Galaxies, Active Galactic Nuclei and Cosmology

Sugata Kaviraj, Garreth Martin (Arizona), Jim Geach

Galaxies are the building blocks of the observable Universe. Studying their evolution enables us to understand the physical processes that shape our Universe from its earliest epochs to the present day. A fundamental property of galaxies is their ‘morphology’ (i.e. their appearance), which encodes their assembly histories and gives us unique insights into galaxy evolution over cosmic time. For example, mergers between galaxies create ‘tidal debris’ around the remnant, composed of material that ‘splashes out’ during the merger episode. The merger remnants themselves are typically ‘elliptical’ galaxies, which are smooth and featureless, as the gravitational torques during the merger wash out structure in the progenitors. On the other hand, spiral galaxies (such as our own Milky Way) are thought to have had a relatively quiet history, free of galaxy mergers, and shaped largely by the gradual accretion of gas from the cosmic web. Studying the evolution of the morphological mix of galaxies is therefore key to deciphering the physical processes that drive the evolution of the observable Universe over cosmic time.

Astrophysics is entering a ‘Big Data’ era, with new surveys like the Legacy Survey of Space and Time (LSST) and the Hyper Suprime-Cam (HSC) Subaru Strategic Program poised to transform our understanding of galaxy evolution. These surveys will not only image tens of billions of galaxies, but their unparalleled depth will give them unprecedented sensitivity to (the often faint) merger-driven tidal features. However, the sheer size of these datasets makes the classification of galaxy morphology using traditional techniques, e.g. visual inspection of galaxy images by humans, prohibitively time-consuming.

This project will combine (1) state-of-the-art data from HSC and LSST, (2) unsupervised machine-learning techniques for classifying galaxy morphology (e.g. Martin et al. 2020) and (3) in-house cosmological simulations (e.g. Horizon-AGN) to perform the first, automated morphological analysis of the Universe using such surveys. These studies will, in turn, quantify the role of processes such as galaxy mergers and gas accretion in driving star formation, black-hole growth and morphological transformation, in unprecedented detail, over at least 90% of cosmic time.

The student will collaborate closely (through visits and conference trips) with colleagues in Arizona, Paris, Oxford and a worldwide network of scientists within the LSST project (in which our team has a leadership role). While this is an astrophysics project, it is also suitable for students who primarily have a background in computer science which they would like to apply to data intensive astrophysics.

Kristen Coppin, Jim Geach, Maximilien Franco

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) during 2020 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, Chiaki Kobayashi

The buildup of the chemical elements in the chart of nuclei is a fascinating story that involves a number of different factors: Most of the elements heavier than helium have been formed in stars and during different kinds of stellar explosions. Direct evidence for this process comes from the measurements of radioactive decay lines shortly after an explosion, which are very rarely close enough for a successful measurement and also from radioactive decay of elements with longer half lifes that accumulate in the interstellar medium. 26Al and 60Fe, both have half lives of the order of a million years and have been found throughout the Milky Way with gamma ray observations. The observations can, however, only be interpreted with the help of computer simulations that predict sky distributions for different assumptions about how the stars shed the radioactive nuclei and how the ejecta evolve in the dynamic interstellar medium. Our group is active in such simulations. The project consists in running new simulations with the existing code to help answer questions like: What is the total mass of 60Fe and 26Al in the Milky Way? This has implications for nucleosynthesis in stars and during stellar explosions. How much of radioactive material makes it into newly formed stellar systems and affects things like water retention on young planets through the released heat? How much of the stellar ejecta leaves the Milky Way for good and does interesting things in the surrounding gas?

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.

Daniel Smith

Starting in 2021, 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. 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 write some of the first WEAVE science papers. Lots of different directions are possible depending on your interests, ranging from studying star formation evolution over the last half of cosmic history, to obtaining the best yet measure of the diversity in the radio source population. The possibilities are endless. If this sounds like something that would interest you - get in touch!