Do supernova Ic's require special environments? 

Metallicity is amongst the key parameters that influence the death of massive stars as supernovae and gamma-ray bursts. Recent studies have measured the metallicities of different kinds of supernovae and GRBs and found that regular Type Ic SNe appear to be in more metal-rich environments than broad-lined SNe Ic, SNe Ib, and GRBs with broad lined SNe Ic. However, existing observations are based on long slit spectra, which only probe a small part of the host galaxy. It remains unknown if the metallicities in SN regions are representative of their host galaxies, or if special conditions are needed to give rise to specific SN types. This master thesis project will use VIMOS - integral field spectra to measure resolved oxygen abundances over the face of the galaxies, and measure the resolved stellar populations and the amount of dust in the HII regions where the supernovae explode.

Supervisor: Lise Christensen

The sizes of intergalactic gas clouds

Since the universe was re-ionized at high redshifts, the majority of hydrogen atoms in intergalactic medium are naturally also ionized. A small fraction of gas remains neutral and is present in filaments or larger clumps, as best illustrated by the similarities between numerical simulations and the hydrogen column density distribution inferred from observations of quasar absorption lines. However, quasar sight lines by definition probe only one dimension, so the transverse size of the clouds or filaments cannot be determined directly from single observations. In rare cases when two quasars are located close to each other, a comparison of their absorption line spectra can be used to determine the sizes of the intervening clouds. This bachelor project will use X-shooter spectra of a double quasar and the sizes of intervening hydrogen and metal-bearing clouds will be measured.

Supervisor: Lise Christensen

Dark energy clustering

Cosmic acceleration has been measured using various cosmological data sets but its underlying cause remains unveiled. A possible explanation for this acceleration is dark energy, a new energy component of the Universe with negative pressure. A plethora of dark energy models have been proposed but their kinematical effects on the background energy density are relatively weakly constrained by current data. Recently, within the context of effective field theory some authors have calculated the dynamical properties of measurable energy density fluctuations for quintessence, a candidate of dark energy. The main goal of the project will be to constrain these properties with measurements of galaxy clusters and other cosmological probes.

Supervisor: David Rapetti

FLRW metric and its implications

The highly symmetric Friedmann-Lemaitre-Robertson-Walker metric is currently used to model our approximately homogeneous and isotropic Universe on large scales. However, this choice of metric implies strong theoretical priors on the interpretation of our cosmological data sets. This project would aim to investigate this topic in depth and in particular to shed light on the interrelation between the metric and the observables used to infer crucial phenomena of the standard cosmological model.

Supervisor: David Rapetti and Radek Wojtak

X-raying massive stars

Gamma-ray bursts are the explosive deaths of very massive stars. Because they are so incredibly bright, they are an important tool in understanding the history of star-formation, especially in the very early universe and to probe the dust, gas and dynamics in the galaxies in which they occur. But apart from knowing that GRBs come from stripped massive stars, we don't know much else about them: why and where do they form, what makes a star explode so violently? X-rays are highly penetrating, which makes them the only way we have to see into the extreme conditions close to the burst. The high-resolution data now exist to try to analyse the medium surrounding GRBs, which should allow us to understand their circumstellar environments, potentially constraining the progenitor star's age, mass, binarity and environment.

Supervisor: Darach Watson

The build up of galaxies over the last 12 billion years

CANDLES: the largest deep survey of the structure and morphology of galaxies over the last 12 Billion years, is currently being undertaken with the Hubble Space Telescope. Large amounts of exquisite data is publicly available, which allows us to study the evolution of the structure of galaxies from their formation in the early universe to the present. The project goal is to retrieve this data, and fit the surface brightness distributions of galaxies, to build a catalog of galaxy shapes and sizes as a function of cosmic time. This catalog will be a treasure trove for studying how galaxies build up their mass and shape from their formation to today.  

 Supervisor: Sune Toft

Spectroscopy of the most distant galaxy clusters in the Universe

Galaxy clusters are the largest bound structures in the Universe, and are believed to form in its highest density peaks, which is also where the first galaxies form. In the local Universe, galaxies in clusters are observed to be older and more evolved than galaxies in lower density regions.
Old evolved galaxies in clusters have been spectroscopically confirmed out to a redshift of 1.3. Beyond that redshift, spectroscopic confirmation of evolved galaxies have to be done in the Near Infrared, as the spectral features are redshifted out of the optical.
Several examples of "proto-cluster" candidates are known out to redshifts of z~5, but spectroscopic confirmation old evolved galaxies in these have yet to been achieved. Finding such galaxies is essential to testing our hypothesis that galaxies form first in dense environment.
Using recently obtained Near Infrared spectroscopic observations from the Hubble Space Telescope and the Japanese 8m Subaru telescope, the project aims at, for the first time, spectroscopically confirm old evolved galaxies in proto clusters at a record redshift of z~2, and investigate if these have different properties, than galaxies in lower density environments at similar redshifts.

Supervisor: Sune Toft

Where Do Galaxies Put On Weight?

Galaxies are known to evolve in mass and morphology, even in the relatively recent past, corresponding to the nearby universe. Using resolved images of galaxies with known redshifts it is possible to trace this evolution. This has been done for galaxies as a whole, meaning, we can tell how much mass is added to galaxies as a function of time. The second question is where in those galaxies is the extra mass being made or deposited. Using these same images we can model the mass in each pixel and then infer when and where within galaxies the mass is increasing. This project is designed to start making these measurements using existing galaxy images with some amount of pre-processing. This project could be a Master's scale project that deals with the data and proceeds to the analysis and eventually to derive the spatially-resolved mass evolution of galaxies over the last gigayear or so.

Supervisor: Andrew Zirm 

Mass estimates of poor clusters of galaxies

Reliable estimates of the total mass and virial radii of groups and low-mass clusters of galaxies are important within a wide variety of extragalactic studies. Unfortunately, traditional methods for the mass estimation of clusters (such as X-rays and gravitational lensing) are often not applicable in the group regime. In this project, we will evaluate and compare different methods for deriving the total mass and virial radius of groups (e.g. based on galaxy richness, velocity dispersion, optical luminosity, etc). A possibility would be to also apply the "caustic method" (arXiv:1011.0372) to galaxy groups, and potentially also to compare and calibrate the various methods using groups formed in cosmological simulations. The project would involve a detailed case study of the 25 groups in the XI Groups Survey (astro-ph/0609107).

Supervisor: Jesper Rasmussen

Galaxy interactions and mergers in groups

A systematic study of the frequency of galaxy mergers in low-redshift groups would provide useful constraints on the role of mergers in driving galaxy evolution within these systems. By establishing the abundance and properties (projected separation, velocity difference, stellar masses) of close galaxy pairs in the XI Groups Survey (astro-ph/0609107), estimates of the galaxy merger rate in the groups can be derived with the aid of calibrations obtained from cosmological simulations (e.g. arXiv:0804.1965). We would then explore how the inferred merger rates depend on global group properties, and whether this is consistent with theoretical expectations, e.g. from galaxy velocity dispersions. A further possibility would be to also investigate how galaxy merging is affecting the properties of the merging galaxies themselves, for example by inducing enhanced star formation activity.

Supervisor: Jesper Rasmussen

Clusters of galaxies: Dark matter laboratories and cosmological tools

Massive clusters of galaxies at intermediate redshifts act as gravitational lenses of background galaxies. The location of the multiply image arcs depend on the dark-matter distribution in the cluster and the cosmological parameters, natably the dark matter and dark-energy density parameters. X-ray observations can be used to map the distribution of most og the baryons, which are in the form of a million-degree, tenuous plasma. Deep Hubble Space Telescope images and ground-based spectroscopy of the most spectacular cluster lenses allow us to map the total matter distribution through gravitational lensing. In this project X-ray and/or gravitational lensing will be used either to study the dark matter distribution in clusters of galaxies or to use clusters for determing cosmological paramteres.

Supervisor: Kristian Pedersen

The European space mission Euclid

Euclid is a European space mission, which will provide the next leap forward in our understanding of dark matter and dark energy by imaging and spectroscopic surveys of the entire extra-galactic sky. Euclid is projected for a launch in 2018, but now is the time to decide on detailed science goals, instrument designs and observing strategies for this mission based on exiting data and models, as well as instrument design. The projects can focus on either the instrumentation aspects or on modeling the properties, distribution and evolution of targets, e.g. quasars, galaxies and clusters of galaxies.

Supervisor: Kristian Pedersen

Estimating metallicity build-up in quasars

Quasars are powered by supermassive black holes and reside in centers of distant galaxies. The quasar phenomenon is thought to be an early evolutionary phase of galaxies during which stars are forming and the black hole becomes active as it accretes matter.
The aim of this project is to trace the chemical evolution in quasars using the FeII and MgII (alpha-element) abundances. FeII is thought to be produced in stars that take about 1 Gyr to mature to the point where significant iron is produced, while MgII is thought to be produced by supernovae with shortlived progenitors. As a result we can use the FeII/MgII line ratio as a cosmic clock to trace the build-up of chemical elements in these interesting, early evolutionary stages of galaxies. We will look at how this ratio changes with age of the universe by measuring these line ratios in spectra of quasars from the Sloan Digital Sky Survey. We will discuss the implications for our understanding of galaxy evolution.
Light to moderate programming is involved.
(30 - 45 ETCS)

Supervisor: Marianne Vestergaard

Dark matter has memory

When gas particles scatter off each other, then they loose track of where they came from, and forget where they were going (just like my grandmother, except for the scattering). The dark matter particles most probably don't scatter off each other, and they should in principle therefore remember their orientation in a large cosmological structure. The goal of this bachelor project is to actually measure this, and hence demonstrate that dark matter has memory.

Prerequisites: some mathematical patience.

Supervisor: Steen H. Hansen

Physical parametrization of cosmic dust extinction laws

Cosmic dust is ubiquitous in the Universe and distorts our view of distant objects. For example, the interpretation of the light curves of high-redshift supernovae used to infer the existence of dark energy critically depends on correction for dimming by dust. The standard 'extinction law' is based on observations of stars in the Milky Way and is purely empirical. This project aims to derive a new parametrization of the extinction law based on (1) devising a new physical parametrization of the Milky Way extinction law and (2) extending the law to other galaxies and higher redshift. The new law will be used to address the nature of the famous 2175 Å feature and the consequences for what we can infer from dark energy based on distant supernovae.

Selection bias in academic positions (and other areas)

There are very few female lecturers and professors at NBI and in the natural or technical sciences in general. This is despite a recruiting pool of about 30 % women at the PhD level. The dramatic decrease in female scientists as a function of seniority is known as the 'leaking pipeline'. The effect is sometimes attributed to a combination of women's lack of competitiveness and personal choice as they get kids and prioritize differently. However, a recent study (Watson, Andersen & Hjorth, Nature 436, 174, 2005) shows exactly the same effect occurring over just a few months resulting from small biases in a series of selections of applicants for the European Young Investigator Awards. The purpose of this project is to create a physical model capturing the main effects of a multidimensional set of skills required for selection, a suitable dispersion in these, small selection biases leading to a leaking pipeline. Armed with this model one can address the causes of gender bias in academia, business, and government.

Physical properties of galaxies hosting gamma-ray bursts

Gamma-ray bursts occur in violently star-forming galaxies, generally at high redshift. Such galaxies can be used to map the star formation in the universe and the fundamental routes to galaxy formation and build-up. This project aims to exploit a huge data set of GRB host galaxies gathered in Copenhagen, the most comprehensive and statistically available worldwide. Scientific questions that can be addressed are: What is the redshift distribution of GRBs and how does this relate to the star formation history of the universe? What is the fraction of obscured star formation in the universe? What is the relation between GRB host galaxies and other populations of high-redshift galaxies such as Lyman-alpha emitters and Lyman break galaxies? What are the ages and dust content of individual galaxies?

Dark energy equation of state, dark matter distribution, and galaxies at the end of the dark ages as probed by massive clusters of galaxies

Massive clusters of galaxies at intermediate redshifts act as gravitational lenses of background galaxies. The locations of the multiply imaged arcs depend on the dark-matter distribution in the cluster and the cosmological parameters, notably the matter and dark-energy density parameters. Ideally, an extremely deep Hubble Space Telescope image of the most spectacular cluster lens can constrain both at unprecedented accuracy. The project will involve modeling of HST and Chandra X-ray observatory, and VLT/X-shooter data on massive clusters.

Supervisors: Jens Hjorth, Kristian Pedersen & Darach Watson

Bibliometrics and other quantitative measures of scientific quality: caveats and new ideas

Quantifying scientific productivity (quality, quantity, impact, etc.) is becoming more and more important. A relatively new academic discipline "scienometrics" is gaining momentum while politicians and science administrators are implementing various measures supposed to be 'simple' and operation, e.g., for distributing funding. This project aims to study current ideas in the field and will address the issue from both the scienometrics point of view and the administrator point of view. The purpose of the project is to help define useful criteria and measures and at the same time help scientists define their 'bottom line'.

Supervisors: Jens Hjorth & Darach Watson

X-raying massive stars

Gamma-ray bursts are the explosive deaths of very massive stars. Because they are so incredibly bright, they are an important tool in understanding the history of star-formation, especially in the very early universe and to probe the dust, gas and dynamics in the galaxies in which they occur. But apart from knowing that GRBs come from stripped massive stars, we don't know much else about them: why and where do they form, what makes a star explode so violently? X-rays are highly penetrating, which makes them the only way we have to see into the extreme conditions close to the burst. The high-resolution data now exist to try to analyse the medium surrounding GRBs, which should allow us to understand their circumstellar environments, potentially constraining the progenitor star's age, mass, binarity and environment.