Welcome to the research section of my homepage!

I'm a Professor and Canada Research Chair (Tier II) at Saint Mary's University (formerly Queen's University, Mcmaster University, and U.C. Berkeley) working on large scale structure and galaxy formation. I also have links with the Virgo, Hydra and Seattle Consortia. My work is focused primarily on using simulations to aid our understanding of the structure formation process. I also have a 304-core computer cluster dedicated to my research group that enables to perform simulations that would not normally be possible in a shared queue environment.

You can link to a list of my publications and presentations .

Former and current students are listed on my "friends" page.

2016 Grad student research opportunities

I am very interested in taking on new students, both at the MSc and PhD level. Although I encourage students to think independently and come up with research projects that are mutually agreeable, I currently have particular interest in the following student projects:

(1) Development of the next generation of parallel cosmological simulation codes (ideally at the PhD level) and large-scale simulations. I've recently starting working with the GIZMO code developed by Phil Hopkins at Caltech and I'm looking to add a number of revised feedback routines to the code.

(2) Ongoing studies into the representation of supernova feedback in galaxy formation, and addressing the "bulge" issue in current numerical simulations of galaxy formation. This project may also tie in closely with observations of disk galaxies.

(3) Connecting local and global dynamics in galaxies. This is an extension of some exploratory work currently underway. If successful there will be a number of new avenues in galaxy dynamics, specifically related to the interstellar medium evolution, to explore.

If you are interested in computational cosmology and galaxy formation, but unsure about a specific research project, or you have general questions about the research topics listed above, feel free to send me an email (click here for address). Saint Mary's actively encourages applications from students who wish to take time to decide their research topic. Interested students are encouraged to submit applications before February 2017, although later applications will still be considered subject to the availability of positions.

Below is a precis of my research interests, at a somewhat technical level. Although slightly out-of-date now, it gives a reasonable background to the research areas I am currently active in.

Formation of galaxies in hierarchical clustering cosmologies

Developing a detailed understanding of galaxy formation is one of the primary goals of modern cosmology. The basic theory of baryons clustering in a hierarchical system of dark matter halos was laid out by White & Rees (1978, MNRAS, 183, 341). In a hierarchical cosmology the cooling time for baryons in halos is much shorter than the free-fall collapse time, which results in star formation being precipitated in cool gas in low mass halos at very early epochs -- the `cooling catastrophe' (White & Frenk 1991, ApJ, 379, 52). Simulations confirm this prediction and introduce a further problem: the dense gas cores lose much of their angular-momentum to the dark matter via dynamical friction (Navarro & Benz 1991, ApJ, 380, 320; Katz & Gunn 1991, ApJ, 377, 365).

The solution to both these problems is believed to be the inclusion of feedback from supernovae (SN) and stellar winds. White & Frenk (1991) have shown that, under reasonable assumptions, the energy input from SN can be sufficient overcome the cooling catastrophe. However, it has proven extremely difficult to implement feedback in hydrodynamic simulations as it is a sub-resolution process. As part of my Ph.D. thesis, I conducted a number of simulations comparing different ways of returning feedback energy to the local gas in Smoothed Particle Hydrodynamic (SPH) simulations of galaxy formation. My results have shown that because the first generation of objects overcool (they must do so before star formation begins), it is almost impossible to circumvent the overcooling phenomenon without drastic feedback (Thacker & Couchman 2001, ApJ, 545, 728). However the low resolution used in these studies (4000 (baryonic) particles per $L^*$ galaxy) and the complex interaction between resolution and feedback rendered this conclusion preliminary. Note that Binney et al (2001, MNRAS, 321, 471) indirectly support this conclusion.

My investigations of star formation within the galaxy formation process show that the best method for preventing drastic (unphysical) radiative losses following a feedback event is to use a brief period of adiabatic expansion. In simulations with 40,000 particles per galaxy (Thacker & Couchman 2001, ApJ, 555, L17, see also Science, July 6, 2001), I have found feedback can lead to a galaxy preserving 40% of its specific angular momentum, which is an improvement over the 10% value typically found in simulations (eg Navarro, Frenk & White 1995, MNRAS, 275, 56), but remains below the value required to match the Tully-Fisher relation for observed galaxies (Navarro & Steinmetz 2000, ApJ, 528, 607). However, the scale length of the resultant galaxy is within 10% of that predicted on theoretical grounds (Mo et al 1998, MNRAS, 295, 319).

Interaction of galaxies with the inter-galactic Medium (IGM)

In the past two years, driven by an influx of new observations, the study of galactic outflows has moved from the fringes of interest to being a ``hot topic''. In collaboration with Scannapieco (UCSB), I have developed an n-body implementation of his semi-analytic galactic wind/outflow model (Scannapieco & Broadhurst 2001, ApJ, 549, 28). We have confirmed a number of conclusions from the semi-analytic work (Scannapieco, Thacker & Davis, 2001, ApJ, 557, 605), the most important being that outflows can strongly suppress galaxy formation on 109 solar mass scales, thereby solving the `missing satellites' problem for CDM. We have recently updated the model to include metal enrichment (Thacker, Scannapieco & Davis, 2002, ApJ, 581, 836). With my colleagues, I have shown that simple, plausible, assumptions for metal production can reproduce volume filling factors and enrichment levels consistent with estimates from analytic and semi-analytic models (Pei et al 1999, ApJ, 522, 604; Scannapieco et al 2002, ApJ, 574, 590). Future plans for these models include updating the early universe physics to include cooling from molecular hydrogen. In an effort to constrain these models further, specifically metal enrichment in the IGM, I recently became part of a N. America-Euro collaboration (headed by Petitjean, CNRS) that has been analyzing quasar spectra obtained by the UVES spectrograph on the European Southern Observatory Very Large Telescope (VLT). The quasar spectra show absorption features of CIV, SiIV and MgII which we have compared to mock spectra produced from one of the highest resolution simulations of large-scale structure in the IGM. Remarkably, indicating that we do not yet fully understand the details of the enrichment process, the correlation function of CIV systems at z~3 suggests that enrichment patterns produced are dominated by large systems which expell metals over very large ($>2$ Mpc comoving) radii (Scannapieco et al, 2004, in prep). This is contrary to initial ideas of comparatively uniform enrichment produced by dwarf systems.

Gravitational theory and simulations of large-scale structure

The spectacular progress in uncovering the large-scale structure of our universe over the past ten years has eclipsed growth in our understanding of the non-linear evolution of structure formation. Almost 30 years ago Peebles (1974, ApJ, 189, L51) postulated the `stable clustering hypothesis' (SCH) which states that, in the highly non-linear regime, small-scale gravitational clustering should exactly counterbalance Hubble expansion. The SCH is important because it underlies a number of methods for determining the non-linear evolution of the correlation function of galaxies (eg Hamilton et al 1991, ApJ, 374, L1). To date, there have been arguments suggesting that the SCH is valid (eg Jing 2001, ApJ, 550, 125), while others argue that it fails (eg Smith et al, 2003, MNRAS, 341, 1311). The primary disagreement stems from the different methods of determining whether the SCH holds; one can use pair-wise velocity statistics, or the correlation function/power spectrum. The problem is further complicated by the fact that while the SCH may not be exact for individual halos, it could still be true in a statistical sense. In work funded by the Pittsburgh Supercomputing Centre, conducted in collaboration with Sheth, Colberg (Pittsburgh) and Couchman, we are currently conducting simulations with sufficient resolution to test the SCH using both pairwise and ensemble methods.

In support of this research, I have also been examining how to create very precise initial conditions for simulations. On `glass-like' initial conditions, standard interpolation methods fail to accurately reproduce the power spectrum near the Nyquist frequency. In collaboration with Couchman and Scoccimarro (NYU), we are working on creating initial conditions that use 2nd order Lagrangian perturbation theory to calculate the velocities of particles (see Scoccimarro, 1998, MNRAS, 299, 1097) rather than the usual first order method. This has particular relevance for higher order statistics, such as skew and kurtosis, where first order methods are known to produce inaccuracies unless the simulation is begun from extremely early epochs. We plan to publically release a parallel implementation of the algorithm.

Programming and load balancing N-body codes on parallel computers

The majority of my research on parallel codes has focused on shared memory hardware (Thacker, Couchman & Pearce, 1998, in ``HPCS1998'', Kluwer Academic). The simple programming model afforded by shared memory has helped alleviate a number of problems stemming from the complexity of the adaptive data structures used in the HYDRA code. I have spent a significant amount of effort in improving the parallel efficiency, primarily by parallelizing a number of previously serial parts of the code, but also by improving the data locality (and thus achieving better cache performance on RISC processors). Over the past year the focus of my work has been the completion of a distributed memory (MPI-2) version of the HYDRA code (see Thacker et al 2003, in ``HPCS2003'', NRC Research Press). This research is conducted in collaboration with the Virgo Computational Cosmology Consortium, and is the culmination of a three year effort. I first used the code as part of the project examining the distribution of metals in the IGM. For this project we conducted a hydrodynamic simulation with 65 million particles, using 64-128 CPUs of the SHARCNET facilities at McMaster. The code is an exceptional piece of technology as it uses a number of novel load-balancing techniques. Parallel scaling tests conducted at Lawrence Berkeley Lab on their IBM SP-3 have shown almost perfect scaling out to 512 CPUs, and there is no reason not to expect good performance for several thousand processors. Also, due to the early release nature of MPI-2, it has proven particularly challenging to implement the code efficiently on different platforms. I have had to work closely with compiler engineers both to update support of the MPI-2 standard and also to improve performance.

Dr. Rob Thacker
319E Atrium Building
Department of Astronomy and Physics
Saint Mary's University
Halifax, Nova Scotia
Canada B3H 3C3

Electronic-mail: (click here for address)
Telephone: 902-420-5636
Facsimile: 902-496-8218