Galaxies, Active Galactic Nuclei and Cosmology

What makes radio galaxies tick? Exploring polarisation and particle acceleration with the AGES-XL survey

Jeremy Harwood, Martin Hardcastle

Powered by the supermassive black holes found at the centre of massive galaxies, radio galaxies comprise of relativistic jets that can extend millions of light years into their surrounding environment. The ability of these powerful sources to suppress star formation has led to them being widely accepted as a key ingredient in one of astronomy's big outstanding questions: how do galaxies evolve to make up the Universe we observe today? However, many of the processes which drive these powerful objects remain poorly understood.

Working initially with a new, cutting edge survey (AGES-XL; PI: Harwood) recently observed with the Jansky Very Large Array, you will aim to unlock these mysteries by exploring the polarisation and particle acceleration properties of the radio galaxy population. Along with analysing and interpreting observations from this new survey you will have the opportunity (depending on your interests) to develop new methods of analysing these sources and/or derive new models to explain the observed behaviour. Throughout your PhD you will also have the chance to work with a variety of next-generation radio telescopes and surveys including the international LOFAR telescope, e-MERLIN, and the newly commissioned MeerKAT telescope to further explore the physics that underlie these important objects.

During the project, you will have the opportunity to travel to key radio astronomy institutes around the world such as Oxford, Manchester, ASTRON (the Netherlands), and the NRAO (USA), as well as other UK and international locations to work with collaborators and present your findings.

Cosmic Chemical Enrichment from the First Stars

Chiaki Kobayashi, Sean Ryan

Just after the Big Bang, the first stars form, and cause the first enrichment in the Universe when they die. What is the nature of the first stars? How massive are they? Which elements were produced? When do we have enough elements to permit life? The student will answer these questions by using computational simulations to follow the chemical and dynamical evolution of the Universe across Cosmic Time. Our simulation code self-consistently includes relevant physics of atomic matter - hydrodynamics, star formation, chemical enrichment from Type II and Ia supernovae, and feedback from supernovae and supermassive black holes - and therefore, the predictions are comparable with observations from nearby to distant galaxies. In the early Universe, the first stars are supposed be more massive and cause pair-instability supernovae or black-hole-forming faint supernovae, and thus have different nucleosynthesis yields. The student will include the chemical enrichment from the first stars into the simulation code, and constrain the nature of the first stars by comparing to the observations of high-redshift galaxies and quasar absorption line systems.

Active galaxies in the LOFAR Two-Metre Sky Survey

Martin Hardcastle, Jeremy Harwood, Dan Smith

The International LOFAR telescope -- a next-generation radio telescope centred in the Netherlands but with stations across Western Europe, including the UK -- is surveying the northern sky at a frequency of 150 MHz, and will provide the best wide-area radio survey in the northern hemisphere for the foreseeable future. Work on processing the LOFAR data, on the optical spectroscopic survey WEAVE-LOFAR which will follow up LOFAR detections, and on the analysis of radio-loud active galaxy populations is led from UH. Radio-loud active galaxies are objects in which accretion onto a central supermassive black hole drives a powerful, relativistic outflow, giving rise to strong low-frequency radio emission; these jets and the energy they carry are key ingredients in cosmological models of galaxy formation and evolution but there are still many unanswered questions about the population of these objects and its evolution. The student will use UH’s excellent access to survey data, along with numerical and analytical models of radio galaxies, to construct samples of radio-loud AGN, and will address key questions including: what are the kinetic powers of radio-loud AGN as a function of host galaxy and environment? How does the kinetic power provided by these sources evolve as a function of cosmic time? What processes drive the birth, death and rebirth of jet activity?

High sensitivity, high resolution imaging at radio wavelengths of four nearby galaxies

Elias Brinks, Luke Hindson

Modern radio telescopes, such as the Very Large Array (VLA) in New Mexico, are revolutionising our understanding of galaxies. Radio emission, rather than picking up emission from stars like at optical wavelengths, instead probes material floating between the stars, the interstellar medium (ISM). This is where on the one hand processes leading to the formation of new generations of stars take place (the cooling and collapse of neutral gas clouds leading to molecular clouds which fragment and collapse further to form stars) and on the other stars exploding as supernovae return matter and relativistic electrons (also known as cosmic ray electrons) to the ISM.

Radio emission is turning out to be a powerful probe for both the formation of new generations of stars, through tracing the thermal emission generated in HII regions, and the death throes of stars, by mapping the relativistic electrons produced in supernova shocks, which lose their energy as they spiral around magnetic field lines, generating non-thermal synchrotron radiation. Unlike optical observations, radio emission is hardly affected by dust which gives radio studies the potential to map star formation out to large redshift. Eventually that will be one of the key areas of research for the Square Kilometre Array, an ambitious “World” telescope, the first phase of construction of which is imminent.

A substantial allocation of observing time was awarded to observe 4 nearby galaxies with the VLA in great detail covering several wavelength and mapping total intensity as well as full polarisation. Observations have started in Sept 2016 and will continue through 2017. Improvements in telescope receivers in recent years means our study will be vastly superior to what has been achieved thus far. The data will enable many different studies. A challenging, but rewarding project that is foreseen is to derive total and polarised intensity maps to derive the total (equipartition) strength of the magnetic field, and the constituent ordered and turbulent fractions. We will also exploit the near contiguous coverage across frequency to apply Faraday Rotation-Measure (RM) Synthesis. We will use the Faraday rotation measure, its azimuthal variation, and RM–Synthesis to obtain a census of axisymmetric mean magnetic field strength and magnetic pitch angle, all crucial for testing the theory of a galactic dynamo.