Galaxies, Active Galactic Nuclei and Cosmology

Chiaki Kobayashi, Sean Ryan

Just after the Big Bang, only very light elements (H, He, Li, Be, and B) can be produced, and heavier elements are all formed in stars and ejected by supernovae. Many elements (from carbon to uranium) have been observed in many stars in the Milky Way with spectroscopic ‘galactic archaeology' surveys, and some of the production sites are also observed as gravitational wave events such as the neutron star merger in 2017. The student will study the origin of elements in the Universe by comparing computational simulations of galaxies to these observational data. Our hydrodynamical simulation code already includes basic physics such as star formation and supernova feedback, and thus it is possible to compare with the observed elemental abundances in the Milky Way and its satellite galaxies. The student will update the code (written in c) to include the detailed effects from binary stars, predict the distribution of elements in galaxies across Cosmic Time, and constrain the theory of galaxy formation.

Kristen Coppin, Jim Geach

Over half of the star formation energy generation in the Universe is extincted at optical wavelengths and enshrouded by dust which absorbs and re-radiates the starlight in the far-infrared/submm; and the sub-mm and mm atmospheric windows allow us to access the redshifted far-infrared emission from this obscured or ``hidden’’ side of galaxy formation and evolution.  The James Clerk Maxwell Telescope (JCMT) Cosmology Legacy Survey (S2CLS; Geach et al. 2017) and its extension via the SCUBA-2 COSMOS survey (S2COSMOS; Simpson et al. 2017) and now S2XLS (PI Geach) and STUDIES (Wang et al. 2017) are the largest and most sensitive and ambitious single-dish surveys at 850 and 450 micron (in the submm wavebands) ever conducted.  In addition, the 50-m Large Millimeter Telescope in Mexico will be conducting unique and transformative imaging of the sky at millimeter wavelengths through a series of public Legacy Surveys (Ultra-Deep and Large Scale Structure surveys in particular) starting in late 2021 using the new TolTEC camera.  These unprecedented legacy surveys have been yielding thousands of high-redshift galaxies selected in the sub-mm/mm wavebands - providing an order-of-magnitude improvement in the sample sizes of previous surveys at these wavelengths!

With so much data now in-hand there are several possible projects that could be carved out using a combination of these legacy (sub)mm surveys with existing ancillary multi-wavelength data to make progress on a key outstanding question in galaxy evolution: How are dust and metals built up in massive galaxies over cosmic time? Some key science that could be explored with these data sets by a keen student include (for example): 1) constraining the (sub)mm source counts beyond the confusion limit (using a statistical P(D) fluctuation analysis); 2) locating and probing the high-redshift tail of the distribution of (sub)mm galaxies (via new mm observations); and 3) exploring new parameter space on the dust content, obscuration fraction, and gas content in galaxies (via (sub)mm observations) as a function of mass out to much higher-redshift than has previously been explored.  The project can be tailored to some degree to match the student’s interests and skill set.

We are also involved in ongoing efforts to perform detailed follow-up of these high-z submm-detected sources at higher resolution with the Atacama Large Millimetre Array (ALMA) situated at 5000m on the Chajnantor plateau in Chile.  It is envisaged that the findings of this work will feed naturally into new ALMA and other telescope proposals, such as the James Webb Space Telescope (JWST).

Martin Krause, Martin Hardcastle

The evolution of galaxies and their central supermassive black holes is closely linked and evidence is accumulating that the mutual interaction is different for different galaxies and in different epochs of the Universe. The black holes produce powerful radio jets that may affect star formation in galaxies, in particular, via preventing cooling gas around the galaxies from arriving in the galaxy in the first place and also by triggering star formation by compressing dense gas already inside the galaxies. Our group has contributed to this progress with simulations in the past. This PhD project would use existing simulation software to study jets in galaxies with a star-forming disc, similar to the Milky Way. The jets are expected to form radio lobes, affect outflow properties of the gas and the rate of star formation. Such results will then be compared to observations available within the group and from the literature. This will result in a better understanding which observations may be explained by jets and how important this jet feedback really is in star-forming galaxies.

Dom Walton

Accretion onto compact objects powers some of the most luminous and extreme phenomena in the universe. Accreting supermassive black holes ultimately power all of the different populations of active galactic nuclei (AGN), while stellar remnant black holes and neutron stars power luminous X-ray binary systems (XRBs) that are scattered throughout all major galaxies. This accretion process also drives powerful outflows (winds, jets) that allow these compact sources to impact their surroundings, and are responsible for driving the 'feedback' processes that dictate the observed co-evolution of supermassive black holes and their host galaxies. Understanding this accretion process (and its by-products) is therefore of critical importance. In particular, there are still major questions to be answered regarding how supermassive black holes came to be as massive as they are today (and, in the most extreme cases, how they were able to rapidly do so while the universe was still in its relative infancy).

Two potential projects are available, on black hole spin and on ultraluminous X-ray sources, both of which will primarily involve analysis of both new and archival X-ray observations from e.g. the XMM-Newton, Swift, Chandra and NuSTAR observatories, including spectroscopic and time-domain analyses. In addition to the immediate scientific progress, both will also serve as key preparation for the Athena X-ray observatory – ESA’s next flagship-class space telescope – and also offer connections with the gravitational wave community.

Black Hole Spin: in astrophysics black holes are expected to be fully defined by just two numbers, their mass and their spin (or angular momentum). Information on the growth history of SMBHs is imprinted on their spin distribution, and so SMBH spin measurements are of particular importance for understanding this process, and in turn galaxy evolution (which is inextricably linked to SMBH growth). Such measurements are possible via detailed X-ray spectral/timing analyses of accreting black holes, but disparate analyses have been presented in the literature. The primary goal here is to use the latest, state-of-the-art spectral models to construct a pipeline to measure BH spin that can be applied uniformly to samples of AGN, a key step in connecting SMBH spin measurements to galaxy evolution models. Furthermore, the same techniques can be used to measure spin in black hole XRBs, offering a means to compare the XRB population with the black holes seen merging via gravitational waves.

Ultraluminous X-ray Sources: these are the most extreme XRBs and are now understood to be the best local examples of accretion above the Eddington limit, which may be required to grow the SMBHs now being seen in the early universe. ULXs were themselves assumed to be black holes for a long time, but remarkably we now know that some ULXs are powered by wildly super-Eddington neutron stars! Only a handful of these neutron star ULXs are currently known, but there is speculation they may make a significant contribution to the broader ULX population. In fact, we are still hunting for the first confirmed black hole ULX, although there is again speculation that such systems exist as evolutionary pre-cursors to the gravitational wave black hole mergers. The primary goal here is to further our understanding of the ULX population, studying the known ULX pulsars to understand how they are able to accrete at such extreme rates, searching for new ULX pulsars, and also for the first black hole ULX.

Daniel Smith, Martin Hardcastle, Alyssa Drake

Starting in 2022, the WEAVE-LOFAR survey will use the new WEAVE multi-object spectroscopic facility on the William Herschel Telescope to obtain spectroscopy of radio sources identified in the new LOFAR Surveys. The LOFAR surveys have the best combination of huge sensitivity, resolution and sky coverage, and they are already revolutionising our understanding of the Universe. However, WEAVE-LOFAR is the key for taking this to the next level, by providing the most accurate redshifts, source classifications and the most inclusive sampling of the active sources in the Universe. Over the coming five years WEAVE-LOFAR will generate a huge library of optical spectra, and this is the first time that spectroscopy of radio sources has been done on this scale - giving WEAVE-LOFAR access to a huge discovery space. This PhD position is ideally-timed to get your hands on the first data from WEAVE-LOFAR, and join the team writing the first publications. Lots of different directions are possible, including studying star formation evolution in galaxies over the last half of cosmic history, searching out and characterising the most star-forming galaxies in the Universe, or making new measurements of the influence of accretion by black holes on galaxy evolution. The possibilities are virtually endless. If this sounds like something that would interest you - please do get in touch.

Emma Curtis-Lake, Kristen Coppin, Jim Geach

With the advent of the James Webb Space Telescope (JWST), we will have unprecedented observations of galaxies in the high redshift Universe.  JWST probes the stellar light, emission from hot, ionised gas, but some of that emission is obscured by dust which itself emits light at much longer wavelengths. The Atacama Large Millimeter Array (ALMA) covers much longer wavelengths, and probes the molecular gas (the direct fuel for star formation) as well as emission by dust.  The past few years have shown that Hubble has not revealed the full extent of high-redshift galaxies, the rest-frame UV emission is offset from the dust emission revealed by ALMA, meaning that we were missing lots of the light.  The student will start by implementing the dust emission model within a Bayesian spectral and SED (spectral energy distribution) code called BEAGLE (BayEsian Analysis of GaLaxy sEds, Chevallard+2016).  BEAGLE already self-consistently models the stellar light, dust attenuation, and the reprocessing of stellar light as it travels through stellar birth clouds (which produces line emission within the hot, ionised gas). The model and implementation for the dust emission will be challenged by fitting to simulated data before fitting to new JWST data and archival ALMA data giving us the most complete view of galaxies in the early Universe.  This work is also likely to naturally lead to new JWST and ALMA proposals.

Emma Curtis-Lake, Daniel Smith, Chiaki Kobayashi

We can trace the build-up and processing of different elements in Galaxies throughout cosmic time by studying spectra of the hot, ionised gas which is excited by young, hot stars or even super-massive black-holes.  It has been possible to estimate the oxygen-abundance and other element abundances in gas excited by star formation for a long time. Recently there have been significant advances in analysing the element abundances of gas excited by the emission from the accretion discs surrounding actively accreting black-holes within AGN (active galactic nuclei).  The emission lines of elements (O, N, H, Si)  within the rest-frame optical have also been routinely used to identify obscured AGN locally but we expect the distinction to be less clear-cut at high redshifts, meaning different techniques are likely required to find new, obscured AGN in the early Universe. With the advent of the James Webb Space Telescope (JWST), and in particular the incredibly sensitive multi-object spectrograph NIRSpec we will have rich data-sets probing to very early periods in the history of the Universe (we will have all the rest-frame optical lines used locally out to z~7, i.e. just ~700 million years after the Big Bang!). The student would use models of the emission from gas within star-forming regions (Gutkin+2016) and the narrow-line regions (Feltre+2016) of AGN within the spectral fitting code BEAGLE (BayEsian Analysis of GaLaxy sEds, Chevallard+2016) to analyse these exciting new data-sets, as well as current data-sets that haven’t yet been explored using BEAGLE.  It will be possible to search for new, obscured AGN, providing constraints on the evolution of AGN at these early times, as well as deriving constraints on the metal abundances (also from known AGN) and comparing them to star-forming galaxies at each epoch. Towards the latter part of the project, the WEAVE-LOFAR dataset will also supply ample AGN to study with BEAGLE.

Obscured AGN: Under the unified model of AGN, what we see of the emission from the central accreting black hole is determined by the viewing angle.  There is a thick torus like structure that can block light from the accretion disc itself, as well as a region called the broad-line region, so-called because it emits very broad emission lines that are generally easy to discern from standard star-formation induced line emission.  The gas producing these lines is very close to the black-hole and rotating incredibly quickly.  When you cannot see the accretion disc emission or the broad-line region, you might still see narrow-line emission powered by the central accretion disc. This is gas further away from the black hole that is not blocked by the dusty torus, and which is not moving at such high speeds.  It is fairly easy to identify AGN via broad-line emission, and so with BEAGLE we have focused on incorporating the model of the narrow-line region, which is not always so easy to dis-entangle from emission from a normal star-forming galaxy.