Research

Our research interests mainly concern the formation and evolution of structure in the Universe. We use a variety of analytical and numerical techniques to address these issues; N-body simulations, analytical models and current observational data-sets allow for focused research programmes intended to shed new light on the old problem of structure formation. Broadly, our research efforts focus on:

1. Large Scale Structure: Theory - Non-Gaussianity - Galaxy Clusters - Cosmography
2. Galaxy Formation and Reionization: IGM and reionization - Galaxy Formation
3. N-body simulations: CDM Halos

A list of our recent publications can be found here.

We acknowledge financial assistance from:

1. Transregional Collaborative Research Centre TRR 33 - The Dark Universe
2. SFB 956 - Conditions and Impact of Star Formation

More details details about our ongoing research can be found below. Opportunities for prospective postdoctoral fellows and students can be found here; potential collaborators are encourages to contact directly any of our members.

Theory

Galaxies and their purported dark matter halos do not uniformly trace the underlying non-linear matter density field; an affect commonly referred to as biasing. Accurate theoretical models describing how the galaxy or halo populations are biased with respect to the matter are essential for extracting cosmological information from galaxy surveys. We investigate these issues using bias models in combination with extensions to linear theory of structure growth: standard perturbation theory; non-linear local bias and resummed perturbation theories, and non-local bias models. We assess the validity of the models by comparing various statistical estimators, such as the dark matter halo power spectrum and bispectrum, directly with the results of fully non-linear cosmological simulations. (To the top)

Non-Gaussianity

As a first approximation, the amplitude of the primordial density perturbations present at the end of inflation follows a Gaussian distribution. At later times, perturbations become increasingly non-Gaussian through non-linear structure formation. However, many inflationary theories predict small deviations from Gaussianity even before the onset of structure formation. This primordial non-Gaussianity can have a measurable impact on the statistical properties of collapsed structures in the universe (e.g. halo mass function, n-point correlation functions, etc.). By modifying the initial conditions of cosmological N-body simulations, one can study the influence of different types of primordial non-Gaussianity on the evolution of density perturbations at later times. (To the top)

Galaxy Clusters

The observed spatial distribution and the abundance of galaxy clusters provides valuable information on both the viability of cosmological models as well as on astrophysical properties encoded in mass-versus-observable scaling relations. The study of galaxy clusters can also provide the cosmological links required to explore the physical processes behind galaxy formation and evolution, particularly in dense environments. We study the clustering properties of these objects using the REFLEX II cataloge; a sample of (how many?) X-ray detected galaxy clusters. Among other findings, we have shown, for the first time, that the clustering pattern of the REFLEX II sample exhibits signatures of non-linear evolution. (To the top)

Cosmography

According to the current cosmological paradigm, all observable structure in the Universe originated from microscopic primordial quantum fluctuations generated in the earliest moments of cosmic history. In the subsequent 13.7 billion years these tiny "seed" perturbations grew, via gravitational amplification, into the presently observed matter distribution. Roughly, the structure formation process follows from the gravitational aggregation of (yet unknown) dark matter particles in to "halos", in which baryonic gas can cool and condense, giving rise to the luminous objects we see in the Universe today. The process of structure formation involves exciting physics: quantum field theory; general relativity; gravitational phsyics; complex hydrodynamics and radiative trasfer. Throughout cosmic history, all these have left their imprint on the three-dimensional large scale matter distribution.

For this reason the major aim of cosmography is to map and analyze the three dimensional cosmic matter field from observational probes of large scale structure. In our work, we aim at building observation-based three dimensional maps of the universal matter distribution and at quantifying their statistical properties in terms of power-spectra and higher order statistics. With these results we can study the origin and evolution of structure in our Universe, as well as many details of galaxy formation by, for example, simulating our local Universe directly. (To the top)

The Physics of the IGM and reionization

We plan to perform a suite of cosmological hydro-simulations of the Intergalactic Medium using RAMSES, a publicly available simulation code based on the adaptive mesh refinement algorithm (Teyssier, 2002). The simulation outputs will be processed using our RADAMESH radiative-transfer code (Cantalupo & Porciani, 2010) in order to follow the Hydrogen and Helium reionization processes. At the same time, we will trace the cosmic evolution of the metal enrichment of the Intergalactic Medium due to star formation. To best take advantage of our simulations we plan to produce mock observational data sets (both spectroscopic and photometric) that may help us understand the possible pitfalls of, and set the strategy for future observational campaigns (such as LOFAR, SKA, JWST) and to directly compare various theoretical models with observations (for instance, for COS onboard HST). (To the top)

Galaxy Formation

Building upon our simulations of the IGM outlined above we plan to implement new, physically-motivated recipes for star formation on sub-resolution scales. The hydrodynamics of the baryonic gas, coupled to the dark matter via gravitational forces, will be followed using Eulerian adaptive mesh refinement techniques. We plan to surpass current implementations of star formation by modelling directly the formation and distruction of several molecular species thought to play a key role in star formation at in low-mass halos. (To the top)

The Origin and Structure of Cold Dark Matter Halos

The large dynamic range probed by current simulations of structure formation allows for robust measurements of the internal structure of large samples of dark matter halos spanning a wide range of masses. The mass profile of CDM halos holds particular interest, mainly because of its direct connection with key observational probes of halo structure, such as disk galaxy rotation curves, gravitational lensing measurements, and more recently because of the possibility of observing the dark matter directly through its self-annihilation signal, or in laboratory detectors.

In recent years the standard hypothesis of a "universal" dark matter halo mass profile has been brought into question, and it is now generally agreed upon that there is a genuine variation in the various halo profile shapes. Although the variation is small, its origin is an issue that can be profitably addressed using currently available cosmological simulation data. We aim to address the origin of the scatter in the dark matter halo density profiles using using large sample of well-resolved and relaxed halos in the Millennium, Millennium-II, and Aquarius simulations (image of Aquarius halo Aq-D, above, credit of Volker Springel). (To the top)