Several PhD places will be available in the astrophysics group this year, with projects chosen from among those listed below. Candidates need not express a preference for project/supervisor before interviews are held. Applications will be reviewed at the end of January and successful candidates invited for interview shortly afterwards. Late applications may be considered. We also offer the possibility to co-host PhD studentships in collaboration with ESO, Europe’s flagship observatory. PhD projects will typically be supervised by a member of staff of the astro group and one at ESO in Garching near Munich (Germany) or Santiago (Chile). Students will have the opportunity to work two years in an international environment in Germany or Chile before finishing their last year at the University of Southampton. Please contact a member of staff if you are interested.
High-energy emission from black hole and neutron star systems using NuStar (Diego Altamirano)
The Nuclear Spectroscopic Telescope Array (NuStar) mission was launched in June 2012, and it is the very first (and so far only) focusing high-energy X-ray telescope in orbit. NuStar operates in the 3 - 80 keV band, extending the sensitivity of focused (hard) X-rays far beyond the ~10 keV high-energy cut-off achieved by all previous X-ray satellites. Using an unprecedented combination of sensitivity and spatial and spectral resolution, NuStar is suited to address long-standing questions on a variety of topics, including obscured active galactic nucleus, ultra-luminous X-ray sources, non-thermal radiation in young supernova remnants, and galactic sources such as black holes and neutron stars (both pulsars and not pulsars). New NuStar data are becoming publicly available every month since August 2013, giving a unique opportunity to analyse and study almost unexplored data from a variety of sources.
The PhD student will use NuStar data to study galactic black holes and neutron stars in binary systems (the so-called low-mass X-ray binaries). Building on our groups recent successful work on complex modelling of X-ray spectra we will
use simultaneous XMM-Newton/NuStar observations to tackle questions related to the relativistic iron emission line found in the energy spectra of black-hole and neutron-star systems. Furthermore, together with the PhD student will explore
alternative methods to test the nature of the so-called quasi-periodic oscillations (QPOs), with particular focus on the Lense-Thirring precession.
Accretion processes in HMXB (Antony Bird & Malcolm Coe)
The accretion processes in HMXB are dominated by two effects: (i) inhomogeneous and time-varying winds from the donor star, and (ii) the magnetic field around the neutron star. These primary effects may also be heavily modified by orbital modulation in eccentric systems. These time-dependent effects modify both the X-ray signature (from accretion) and the optical/IR signature (from the mass as it transfers between the binary components), and multi-wavelength observations allow us to study the geometry and physical processes of the detected systems. Models of the mass transfer processes can now be implemented in SPH simulation, providing a new level of support in the interpretation of the observational data, and new source detections can shed new light on individual systems, challenging and refining existing models.
Hunting new hard X-ray transients (Antony Bird)
Current generation hard X-ray telescopes rely on long integration times, of typically 10-100 ks, to make an observation of a field with the intention to detect faint persistent sources. This has been the basis of catalogs produced by the INTEGRAL and Swift teams, surveying the hard X-ray sky. It is much more challenging to look for short-lived emission which may be lost in long observations, but the event-mode data collected by modern telescopes contains much more information which is currently not being fully exploited. The aim of this project is to radically improve existing source detection algorithms by implementing a system that searches all points on the sky on all timescales for significant emission. This is a computationally intensive problem, and also requires statistical analysis to rigorously establish the significance of results and ensure resulting catalogs are robust. Early version of these analyses already found a new class of objects - the SFXTs - but those methods are too expensive to employ on the new, much larger datasets being produced today. We know that the hard X-ray sky is a tremendously variable place, and this project aims to fully explore those properties in an unbiased way for the first time.
Near field 3-D gamma-ray imaging with coded apertures for medical and security applications (Antony Bird)
The coded aperture imaging technique has become standard for hard X-ray astronomical imaging in missions such as INTEGRAL and Swift, providing good imaging over a wide field of view. In the context of astronomy, the technique has been optimised for point sources at infinity in a low signal-to-noise environment. The aim of this project is to transform the coded aperture technique to address the much more challenging near-field case found in medical and security applications. Here, sources may be at an unknown distance (or same order as the detector-aperture distance, and may be extended in both lateral and depth axes). In such a case, the aperture design may differ dramatically from those used for astronomy, and must take into account recent advances in detector technology which make depth-sensing practical for the first time. This project will aim to develop new reconstruction algorithms for near-field imaging of complex sources, and investigate the optimum design of an aperture for this application. The project is in collaboration with an industrial partner that is currently developing the detector technology, and there will be an opportunity to develop and test a prototype imaging system if progress allows it.
Thermonuclear Supernovae One Year After Explosion (Mike Childress)
Type Ia supernovae are the explosive deaths of carbon-oxygen white dwarf stars whose quantum stability has been disrupted by interaction with a companion star. One year after explosion, the innermost supernova material is being lit up by the nuclear decay of radioactive cobalt -- which itself resulted from decay of radioactive nickel that powered the brightest phases of the supernova. These late epochs give us our first glimpse at this innermost material, whose light emitted in the first few weeks after the explosion was absorbed or scattered by the outer layers of ejected material. This PhD project will conduct observations of Type Ia supernovae in the year(s) after explosion to examine the physics of radioactive explosion products. The project will make use of the extensive observational facilities available through the European Southern Observatory (ESO) and other UK facilities. These data will reveal the details of the white dwarf explosion events themselves, and potentially the nature of the binary companion stars.
Environments of Massive-Star Supernovae (Mike Childress)
Massive stars end their short lives as "core-collapse supernovae" (CCSNe) which produce compact remnants (neutron stars or black holes) and expel heavy elements (notably oxygen and other "alpha elements") into the interstellar medium. These CCSNe come in a wide variety of flavours which represent the corresponding diversity of evolutionary histories of the pre-explosion massive stars. The chemical enrichment of the pre-explosion star is believed to be a critical factor in determining the way the star explodes -- and thus the elements it ejects at the time of death. This PhD project will explore the environments where these CCSNe explode in distant galaxies using integral field spectroscopy. This spatially-resolved spectroscopy of the SN host galaxies will produce maps of composition and star-formation intensity throughout the SN host galaxies, and these maps in turn will reveal the regions where the CCSN flavours do (or do NOT) preferentially occur. These insights will held build a picture of how heavy elements in the Universe have been produced throughout cosmic time.
Supernova Discovery and Classification with the SkyMapper Telescope (Mike Childress)
Stars die in a myriad variety of ways, and large-area optical imaging surveys are now finding thousands of extragalactic transients every year. The SkyMapper telescope in Australia has recently (from mid-2015) begun such a search of the southern sky, and is poised to discover hundreds of supernovae over the course of its three-year survey. In this PhD project the student will participate in all aspects of this exciting survey, including supernova discovery using image subtraction and supernova classification using spectrographs on other telescopes. Science topics will focus on interesting and exciting new discoveries, as well as detailed study of more common thermonuclear or core collapse supernovae.
Zooming into the hearts of active galactic nuclei (Sebastian Hoenig)
How do black holes grow and how do they influence their host galaxies in the process? All big galaxies in the universe host a supermassive black hole with millions to billions of solar masses in their centre. We know now that these black holes are fed by accretion of mass from their surrounding and that the growth is tightly connected to the evolution of the host galaxy. However, the exact mechanisms are not fully understood, in part because the mass accretion takes place on very small spatial scales. The resolution power to see these processes is equivalent to resolving the distance to the nearest stars of our sun in galaxies tens of millions of light years away. Since few years, this resolution power is available in the infrared (IR) by making use of the Very Large Telescope Interferometer (VLTI) at Paranal in Chile, where up to four 8m-class telescopes are combined to provide the resolution power of a 130m telescope. Such observations revealed how dusty gas is distributed around the black hole and led to the discovery of a new dusty wind structure that is responsible for the bulk of the IR emission.
In 2015/2016, new instruments will be commissioned at the VLTI, which will enable us to reconstruct first IR images of the accreting dust and gas. With these instruments, we will get a first panchromatic view of the accretion and outflow of hot and cold gas. The post-graduate student in this project will work with the team of the MATISSE instrument. In the course of the PhD project, the student will help with commissioning of the instrument and get access to the first science data. The project will involve training on interferometry data reduction and modeling, including further development on radiative transfer models. The student will be part of an international team of scientists from France, Japan, the US, and the UK.
More on VLTI's impact on active galactic nuclei research:
More on the new MATISSE instrument:
Active galactic nuclei as cosmological probes (Sebastian Hoenig)
Distances to extragalactic objects form the basis for tests of our cosmological model, which describes the age and evolution of the universe. These distances are usually inferred from "standard candles", i.e. objects with known absolute brightness that can be compared to the observed brightness. The standard
candles most commonly used are special types of supernovae and Cepheids, a particular class of variable stars. It was recently proposed that the infrared emission of active galactic nuclei (AGN) --- actively growing supermassive black holes in the heart of galaxies --- can also be used as a standard candle. This opens a new and independent way to constrain cosmological parameters and test for systematic effects that may affect the currently favoured objects. In the course of the PhD project, the viability of active galaxies as standard candles will be
tested. This involves a feasibility study, as well as planning, execution, and reduction of AGN monitoring observations. The student will be trained on statistical methods to analyze these observations and refine models for the infrared emission. The project will also involve preparation for the Large Synoptic Survey Telescope (LSST), an upcoming "big data" survey facility to which the UK will have data right access.
More information on LSST:
More information on AGN as standard candles using LSST:
Accretion Disk Winds in Quasars (Christian Knigge)
All quasars are powered by the same central engine: a supermassive black hole that is surrounded and fed by a luminous accretion disk. Approximately 15% of all quasars exhibit clear evidence for powerful outflows driven from these disks, in the form of broad, blue-shifted absorption lines. However, these so-called "broad absorption line quasars" (BALQSOs) are just the tip of the iceberg: since disk-driven winds cannot be spherical, BALQSOs are just the sub-set of quasars viewed at a particularly favourable orientation. In reality, *all* quasars are likely to drive such winds. This is important, because these outflows provide a key feedback mechanism: they can remove significant amounts of mass, energy and angular momentum from the quasar and inject it into the surrounding (inter-)galactic medium. However, despite their importance, we know almost nothing about these accretion disk winds. For example, the geometry, kinematics, and even the basic driving mechanism responsible for launching them are still basically unknown. The aim of this PhD project will be to remedy this situation by modelling the wind-formed features in the spectra of quasars. This work will be carried out in the context of an established collaboration (which includes two other PhD students and one postdoctoral fellow at Southampton) and will use an existing, state-of-the-art Monte Carlo radiative transfer code. The ultimate goal we are pursuing is to determine the fundamental parameters of quasar accretion disk winds and thus shed light on how they regulate the fueling of supermassive black holes and the feedback of energy into their environment. In addition, we aim to shed light on quasar unification: is it possible that *most* observational signatures we associate with (even non-BAL) quasars are actually shaped by disk winds?
The Evolution of Accreting White Dwarfs (Christian Knigge)
Cataclysmic variables (CVs) are interacting binary systems in which a white dwarf siphons material off a companion star. During the early part of a CV's evolution, the mass donor is an ordinary main sequence star, and the mass exchange causes the CV evolve from long to short orbital periods. However, gigayears of mass transfer eventually whittle the mass donor down to a brown dwarf. At this stage, the orbital period of a CV should start to increase again. Thus there is a well-defined period minimum through which all CVs are expected to pass. However, even though 70% of CVs are theoretically predicted to have evolved through the period minimum already, only a handful of potential "period bouncers" are actually known. Thus there is a serious conflict between binary evolution theory and observations, with ramifications that go well beyond the CV setting -- the key processes that govern CV evolution also drive the evolution of virtually all close binary stars. The goal of this project will be to resolve this conflict. This may involve theoretical work (e.g. using theory to predict what we *should* see, via binary population synthesis), statistical analysis (e.g. exploiting CV samples found in large surveys), but also new ground-based and space-based observations of the most exotic and interesting types of CVs.
Accreting White Dwarfs as Universal Accretion Laboratories (Christian Knigge)
Accreting white dwarfs (AWDs) are numerous, bright and nearby, making them excellent laboratories for the study of accretion physics. Since their accretion flows are unaffected by relativistic effects or ultra-strong magnetic fields, they provide a crucial "control" group for efforts to understand more complex/compact systems, such as accreting neutron stars (NSs) and black holes (BHs). Surprisingly, it has recently become clear that these superficially simple systems actually exhibit the full range of accretion-related phenomenology -- outbursts, disk winds, jets, variability -- that is also seen in accreting NSs and BHs. Given this rich set of shared behaviour, it is reasonable to hope that much of accretion physics is universal. The goal of this project will be to test and develop this emerging picture. This will involve gathering, analysing and interpreting observational multi-wavelength data, using both ground-based and space-based observatories (such as Hubble and Chandra). If AWDs really do provide a viable and accessible model for disk accretion in general, such observations will yield qualitatively new insights into the nature of accretion physics and associated outflows.
Probing the Co-evolution of Super-massive Black Holes and their Hosts via Semi-empirical Models (Francesco Shankar)
Massive black holes (BHs) of the order of million to billion solar masses have been detected at the centre of the most massive spheroids, with their mass tightly correlated with the large scale properties of the galaxies they live in. Understanding the evolution of massive, bulge-dominated galaxies thus contemporarily implies probing the origin and evolution of BHs and of their tight scaling relations with their hosts. “Hierarchical” models describe the evolution of the stellar mass and size of massive spheroids as a sequence of major and minor mergers. In this scenario, BHs are predicted to have accreted gas and merged with other BHs during galaxy-galaxy mergers. However, the situation is still quite unclear. 1) The actual role played by mergers is still highly uncertain; 2) there is increasing evidence that a large portion of BHs have accreted their mass in secular processes, not merger-driven; 3) overall, several other physical mechanisms may have triggered accretion onto BHs, such as bar instabilities, stellar winds, radiation drag during star formation, etc.
The basic idea of the modelling put forward here, is to follow the hierarchical merger histories of dark matter haloes, and at each timestep during the evolution assign galaxies to haloes via state-of-the-art Sub-Halo Abundance Matching (SHAM) models. A central seed BH is then assigned to each central galaxy at the time of initialization. At each merger event, the BH is allowed to grow following a given light curve, inspired by high resolution simulations and/or basic theoretical arguments. Analogously to bulges, besides mergers, also other in-situ triggering mechanisms will be considered, e.g., disc instabilities, radiation drag, stellar winds. Knowing the gas fractions competing to each star-forming galaxy, as well as the mass accretion on each BH (from the light curve), detailed predictions for the small and large scale obscuration will be computed. While the modelling of BHs in a cosmological setting has already been proposed several times in the literature, it was never performed before onto a refined semi-empirical model which, by construction, already satisfies all the basic statistical, structural, and spectral host galaxy properties.
Clear, detailed predictions will be made for the optical/X-ray AGN luminosity functions, their clustering properties at all scales, the BH mass function at all cosmic epochs, the evolution of the BH spin (important for future gravitational wave detectors), and extrapolations to high redshifts to probe the still debated contribution of AGNs to Reionization (fundamental for, e.g., LOFAR, SKA).
Constraining the key physical processes governing the formation and evolution of lenticular galaxies (Francesco Shankar)
Lenticular (S0) galaxies are a special class of bulged galaxies in between ellipticals and spirals. They are often characterized by lower specific star formation rates and no clear spiral patterns, with a broad variety of structural properties, going from galaxies with very big bulges to others dominated by the disc component. After almost one century since their definition, the origin of lenticular galaxies is still a matter of debate. This proposal aims at probing the formation and evolution of bulged galaxies, with a specific focus on lenticulars/S0s, using a cutting-edge methodology based on extensive, advanced semi-empirical models which make use of sub-halo abundance matching and halo occupation distribution techniques.
The main objectives of this proposal are the following:
- Analyse in a cosmological context through advanced semi-empirical models, an array of key physical processes to form and evolve lenticulars, such as mergers, bar/disc instabilities, disc regrowth, clumpy accretion, morphological and/or halo and/or environmental quenching. Several of the latter physical processes are still nearly unexplored in this context.
- Compare the outputs of each different model with the statistical, spectral, morphological, structural, and environmental properties of lenticulars.
We will make extensive use of new, unique, and comprehensive data sets available to our group from, e.g., SDSS and COSMOS, specifically catalogued for S0 galaxies in different environments and redshifts.
Fine Structure, Dynamics, and Atmospheric Impact of Shock Aurora (Daniel Whiter)
(Please note that the application procedure for this NERC-funded position is different - for this project, please apply as detailed here. Please also note that the deadline for applications for this project is *4th January 2016*)
The aurora is a result of energised charged particles (electrons and protons) travelling down the Earth's magnetic field lines and colliding with the neutral atmosphere. It typically occurs in ovals surrounding the magnetic poles, at latitudes of about 65-75º. The ultimate source of energy for the aurora is the continuous flow of solar wind interacting with the Earth's magnetic field. The solar wind is highly variable, with a speed of about 300-1000 km s-1 and particle concentration of about 1-10 particles cm-3. Interplanetary shocks or pressure pulses can form and travel in the solar wind, and occasionally impact upon the Earth. These impacts cause “shock aurora”, characterised as an intense brightening of the aurora which begins on the dayside (at noon) and rapidly propagates around the dawn and dusk sides of the auroral oval. If the energy of auroral particle precipitation is high enough significant ionisation can occur in the lower thermosphere and mesosphere, which can have important implications for the chemistry and climate of the region. The objective of this project is to investigate the fine structure and dynamics of shock aurora, and how it can affect the upper atmosphere.
Spatial and temporal structure of the Earth's magnetotail (Robert Fear)
(At this time only applicants with external funding can be accepted for this project)
The Earth's magnetic field forms a cavity in the solar wind called the magnetosphere; the interaction between the solar wind and the magnetosphere is ultimately responsible for dynamics of near-Earth space, including variations in the intensity of the radiation belts and the most spectacular displays of the aurora (the northern and southern lights). The night side of the Earth's magnetosphere forms an extended magnetotail, which consists of a plasma sheet sandwiched between two low density regions called the lobes. However, several aspects of its structure are unclear. First, whilst it is known that the interplanetary magnetic field affects the orientation of the magnetotail magnetic field through a process of "penetration" into the tail, the timescale for this process is hotly debated. Secondly, in textbook picture of the magnetotail, the low latitude "plasma sheet" is hot, whereas the plasma in the lobes is very cold and usually low in density. However, a recent series of papers have begun to challenge this paradigm, and have found that under certain conditions uncharacteristically hotter/higher density plasma can be observed in the lobes. The mechanisms causing this are at times unclear. The aim of this project is to use in situ satellite data from spacecraft such as the European Space Agency's Cluster mission to investigate the timescales on which the magnetotail is controlled by the interplanetary magnetic field, and to determine and explain its structure during more complex intervals. At this time only applicants with external funding can be accepted for this project.
What powers outflows in stellar-mass compact objects? (Poshak Ghandi)
(At this time only applicants with external funding can be accepted for this project)
Powerful outflows appear to be a ubiquitous characteristic of compact object growth, and are seen across the full mass spectrum from stellar-mass to super-massive sources. Outflows can take the form of relativistic collimated jets, radiative feedback or winds. Their origin and transition from one form to another is poorly understood. High time resolution multi-wavelength observations provide unique insight into the interplay between accreting and outflowing matter in stellar-mass compact objects, and this field is set to open wide soon with a multitude of time-domain observatories coming online this decade. The aim of this project is to analyse multi-wavelength, multi-timescale data of Galactic accreting sources to build a picture of the physical processes that drive outflows. Scale-invariant connections to outflows in accreting super-massive black holes will also be sought.