Possible Research Projects for Students
This list is ceratinly not exhaustive and projects can be agreed upon individually! Therefore, if you are interested to pursue a project do not hesitate to contact me!
(Semi-Analytical) Galaxy Formation
Understanding galaxy formation within a full cosmological context is one of the prime fields of research in astrophysics. While a lot of effort is going into directly modeling galaxies by means of hydrodynamical simulations, another route to the subject is to defer to dark matter only cosmological simulations, and then populating the haloes emerging in them with galaxies in a semi-analytical fashion. And there are currently various such models out there all aiming at producing the same observational predictions.Besides of performing direct simulations of galaxies, we have also used 13 different semi-analytical codes (basically all existing) for running their model over the same cosmological simulation. The question now is whether they all give the same results when compared amongst each other or against direct simulations. For some of the proposed projects the existing and unique dataset of galaxy catalogues should be used to investigate various scientifically interested topics. But we also have at our disposal a suite of cosmological simulation that allow the study of galaxies in a statistical sense.
Signing up for one of these projects will further open the possibility to join an existing international group of researchers working on both this comparison and galaxy clusters, respectively; see http://popia.ft.uam.es/nIFTyCosmology/week2.html for more details.
Intra-Halo Light:
Galaxies falling into and then orbiting a larger host galaxy experience tidal forces which will tear them apart (see, for instance, the two Magellanic clouds are orbiting our Milky Way). First their dark matter halo will be stripped, but eventually also stars will be removed and deposited into the halo of the host galaxy. These stars are then free-floating and constitute the so-called "intra-cluster light". The aim of this project is to investigate how different semi-analytical models for galaxy formation treat and predict these halo stars. Will we be able to distinguish different models via the intra-cluster light? Or will we be able to even improve models?Environmental Effects:
We aim to study the difference (between different SAMs) in the degree of environmental effects on galaxy properties. Environmental effects are observed and predicted, but whether its degree is in quantitative agreement with SAM prediction and how robust the SAM prediction is have not been addressed in good detail. This is part of the goal of this project.Cold/Hot Gas in Galaxies:
Gas cooling and heating are competing processes during the formation of galaxies: cool gas is able to form stars, but feedback mechanisms heating part of the gas again are required to prevent an overproduction of stars. For this project the evolution of hot and cold gas shall be studied in the various different semi-analytical models.Mbh-sigma Relation:
The Mbh-sigma (or simply M-sigma) relation is an empirical correlation between the stellar velocity dispersion of a galaxy bulge and the mass M of the supermassive black hole at the galaxy's centre. Here we plan to study how this relation differs amongst the various semi-analytical models of galaxy formation. There have recently been claims that this relation is different at higher redshift. As different SAMs might use different prescriptions on the growth of BH and bulge (AGN FB, merger, starburst prescriptions, etc) we can check how this very important relation varies with redshift in all the models available to us. Will we be able to use this rule out or confirm some of the models?Angular Momentum of Galaxies
The origin of the distribution of mass and angular momentum in galaxies is still an open issue, despite its long history. Angular momentum is presumably acquired by the dark matter (and gas) through tidal interactions with neighbouring objects (Peebles 1969). And the particular way in which the angular momentum of a halo is advected through the virial sphere as a function of time is expected to play a key role in rearranging the gas and dark matter within dark matter haloes.In this project we aim at using (existing) cosmological simulations to investigate the origin of angular momentum of dark matter haloes found in them. To this extent we plan to examine how the angular momentum of all material making up the final halo relates to the angular momentum of the same material at the time of accretion; something not directly studied before. We will further not restrict ourselves to the commonly accepted/favoured LCDM model of structure formation but primarily utilize simulations of a so-called scale-free nature: this will allow us to make much more general predictions.
Galaxy Mergers
The project aims at using one of the best-resolved simulation of cosmic structure formation to date (CURIE simulation, http://curiehz.ft.uam.es) to shed light into the subject of galaxy mergers. The simulation covers a (comoving) volume of (285 Mpc)^3, sufficiently large to statistically study mergers in different environments (e.g. voids, filaments, and high-density regions where galaxy clusters form). It further contains modelling of all the relevant hydrodynamical and radiative physics leading to observable galaxies (Springel & Hernquist 2003 model). The data generated by the simulation should now be analysed with the specific question in mind of following individual objects and their merger history. But one shall not only derive merger rates (as a function of environment) but also investigate the merger product as a function of the merger parameters such as impact parameter, mass ratio of the participating dark matter haloes, morphology of the merging galaxies. This study will then be compared against observational data and hence various definitions for 'merger' need to be applied: observations only provide a snapshot of the merger and hence utilize 'close pairs counts' (Xu et al. 2011), 'morphology distortions' (Lotz et al. 2008), etc. But the simulation data provides us with the actual temporal evolution of the merger process and we can then additionally gauge the observationally applied methods comparing them to the actual merger scenario.Semi-Analytics against direct Simulations:
The project aims at using one of the best-resolved simulation of cosmic structure formation to date (CURIE simulation, http://curiehz.ft.uam.es): this simulation models the formation of galaxies in a full cosmological context. But we also have a semi-analytical model available for this simulation that was constructed using its dark matter component only. The question now is how do the galaxy properties in these two different approaches compare. Is it really necessary to run a full physics simulation or do we really get sufficiently accurate results with the semi-analytical approach?Halo Mass Definition:
Whether you are performing a cosmological simulation of galaxy formation or use (semi-)analytical modelling you have to make an assumption about the definition of the galaxy and halo mass. But are the results independent of this theoretical definition or do they vary? Galaxies in the real Universe certainly do not care about our preferred choice for defining the mass. All the semi-analytical galaxy formation models here were run using a suite of 5 different yet commonly applied mass definitions and the aim of this project is to systematically study its influence on the various galaxy properties. Can we define the best mass definition for the field of galaxy formation?Influence of Halo Finder:
Semi-analytical models are not only influenced byt the definition of mass, but also by the underlying catalogue of dark matter haloes: previous studies (see Knebe et al. 2013) have shown that there are non-negligible differences in the dark matter halo properties when applying different halo finders to cosmological simulations of structure formation. This project now aims at investigating how such differences propagate to galaxy properties. Will we be able to define the best halo finder choice? Or will we be able to find the best semi-analytical model that is independent of this rather technical issue?In this project we aim at using (existing) cosmological simulations to investigate the origin of angular momentum of dark matter haloes found in them. To this extent we plan to examine how the angular momentum of all material making up the final halo relates to the angular momentum of the same material at the time of accretion; something not directly studied before. We will further not restrict ourselves to the commonly accepted/favoured LCDM model of structure formation but primarily utilize simulations of a so-called scale-free nature: this will allow us to make much more general predictions.
Galaxy Clusters
Galaxy clusters are the largest gravitational bound objects in the Universe and lie on the cross-roads of astrophysics and cosmology. On the one hand, by studying their masses and number density they provide insights of the cosmological background. On the other hand they constitute an ideal astrophysical environment to study all the processes that take place during the formation of galaxies.We have used 11 different simulation codes for modelling the same galaxy cluster in a cosmological context in- and excluding all the relevant physical processes. For some of the proposed projects this existing and unique dataset should be used to investigate various scientifically interested topics. But we also have at our disposal a suite of cosmological simulation that allow the study of galaxy clusters in a statistical sense.
Signing up for one of these projects will further open the possibility to join an existing international group of researchers working on both this comparison and galaxy clusters, respectively; see http://popia.ft.uam.es/nIFTyCosmology/week1.html for more details.
Cluster Outskirts:
"Backsplash galaxies" are galaxies that lie on the outskirts of galaxy clusters after having previously crossed the interior regions of that cluster. They are galaxies that - although found in low-density environments - have suffered the effects of the dense environment of the cluster. Cosmological simulations (e.g. Gill et al. 2005) find that this population is quite large. Specifically, they find that half of the galaxies that are now in a radius between 1-2 virial radius of the cluster are backsplash galaxies who have already gone through the most densely cluster. If this is true, it is essential to take this population into account when interpreting the morphology-density relation and decouple the degeneracy between "nature" and "nurture".The project aims at using one of the best-resolved simulation of cosmic structure formation to date (CURIE simulation, http://curiehz.ft.uam.es) to statistically study the existence of backsplash galaxies and put them into a cosmological context. Previous studies (like aforementioned Gill et al. 2005) based their results only on a handful of simulations that focused on the formation of individual galaxy clusters. But the use of the CURIE simulation will allow us to relate the existence and properties of backsplash galaxies to the underlying properties of the clusters and its environment.
The Influence of Baryons:
This project aims at investigating how the baryons included in the simulation affect the shape, angular momentum, and general distribution of matter when compared against dark matter only simulations. For this project we should again use the suite of single cluster simulations as this is available to us with and without gas physics.Brightest-Cluster Galaxy + Intra-Cluster Light:
The plan for this project is to study differences in the properties of the brightest cluster galaxy (i.e. the central galaxy), the luminosities of the satellite galaxies as well as any intra-cluster light. For this purpose it appears adequate to make use of the single cluster simulation performed with aforementioned 11 different simulation code: how stable are results with respects to the numerics?The Dark Universe
There is mounting evidence that the Lambda-cold dark matter (LCDM) structure formation scenario provides the most accurate description of our Universe (cf. Komatsu et al. 2009). However, the currently favoured model rests upon some important assumptions, i.e. the Universe consists of 72% dark energy and 24% dark matter. While we have no conclusive theory for either of them, there is the additional possibility that these two putative ingredients of the Universe also interact with each other.We just finished performing state-of-the-art simulations of cosmic structure formation where we ran - besides of the fiducual LCDM model - models including said interactions. We are now in the process of analysing these simulations with an emphasis on the cosmic web and in particular void regions in the Universe: first results indicated that we might find the greatest differences in those zones that have so far been ignored both observationally and theoretically.
The work to be undertaken is to analyse our set of simulations studying the formation and evolution of cosmic structures (e.g. galaxy clusters) with a focus on the orbits and dynamics of galaxies inside of them.
Gravitational Lensing
The deflection of light by gravity, i.e. gravitational lensing, has established itself as an extremely useful astrophysical tool with remarkable successes. Gravitational lenses have been a precious mean of probing the matter distribution in the Universe: from the search of matter in the Galactic halo to the study of the large-scale structures of the Universe, the gravitational lens effects offer a unique alternative to light surveys and are now widely used.For this project the student should gain a basic understanding of the lensing theory by studying the relevant literature. He should further investigate several very simple lens systems by numerically solving the lens equation: the idea is to generate the lensed picture of a certain image in the source plane by shooting rays backwards from the observer passing the lens into the source plane. Each of these rays will be deflected by the lens and hence 'move' a source pixel to a different position. Only simple lenses (e.g. a single (or several) point masses) shall be considered for this project. This method is called the inverse ray-shooting method described in more detail in Refsdal & Stabell (1986) and Schneider & Weiss (1986).