## Overview

Andrew Benson's research focuses on modeling the formation and evolution of galaxies. The primary aim of Benson’s research program is to develop a detailed and, most importantly, quantitative model of galaxy formation based upon known physical laws rather than empirical rules.

Our empirical knowledge of galaxy formation is rapidly becoming quantitatively precise, and so has the potential to strongly discriminate between theoretical ideas regarding the formation and evolution of galaxies. Unfortunately, the required confrontation of theory with observations cannot occur at present as our ability to analytically model galaxy formation is currently restricted to making predictions accurate only to within "factors of a few."

Progress can only be made therefore by developing a model of galaxy formation which incorporates the relevant physics in detail and which strives to solve that physics to high accuracy. These goals require the use of state-of-the-art models of galaxy formation, both N-body and phenomenological (a.k.a. "semi-analytical").

Over the past few years he has developed a novel, open source semi-analytic model of galaxy formation, Galacticus, which represents a new and unique approach to the problem. Galacticus is now arguably the most advanced and detailed analytic model of galaxy formation available.

Progress can only be made therefore by developing a model of galaxy formation which incorporates the relevant physics in detail and which strives to solve that physics to high accuracy. These goals require the use of state-of-the-art models of galaxy formation, both N-body and phenomenological (a.k.a. "semi-analytical").

Over the past few years he has developed a novel, open source semi-analytic model of galaxy formation, Galacticus, which represents a new and unique approach to the problem. Galacticus is now arguably the most advanced and detailed analytic model of galaxy formation available.

## Timeline

### Andrew Benson's Astronomy Lecture at the Huntington

## CV

- Ph.D in Astrophysics, 2000, Durham University
- M.Phys in Physics and Astrophysics, 1997, University of Leicester

## Recent Publications

Abstract

The nature of dark matter is one of the most important unsolved questions in science. Some darkf matter candidates do not have sufficient nongravitational interactions to be probed in laboratory or accelerator experiments. It is thus important to develop astrophysical probes which can constrain or lead to a discovery of such candidates. We illustrate this using state-of-the-art measurements of strong gravitationally lensed quasars to constrain four of the most popular sterile neutrino models, and also report the constraints for other independent methods that are comparable in procedure. First, we derive effective relations to describe the correspondence between the mass of a thermal relic warm dark matter particle and the mass of sterile neutrinos produced via Higgs decay and grand unified theory (GUT)-scale scenarios, in terms of large-scale structure and galaxy formation astrophysical effects. Second, we show that sterile neutrinos produced through the Higgs decay mechanism are allowed only for mass >26keV, and GUT-scale scenario >5.3keV. Third, we show that the single sterile neutrino model produced through active neutrino oscillations is allowed for mass >92keV, and the three sterile neutrino minimal standard model (nuMSM) for mass >16keV. These are the most stringent experimental limits on these models.

Abstract

We describe a simple extension to existing models for the tidal heating of dark matter subhaloes which takes into account second-order terms in the impulse approximation for tidal heating. We show that this revised model can accurately match the tidal tracks along which subhaloes evolve as measured in high-resolution N-body simulations. We further demonstrate that, when a constant density core is introduced into a subhalo, this model is able to quantitatively reproduce the evolution and artificial disruption of N-body subhaloes arising from finite resolution effects. Combining these results we confirm prior work indicating that artificial disruption in N-body simulations can result in a factor two underestimate of the subhalo mass function in the inner regions of host haloes, and a 10-20 per cent reduction over the entire virial volume.

Abstract

We describe a semi-analytic model to predict the triaxial shapes of dark matter haloes utilizing the sequences of random merging events captured in merger trees to follow the evolution of each halo's energy tensor. When coupled with a simple model for relaxation toward a spherical shape, we find that this model predicts distributions of halo axis length ratios that approximately agree with those measured from cosmological N-body simulations once constrained to match the median axial ratio at a single halo mass. We demonstrate the predictive and explanatory power of this model by considering conditioned distributions of axis length ratios, and the mass dependence of halo shapes, finding these to be in good agreement with N-body results. This model provides both insight into the physics driving the evolution of halo triaxial shapes, and rapid quantitative predictions for the statistics of triaxiality connected directly to the formation history of the halo.

Abstract

A key obstacle to developing a satisfying theory of galaxy evolution is the difficulty in extending analytic descriptions of early structure formation into full non-linearity, the regime in which galaxy growth occurs. Extant techniques, though powerful, are based on approximate numerical methods whose Monte Carlo-like nature hinders intuition building. Here, we develop a new solution to this problem and its empirical validation. We first derive closed-form analytic expectations for the evolution of fixed percentiles in the real-space cosmic density distribution, averaged over representative volumes observers can track cross sectionally. Using the Lagrangian forms of the fluid equations, we show that percentiles in delta - the density relative to the median - should grow as delta(t) proportional to delta(alpha)(0) t(beta), where alpha 2 and beta 2 for Newtonian gravity at epochs after the overdensities transitioned to non-linear growth. We then use 9.5 square degress of Carnegie-Spitzer-IMACS Redshift Survey data to map galaxy environmental densities over 0.2 < z < 1.5 (similar to 7 Gyr) and infer alpha = 1.98 +/- 0.04 and beta = 2.01 +/- 0.11 - consistent with our analytic prediction. These findings - enabled by swapping the Eulerian domain of most work on density growth for a Lagrangian approach to real-space volumetric averages - provide some of the strongest evidence that a lognormal distribution of early density fluctuations indeed decoupled from cosmic expansion to grow through gravitational accretion. They also comprise the first exact, analytic description of the non-linear growth of structure extensible to (arbitrarily) low redshift. We hope these results open the door to new modelling of, and insight-building into, galaxy growth and its diversity in cosmological contexts.

Abstract

We introduce LATIS, the Ly alpha Tomography IMACS Survey, a spectroscopic survey at Magellan designed to map the z = 2.2-2.8 intergalactic medium (IGM) in three dimensions by observing the Ly alpha forest in the spectra of galaxies and QSOs. Within an area of 1.7 deg(2), we will observe approximately half of greater than or similar to L* galaxies at z = 2.2-3.2 for typically 12 hr, providing a dense network of sightlines piercing the IGM with an average transverse separation of 2.5 h(-1) comoving Mpc (1 physical Mpc). At these scales, the opacity of the IGM is expected to be closely related to the dark matter density, and LATIS will therefore map the density field in the z similar to 2.5 universe at similar to Mpc resolution over the largest volume to date. Ultimately, LATIS will produce approximately 3800 spectra of z = 2.2-3.2 galaxies that probe the IGM within a volume of 4 x 10(6)h(-3) Mpc(3), large enough to contain a representative sample of structures from protoclusters to large voids. Observations are already complete over one-third of the survey area. In this paper, we describe the survey design and execution. We present the largest IGM tomographic maps at comparable resolution yet made. We show that the recovered matter overdensities are broadly consistent with cosmological expectations based on realistic mock surveys, that they correspond to galaxy overdensities, and that we can recover structures identified using other tracers. LATIS is conducted in Canada-France-Hawaii Telescope Legacy Survey fields, including COSMOS. Coupling the LATIS tomographic maps with the rich data sets collected in these fields will enable novel studies of environment-dependent galaxy evolution and the galaxy-IGM connection at cosmic noon.