Galaxies

DPhil Projects 2020: Galaxies

Weighing Supermassive Black Holes
Martin Bureau

Understanding the formation and evolution of galaxies is central to much of contemporary astrophysics. The relations between black hole mass and various galaxy properties imply a tight connection between the growth of central supermassive black holes (SMBHs) and that of galaxies, and these relations now underlie a staggering number of observations and simulations. However, the number of reliable SMBH mass measurements is small, and the number of independent measuring methods even smaller. A team led by Prof Bureau has recently shown that the dense molecular gas of galaxies is the best tracer of their circular velocities, and thus of their masses. Most importantly,
following the first SMBH mass measurement published in Nature, the team has now shown that these measurements are both much more accurate and much easier to carry out than with other methods. It is thus time to scale up those efforts and renew our knowledge of SMBHs.
As part of the WISDOM team (mm-Wave Interferometric Survey of Dark Object Masses), the student will use current mm/sub-mm telescopes to pursue a programme of SMBH mass measurements in a large sample of local galaxies spanning a range of morphological types, masses, and nuclear activities. This will primarily use ALMA, the largest ground-based telescope project in existence, on which WISDOM has received large allocations of observing time. There are thus much data in hand already, with more to come, and the tools necessary to model the velocity fields and estimate uncertainties have already been developed. The student will thus exploit a well-oiled machinery to make multiple measurements, and thus explore how SMBH masses and galaxy properties correlate, in addition to probing the nuclear-scale gas dynamics that allows SMBHs to be fed. The project will thus significantly increase the number of reliable SMBH masses available, and it will revolutionise our understanding of the co-evolution of SMBHs and galaxies.

Unravelling Giant Molecular Clouds.
Martin Bureau

Understanding how interstellar gas turns into stars is arguably the greatest remaining puzzle in galaxy formation. Stars form in dense gas clouds known as giant molecular clouds (GMCs), but how these emerge, what their structure is, or even whether they are long-lived or transient remains unclear. Previously restricted to our own Milky Way and nearby late-type (i.e. spiral) galaxies, with a new generation of telescopes studies of GMCs can now take an immense leap forward. By probing more diverse galaxies and hitherto inaccessible environments, new laboratories to study star formation are now available.
As part of the WISDOM team (mm-Wave Interferometric Survey of Dark Object Masses), the student will use current mm/sub-mm telescopes to study GMC populations in a large sample of galaxies spanning a range of morphological types, masses, and nuclear activities. This will primarily use ALMA, the largest ground-based telescope project in existence, on which WISDOM has received large allocations of observing time. There are thus much data in hand already, and the project aims to refine and develop the tools required to characterise individual GMCs and thus infer the properties of entire GMC populations. In particular, the student will for the first time probe individual clouds orbiting SMBHs, measuring their sizes, luminosities, and dynamics, and constraining their evolutionary histories. This is essential to establish whether the properties of GMCs are universal, or whether they depend on the SMBH and galaxy properties, in turn affecting their ability to turn into stars or otherwise feed the SMBHs. By significantly increasing the number of galaxies with GMC censuses, the project will revolutionise our understanding of GMC formation and evolution.

Measuring the Star Formation Rate when the Universe was Young with the James Webb Space Telescope.
Andrew Bunker

There is a project available to work with Andy Bunker on measuring the star formation rate in very distant galaxies (at redshifts well beyond one), using data fromthe forthcoming James Webb Space Telescope (JWST, to be launched in early 2021) and also existing data from the Hubble Space Telescope (HST). The science goal is to map the average rate at which the Universe forms stars as a function of time, using the H-alpha line of hydrogen which can be powered by hot, massive and short-lived stars. This line is in the red region of the spectrum, and is less susceptible to extinction by dust than other potential indicators of star formation (such as the UV continuum), but its long wavelength means that it is inaccessible from the ground at redshifts beyond 2. The Near Infrared Spectrograph (NIRSpec) on JWST can reach wavelengths as long as 5 microns, and potentially can measure star formation rates from H-alpha at redshifts up to 7. Prof Bunker is on the European Space Agency Instrument Science Team for NIRSpec, the near-infrared spectrograph on JWST, and we will have guaranteed time observations from the start of the mission. Prof Bunker is also a member of the WFC3 Infrared Spectroscopic Parallel Survey (WISPS) on HST, and we will use existing data from the WISPS survey to constrain the luminosity function of H-alpha at lower redshift (0.4 ≤ z ≤ 1.7) We will compare our H-alpha luminosity functions from targetted multi-object spectroscopy with JWST-NIRSpec at 2 ≤ z ≤ 7 to that from untargetted slitless spectroscopy from WISPs at low redshift to chart galaxy evolution.

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Studying the Physical Conditions in Galaxies at High Redshift with the James Webb Space Telescope.
Andrew Bunker

This project is associated with Andy Bunker and the Instrument Science Team of the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope - the successor to the Hubble Space Telescope to be launched in early 2021. The science goal is to study the spectra of galaxies out to unprecedentedly high redshifts, in particular using emission lines at rest-frame optical wavelengths, which are well studied at lower redshifts, to explore galaxy evolution.

The ratios of emission lines can lead to measurements of the chemical enrichment of galaxies (all elements heavier than lithium are formed in later stellar processes rather than the Big Bang), as well as the temperature and ionization conditions of galaxies in the early Universe, along with an estimate of the effect of extinction of light by dust grains.
Hence we can study the physical conditions during the early stages of galaxy formation, and we will search for candidate “Population III” galaxies (with no signatures of heavy elements, consistent with being the first generation of stars).

Andy Bunker is on the European Space Agency Instrument Science Team for NIRSpec on JWST and we will have guaranteed time observations from the start of the mission.

The environment of massive galaxies in the early universe.
Rebecca Bowler, Matt Jarvis & Peter Hatfield

Galaxies are formed and live in dark matter halos, and the environment of the host halo is believed to be of critical importance for the formation of the resident galaxies. A popular approach to determine the large scale environment of galaxy populations is to measure the clustering (2-point statistics) alongside the number counts (1-point statistics). The goal of this project is to bring together these key techniques in order to understand the formation and evolution of the brightest galaxies within the epoch of reionisation (z~>6), by linking the galaxies to their underlying dark matter halos in a complete statistical framework.
The student will perform a clustering and ‘Halo Occupation Distribution’ modelling analysis (e.g. Hatfield, Bowler, Jarvis et al 2018; https://arxiv.org/pdf/1702.03309 ) of the samples of z > 6 galaxies from ongoing near-infrared VISTA surveys (Figure below) coupled with the best optical data from Subaru and the Dark Energy Survey. This approach accounts for the non-linear clustering of galaxies within individual halos, and the large scale clustering of the halos simultaneously, giving information about how many galaxies are in each halo as a function of halo mass. The student will become an expert in this approach to connect the observed galaxy properties to the underlying cosmic web. In the later part of the project the student will apply their analysis to mock data from the Horizon-AGN cosmological simulation, and hence consistently compare the results of observations and simulations, and be ideally placed to fully exploit the next generation of surveys from Euclid and LSST.

The evolution of neutral hydrogen in the Universe.
Matt Jarvis, Anastasia Ponomareva & Ian Heywood

Neutral hydrogen is the building block of all the structure we see in the Universe. The new generation of radio telescopes are about to open up a new view on this gaseous phase within the Universe, as they have the sensitivity to detect this neutral hydrogen through the 21cm hyperfine transition. Oxford is leading of one of the Large Survey Projects (MIGHTEE; https://arxiv.org/pdf/1709.01901.pdf) on the new MeerKAT radio telescope in South Africa. One of the key aims of this survey is to understand how this neutral hydrogen evolves from the present day through to a time when the Universe was around half of its current age (z~0.6). The student will lead work in this area to conduct studies of the mass and dynamics of HI in galaxies, determine how this is linked to the stellar mass and environment in which they reside, along with the redshift evolution. The student will have the opportunity to work with optical, near-infrared imaging and spectroscopy, as well as the new radio data to define new samples that are most appropriate for the science. The experience would put the student in an excellent position for future work with the SKA, Euclid and LSST.

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The cool gas fuelling distant radio galaxies with MeerKAT.
James Allison & Matt Jarvis

Cool neutral gas is crucial to star formation and supermassive black hole growth in galaxies. Globally these processes peaked 10 billion years ago and have decreased by an order of magnitude to the current epoch. A similar decrease in either the efficiency and/or available cold gas in galaxies is expected. The goal of this project is to use the unprecedented spectral sensitivity of the South African MeerKAT and Indian uGMRT radio telecopes to determine how the cold gas content of galaxies hosting active galactic nuclei (AGN) has evolved over the past 10 billion years. The student will use data from the MIGHTEE survey (Jarvis et al. 2016) to detect and model the 21-cm line of neutral hydrogen (HI) gas in absorption against the radio emission from AGN. This approach allows both the abundance and kinematics of cold gas to be determined on sight-lines towards the radio source,
establishing in individual active galaxies the existence of infalling clouds, feedback-driven outflows and circumnuclear discs. The MIGHTEE fields have a wealth of multi-wavelength spectroscopic and photometric information that will aid interpretation. The results will be compared with surveys carried out in the nearby Universe (e.g. Maccagni et al. 2017) allowing evolutionary models to be established. The student will have a unique opportunity amongst their peer-group to gain hands-on experience with real SKA-era data and publish high impact science that will have direct relevance and crucial importance in the next two decades of radio astronomy.
References: Jarvis et al. (2016), Maccagni et al. (2017)

Modelling the most extreme high redshift galaxies: from star formation rates to supermassive black hole growth.
Julien Devriendt & Adrianne Slyz

Extreme galaxies are routinely detected with star formation rates in excess of 1000 solar masses per year at high redshift (z>2). These galaxies are also thought to host the most massive supermassive blackholes (SMBH) in the Universe, at a time when their energy input into the circum-galactic medium (so called Active Galactic Nuclei (AGN) feedback) is the largest.
However, these objects have proven notoriously challenging to model, as they are very rare and hence necessitate running very large volume cosmological simulations whilst still resolving the interstellar medium (ISM) of the galaxy and the central region surrounding their SMBH.
To overcome these difficulties, this DPhil project proposes to extract a sample of rare objects from a gigaparsec cube dark matter only simulation and resimulate them with resolution fine enough to resolve the giant molecular clouds that form in their ISM. Analysing these zoom simulations, the student will focus on understanding what physical conditions are necessary to reach the extreme star formation rates observed in these galaxies and feed their central SMBH engine. In particular, they will focus on disentangling the contribution of AGN feedback and star formation to their luminosties and assess their role in driving the evolution of these galaxies.

Although no prior knowledge of numerics is required to carry out the project, a strong taste for theoretical physics and the numerical implementation of
physical problems is mandatory.

Dynamics of the inner Milky Way.
John Magorrian

How is matter distributed in the inner regions of our Galaxy? The potential of the innermost few parsec is dominated by the central black hole. However, beyond the massive black hole, little is known and less understood about the structure and potential of the central nuclear cluster and its surrounding bulge: these central regions are obscured by dense gas and dust, so that our knowledge of them comes mostly from the very brightest stars. With new infra-red surveys, observations have finally started to probe this area. This project will involve developing new applications of techniques for using gas and/or stars as dynamical tracers to understand what is going on in the Galactic centre, and construct a coherent picture of the matter distribution within the innermost kpc of the Milky Way. A firm grasp of the fundamentals of Hamiltonian dynamics and of fluid mechanics is essential.

Galaxy evolution with next-generation multi-wavelength sky surveys.
Ian Heywood and Matt Jarvis

We are seeking a student to work with us on one of the key issues in modern astrophysics: namely understanding the formation and evolution of galaxies across cosmic time. This project will make use of data from two state of the art surveys: (i) near-infrared observations from the VIDEO+VEILS survey, and (ii) radio data from the MIGHTEE survey using the recently completed MeerKAT telescope in South Africa. The breadth and depth of these combined observations allows the stellar mass and active star-formation to be probed in many thousands of galaxies, as a function of their environments, from the present day right back to the cosmic epoch of reionization. Radio observations are arguably also the best tracer for activity associated with central supermassive black holes and star formation in galaxies out to z~6, and uniquely allow simultaneous measurements of the 21 cm hydrogen line which measures the size and dynamics of the gas reservoir from which stars eventually form. Data acquisition for the MeerKAT-MIGHTEE survey began in 2018 and is still on-going so there is scope in this project for working with colleagues in South Africa to solve some of the data challenges associated with next generation radio telescopes, and produce some of the first transformational science results from this new facility.

The molecular Universe: Tracing Star Formation and AGN activity across cosmic time.
D. Rigopoulou & I. Garcia Bernete

Determining star formation rates, the growth of supermassive black holes (SMBH) and how these two processes evolve with time and galaxy mass is fundamental to our understanding of how galaxies form and evolve. In many galaxies these processes occur behind copious amounts of dust which often absorb a large fraction of the UV/optical light and re-emit it in the infrared. Spectral features from polycyclic aromatic hydrocarbon (PAH) molecules, observed in the mid-infrared can be used to infer the amount of recent and ongoing star formation in galaxies and around active galactic nuclei (AGN) where more traditional methods may fail. Using PAH spectra of large samples of galaxies taken with NASA’s Spitzer satellite and the upcoming James Webb Space Telescope, this project will test rigorously the role of PAHs as star formation rate indicators. New evidence from ground-based studies suggests that an AGN may excite their surrounding clouds resulting in PAH emission. If this found to be the case then the use of PAH emission as a universal star formation rate tracer will come under question. The work may involve use of additional ALMA CO observations to probe the physical properties of the gas in and around galaxies and AGN.

The project is particularly timely since the James Webb Telescope (JWST) due for launch in 2021 is expected to probe PAH emission from nearby and distant galaxies and a variety of environments.

For more information and publications related to the topic please contact:
D. Rigopoulou (dimitra.rigopoulou@physics.ox.ac.uk)
I. Garcia-Bernete ismael.garciabernete@physics.ox.ac.uk

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Dissecting galaxies: Understanding the structure of galaxies using the rare phenomenon of strong gravitational lensing.
Aprajita Verma, Matthias Tecza, Dr Anupreeta More (ICUAA, India), Dr Phil Marshall (SLAC/Stanford USA, Visiting Lecturer at Oxford), Chris Lintott

The phenomenon of gravitational lensing, predicted by general relativity, produces some of the most spectacular astronomical images seen. A single galaxy, group or cluster of galaxies can act as "cosmic telescopes" amplifying and magnifying the light of distant galaxies lying behind them into multiple images, rings or arcs. The separation or deflection of the lensed images is determined by the total mass (dark and light) of the foreground lensing galaxy. Strong gravitational lenses have a wide range of astrophysical and cosmological applications.

In this DPhil project, we focus on understanding the distribution of mass in the lenses, and the nature of the distant lensed galaxies. We still lack a full physical picture of the dark and baryonic matter in galaxies. Gravitational lensing combined with full stellar kinematic information can constrain the dark matter distribution in the centres of galaxies. The student will work on large samples of new strong lenses discovered by members of the public as part of the Galaxy Zoo and Space Warps (spacewarps.org) Zooniverse projects. Using data from large telescopes that we already have in hand, we will extract stellar dynamical information of the lensing galaxy and investigate the nature of the lensed, high redshift, background galaxies. The magnification and amplification due to lensing allow fainter galaxies to be studied on smaller scales than would be normally possible. This is a preview to the detailed galaxy science that will be achievable by the next generation of extremely large telescopes (https://www.eso.org/public/unitedkingdom/teles-instr/elt/).

The project is designed to allow flexibility between modelling, lens and background galaxy studies according to the interests of the student. The student will be expected to assist observing runs, write observing proposals and liaise with citizen lensing enthusiasts participating in Galaxy Zoo & Space Warps. In addition, the discovery of gravitationally lensed systems remain a challenging problem; automated discovery methods (e.g. machine learning) still yield samples of high impurity requiring visual inspection. Human pattern recognition currently outperforms automated discovery algorithms, that motivates our citizen scientist programmes. Looking forward to the wide, sensitive surveys to be conducted by the Large Synoptic Survey Telescope (LSST), we will also work on developing on an efficient combined machine learning assisted and citizen-powered visual classification discovery pipeline for LSST using ongoing and forthcoming large surveys.

For further information please contact aprajita.verma@physics.ox.ac.uk

More information regarding the results of the Space Warps project can be found at
- Space Warps Paper I http://arxiv.org/abs/1504.06148
- Space Warps Paper II http://arxiv.org/abs/1504.05587
- Red Radio Ring: http://arxiv.org/abs/1503.05824
- Citizen Lens Modelling http://arxiv.org/abs/1502.00008
- https://www.sciencefriday.com/person/aprajita-verma/

For information on the telescopes related to this project
- Extremely Large Telescope https://www.eso.org/public/unitedkingdom/teles-instr/elt/
- Large Synoptic Survey Telescope https://www.lsst.org/
- Hale 200" Telescope http://www.astro.caltech.edu/palomar/about/telescopes/hale.html
- Dark Energy Survey on the Victor M. Blanco 4-meter Telescope https://www.darkenergysurvey.org/
- HyperSuprimeCam Survey on the Subaru Telescope https://www.naoj.org/Projects/HSC/

Studying galaxies in 3D
Michele Cappellari

Oxford pioneered the study of galaxy evolution using integral-field spectroscopy, which provides a 3D view of galaxy structure. The first surveys provided a new paradigm for the decades-old view of galaxy structure and formation (see this review: http://arjournals.annualreviews.org/eprint/rqUatQN2NVUqeusFv4hJ/full/10.....
Now Oxford is part of the MaNGA survey ( https://www.sdss.org/surveys/manga ) which introduces the 3D spectroscopy technique to the very productive Sloan Digital Sky Survey (SDSS) telescope. MaNGA extends the size of previous galaxy samples by nearly two orders of magnitude, by observing 10,000 galaxies.In the proposed DPhil project the student will be involved in the exciting MaNGA survey. The student will develop a new dynamical modelling technique to measure the masses of galaxy bulges by combining galaxy photometry and 3D stellar kinematics. The student will then use this new kinematic bulge/disk decomposition on the MaNGA data to the link between bulge growth and galaxy evolution.

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Measuring masses of supermassive black holes.
Michele Cappellari

In the past two decades, supermassive black holes have changed from being a remarkable confirmation of general relativity, to representing a key element of our understanding of how galaxies evolve and one of the hottest topic in astrophysics (e.g. this paper : https://ui.adsabs.harvard.edu/abs/2013ARA%26A..51..511K ). This revolution has happened thanks to a relatively small number of dynamical determinations of the masses of these exotic objects in the centres of galaxies, using the dynamics of the stars or the gas moving under the gravitational influence of a central dark mass.
In the proposed DPhil project the student will use dynamical models of the stellar and gas kinematics to measure masses of supermassive black holes in galaxies using state-of-the art modelling techniques (e.g. this paper : https://ui.adsabs.harvard.edu/abs/2019A%26A...625A..62T). These masses will be used to study black hole demographics and the link between black holes and galaxy evolution.

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Black hole disks embedded in globular clusters.
Bence Kocsis

The recent discovery of gravitational waves opened new horizons for understanding the Universe and further developments are expected in the near future with new Earth and space-based instruments. The measurements have unveiled an abundant population of stellar mass black hole mergers in the Universe. The great challenge is to understand the possible astrophysical mechanisms that may lead to mergers. Existing theoretical models of the astrophysical origin of the observed sources are currently either highly incomplete or in tension with data (Barack+ 2018).
One possibility is that the observed black hole mergers are generated in dense stellar systems. In these systems, black holes become the most massive objects and sink to the center of the cluster and undergo frequent dynamical encounters. These encounters lead to the formation of black hole binaries, whose separation decreases during subsequent encounters until eventually the binary merges due to gravitational wave emission. Recent developments showed that the black hole subclusters do not necessarily decouple and evaporate at short time scales and that globular clusters can retain black holes up to a Hubble time (Morscher+ 2015),
A common simplification in globular cluster models is the assumption of spherical symmetry. This approximation is motivated by the approximately spherical appearance of these systems. However, observations show deviations from spherical symmetry both spatially and in velocity space (e.g. Van de Ven+ 2006, Bianchini+ 2018, Zocchi+ 2019). The geometry of the embedded black hole population in these systems may be even more anisotropic. This possibility is supported by the recent finding that the most massive objects dynamically settle to a disk-like configuration (Szolgyen & Kocsis 2018, Szolgyen, Meiron, Kocsis 2019).
In this project, the student will work with Prof. Bence Kocsis and examine the consequences of black hole disks embedded in globular clusters using direct N-body simulation methods and analytical techniques. The student will determine if such subsystems may be long-lived, how it affects the evolution of the cluster, the formation and evolution of binaries, and examine the implications for electromagnetic and gravitational wave observatories.

Links to further reading:
Barack L. et al., 2019, Classical and Quantum Gravity, Volume 36, Issue 14, article id. 143001, arxiv:1806.05195
Bianchini P. et al. 2018, MNRAS, 481, 2125, arXiv:1806.02580
Morscher M. et al.. 2015, Astrophysical Journal 800, 9, arxiv:1409.0866
Samsing J, 2017, Physical Review D, Volume 97, Issue 10, id.103014, arxiv:1711.07452
Szolgyen A, Kocsis B., 2018, PRL, 121, 101101, arxiv:1803.07090
Szolgyen A, Meiron Y, Kocsis B., 2019, arXiv:1903.11610
van de Ven G. et al,, 2006, A&A 445, 513, arXiv:astro-ph/0509228
Zocchi A., Gieles M., Hénault-Brunet V., 2019, MNRAS, 482, 4713, arXiv:1806.02157