Research Summary

My research focuses on modelling the formation and evolution of galaxies. The primary aim of my 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 five years I have 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.


Building a Predictive Model of Galaxy Formation

[2014] In this paper, I use an extremely simple galaxy formation model, and a single constraint - the z=0 stellar mass function of galaxies. What sets this study apart from previous, similar studies, is that I attempt to carefully quantify sources of random and systematic error in both the data and the model, and to account for these when constructing the model likelihood function. For example, I compute the full covariance matrix of the stellar mass function, which turns out to look like this (well, technically this is the correlation matrix).


Assessing the Precision of Galaxy Formation Calculations

[2012] Often, merger trees are extracted from N-body simulations of large scale structure. The usual approach in such simulations has been to dump out all of the particle information at a number of "snapshot" times. These are then post-processed to find dark matter halos which are then linked together to make merger trees. Those merger trees are fed into semi-analytic models which populate them with galaxies. An important question is, "How many snapshots do I need to get an accurate answer?" Galacticus is ideally suited to answering this question. Using Galacticus, I was able to construct merger trees with effectively infinite time resolution, and then degrade these to appear as they would if they'd been extracted from an N-body simulation with a finite number of snapshots. Quantitatively, the answers are that 128 or more snapshots is sufficient to get most average galaxy properties to an accuracy of 5%. Fewer snapshots and errors begin to increase rapidly.


The Growth of Cosmological Ionization Fronts

[2010] This image shows the ionization state and thermal structure of an expanding cosmological ionization front which is growing around a proto-galaxy at z=10. The galaxy has active Population III star formation and a small AGN in its center. The UV and X-ray photons produced by the stars and AGN are rapidly ionizing the intergalactic medium around the galaxy and heating it to a high temperature. Utilizing the Galacticus code, we can solve the equations governing the growth of these ionization fronts for cosmologically representative samples of galaxies and so predict what will be observed by future 21cm experiments that aim to survey the Epoch of Reionization.


Feedback from Active Galactic Nuclei

[2010] With Arif Babul, I am developing a more sophisticated model of black hole growth and jet production which is embedded within a hierarchically growing population of galaxies. As part of this work we have developed a relatively simple model to compute the rate of spin up (or down) of an accreting black hole and the power of the jets it produces while accreting. The figure shows the efficiency (measured relative to the accretion luminosity) of jets driven out of supermassive black hole plus accretion flow systems as a function of the spin (j) of the black hole. The jet efficiency is divided into contributions from winds driven from the accretion disk, and jets launched from close to the black hole horizon. Points show results from relativistic magnetohydrodynamics simulations, while the lines indicate the results from my model of jet efficiency. This model is simple to implement in analytic calculations but agrees extremely well with relativistic magneto-hydrodynamic calculations.


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Page last updated Saturday, July 5, 2014.