Galaxies

DPhil Projects 2016 : Galaxies

Discovering Galaxies in the Early Universe with the Hubble Space Telescope
Prof Andrew Bunker

JWST: The James Webb Space Telescope

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 Hubble Space Telescope images and follow-up spectroscopy from large ground-based telescopes in Chile and Hawaii. The science goal is to map the average rate at which the Universe forms stars as a function of time, and to assess whether the ultra-violet photons from the most massive stars could have produced the reionization of the Universe, which we know occurred at high redshift. These galaxies will be important targets for future study with the James Webb Space Telescope, the successor to Hubble to be launched in 2018.

Weighing Supermassive Black Holes
Supervisors: Martin Bureau (primary), Michele Cappellari (secondary)

NGC 2526

As part of the WISDOM team (mm-Wave Interferometric Survey of Dark Object Masses), the student will use current mm/sub-mm telescopes to pursue a programme of supermassive black hole mass measurements. This will primarily use ALMA, the largest ground-based telescope project in existence, on which we recently received a large allocation of time for such a purpose. With the revolutionary sensitivity and angular resolution of ALMA, the student will probe the physical conditions and kinematics of the molecular gas in galaxies to the smallest spatial scales yet, well within the spheres of influence of the SMBHs. The SMBH mass measurements are then straightforward and robust, and the tools to model the velocity fields obtained have already been developed. In parallel, the student will use lower resolution observations from ALMA and other telescopes to survey nearby galaxies and identify promising targets for future measurements. The project will thus significantly increase the number of reliable SMBH masses available, and will revolutionise our understanding of the co-evolution of SMBHs and galaxies.
Link to further resources: http://www-astro.physics.ox.ac.uk/~bureau/DPhil.html

Studying galaxy formation in 3-dimensions with 10,000 galaxies
Michele Cappellari

Oxford has been leading the study of galaxy evolution using integral-field spectroscopy. This technique gives a 3-dim view of galaxies structure, kinematics and chemical composition. Our group provided the first 3-dim view of galaxies with the SAURON and Atlas3D surveys. This work lead to a number of discoveries, including a new paradigm for the decades-old view of galaxy structure and formation (see http://www-astro.physics.ox.ac.uk/atlas3d/).

Now Oxford is part of the MaNGA survey which will provide a major advance in our understanding of galaxies by applying the 3-dim spectroscopy technique to the very successful Sloan Digital Sky Survey (SDSS) telescope (http://www.sdss.org/). MaNGA will extend galaxy sample size by nearly two orders of magnitude, being able to study 10,000 galaxies in 3-dim (see https://www.sdss3.org/future/manga.php).

In the proposed DPhys project the student will be involved in the exciting MaNGA survey which will study the connection between the stellar and gas kinematics, supermassive black holes, the stellar population and the local galaxy environment. The data will provide a local benchmark for any future study of galaxy formation.

The impact of turbulence on star formation and super massive black hole growth in high redshift galaxies
Co-Supervisors: Julien Devriendt & Adrianne Slyz

Physical processes that play a key role in driving galaxy formation and evolution, such as star formation and stellar feedback, and super massive black hole formation, accretion and feedback, take place on extremely small, sub-galactic scales. For this reason, it is a tremendous challenge for numerical hydrodynamics simulations to capture both the environment of galaxies and meaningfully resolve their internal structure. In particular, the role played by compressible turbulence in shaping galaxy properties has received relatively little attention, even though it has long been known (Larson, 1981) that the cradles of star formation, i.e. molecular clouds, are supersonically turbulent. We propose to remedy this situation using a new suite of state-of-the-art cosmological zoom-in simulations called the Nephthys suite, with enough resolution to partially resolve the turbulent cascade of galactic gas in the explicit context of a high-redshift, cold dark matter expanding Universe.

This DPhil project consists of two stages. At first, the student will mostly analyse (but also run several) high-resolution cosmological simulations of the formation and evolution of galaxies of different masses. She/He will then study the properties of the turbulence which develops in the galaxy sample and how it influences its star formation rate and the growth of a putative supermassive black hole located at its centre. 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.

Dissecting galaxies during the peak of the star formation activity in the Universe
Prof Dimitra Rigopoulou

Over the past two decades a consistent picture has emerged whereby the star formation rate density of the Universe peaked at about 3.5 Gyr after the Big Bang (at z~2) and declined exponentially at later times. Until recently, the high star formation rates and the associated stellar mass growth of galaxies at early cosmic epochs were attributed to violent major mergers. However, recent observational evidence suggests that mergers might not be as ubiquitous as once thought and that more efficient modes of gas accretion may dominate the star formation activity at z~2.
Spatially resolved observations of high redshift galaxies are revealing a large diversity in their dynamical properties, ranging from orderly rotating disks to fully dispersion dominated and merging/interacting systems. The origin of this diversity is not clear. Using a sample of the most intensely star-forming galaxies at z~2 we will trace their gas kinematics and dynamical properties and investigate the main mechanism that drives their star formation activity. Using observations taken with the Herschel Space Observatory, The Atacama Large Millimetre Array (ALMA) and the VLT multi-object near infrared spectrograph (KMOS) we will investigate the processes that drove the star formation activity during the epoch of massive galaxy formation and determine how stellar mass is assembled at the peak of the cosmic star formation rate density of the Universe. The project would suite a student wishing to pursue research in Observational Astrophysics and Cosmology.
Please contact Prof. D. Rigopoulou (d.rigopoulou1@physics.ox.ac.uk) for further information.

Tracing metallicities across cosmic time
Prof. Dimitra Rigopoulou

In astronomy the metallicity of an object is the fraction of its matter that is made up of chemical elements heavier than hydrogen and helium. The term `metal’ is used for convenience to describe all other elements collectively. The metallicity of gas and stars in a galaxy is determined by the cumulative effect of its star-forming activity and its gas inflow/outflow history. Studies of gas metallicity in local and distant galaxies have resulted in strong constraints on galaxy evolution scenarios.

Traditionally, studies of gas metallicities have relied on optical and near infrared diagnostics. However, rest-frame optical metallicity diagnostics cannot be applied to heavily dust-obscured galaxies such as ultraluminous infrared galaxies (ULIRGs) and submillimetre galaxies (SMGs). In these galaxies, the rest-frame optical lines are significantly affected by dust extinction and accordingly the optical metallicity diagnostics could lead to large systematic errors of the true metal content of galaxies.

Mid and far-infrared lines provide an alternative way of determining metallicities avoiding the problem of extinction that plagues optical measurements. The mid to far-infrared wavelength regime contains many fine-structure emission lines that can be used to determine metallicities in dust-obscured galaxies. These lines arise in HII regions and Photodissociation regions (PDRs) around the stars and can be understood and modeled via sophisticated photoionization codes such as CLOUDY.

The goal of the project is to establish a new set of metallicity indicators based on mid and/or far-infrared fine structure lines. The empirical relations of the new set of indicators will be derived based on observations acquired with the Herschel Space Observatory and/or the newly commissioned ATACAMA Large Millimetre Array (ALMA). These will be benchmarked against local samples of galaxies before applying the new indicators to distant galaxy samples.

We are looking for an enthusiastic DPhil student to join this project. For more details on the project please contact Prof. D. Rigopoulou (d.rigopoulou1@physics.ox.ac.uk)

A 3-D view of environmental quenching across cosmic time
Dr John Stott and Prof. Roger Davies

As the Universe ages galaxies find themselves drawn together into filaments, groups and clusters. When galaxies enter such dense environments they experience processes which can ultimately lead to a dramatic change in their appearance and internal properties. This project will discover how galaxies are transformed from blue star-forming spiral discs, like our own Milky Way, into passive red elliptical galaxies, through their interactions with the cluster environment.
To achieve this we need to resolve and examine the processes that take place within the galaxies themselves. This is done with Integral Field Units (IFUs), which can measure a spectrum at each spatial position of a galaxy, giving a 3-dimensional picture of its gas dynamics, star formation and chemical composition.
This PhD project will be a detailed study of galaxy transformation with environment, which will encompass some or all of the following:

1. by measuring the spatial extent of the star formation within cluster galaxies compared with those in low density environments i.e. is the star formation being truncated from the outside-in, as theory predicts for an environmental process that removes gas?

2. searching for signatures in the gas spectrum which indicate shocks and seeing whether they correlate spatially with direction towards the cluster centre, like the bow shock of a boat;

3. Exploring the dynamical signatures of ongoing or past mergers with other galaxies.

You will utilise Oxford's involvement in the state-of-the-art IFUs: KMOS, a revolutionary instrument on the Very Large Telescope in Chile, which can perform simultaneous near-infrared observations of 24 galaxies; and both SDSSIV MaNGA and SAMI, optical surveys which will study thousands of relatively nearby galaxies across all environments. KMOS will tell us how clusters were transforming galaxies at early times, MaNGA and SAMI will probe these processes now.

Search terms for further reading: galaxy clusters, red sequence, Hubble tuning fork, Butcher Oemler effect, galaxy mergers, Integrated Field Units, KMOS VLT, SDSSIV MaNGA, SAMI galaxy survey, galaxy spectra.

Dissecting galaxies: Understanding the structure of galaxies using strong gravitational lenses discovered by citizen scientists

Supervisors: Dr Aprajita Verma (Oxford), Dr Matthias Tecza (Oxford), Dr Anupreeta More (Kavli-IPMU, Japan), Dr Phil Marshall (SLAC/Stanford USA, Visiting Lecturer at Oxford), Prof Chris Lintott (Oxford)

Gravitational Lenses from the SpaceWarps project

The phenomenon of gravitational lensing, predicted by general relativity, produces some of the most spectacular astronomical images seen. A single galaxy, group or cluster of galaxies can act as "cosmic telescopes" amplifying and magnifying the light of distant galaxies lying behind them into multiple images, rings or arcs. The separation or deflection of the lensed images is determined by the total mass (dark and light) of the foreground lensing galaxy. The student will work on a large sample of new strong lenses discovered by members of the public as part of the Galaxy Zoo and Space Warps (spacewarps.org) Zooniverse projects. Strong gravitational lenses have a variety of astrophysical and cosmological applications, in this DPhil project we focus on understanding the distribution of mass in the lenses, and the nature of the distant lensed galaxies. We still lack a full physical picture of the dark and baryonic matter in galaxies. Gravitational lensing combined with full stellar kinematic information can constrain the dark matter distribution in the centres of galaxies. To address this we will model the lens systems and, using data from large telescopes that we already have in hand, extract stellar dynamical information of the lensing galaxy. We will also investigate the nature of the lensed, high redshift, background galaxies. The magnification and amplification due to lensing allow fainter galaxies to be studied on smaller scales than would be normally possible. This is a preview to the detailed galaxy science that will be achievable by the next generation of extremely large telescopes. The project is designed to allow flexibility between modelling, lens and background galaxy studies according to the interests of the student. The student will be expected to assist observing runs, write observing proposals and liaise with citizen lensing enthusiasts participating in Galaxy Zoo & Space Warps. In addition, there is the opportunity to contribute to the planning of future Space Warps projects with ongoing and forthcoming large surveys. It may be possible to spend time at Kavli-IPMU (Japan) with co-supervisor Dr Anupreeta More (TBC).

For further information please contact aprajita.verma@physics.ox.ac.uk

More detailed information regarding the results of the Space Warps project can be found at
- Space Warps Paper I http://arxiv.org/abs/1504.06148
- Space Warps Paper II http://arxiv.org/abs/1504.05587
- Red Radio Ring: http://arxiv.org/abs/1503.05824
- Citizen Lens Modelling http://arxiv.org/abs/1502.00008