Edinburgh PhD Projects 2005

Simulating gravitational lensing mass reconstruction

Supervisors: J.A. Peacock, D.J. Bacon, A.F Heavens

Gravitational lensing is a central tool in cosmology, which reveals the distribution of dark matter directly. Edinburgh has one of the largest research groups in this area in the world. This project is primarily to calculate predicted lensing properties of systems seen in the largest existing N-body simulation of cosmological structure, a 10-billion particle run carried out by the Virgo Consortium http://www.mpa-garching.mpg.de/Virgo/. There is scope for studying several different aspects of the lensing phenomenon:

Large-scale cosmic shear. Edinburgh is actively planning for major imaging surveys of gravitational lensing, which will cover hundreds of square degrees. These should start taking data in 2006. Realistic simulations of lensing in this "weak" regime will show how accurately the dark-matter power spectrum and its evolution can be recovered.

Dark matter substructure in clusters. N-body simulations predict a characteristic tidal truncation of galaxy-scale dark-matter haloes inside clusters, which increases with time since the cluster formed. Using higher-order lensing distortions beyond pure shear, simulated clusters will be studied to see how this key prediction of hierarchical structure formation can best be detected.

The Cosmic Microwave Background and the Early Universe

Supervisor: Andy Taylor

The CMB is one of the main windows on the very Early Universe. In addition, the polarization of the CMB could hold the key to the detection of gravitational waves emitted during the Inflationary Era, when structure was laid down. This project will explore methods for digging the gravitational wave signal out of CMB data, apply them to the Anglo-American QUaD CMB survey, and probe the physics of inflation.

Mining the Resolved Fossil Record in the z=0 Universe

Supervisor: Annette Ferguson

The fossil record of galaxy formation and evolution is written in the spatial distribution, ages, kinematics and metallicities of their stellar populations. This record is best deciphered when galaxies are close enough that they can be resolved into individual stars. Until recently, detailed studies of resolved stellar populations in large Milky Way-like galaxies were severely hampered by the lack of suitable instruments and/or telescopes. Fortunately the situation is now changing and we can begin to probe large-scale properties of resolved populations in galaxies residing beyond the Local Group.

The project will involve using wide-field imaging data from ESO's VLT and Subaru to study the fossil record in at least five nearby galaxies. One of the key goals will be to search for signatures of tidal debris in the halos of these galaxies. Current models predict such substructure should be common, arising from the continual accretion and disruption of small satellite companions. There will also be opportunities to pursue additional observations with the Hubble Space Telescope and/or to pursue detailed modelling of the star formation and chemical evolution histories of individual galaxies.

Title: Galactic structure from stellar proper motions

Supervisors: N Hambly & J Cooke

Stellar proper motions measured to faint limits over wide angles enable detailed studies of the structure of our Galaxy. Luminosity functions (and hence mass functions) of various kinematic populations and different luminosity types can provide clues as to the contribution to total mass made by different populations, and also to the possible contribution of stellar remnants to dark matter problems. We have a number of wide field datasets available at Edinburgh that can be employed in such work. Example topics to be studied may include:

(i) There is some evidence for a population of cool white dwarf stars in the halo of our Galaxy. This has important implications for halo dark matter models, disk rotation curves and the evolution and structure of spiral galaxies. White dwarfs provide a natural candidate for the MAssive Compact Halo Objects (MACHOs) that are observed in microlensing experiments. The wide field datasets will be used to measure the space density of these stars. It will then be possible to measure to what extent cool white dwarfs contribute to the total halo mass density required to explain the MACHO and rotation curve results.

(ii) The hypothesis detailed in i) is naturally controversial. The contribution made by old disk and thick disk white dwarfs to the population of high velocity, cool white dwarfs is currently unknown. The same wide field datasets can be used to examine the cool stellar members of these respective populations.

It will be possible to draw some conclusions as to the star formation history of the distinct kinematic components of the Galaxy, and to refine their age estimates, for example by removing thick disk and halo contamination from the disk WD luminosity function.

This project will involve extensive use of computers to process large catalogue datasets. It is probable that some experience will also be gained in follow-up observations of stellar objects using large aperture telescopes.

Title: Exploiting the WFCAM and HARP Galactic Plane Surveys

Supervisors: Andy Longmore, Bill Dent

Why are some dark clouds and globules populated with protostars and YSOs whereas other otherwise identical clouds bereft of star formation? Astronomers have been remarkably unsuccessful to date in identifying the collapse status and young star content of low-mass, isolated dark clouds, frequently calling clouds "starless" only to find signs of activity on more careful examination. This project is to exploit the data from the WFCAM (Wide Field Infrared Camera) and HARP Galactic Plane Surveys (GPS) to answer this question. A particular aim is to understand the nature of dark globules, their density profiles, molecular structure, turbulent line widths and their role in the formation of low mass stars and brown dwarfs.

WFCAM is just being commissioned on UKIRT in Hawaii and HARP will be commissioned on the JCMT in summer 2005. Over the next two years WFCAM will complete the first stages of several surveys including the GPS, using the largest area of near-infrared array detector real-estate ever assembled.

The deep but wide area (780 sq.deg) J,H and K band WFCAM GPS, combined with results from the Spitzer GLIMPSE survey, will enable the young stellar and sub-stellar populations of dark globules and their density profiles to be measured on a statistically significant basis, in a range of environments. Simultaneous molecular line studies using wide-field mapping with HARP will then allow these results to be interpreted in the light of dynamical properties, for the first time giving a reliable picture of how these clouds collapse, and why they form stars. The opportunities for exciting serendipitous discoveries in such a survey are also extremely promising.

Title: The Ages of Galaxies Detected by the Sloan Digital Sky Survey

Supervisor: Avery Meiksin

The formation of galaxies is an outstanding mystery in our knowledge of the evolution of the cosmos. Numerical simulations are able to predict the collapse of halos of gas and dark matter, but the process by which this material is converted into stars is still poorly understood. The key to solving the riddle is the age of the stars within the galaxies, as the process by which the galaxy was assembled is encoded in the distribution of the stellar ages. The most reliable method known for estimating the ages of the stars that compose a galaxy is to re-create the expected spectrum of the galaxy based on an assumed star formation history, a method known as 'population synthesis.' The most accurate age determinations are based on the measurments of characteristic absorption features produced in the spectra of the stars. Young stars, for instance, show much deeper Hydrogen absorption features than older stars, while the absorption in old stars is dominated by metallic ions or molecules. In the past, using these absorption features to determine the ages of galaxies could only be applied to small samples of galaxies. The Sloan Digital Sky Survey has dramatically changed this, now permitting the ages of hundreds of thousands of galaxies to be estimated from their absorption features, and so test models of galaxy formation.

This project builds on a population sythesis code developed at the Institue for Astronomy specifically to estimate the ages of galaxies measured by the Sloan.

Title: The star-formation history of obscured Active Galactic Nuclei

Supervisors: Rob Ivison, Jason Stevens

This project offers the chance to determine the star-formation history of obscured AGN, a subject closely linked to galaxy formation. The first phase will involve cross-correlating deep surveys made with the Very Large Array (radio) and XMM-Newton (X-ray) observatories. You will be provided with one of the deepest 1.4-GHz VLA maps ever taken, allowing us to probe star-formation rates down to a few tens of Solar masses per year. You will also be given access to a 1-Ms X-ray image of the same region. This is currently the deepest dataset taken at hard X-ray energies where buried AGN are best detected. The plan is to study the star-formation rates of AGN as a function of their absorbing column densities in the X-ray (and redshift when measurements become available). The results are potentially very interesting because current investigations along similar lines have not been able to probe such low star-formation rates. It is anticipated that the project will be closely linked to submillimetre studies of galaxy formation. For the latter, there will be plenty of opportunity to observe at radio, submillimetre, infrared and optical telescope facilities.

Title: Gas in the planet-forming region of young discs

Supervisors: Suzanne Ramsay Howat, Bill Dent

Although >140 extra-solar planets are now known, their formation process is poorly understood, and has yet to be clearly observed. It is certain that the formation mechanism of Jupiter-like planets depends on the gas content in the inner ~10AU of discs around young stars, but this parameter is difficult to measure with current telescopes. Initial results in small numbers of well studied sources have produced controversial estimates of the mass and physical conditions of this gas.

In this project, we plan to select a sample of discs based on their geometrical properties and then to use high-resolution infrared spectroscopy as an indirect probe of the disc. Recently available near-infrared instrumentation on 8m class ground-based telescopes, and far-infrared spectroscopy with the Spitzer space telescope can now provide enough sensitivity to allow some of the most basic questions to be answered. The several steps to the project will include:

- selecting an appropriate sample of objects from an observational survey
- devising a spectroscopic campaign informed by existing models of gas emission in discs (e.g. H2 and CO)
- carrying out the spectroscopic observations
- analysis of the results and development of models to explain the findings

The result of this work will place important new constraints on planet formation models.

Title: Understanding Star Formation in Nearby Galaxies

Supervisors: Tim Hawarden (ATC), Gillian Wright (ATC), Suzanne Ramsay Howat (ATC), Peter Brand (IfA)

How can we understand the physical nature of distant galaxies? Only by interpreting correctly the diagnostic properties of the radiation that we receive from them. This radiation comes from the stars of the galaxy themselves, the ISM of the galaxy (both its excited gas and its heated dust) and the structures surrounding its nuclear black hole. We have amassed a unique suite of high quality long-slit JHK (near-infrared) spectra across the nuclei of 29 nearby IR-bright galaxies. Our immediate, ready-to-go objective is to catalogue and interpret these spectra, and thereby to acquire a realistic understanding of the circumnuclear processes in these galaxies. The data will be compared with existing models of starburst galaxies with the additional aim of expanding those models to include interpretive tools based on (rest-wavelength) near-IR spectroscopy that will help understand high-redshift star formation as it will be observed with existing ground based large telescopes (Gemini and the European Southern Observatory VLTs) and, in the longer term, the James Webb Space Telescope.

Title: MIRI: a mid-infrared spectrometer for the James Webb Space Telescope

Supervisor: Gillian Wright

We seek a student to work on instrumentation for the James Webb Space Telescope (JWST), the successor to the Hubble Space Telescope. Scheduled for launch in 2011, the JWST's primary science objectives are detecting the first generation of starts (first light), formation of stars and planetary systems, and evolution of planets and the conditions for life. To achieve these goals, the JWST will require much more light-gathering capability than Hubble, and will be equipped with near and mid-IR instruments. At approximately 6 meters in diameter, JWST's primary mirror will be more than two-and-a-half times as large as the Hubble telescope, and it will be the largest space telescope ever flown.

Gillian Wright is leading an international partnership to provide the European Optics Module for the mid-infrared instrument, MIRI. Here in Edinburgh we will be responsible for designing and building a spectrometer pre-optics unit containing a set of four image slicers. Image slicers are a relatively new technique in spectrometers which enable the study of spatial and spectral information simultaneously. The image slicers will be based on ones developed at the ATC for the UIST instrument, but no image slicer has ever been shown to work at mid-IR wavelengths.

The student will work closely with the JWST team at the ATC on the design and testing of a mid-IR image slicer. In the process you will learn about the techniques for building astronomical instruments in space, optical design, diffraction, cryogenics and instrument evaluation techniques. You will also take part in research at Gemini and UKIRT on science that lays the foundations for future research with the JWST, using the UIST image slicer to study nearby galaxies in infrared absorption lines, coupled with mid-IR spectroscopy using Michelle on Gemini.

Title: Oscillations in Galaxy Cluster Cores

Supervisors: Eric Tittley & Avery Meiksin

Galaxy clusters are the largest bound objects in the universe. X-ray observations reveal them to be the most luminous objects, as well. Many are in a dynamic state indicating exposure to disruptive forces. Interactions and mergers with other galaxy clusters are a likely cause of much of the unrest. Recent numerical simulations have shown that interactions with other clusters induce quasi-oscillations in the distribution of matter. Features of the oscillations predicted by the simulations are observed in clusters. A PhD student would be entrusted to determine (1) the link between the state of the clusters observed and the timescale and geometry of the instigating merger; (2) the link between the morphology of the oscillations and the dark matter distribution.

Title: Numerical simulations of the IGM

Supervisors: Avery Meiksin & Eric Tittley

Observations reveal an intimate relation between galaxies and their environment, the Intergalactic Medium (IGM). Establishing the origin of the relation and its connection to the formation and evolution of galaxies is one of the grand theoretical challenges of modern cosmology. Detailed numerical modelling is proving increasingly essential to tie the theoretical expectations to the observed universe. This project will involve the student in the numerical modelling of the impact of galaxies and quasars on their environment using simulations on high performance computers.

Title: Revealing the central engine of quasars and active galactic nuclei

Supervisors: Makoto Kishimoto, Andy Lawrence

A supermassive black hole is quite certainly sitting at the center of quasars and AGNs. The big unsolved paradigm is now the energy production mechanism. Even after many years of research, we still don't understand how this black-hole engine works, which is in many cases producing huge radio jets. The hole is accreting mass, and this accretion is quite likely to be playing a key role in the engine. This can directly be investigated by observing its accompanying radiation, i.e. the UV/optical continuum of quasars, which dominates their radiative output. However, this has been very difficult, since the spectrum looks quite featureless, or otherwise any key spectral features are heavily contaminated by many emission components (e.g. broad emission lines) from outside the nucleus.

We are now developing and exploiting a new method which removes off all the contaminations, specifically by taking the polarized flux spectrum of normal quasars. In fact, by this method, we recently discovered with Keck and VLT a Balmer edge of the hydrogen opacity in several quasars for the first time, which we think is an intrinsic signature from the "atmosphere" of the accretion flow (just like in stellar atmospheres, but has never been seen! - see the recent ROE press release "Polaroid Sunglasses"). This opens up a totally new way to look into the physical state of the 100 Schwarzschild radii of the engine.

We offer a project which will exploit this new unique probe, including its variability, to investigate the accretion flow in many quasars with and without jets and their distinctions, specifically by:

(1) implementing an unprecedented spectropolarimetric survey of all the bright quasars with 4m-class telescopes, to list up the suitable candidates (roughly a half of the desired data has recently been acquired).

(2) continuing to use 8-10m telescopes (VLT, Keck, Subaru; we have secured VLT nights in the coming autumn 2005) to follow up excellent candidates for superdeep observations.

(3) extending this method to near-infrared to remove off the dust emission - we can investigate the outer edge of the engine and the possible disk self-gravity effect there for the first time, which will also be a fundamental test for models (initial NTT nights secured also in autumn 2005).

Title: Host galaxies and interactions of AGN

Supervisor: Philip Best

A small fraction of galaxies exhibit extremely powerful radio emission, associated with accretion of material onto a supermassive black hole at the centre of the galaxy. These are known as radio galaxies, and possess jets of relativistic particles which drill their way out from the centre of the galaxy through the interstellar and intergalactic media.

At high redshifts, radio galaxies are found to display an excess of ultraviolet emission, which is elongated and aligned along the radio axis. This is believed to arise from a combination of scattered light from the obscured active galactic nucleus and more local emission mechanisms arising from the interaction of the radio jets. The aims of this project are to study in detail the nature of this ultraviolet emission, and to investigate the feedback of energy that active galaxies provide into their surroundings and how this may influence the process of galaxy formation and evolution in general.

The initial part of the project will be concerned with analysing very deep (~6 hour) spectropolarimetric observations using ESO's Very Large Telescope, of a sample of 9 radio galaxies with redshifts z~1.5. These are some of the deepest observations ever taken of distant active galactic nuclei.