Research
Summary of Research Topics and Projects
Primary Research Topics
Summary: Both black hole growth (AGN) and star formation are fuelled by cold gas that originates from a shared (potentially hot) gas reservoir inside the galaxy halo. The amount of gas and the ability for this gas to cool determines the amount of usable fuel that can be used for feeding black hole growth and star formation. Both processes are also known to inject energy and momentum (via radiation, winds and jets) that can reduce the availability of usable fuel through ionising, heating, shocking or expelling material, and hence provide self-regulatory feedback mechanisms. A key component of most galaxy formation models is that these two processes can also have a positive or negative impact on the usable fuel supply for the other process (black and grey arrows).
I have written some of the key literature that observationally characterises how black holes grow and the relationship between star formation and black hole growth. These works use multi-wavelength observations from multiple observatories (e.g., ALMA, Herschel, Chandra) .
See some of my highlighted results
Summary: Outflows of gas driven by growing supermassive black holes (i.e., Active Galactic Nuclei; AGN) are a key component of our current models of galaxy formation. AGN are expected to drive material away by injecting mechanical and/or radiative energy into their surrounding gas. Such outflows have been detected; however, a significant challenge has been assessing their prevalence, properties and impact in galaxies over cosmic time. Crucial measurements can be made by spatially resolving the gas kinematics, using spatially-resolved spectroscopy, in order to establish the mass, energy and momentum carried by these outflows. Furthermore, as these outflows are multi-phase (i.e., ionised, atomic and molecular phases), it is necessary to use multiple observational facilities in order to establish the full physical properties.
I have been pioneering approaches of using large surveys to establish the prevalence of AGN-driven outflows and then using multiple observational facilities in order to establish their detailed properties. This involves data from ground-based and space-based observatories, covering the whole electromagnetic spectrum.
See some of my highlighted results.
Summary: Sonification is the conversion of data to non-speech audio. Challenging the idea that we should solely use visualisations to represent astronomical data, there has been an emerging research interest in converting astronomical phenomena into sound. There are now several astronomers performing such “sonification” of astronomical data to make academic research, outreach and education accessible to people who are blind or visually impaired (BVI) or to assess if data sonification, with visualisations, can enable a deeper understanding of the underlying data.
I am working with multi-disciplinary teams across the world to understand how best to apply sonification in an astronomy context for both scientific discovery and improved accessibility. In 2021 and 2022 I chaired (with Anita Zanella) the Audible Universe workshops (sonified poster on the left!). These brought together experts in sound design, sound perception and astronomers (as well as others) to discuss current attempts and future directions.
See some of my highlighted results.
Summary: A picture is emerging in which the mode of galaxy formation in the distant Universe is very different to that in the local Universe. Rather than the quiescent formation of stars that is the norm in today's Universe, violent episodes of star formation are dominated by the formation of super-star clusters. However, the origin of these differences is controversial: the conventional picture is that they are driven by an increase in the galaxy merger rate, but some recent theories have suggested that the difference is driven by the higher rate of gas accretion expected in the distant Universe. How then, did young galaxies, with irregular morphologies, disturbed kinematics and violent episodes of star formation evolve into the quiescent dynamically stable galaxies we see in the local Universe? Addressing this requires us to look inside galaxies by resolving the motion of the stars and gas clouds within them (to understand their structure, whether they are chaotic mergers or rotating disks) and map the distribution of new stars and metals within these galaxies.
With my collaborators, I have been designing, implementing and exploiting the largest ever spatially-resolved spectroscopic surveys of galaxies designed to investigate billions of years of galaxy evolution.
See some of my highlighted results.
Summary: Growing supermassive black holes (Active Galactic Nuclei; AGN) are broadly split into two types based on their dominant energy output: (1) those which are radiatively [i.e., photon] dominated and (2) those which are mechanically [i.e., jets of particles] dominated. Traditionally radio emission has been used to study mechanically dominated AGN and powerful jets of charged particles have been observed in such systems for decades. However, in radiatively dominated AGN, which make up the majority of the population, the origin of the radio emission is much less clear: star formation in the host galaxy, a corona around the accretion disk, shocked winds and radio jets could all be dominating this emission. Disentangling these possibilities is important to establish the energetics of different physical processes inside these galaxies, and also to determine how black hole growth and star formation are related.
By performing deep and high spatial resolution radio observations on powerful AGN (i.e., quasars) I have provided some of the first evidence that radio jets are common in radiatively-dominated AGN and quasars appear to interact with their host galaxies in a similar manner to their mechanically dominated counterparts.
See some of my highlighted results.
Star-formation in the distant Universe
SMGs: Galaxies that are bright at sub-mm wavelengths typically represent distant dust-obscured galaxies. These "sub-mm galaxies" (SMGs) are forming stars at incredibly high rates, up to 100s per year, and are prevalent 10 billion years ago. However there remain open questions on how such activity is triggered, and if they are truly different to star-forming galaxies in the nearby Universe. I have been involved in teams of astronomers using multi-wavelength observations to characterise the properties of these distant galaxies.
Clumps: Distant galaxies appear clumpy in nature and appear to have giant regions of star-formation. I have worked with multi-wavelength observations to try and understand the properties of these star-forming clumps and their role in galaxy evolution.
Projects, Consortia & Collaborations
My Role: Project Leader
The most powerful AGN, called quasars, release incredible amounts of energy - more energy than all of the stars in their host galaxies put together. It is expected that this energy has a dramatic impact on the host galaxies, by heating or removing gas and even preventing the formation of stars. However the observational constraints are loose.
Using 1D spectroscopy we have constrained the prevalence of ionised outflows in low-redshift z<0.4 AGN and created a well understand parent sample of 1000s of quasars. I have consequently been leading a multi-wavelength, spatially-resolved follow-up observational campaign in order to determine the properties and drivers of the outflows. These observations include spectroscopy and radio imaging that have revealed galaxy-wide ionised outflows and radio jets. Our X-ray observations and ongoing CO observations are revealing the multi-phase gas content in these outflows. The power of our approach is a detailed understanding of how quasar feedback works in the context of the overall parent population - providing crucial tests of theoretical predictions.
Find out more at the project website.
The KMOS AGN Survey at High Redshift (KASHz)
My Role: Project Leader/Co-ordinator
ESO messenger articles on this survey:
Summary: Outflows of gas driven by supermassive black holes are a key component of our current models of galaxy formation. Nonetheless, the prevalence of these outflows is poorly constrained, particularly in the distant Universe. Although significant progress has been made using 1-dimensional spectroscopy to search for high velocity gas in the host galaxies, in order to test model predictions it is critical to assess the spatial distribution of these outflows. This requires spatially-resolved spectroscopy with integral field units (IFUs). Some early work using this technique with single object IFUs established that outflows driven by supermassive black holes can be found over galaxy-wide scales. However, this typically was on small and biased samples of galaxies.
The KMOS instrument on ESO's Very Large Telescope provided a revolution by enabling IFU observations of 24 galaxies simultaneously. Using KMOS Guaranteed Time Observations, I have been leading an extensive survey of 250 high-redshift AGN (z~0.6-3.2; see introduction in Harrison et al. 2016). This is designed to assess the warm ionised gas kinematics inside the host galaxies during the peak cosmic epoch of star formation and black hole growth. For example, we have looked into the connection between the outflows and host galaxy properties in Scholtz et al. (2020). This work is complementary to optical spectroscopic surveys (such as QFeedS above) that cover the same emission lines but in lower redshift systems.
My Role: I have been part of the target selection and the host galaxy characterisation. I designed an experiment to characterise the pros and cons of using adaptive optics (being carried out by my student). I am leading follow-up observing proposals designed to fully constrain the impact of the outflows on their host galaxies using dust emission.
Summary: Outflows of gas driven by supermassive black holes are a key component of our current models of galaxy formation. Nonetheless, the properties of these outflows are poorly constrained, particularly in the distant Universe where such processes are likely to be most prevalent. The SUPER survey complements the KASHz survey, by targeting a smaller number of galaxies hosting powerful growing black holes but with improved spatial resolution - resulting in higher precision outflow measurements. SUPER is a 280 hour Large Program at ESO's Very Large Telescope, using adaptive optics in order to reach a spatial resolution of ~1-2 kiloparsec at z~2. The program is designed to perform high-resolution, spatially-resolved spectroscopy of multiple emission lines (Hb, [OIII], Ha) of a carefully selected sample of ~30 AGN. The data will be used to map ionised outflows, constrain their impact on star formation and investigate the variation of outflow properties as a function of the host galaxy properties.
My Role: I have taken a leading role across each of the projects, including: leading the analyses that lead to the published catalogued values for KROSS; designed, prepared and performed the observations for KGES and supervised the analyses of the KDS data that is part of Owen Turner's PhD thesis.
Summary: The peak in the volume averaged star-formation rate in galaxies occurs in the redshift range z~1-2 (i.e., ~9 billion years ago). At this epoch, the star formation rate in typical galaxies is an order of magnitude higher than in the local Universe. This is the era when most of the stars (and also black holes) in the Universe were formed. The task is now to address "how" and "why" the Universe is so different in recent times. This requires us to look inside galaxies by resolving the motion of the stars and gas clouds within them (to understand their structure, whether they are chaotic mergers or rotating disks) and to map the distribution of new stars and metals within these galaxies. These observations would in turn determine the dynamical state of galaxies and hence directly address the cause of their elevated star formation and so test the scenarios by which early systems mature into the regular galaxies we see today. To undertake such a study requires spatially-resolved spectroscopy of hundreds of faint galaxies, beyond the capabilities of standard instruments until the recent development of the KMOS near-infrared multi-integral-field-unit (IFU) spectrograph. I have been a leading player in three KMOS surveys targeting >1000 galaxies from z=0.5-3.8 (i.e., 7 billion years of cosmic time), mostly using Guaranteed Time observations from the KMOS consortium. These three surveys are: KROSS (z~0.8); KGES (z~1.5) and KDS (z~3.5).
My Role: I have worked closely with the EAGLE builders, testing the simulation by comparing to observations (e.g., Scholtz et al. 2018; McAlpine et al. 2020; Ward et al. 2022). I also worked with them on the Galaxy Makers exhibition (see Engagement pages).
Summary: EAGLE (Evolution and Assembly of GaLaxies and their Environments) is a simulation aimed at understanding how galaxies form and evolve. This computer calculation models the formation of structures in a cosmological volume, 100 Megaparsecs on a side (over 300 million light-years). The EAGLE simulation is one of the largest cosmological hydrodynamical simulations ever, using nearly 7 billion particles to model the physics. It took more than one and a half months of computer time on 4000 computer cores of the DiRAC-2 supercomputer in Durham.
My Role: Developing software infrastructure and survey planning for the galaxy evolution consortium as well as investigating at galaxy-AGN science.
Summary: MOONS is a revolutionary multi-object optical--infrared spectrograph situatation on the Very Large Telescope. MOONS will enable the study galaxy formation and evolution over most of the history of the Universe with unprecedented accuracy and statistics for such spectroscopy. The MOONRISE survey, of which I am a member, is the gauranteed time extragalactic project, expected to take place for five years from 2024.
My Role: I am on the science advisory team for the instrument. I have written several science cases to guide the design of the instrument.
Summary: The Extremely Large Telescope (ELT), i.e., ESO's future 39 meter telescope, will probably be the most ambitious ground-based optical astronomical facility of the century. A major challenge is awaiting astronomers: the Universe contains hundreds of billions of galaxies, each of which consists of hundreds of billions of stars! Surveying these systems efficiently calls for a multi-object spectrograph (MOS). The international MOSAIC Consortium, coordinated by Paris Observatory, is gathering together efforts across Europe and Brazil to build this ELT survey machine. MOSAIC is a multi-object and multi-integral field spectrograph that will use the widest possible field of view provided by the ELT.
Previous Collaborations/Projects
SCUBA2 Cosmology Legacy Survey; My Role: One of the observers on the James Clark Maxwell Telescope and scientific contribution.
I have made a series of schematic diagrams and figures to help explain some of the basic concepts of my research. These have been used extensively by people in talks at conferences across the world, in PhD theses and even in published review papers by other authors. These images can be found here and can be used freely with appropriate credit.
My PhD Thesis (2014), titled "Observational constraints on the influence of active galactic nuclei on the evolution of galaxies" can be downloaded here. My thesis also appears in an updated form as part of the Springer Thesis series.