Below are a selection of research projects from the past, present and future that have (or will be) carried out by my research group.

Future Projects

Physics Inference By Exploring High-Dimensional Parameter Spaces
The parameter space of galaxy formation models is large. This is an inevitable consequence of the fact that galaxy formation is a complex, non-linear process that involves many different physical mechanisms. Building on earlier work that I have carried out in exploring the parameter space of galaxy formation models (Bower et al. 2011), this project will develop new methods to increase the speed of such calculations while simultaneously improving the statistical accuracy of the results. Additionally, it is crucial to identify the optimal datasets to use when constraining models. Initial studies using the measurements of Leauthaud et al. (2011), who measure the conditional stellar mass function (the distribution of galaxy masses in a halo as a function of that halo's mass), at a range of redshifts suggest that this is a very constraining property. This would greatly limit the allowed range of model parameter space.
Understanding the Sub-mm Universe
It is well established that a large fraction of the star formation in the Universe is hidden due to extinction of starlight by dust. That absorbed starlight is re-emitted at longer wavelengths as thermal emission from dust grains. Previous attempts to model this population (e.g. Baugh et al. 2005) have met with significant problems, and have been forced to adopt very top-heavy initial mass functions and implausible star formation efficiencies in order to match observed data. Initial studies using the Galacticus model do not show these same problems, and are able to match observations using a standard IMF and star formation efficiencies. Understanding why this is the case is challenging, as the emergent sub-mm flux from a galaxy depends on many factors (the star formation history of the galaxy, its dust content, geometry, etc.). Fortunately, the Galacticus code's highly modular nature allows it to recreate other models - for example it can reproduce the results of the Baugh et al. (2005) model with high precision. This project will explore the differences between that model and a successful one to ascertain what physics is key for explaining the sub-mm population with a standard IMF and star formation efficiency, and so produce detailed predictions for future sub-mm surveys.
Robust Predictions for Next Generation Telescopes and the High-z Universe
The next decade will see the arrival of several next generation observational facilities (e.g. JWST, TMT, GMT), for many of which observing the formation of the earliest generation of galaxies is a key science goal. Observations of these earliest galaxies will provide very strong constraints on theories of galaxy formation. Attaining a correct theoretical understanding of these earliest galaxies is crucial as they affect all later galaxy formation via chemical enrichment of the IGM and by acting as the building blocks of later generations. There is a real opportunity here to make detailed predictions in advance of the observations instead of, as often happens, simply confirming that a model fits pre-existing data. This would provide a very strong and necessary test of our theory of galaxy formation. Galacticus is extremely well suited to predicting the properties of unprobed regimes of redshift and galaxy mass, incorporating physics such as: cooling due to molecular hydrogen and Compton cooling; modeling of the IGM and intergalactic radiation field and its back reaction on galaxy formation during the reionization epoch; feedback from Type II and Ia supernovae; and chemical enrichment of an arbitrary number of elements. This project will make predictions for the detailed properties of the galaxy population across the redshift range z=6 to 20 tuned to specific proposed instruments such as JWST and GMT.

Current Projects

Building Merger Trees in Warm Dark Matter Universes
Arya Farahi
Dark matter halo merger trees provide the "backbone" within which calculations of galaxy formation take place. To understand how alternative models of dark matter, such as Warm Dark Matter, influence the process of galaxy formation and the abundance of dark matter structures around our own and other galaxies, it is necessary to build detailed merging histories of large numbers of dark matter halos. This will lead to the prospect of quantitative predictions for Warm Dark Matter universes which will be observationally testable by Omega. This project will develop an accurate merger tree algorithm for non-Cold Dark Matter universes, with an initial application to Warm Dark Matter. This algorithm will be built within the Galacticus galaxy formation code and will be calibrated against N-body simulations.

Past Projects

Merging and Migration of Black Holes in Galaxies
Stéphane Mangeon (University of Nottingham, UK)
Supermassive black holes form in the centers of galaxies and are crucial for shaping the growth and evolution of their host galaxies. But, these black holes do not exist in isolation. When galaxies merge, multiple black holes will inhabit a single galaxy. This project added extensive functionallity to the Galacticus code to allow it to follow the evolution of systems of black holes, as they migrate toward galactic center, driven by dynamical friction, scattering of stars and emission of gravitational waves. Also added were calculations of triple black hole interactions and gravitational recoils which can eject black holes from galaxies.
Testing the Validity of the Adiabatic Contraction Approximation in Dark Matter Halos
Laura Book (Caltech)
The formation of galaxies within dark matter halos leads to compression of the central regions of the halo due to the galaxy's gravitational pull. This process is usually modelled as an adiabatic process in a spherical halo. However, numerical simulations show that this approach does not work as accurately as we might like. We are investigating some of the approximations which enter into the adiabatic contraction calculation to assess which of these may be responsible for its failures.
Cosmological Shock Growth and Environmental Effects
Laura Book (Caltech)
Galaxies show a transition in their properties from the field into clusters. This transition begins to become apparent significantly beyond the virial radii of clusters. We are investigating if the formation shock surrounding each cluster provides a more natural radius for various environmental effects to come into play and whether this can quantitatively explain the observed transitions in galaxy properties.
A Model of the Internal Structure of NFW Halos
Dan Grin (Caltech)
We're attempting to construct a model which explains how the internal structure (basically the scale length) of NFW dark matter halos is changed as a result of mergers between halos. Such a model could be applied to dark matter halo merger trees to predict how the NFW concentration parameter should depend on halo mass (and formation history).
Testing Massive Galaxy Growth Rates in Hierarchical Models with AGN Feedback
Martin Stringer (University of Oxford/Caltech)
In collaboration with Richard Ellis and Kevin Bundy, we are making the most careful and detailed comparison to date between theoretical predictions for the rate of massive galaxy growth and observations of the same. The Galform semi-analytic model of galaxy formation is being used to predict galaxy growth rates, and to determine the effects of cosmic variance and mass-estimation uncertainties in the observational data to provide the most accurate test possible of the theory.
Clustering of Luminous Red Galaxies
Lauren Porter (Caltech)
We are computing the expected clustering signal of luminous red galaxies at high redshifts with the aim of using observational data to place constraints on the distribution of such galaxies between dark matter halos.
Predictions for the JWST Galaxy Formation Science Goals
Tom Fox (University of Nottingham, UK)
This project began the first phase of adapting Galform to the high-redshift Universe (including following the thermal and ionization state of the IGM) to permit calculations to be made for JWST science.
Effects of Stochastic SNe Events on Early Galaxy Formation
Justin Chen (Caltech)
Semi-analytic models of galaxy formation have typically employed simple rules to describes the rates of star formation and feedback in forming galaxies. Typically, feedback (assumed to be due to SNe and stellar winds) is modelled as an outflowing of gas at a rate proportional to the current rate of star formation. This project is investigating what happens if we apply feedback due to individual, stochastically occuring SNe events and how this affects the properties of the first generations of galaxies to form.

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Page last updated Thursday, January 26, 2012.