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Astrophysics Group Graduate Projects for Oct 2004 entry | ASTROPHYSICS |
The titles of the projects available for graduate students commencing their research in October 2004 are listed here - the ordering of the projects in the list is random. Further details are given below; in each case there is a brief description of the project and the names of the primary and secondary supervisors and their contact details.
You may also wish to look at an overview of the current research programmes being carried out in the group through the Astrophysics Group Research Interest web-pages. These give additional background to those research groups offering projects this year.Studies of radio-wave propagation in wide-band cellular networks in urban environments
Wide-field surveys and high-resolution imaging of protostellar outflows
Theoretical THz optics and imaging processing for astrophysics and the applied sciences
Superconducting VLSI detectors for astrophysics
The effect of foregrounds on CMB polarisation measurements
Investigating Grid-enabled reduction of interferometric data
Feedback in AGN and galaxy evolution
B-mode polarization of the Cosmic Microwave Background radiation
Beam combiners for next-generation interferometric telescopes
Fringe tracking and spectro-interferometry with a photon-counting CCD
Multi-way interferometry with photonic crystal fibres
This experimental project is being funded by Cambridge Positioning Systems Ltd. (CPS).
The company was set up in 1993 to develop radio positioning systems based on radio astronomy techniques arising from research carried out at the Cavendish Laboratory. The technology developed by the company (code-named 'Cursor' and 'Matrix') is being applied currently to GSM mobile-phone systems (so-called second-generation systems, '2G'), and is now being developed further for the new wide-bandwidth third-generation systems ('3G'). Cursor and Matrix make use of the telephone signals transmitted by the towers of the communications network to measure the position of a mobile phone. The times taken for messages to travel from mast-head transmitting antennas to mobile-phone receiving antennas are deduced from measurements made inside the handsets. These times can then be converted to distances and the positions of the mobile phones calculated from the intersections of circles centred on the transmitting antennas (see www.cursor-system.com for more information).
As with any other radio-based location system (such as radar, LORAN-C, GPS etc.) perturbations in the wave-fields cause errors in the calculated position. Such perturbations are caused by scattering from near-by objects, refraction, multi-path wave propagation etc. The effects of all these are understood and reasonably well modelled for their effects on communications in general, but little work has been done specifically on their effects on radio positioning.
The project combines experimental, observational, and theoretical facets. The student will need to adapt radio receiving equipment supplied by CPS in order to enable it to make 'snapshots' of the received signals over a bandwidth of about 10 MHz centred at around 2 GHz. The whole apparatus will be portable and used to make a series of tests in various environments - e.g. central London, Cambridge, etc., outdoors and in. The idea here is to build up a sufficiently-large database to support the creation of a set of generalised models which will allow the effects of the respective radio environments on various radio positioning systems to be calculated. In addition, it should be possible to devise ameliorating measures by which to improve accuracy and reliability in such systems. This project therefore requires self-motivation and drive, expertise in programming and soldering irons, a pair of Wellington boots, suiting someone who is practical and looking for variety. The theoretical/modelling aspects will be challenging.
The student will get to know everyone at CPS, and will have the opportunity to make use of the existing expertise in the company.
The dramatic molecular outflows from protostars give unique information on the history of mass loss and accretion during star formation. These fast outflows often show spectacular structures, usually bipolar, and extend over scales up to several parsecs. They may play a significant role in delaying star formation, by the injection of turbulence into molecular clouds, and also in promoting star formation through the generation of shocks. Large-scale, unbiased surveys of such objects in molecular clouds are essential to understand the energy balance in these clouds, as well as the factors controlling outflow structure and evolution. In addition, to understand the basics physics of jet formation and propagation, high-resolution interferometric imaging of the driving protostars is required.
This project will combine both of these techniques, using state-of-the-art mm and submm telescopes. For the wide field surveys, we will map the CO 3-2 emission in molecular clouds using HARP on the James Clerk Maxwell telescope in Hawaii. This 325-375GHz imaging system will permit us to map outflows an order of magnitude more quickly than previous instruments, and so open up a new field of studies of these enigmatic objects: samples can include many tens to hundreds, rather than a few, objects. For the interferometric measurements, there are opportunities to use new and upgraded millimetre and sub-millimetre interferometers, including SMA, CARMA and Plateau de Bure. These are all capable of sub-arcsecond images. This PhD project will allow the student to get involved with HARP at first light on the telescope, and pursue a programme of observations which will generate the largest, most complete survey of protostellar outflows ever undertaken. There is also the opportunity, subject to time allocation requests, to do interferometric observations at (sub)millimetre wavelengths on the SMA and other instruments. There will also be opportunities to take part in the refinement of observing strategies and data reduction with HARP, as well as undertaking a significant science programme which will help us understand the earliest phases of star formation. This project is an ideal project to develop an individual's career and group expertise in the run up to ALMA. The Atacama Large Millimetre Array (see www.alma.ac.uk) is expected to start operations in late 2007 and will rapidly become the most powerful instrument in the world for a wide range of observing programmes at frequencies between 90 and 700GHz. This Ph.D. project will contain a combination of astronomical, observational and technical work, all of which will be highly relevant to ALMA.
This project has a clear scientific goal, namely pushing our understanding of the earliest phases of protostellar accretion and outflow, but will also contain technical elements which are relevant to ALMA. These will include some of:
The THz region (300GHz-3THz, 1mm-100um) of the electro-magnetic spectrum is of considerable importance for astrophysics and the applied sciences. The lowest frequency part of the THz region contains the blackbody peak of the cosmic microwave background radiation, while the highest frequency part contain thousands of spectral lines from nearby star-forming regions and highly-redshifted distant galaxies. THz radiation is also now being used in areas as diverse as dental and soft-tissue medical imaging, spectroscopy of biological fluids, atmospheric physics, combustion, and antiquities.
To support our work on THz science, we have been developing advanced theoretical techniques, and software, to enable the optical behaviour of complex THz instruments to be simulated. THz optics is a fascinating subject because the physical concepts lie midway between the radio and optical extremes. For example, at THz wavelengths diffraction is important and therefore ray tracing cannot be used for designing instruments, but equally optical systems are too complex to enable Maxwell's equations to be solved, even numerically, in any rigorous manner. The solution, therefore, is to understand the way in which electro-magnetic fields propagate in the form of spatial modes. More fundamentally, fields at THz frequencies cannot be considered to be fully coherent or fully incoherent, and therefore the complexities of partial coherence must be accommodated in an elegant way. At THz frequencies it is also often desirable to use a full quantum mechanical description of fields when designing instruments, but to date the quantum behaviour of THz systems has not been studied in detail or incorporated in software. Again, the THz region is unusual because the photon occupation numbers of optical modes are neither very high nor very small.
We now wish to add to our recognised expertise in this area. We have two PhD students finishing this year: one student has opened up a whole new range of possibilities by developing techniques for modelling the behaviour of THz systems in terms of highly unusual optical modes. This work is also being applied to astronomical image processing, and is closely related to the subject of Wavelets. The other student is working on the important topic of bolometric interferometry, which is the technique currently used for astronomical optical interferometry, but has major applications in ground-based and space-borne sub-millimetre-wave and infrared astronomy. We have for the first time internationally developed a rigorous theoretical method for modelling these important instruments, and now wish to write a software package that can be used use to study their performance and design.
The new PhD student will concentrate on the development of theoretical THz optics, and will use newly discovered mathematical techniques to produce software for modelling and designing instruments. The precise project can be tailored to suit the interest of the student, but in general terms will appeal to anyone who enjoys exploring the forms of mathematical structures, and applying these forms to produce software that describes the behaviour of real systems. The primary product of the PhD will be software packages that enable the physics of sub-millimetre-wave and far-infrared astronomical instruments, such as bolometric interferometers, to be studied.
The Astrophysics Group has recently completed major new facilities for designing and fabricating state of the art superconducting detectors. We have programmes relating to the development of optical photon counting detectors (Kinetic Inductance Detectors KIDS), superconducting heterodyne mixers for spectroscopy and interferometry (Superconductor-Insulator-Superconductor SIS tunnel junctions), bolometric power detectors (Transition Edge Sensors TES) and (Superconducting Tunnel Junctions STJ). The opportunities in this field are immense, but always our primary goal is to develop detectors that will enable a new generation of astronomical instruments to be constructed.
A major challenge in superconducting detector technology is to fabricate Very Large Scale Integrate Circuits (VLSI); indeed in semiconductor technology, a technical revolution only occurred once thousands of devices could be integrated onto single chips. Superconducting detector technology is poised in a similar way, in that all of the above devices have the potential to be engineered in to large integrated circuits. For example, it should be possible to produced extremely low-noise, quantum-limited in fact, integrated receivers that operate throughput the whole of the THz region; it should be possible to produce large-format imaging arrays for the sub-millimetre, far-infrared, optical, and x-rays wavelength ranges; it should be possible to produce single-chip spectrometers, and single-chip bolometric interferometers.
We are looking to introduce a new PhD student to this important technology. The project can be tailored to suit the student's interests, and can be in any one of the above areas. We are particularly keen, however, to strengthen our work in sub-millimetre-wave and far-infrared detectors. The project is of an applied nature, in that it would consist of theoretical device modelling and numerical design work, working in the clean room using our new facilities to develop the chosen technology, and carrying out laboratory-based low-temperature, 4K-10mK experiments. Full training would be given, and no previous knowledge of device processing techniques is needed.
Accurate measurements of anisotropies in the cosmic microwave background (CMB) radiation are an important tool in observational cosmology. Their measurement has only recently become possible, and has transformed the subject. The anisotropies contain information about the matter and energy content of the universe and its rate of expansion. They can also constrain the physics of the era of inflation in the very early universe, 10^{-34} seconds after the Big Bang. There has been much interest recently in measurements of the polarisation of the CMB radiation.
CMB polarisation anisotropies can be divided into an E-mode (curl-free) component and a B-mode (curl) component. E-modes are caused by scalar and tensor fluctuations in the last scattering surface while B-modes are caused by tensor fluctuations alone. E-mode fluctuations have recently been detected at a level of about 10% of the temperature fluctuations. A surprisingly large cross-correlation of the E-mode with the temperature fluctuations at large angular scales seems to indicate that the universe was re-ionised earlier than previously thought, at a redshift z = 17. B-mode fluctuations are predicted by inflationary theory to be a number of orders of magnitude smaller. Detection of B-modes at large scales would be evidence for tensor fluctuations - gravity waves - in the early universe and could be used to constrain the theory of inflation.
This neat picture is confused by the microwave radiation emitted by our Galaxy and extragalactic sources - these form "foregrounds" to the cosmic microwave background. As experiments to measure the fluctuations in the microwave sky become more sensitive, the accuracy of our measurements the CMB polarisation fluctuations will become limited by our knowledge of the foregrounds. However, their effects can be removed by using observations at a number of frequencies. If the foregrounds have spectra different from that of the CMB, this knowledge can be used to separate CMB fluctuations from the foreground emission.
This project would be to work on improving our knowledge of the foregrounds and on improving the techniques used to analyse the data. This is especially important in the light of a number of forthcoming experiments including the Planck satellite and the Cambridge-Cardiff B-mode experiment. This project would involve a number of parts:
A new generation of radio telescopes is planned or being constructed which will increase the data rate by many orders of magnitude compared to current instruments. In the short term these include the eVLA, eMERLIN and ALMA. In the longer term new instruments such as the Low Frequency Array (LOFAR) and the Square Kilometre Array (SKA) are being planned which rely for their successful operation on computing power continuing to increase at the same rate as in recent years. What is common to all of these telescopes is that they are interferometers in which the data are collected in Fourier space and reduction requires the following basic steps:
The last step is the most time consuming. In its simplest form it involves gridding the data onto a regular grid and taking a Fourier transform. More generally we seek more sophisticated methods for reconstructing a sky image from the raw data which involves both self-consistent additional calibration and also deconvolution of the instrumental response.
There is a great deal of expertise within the radio astronomical community concerned with these issues, but the new generation of instruments will require that the analysis is performed in a distributed fashion over a large heterogeneous network of systems (the Grid). Taking existing methods and translating them to this environment is a non-trivial task. The aim of this Ph.D. is to investigate and develop methods for reducing these extremely large data sets on the Grid.
More detailed information on this project will be available in February 2004.
There is now a well-established model for the evolution of the galaxy population based on the general idea of hierarchical structure formation. In this picture larger structures such as galaxies and clusters form via the sequential merger of smaller galaxies. At each merger fresh gas is injected into the system which is presumed to lead to a burst of star formation. This model has had very many successes, but there are still many problems. In particular the simplest view of star formation is too efficient. A mechanism is needed to stop all of the baryons cooling and forming stars. One very likely possibility is that Active Galactic Nuclei provide a feedback mechanism. However it is not clear precisely how the energy output from the AGN couples to the surrounding gas providing efficient feedback. One possibility which we are actively examining is that the coupling is by radio AGN where two light jets propagate out through the interstellar medium of the galaxy.
The aim of this project is to examine in particular the possible role of young and low-powered radio sources. We have an excellent way of identifying young radio sources in the 9C survey from their spectral shape. Such young radio sources provide a means both of probing directly the interaction of the radio source with the galactic environment and of studying the mechanisms by which the activity is triggered and the fuelling of the central black hole moderated. Depending on the interests of the student this can be mainly a theoretical or observational project or - ideally - a combination of both. In particular we will:
Use SIRTF surveys coincident with regions of 9C to investigate whether star formation is occurring at the same time as the AGN activity and determine which came first.
Extend our existing models of the small-scale evolution of the radio source to model the interaction with its environment and the ways in which AGN triggering and star formation may be linked.
Extend our modelling of the sorts of instabilities triggered by the radio source in the galaxy and hence investigate the efficiency of the radio source in promoting star formation.
The partial linear polarization of the cosmic microwave background radiation (CMB), predicted over thirty years ago, has finally been detected by the Degree Angular Scale Interferometer and the Wilkinson Microwave Anisotropy Probe. Polarization in the CMB is only generated by scattering, and hence is a direct probe of physical conditions when neutral atoms first formed in the universe, and when these atoms were subsequently re-ionised after the first generation of stars formed. Much of the cosmological information encoded in the polarization of the CMB is complementary to that in the temperature anisotropies. This allows polarization data to break a number of important degeneracies that would otherwise hinder attempts to measure accurately cosmological parameters using CMB data alone.
Arguably the most exciting prospect for CMB polarimetry is the detection of the stochastic background of gravitational waves predicted by most theories of inflation in the early universe. The pattern of CMB polarization on the sky can be decomposed into two geometric modes, called E and B, like the decomposition of a vector field into a gradient and curl part. In linear theory, B-modes cannot be produced by the usual density fluctuations that give rise to large-scale structure, but are produced by primordial gravitational waves. Detection of large-scale, primordial gravitational waves would significantly increase our ability to distinguish between the plethora of inflation models, as well as providing a direct measure of the energy scale at which inflation occurred. Unfortunately, the predicted amplitude of the B-mode signal is very small thus placing strong demands on the sensitivity of any instrument trying to detect it. Furthermore, secondary processes, such as weak gravitational lensing by the intervening mass distribution, modify the pristine curl-free geometry of polarization from density fluctuations leading to the generation of B-modes. Probably even more troubling is the polarized foreground emission from astrophysical sources, which will generally have a B-mode component that must be separated from the primordial signal on the grounds of their characteristic frequency spectra and statistical properties. Instrumental effects, such as cross polarization, and survey effects, such as incomplete sky coverage, will further complicate the task of isolating B-mode polarization.
Despite the difficulties in searching for B-mode polarization, the potential scientific returns are sufficiently high that a number of ambitious experiments are currently being planned that may reach the required levels of sensitivity and control of instrumental effects to detect the B-mode signal. One such experiment is being proposed jointly by the Cavendish Astrophysics Group (specifically the Detector Physics and CMB subgroups) and Cardiff University, and we plan to apply for funding in early 2004. We anticipate that the student will take an active role in the theoretical and analysis work for this experiment, as well as contributing on the theoretical side to our longer-term programme of developing bolometric interferometry for CMB polarimetry. Examples of the work that the student might be involved with include:
This project should suit a theoretically-inclined student, with strong analytic and computing skills, who desires to work at the interface between theoretical and observational cosmology.
The Cambridge Optical Aperture Synthesis Telescope (COAST) is the world's first operational optical synthesis telescope. It uses methods pioneered in radioastronomy to provide angular resolutions some 30 times better than the Hubble Space Telescope. The COAST team in the Astrophysics Group are currently engaged in a collaboration to design and prototype a new interferometer at the Magdalena Ridge Observatory (MRO) in New Mexico. This $40M project aims to deliver a unique interferometer, capable of imaging faint objects at very high angular resolution. The COAST team are also the leading UK group involved in the exploitation of interferometry for astrophysics, both with COAST and with the Very Large Telescope Interferometer (VLTI) in Chile.
The aim of this project is twofold: to participate in the design and prototyping of one of the critical optical subsystems of the MRO interferometer and to make use of data from the VLTI to do astrophysical research.
The MRO interferometer as currently envisaged generates two parallel sets of interferometric signals: the "science" beam combiner produces the interferometric signal needed to make images of the object being observed, and the "tracking" combiner generates the signals necessary to stabilise the interferometer to millisecond-timescale disturbances that affect the alignment of the interferometer. The COAST group expects to take the lead in the design and prototyping of both these beam combiners, which are a significant step beyond any currently implemented designs.
The student taken on for the project could work on one or more of the optical, mechanical and software aspects of the design of these beam-combiners. These include:
In parallel with this experimentally-focussed work, the student would have the opportunity to use existing interferometric arrays (e.g. COAST and the VLTI) to undertake programmes in observational astrophysics which take advantage of the exceptional angular resolution these offer. Existing observational programmes include characterisation of "starspots" on super-giant stars, imaging the motion of dust around evolved stars and directly measuring the pulsation of Cepheid variables to determine their distances, but we also envisage other programmes being developed to exploit the new instruments being delivered at the VLTI.
This project would suit a student who is interested both in hands-on development of innovative instrumentation and in observational aspects of interferometric astronomy. We are looking for a student with a good mix of skills and interests, to form part of our multi-disciplinary team.
As part of our collaboration with the $40M Magdalena Ridge Observatory Interferometer (MROI), the optical interferometry team in the Astrophysics Group have been working on the development of a new low-noise CCD spectroscopic instrument for the COAST array. This L3CCD spectrometer is based on a recent innovation in detector technology - the electron-multiplying CCD - which allows high-speed high-efficiency photon counting in a device with thousands of detector elements. This offers an unmatched capability as compared with all other optical detectors used for interferometric instruments, and will immediately lead to a factor of 25 increase in the number of wavelength channels that can be observed simultaneously with COAST. Since COAST will then be the only interferometer in the world with a high-efficiency optical spectroscopic capability, we will immediately have access to both fainter targets than other arrays and the ability to perform detailed multi-wavelength observations of brighter sources.
The broad goals of this project will be two-fold:
Throughout the project, the focus will be on underpinning research for the MROI L3CCD-detector systems so that the highest sensitivity and performance can be delivered with low technical risk. This project would suit a student who is interested in doing a roughly equal mixture of instrumental work, software algorithm development, and astronomical observation and interpretation.
Photonic crystal fibres (PCF) are optical waveguides which remain single mode over an extraordinarily large wavelength range. This makes them ideal for use in astronomical interferometry where using a large wavelength range is critical to realizing the highest sensitivity. The technology required to fabricate PC fibres and their associated components (e.g. couplers and splitters) is very new, and as a part of our strategic development program for astronomical interferometry, we have initiated a collaboration with the University of Bath who are world leaders in PCF fabrication.
Our particular interest is in exploring the feasibility of using PCF technology to construct a high-sensitivity multi-way beam combiner. This would bring together beams from 4-6 telescopes and produce multi-baseline interference patterns which could then be spectrally dispersed to produce multi-wavelength fringe patterns containing an unprecedented amount of high-precision interferometric information. The successful design and construction of such a novel beam combiner would have wide reaching repercussions, especially for arrays like the $40M Magdalena Ridge Observatory Interferometer whose beam combiners we are responsible for delivering and which will need to combine signals from up to ten 1.4m telescopes
The objective of this project is to design and build a prototype PCF beam combiner in the laboratory and thereby to demonstrate the unique capabilities of this technology for astronomy, especially for the new MRO array. The work would involve characterising the interferometric properties of PCF designs produced by our collaborators at the University of Bath, designing beam combination optics and subsequent interferometric testing of the integrated assembly. This project would interest a student who is interested in optical analysis, design and experiment, perhaps with a preference for hands-on research work.
Last Modified 13 January 2004
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