular velocity ( max) functions that are valid for a broad range in host halo mass, redshift and CDM cosmology. The remainder of the dissertation makes use of the model, together with a number of state-of-the-art N-body simulations, to study the statistics of halo substructure. Recent high-resolution CDM simulations reveal ~10 massive Galactic subhalos whose central potential wells are too deep to be consistent with those of the ~10 brightest Milky-Way (MW) satellite galaxies. This inconsistency, dubbed the `too-big-to-fail' problem (TBTF), has become a persistent challenge to the standard ACDM cosmology. However, the number of well resolved Galactic halos in simulations is too small to fully capture the halo-to-halo variance in substructure content, which hinders the interpretation of the inconsistency. Unleashing the power of the semi-analytic model, we generate thousands of MW-size halos with well-resolved subhalo populations, and explicitly demonstrate that a reliable assessment of TBTF requires such large samples. We argue that existing statistics used to address TBTF suffer from the look-elsewhere effect and/or disregard certain aspects of the data on the MW satellite population. We devise a new statistic that is not hampered by these shortcomings, and, using data of the MW satellites with vmax > 15 km s-1, demonstrate that 1.4+3.3-1.1% of MW-size host halos have a subhalo population in statistical agreement with that of the MW. We also discuss how the severity of TBTF depends on halo mass and cosmology. We conclude the thesis with a study of unprecedented statistical power regarding the halo-to-halo variance of substructure. First, we study the mass fraction (fsub) in subhalos as a function of host halo mass, formation redshift, and halo-centric distance. We note that recent measurements of fsub from gravitational lensing are much higher than the average but within the 90th percentile of the fsub distribution. Second, we quantify the deviation of the occupation statistics of subhalos from Poissonian statistics, which is widely assumed in halo occupation distribution (HOD) models. In particular, we clearly reveal the sub-Poissonian statistics at [special characters omitted] ≤ 3, aside from the already-known super-Poissonity at [special characters omitted] » 1, with [special characters omitted] the average number of subhalos. we also quantify the effect of the sub-Poissonity on the galaxy clustering predictions from HOD models. We further show that the extent of nonPoissonity depends on subhalo selection and on halo formation time - selecting subhalos by their mass or vmax at accretion yields weaker super-Poissonity for large [special characters omitted] but stronger sub-Poissonity for small [special characters omitted], compared to selecting by their present-day mass or vmax; earlier-formed halos exhibit less non-Poissonity than later-formed ones. Finally, we use the occupation statistics of the most massive satellites of the MW to put constraint on the mass and formation redshift of the MW halo. In particular, the ` max gap' of MW satellites between ~ 30 km s-1 and 60 km s-1 favors a low-mass, late-formed MW halo, with 0.25 < Mvir/1012 h-1M[special characters omitted] < 1.4 and 0.1 < zf < 1.4 at 90% confidence.

• Publisher: ProQuest Dissertations & Theses
• Subject: Astronomy / Astrophysics / Physics
• ISBN: 9781369635362, 1369635362