Star Formation in N-Body + SPH Simulations

Abstract

The primary focus of my thesis work is to study star formation using a series of high resolution cosmological N-body Simulations. Specifically, I have studied the total stellar-to-halo mass ratio as a function of halo mass for a new sample of simulated field galaxies using fully cosmological, LCDM, high resolution SPH + N-Body simulations carried to the present time. I find there is extremely good agreement between the simulations and predictions from the statistical Halo Occupation Distribution model presented in Moster et al. (2012). This is due to a combination of systematic factors: a) gas outflows that reduce the overall SF efficiency and b) estimating the stellar masses of simulated galaxies using artificial observations and photometric techniques similar to those used in observations. My analysis suggests that stellar mass estimates based on photometric magnitudes underestimate the contribution of old stellar populations to the total stellar mass, leading to stellar mass errors of up to 50% for individual galaxies and highlight the importance of using proper techniques to compare simulations. Additionally, my work examines the pressure of the star-forming interstellar medium (ISM) of simulated high-resolution Milky-Way sized disk galaxies, using a kinematic decomposition of these galaxies into present-day bulge and disk components. I find that the typical pressure of the star-forming ISM in the present-day bulge is higher than that in the present-day disk by an order of magnitude. Additionally, the pressure of the star-forming ISM in the early protogalaxy is on average, higher than ISM pressures after z = 2. This explains the why the bulge forms at higher pressures: the disk assembles at lower redshift, when the ISM is generally lower pressure and the bulge forms at higher redshift when the ISM is at higher pressures. If ISM pressure and IMF variation are tied together as suggested in studies like Conroy van Dokkum (2012), these results could indicate a time-dependent IMF in Milky-Way like systems. Finally, my thesis work addresses the question of how well observational star formation indicators measure the true underlying star formation in dwarf galaxies. In particular, I examine Halpha and the UV continuum as star formation indicators and study the timescales upon which these indicators accurately measure the underlying star formation of the galaxy. Additionally, I examine the effects of star formation prescription and resolution on applying observational indicators to simulations.