Cosmology

DPhil Projects 2020: Cosmology

Reconstructing structure growth across cosmic time.
David Alonso & Pedro Ferreira

Description: The growth of structure on large scales is one of the most sensitive observables in cosmology to detect deviations from General Relativity and to constrain the properties of Dark Energy. Currently a number of different low-redshift probes have individually found low-significance tensions with the measurement of the Cosmic Microwave Background anisotropies, which seem to indicate a slower growth at late times than predicted in the standard cosmological model. The main goal of this project will be to unify the constraints of all available datasets and build a model-independent reconstruction of the Universe's growth history that can then be used to constrain alternatives Dark Energy models.

Links to articles: https://arxiv.org/abs/1902.10503, https://arxiv.org/abs/1906.09262, https://arxiv.org/abs/1901.05289

Weak gravitational lensing with the Euclid mission.
Lance Miller & Chris Duncan

Measurements of weak gravitational lensing are one of the key ways of testing the standard cosmological model and the accuracy of general relativity on cosmological scales. The European Space Agency’s Euclid mission will break new ground in making accurate measurements over about one-third of the entire sky. But to achieve that, we must first accurately model the distorting effect of the Euclid telescope, and at Oxford we have been leading the effort to model the telescope and correct the lensing measurements. Euclid is due to launch in 2022 and the survey will take five years to complete. The D.Phil student will work on the modelling and shear measurement, including analysis of the first in-orbit calibration measurements, and should have access to the first of the exciting new survey data to be obtained. The work will involve using scientific programming and simulations to test and improve our physical models of the telescope and its detectors, plus statistical analysis of observational data, and the student should have a role in the first analyses of the early data from the mission.

see: https://www.euclid-ec.org/

Modelling the most extreme high redshift galaxies: from star formation rates to supermassive black hole growth.
Julien Devriendt & Adrianne Slyz

Extreme galaxies are routinely detected with star formation rates in excess of 1000 solar masses per year at high redshift (z>2). These galaxies are also thought to host the most massive supermassive blackholes (SMBH) in the Universe, at a time when their energy input into the circum-galactic medium (so called Active Galactic Nuclei (AGN) feedback) is the largest.
However, these objects have proven notoriously challenging to model, as they are very rare and hence necessitate running very large volume cosmological simulations whilst still resolving the interstellar medium (ISM) of the galaxy and the central region surrounding their SMBH.
To overcome these difficulties, this DPhil project proposes to extract a sample of rare objects from a gigaparsec cube dark matter only simulation and resimulate them with resolution fine enough to resolve the giant molecular clouds that form in their ISM. Analysing these zoom simulations, the student will focus on understanding what physical conditions are necessary to reach the extreme star formation rates observed in these galaxies and feed their central SMBH engine. In particular, they will focus on disentangling the contribution of AGN feedback and star formation to their luminosties and assess their role in driving the evolution of these galaxies.

Although no prior knowledge of numerics is required to carry out the project, a strong taste for theoretical physics and the numerical implementation of
physical problems is mandatory.

Measuring the Star Formation Rate when the Universe was Young with the James Webb Space Telescope.
Andrew Bunker

There is a project available to work with Andy Bunker on measuring the star formation rate in very distant galaxies (at redshifts well beyond one), using data fromthe forthcoming James Webb Space Telescope (JWST, to be launched in early 2021) and also existing data from the Hubble Space Telescope (HST). The science goal is to map the average rate at which the Universe forms stars as a function of time, using the H-alpha line of hydrogen which can be powered by hot, massive and short-lived stars. This line is in the red region of the spectrum, and is less susceptible to extinction by dust than other potential indicators of star formation (such as the UV continuum), but its long wavelength means that it is inaccessible from the ground at redshifts beyond 2. The Near Infrared Spectrograph (NIRSpec) on JWST can reach wavelengths as long as 5 microns, and potentially can measure star formation rates from H-alpha at redshifts up to 7. Prof Bunker is on the European Space Agency Instrument Science Team for NIRSpec, the near-infrared spectrograph on JWST, and we will have guaranteed time observations from the start of the mission. Prof Bunker is also a member of the WFC3 Infrared Spectroscopic Parallel Survey (WISPS) on HST, and we will use existing data from the WISPS survey to constrain the luminosity function of H-alpha at lower redshift (0.4 ≤ z ≤ 1.7) We will compare our H-alpha luminosity functions from targetted multi-object spectroscopy with JWST-NIRSpec at 2 ≤ z ≤ 7 to that from untargetted slitless spectroscopy from WISPs at low redshift to chart galaxy evolution.

JWST.jpg

Studying the Physical Conditions in Galaxies at High Redshift with the James Webb Space Telescope.
Andrew Bunker

This project is associated with Andy Bunker and the Instrument Science Team of the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope - the successor to the Hubble Space Telescope to be launched in early 2021. The science goal is to study the spectra of galaxies out to unprecedentedly high redshifts, in particular using emission lines at rest-frame optical wavelengths, which are well studied at lower redshifts, to explore galaxy evolution.

The ratios of emission lines can lead to measurements of the chemical enrichment of galaxies (all elements heavier than lithium are formed in later stellar processes rather than the Big Bang), as well as the temperature and ionization conditions of galaxies in the early Universe, along with an estimate of the effect of extinction of light by dust grains.
Hence we can study the physical conditions during the early stages of galaxy formation, and we will search for candidate “Population III” galaxies (with no signatures of heavy elements, consistent with being the first generation of stars).

Andy Bunker is on the European Space Agency Instrument Science Team for NIRSpec on JWST and we will have guaranteed time observations from the start of the mission.