Galaxies, Active Galactic Nuclei and Cosmology

The low-surface-brightness Universe: a new frontier in the study of galaxy evolution

Sugata Kaviraj, Jim Geach

New astronomical surveys that are both deep and wide - from revolutionary instruments like the Large Synoptic Survey Telescope (LSST) and the Hyper Suprime-Cam (HSC) - are poised to transform our understanding of galaxy evolution. Previous work has been dominated by bright/massive galaxies that lie above the surface-brightness limit of past surveys (e.g. the SDSS). However, ‘low-surface-brightness’ (LSB) galaxies, i.e. ones that are too faint to be detectable in past surveys, actually dominate the galaxy number density. Furthermore, key LSB components, such as tidal features induced by galaxy mergers, strongly constrain our structure-formation paradigm. A complete comprehension of how the Universe evolves therefore demands a detailed understanding of the LSB regime – without it our understanding of galaxy evolution is likely to be highly incomplete!

This project will combine state-of-the-art data from HSC and LSST, with in-house cosmological simulations (Horizon-AGN) and advanced machine-learning techniques (Martin et al. 2019), to perform the first statistical studies of the LSB Universe. It will (1) open up the vast discovery space in this regime by mapping the properties of LSB galaxies in unprecedented detail, (2) quantify the role of galaxy mergers in driving star-formation, black-hole growth and morphological transformation over cosmic time and (3) study the poorly-understood role of black-hole feedback in LSB/dwarf galaxies over at least half the lifetime of the Universe.

The student will collaborate closely (through visits and conference trips) with colleagues in Paris, Oxford and a worldwide network of scientists within the LSST project (in which our team has a leadership role). The project will give the student a broad skillset in astronomical observation, theory and machine-learning that is well-aligned with this new era of Big Data astronomy.

Simulations of the Fanaroff-Riley Dichotomy

Martin Krause, Martin Hardcastle

Extragalactic radio sources are produced in the vicinity of supermassive black holes and accelerated to relativistic velocities on the parsec scale. Their appearance in radio maps remained enigmatic for the last 45 years since the discovery of a fundamental dichotomy: Brighter jets tend be well collimated and are most luminous towards their edges where they convert part of their bulk energy into synchrotron radiation at so-called hot spots. Fainter sources produce a bright (flaring) point near the nucleus of the host galaxy and then dim outwards. Two mechanisms have been suggested to explain this behaviour. One is mass loading by stellar winds, the other a transition in the jet engine that produces wider flows that have been shown not to be able to collimate to a stable large-scale jet. The idea of this project is to use the relativistic magnetohydrodynamics code PLUTO that is user-friendly and extensively used in our group to simulate the radio appearance of jets testing the aforementioned ideas for Fanaroff-Riley class I radio sources. The results will have implications for our understanding of jet formation by supermassive black holes.

The Dusty Universe Unveiled: state-of-the-art (sub)millimetre surveys of galaxies across cosmic time

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).

Cosmic Chemical Enrichment from the First Stars

Chiaki Kobayashi, Martin Krause

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 form a 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, 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, such as with James Webb Space Telescope.