Galaxies, Active Galactic Nuclei and Cosmology


Unveiling the hidden Universe with the Large Synoptic Survey Telescope: the role of mergers in galaxy evolution

Sugata Kaviraj, Marc Sarzi, Robert Lupton (Princeton), Julien Devriendt (Oxford)

The Large Synoptic Survey Telescope (LSST), the most ambitious optical telescope ever conceived, will produce an observational survey of unparalleled depth and area (5 mags deeper than current surveys over half the sky, yielding a database of 20 billion galaxies). Its unique depth will revolutionize the study of the low-surface-brightness (i.e. faint) Universe, which is inaccessible to past surveys because they are too shallow.

Merging is the principal driver of galaxy evolution, triggering strong star-formation episodes, building black holes and inducing morphological transformations. Since low-mass galaxies outnumber their massive counterparts, galaxy assembly proceeds primarily via mergers with unequal mass ratios (‘minor mergers’). However, while clearly fundamental, minor merging remains virtually unexplored, as the low-surface-brightness tidal features it produces are invisible in today’s surveys. A complete understanding of merging (and therefore galaxy evolution in general) requires exploration of the low-surface-brightness Universe, using a telescope like LSST.

While LSST will usher in a new era of low-surface-brightness astronomy, the algorithms required to detect and characterize low-surface-brightness tidal features need to be developed.  The student will use the state-of-the-art Horizon-AGN cosmological simulation to create mock galaxy images, build/train algorithms on these images, test them on available precursor datasets and build a low-surface-brightness pipeline for LSST, in time for start of operations in 2019. He/she will sit at the centre of a new collaborative network that is led by Hertfordshire, with nodes at Princeton, Oxford and the Institut d’Astrophysique de Paris. The student will gain expertise in a broad range of theoretical and observational techniques and write benchmark papers, using LSST, on low-surface-brightness science and the role of mergers in driving galaxy evolution across cosmic time.


Simulations of superbubbles from spiral arms

Martin Krause, Elias Brinks

Stellar outbursts are perhaps the most important driver of the dynamics of the interstellar medium. Yet, the interrelation between dense streams of gas like spiral arms or colliding gas in merging galaxies, their destruction - quite possibly by stellar outbursts - and the effect of the bubbles and superbubbles the stellar outburst drive into the surrounding interstellar medium is not well understood.

Plenty of data is available on scales from star clusters, spiral arms and galaxies. Here we aim to simulate the gas dynamics first around a spiral arm and characterise observational signatures in the different gas phases. Later, colliding filaments in merging galaxies should be investigated.

The student will receive an introduction into a hydrodynamics code and how to operate it on a supercomputer. Simulations and their analysis il then be carried out by the student under supervision. The project will be carried out within a team with observational and theoretical expertise.


Simulations of precessing jets

Martin Krause, Martin Hardcastle

A prediction of the currently favoured hierarchical galaxy evolution scenario is the presence of binary supermassive black holes in some galactic nuclei. Their gravitational wave emission from in-spiral before the final merger might be detectable with upcoming experiments. If at least one of the black holes produces a jet, we might be able to infer the presence of the secondary from a precession of the jet axis.

The interpretation of the relevant radio data is, however, complicated by the interaction of the jet with its environment. This project shall produce new simulations of precessing jets with their environment and compare the results to observational data in order to assess the presence of a secondary supermassive black hole in celestial radio sources. The student would receive an introduction into a hydrodynamics code and how to operate it on a supercomputer. Simulations and their analysis would then be carried out by the student under supervision. The project will be carried out within a team with observational and theoretical expertise.


Do we understand galaxies, really?

Dan Smith, Jim Geach, Chris Hayward (Simons Foundation CCA, New York City, USA)

Astronomy is unlike many other sciences since we are unable to frame observational experiments; our view of the dynamic processes shaping galaxies is fixed in both direction and time (at least on human timescales). The best way to circumvent these potentially crippling issues is through using state-of-the-art simulations, built using realistic astrophysics. These simulations are now sufficiently advanced that we can study them using the same observational tools that we use for analysing real galaxies, providing an unique insight on what our observations are really able to tell us about their (and ultimately, our) place in the cosmos. The student will lead the effort to confront some of the latest galaxy modelling tools with the simulations, and publicise the results of this analysis worldwide. The successful applicant will also have the opportunity for extended visits to collaborate with the team that built the simulations (led by Dr Chris Hayward in New York).


Weighing supermassive black holes – robotically!

Dan Smith, Jim Geach

It is now known that a supermassive black hole lives at the centre of every evolved galaxy, and many lines of reasoning suggest that the evolution of the galaxy is intimately related to the growth of the black hole itself. However, even the basic properties of the supermassive black holes that play such an important role in regulating the growth of mass galaxies are hard to directly measure, since the black holes are extremely small, and enshrouded by dust, which can make their observational signatures very subtle. Indirect means of weighing black holes are therefore often used, despite being virtually untested in all but the most local sources.

In this project, the student will lead the analysis of a new data set, obtained robotically by the Liverpool Telescope located in the Canary Isles, to directly measure the masses of the nuclear sources in a sample of high redshift quasars using an innovative technique called "reverberation mapping". This technique relies on studying the temporal variation of the broad emission lines relative to the central continuum source, to efficiently measure the most robust black hole masses at high redshifts. This project will be an important proving ground for the use of this technique on data sets produced by future surveys (e.g. J-PAS and LSST), which will enable us to expand out to much larger samples of ever more distant sources.


Formation, Evolution and Survival of Dwarf Galaxies

Sean Ryan, Chiaki Kobayashi

The student will study dwarf galaxy formation, evolution and survival using computational simulations. Recent N-body and hydrodynamic simulations provide different, complementary insights in to the formation, evolution and survival of dwarf galaxies in the Local Group. In addition, one of us (CK) maintains a simulation code that includes relevant physics of atomic matter - hydrodynamics, star formation, chemical enrichment from Type II and Ia supernovae, and feedback from supernovae and supermassive black holes. The predictions of these codes can be compared with observations of dwarf galaxies in the Local Group today to ascertain the range of histories that might be associated with these galaxies and with counterparts that have been destroyed or devoured. The student will study the results of existing simulations and may run additional simulations to understand the possible histories of observed systems, for example using observations of Local Group galaxies and/or exploiting the Gaia satellite dataset.


Using dust to explore the evolution of star formation and gas consumption across cosmic time

Kristen Coppin, Jim Geach, Jason Stevens, Maciej Koprowski

This project will exploit data from the James Clerk Maxwell Telescope (JCMT) Cosmology Legacy Survey (S2CLS) to investigate the link between accretion on to black holes and the formation of massive galaxies in the early Universe. The S2CLS is the largest of the JCMT Legacy Surveys and is the largest and most sensitive survey at 850um ever conducted, producing the first samples of thousands of extragalactic sources selected in the submillimetre (submm) wavebands, an order-of-magnitude improvement in the sample sizes of previous surveys at these wavelengths.

The submm atmospheric window allows us to access the redshifted far-infrared emission from high-redshift galaxies; this emission originates from cold interstellar dust heated by the starlight. We can use the submm emission to estimate not only the star formation rate of galaxies but also the mass of cold gas (the fuel for star formation) in a remarkably clean way over a large span of cosmic history. This project will adopt a unique approach, cross-correlating the S2CLS data with very deep optical/near-infrared galaxy catalogues to explore the evolution of star formation and gas consumption across cosmic time. A key theme will include measuring the evolution of the ISM mass over redshift and how this depends on galaxy properties such as mass and environment, as well as other galaxy properties. The findings of this work will feed into detailed follow-up work with the sensitive Atacama Large Millimetre Array.


High sensitivity, high resolution imaging at radio wavelengths of four nearby galaxies

Elias Brinks, Luke Hindson

Modern radio telescopes, such as the Very Large Array (VLA) in New Mexico, are revolutionising our understanding of galaxies. Radio emission, rather than picking up emission from stars like at optical wavelengths, instead probe material floating between the stars, the interstellar medium (ISM). This is where on the one hand processes leading to the formation of new generations of stars take place (the cooling and collapse of neutral gas clouds leading to molecular clouds which fragment and collapse further to form stars) and on the other stars exploding as supernovae return matter and relativistic electrons (also known as cosmic ray electrons) to the ISM.

Radio emission is turning out to be a powerful probe for both the formation of new generations of stars, through tracing the thermal emission generated in HII regions, and the death throes of stars by mapping the relativistic electrons produced in supernova shocks, which lose their energy as they spiral around magnetic field lines, generating non-thermal synchrotron radiation. Contrary to optical observations, radio emission is not or hardly affected by dust which hints to the potential for radio studies to map star formation out to large redshift. Eventually that will be one of the key areas of research for the Square Kilometre Array, an ambitious “World” telescope the first phase of construction of which is imminent.

A substantial allocation of observing time was awarded to observe 4 nearby galaxies with the VLA in great detail covering several wavelength and mapping total intensity as well as full polarisation. Observations have started in Sept 2016 and will continue through 2017. Improvements in telescope receivers in recent years means our study will be vastly superior to what has been achieved thus far. The data will enable many different studies. A challenging, but rewarding project that is foreseen is to derive total and polarised intensity maps to derive the total (equipartition) strength of the magnetic field, and the constituent ordered and turbulent fractions. We will also exploit the near contiguous coverage across frequency to apply Faraday Rotation-Measure (RM) Synthesis. We will use the Faraday rotation measure, its azimuthal variation, and RM–Synthesis to obtain a census of axisymmetric mean magnetic field strength and magnetic pitch angle, all crucial for testing the theory of a galactic dynamo.


Formation and Chemodynamical Evolution of Early-type Galaxies

Chiaki Kobayashi, Martin Hardcastle

The student will study galaxy formation and evolution across cosmic time using computational simulations. Our simulation code self-consistently includes relevant physics of atomic matter - hydrodynamics, star formation, chemical enrichment from Type II and Ia supernovae, and feedback from supernovae and supermassive black holes - and therefore, the predictions are comparable with observations from nearby to distant galaxies. The observational data suggest that star formation took place early in more massive galaxies than in less massive galaxies (the down-sizing effect). Such massive galaxies contain more heavy elements than small galaxies (mass-metallicity relations). Inside galaxies, central parts are more chemically enhanced than outer parts (metallicity gradients).  The student will update the model of supernovae and supermassive black holes in order to reproduce these observations, and understand the physical origins of these observations.


On-going and Recent star formation in the Fornax cluster galaxies

Marc Sarzi, Sugata Kaviraj

The Fornax 3D survey (F3D) is an on-going survey of the Fornax cluster with the state-of-the-art MUSE integral-field spectrograph that will allow to map in great detail the properties of the stellar and gas components of over 30 galaxies both across their central regions and in their halos. Such a unique spectroscopic dataset will be accompanied by a plethora of ancillary measurements across a wide spectral range (from the UV to the radio) and at differing spatial resolutions, some even surpassing that of the MUSE data (e.g. from the Hubble Space Telescope).

An international group of researchers have already gathered behind this project and is set upon using these data to answer a number of key question in extra-galactic astronomy. An enthusiastic and proactive Ph.D. student will have the opportunity join such a collaboration to study in particular the on-going and recent star-formation activity across the F3D sample, in the context of understanding how star formation is quenched in crowded galactic environment and how this helps driving the evolution of spiral galaxies into lenticular or elliptical galaxies. All needed MUSE and ancillary data will be at hand at the beginning of this project.


A complete census of the planetary nebulae population in the Fornax galaxies

Marc Sarzi, Sugata Kaviraj

The Fornax 3D survey (F3D) is an on-going survey of the Fornax cluster with the state-of-the-art MUSE integral-field spectrograph that will allow to map in great detail the properties of the stellar and gas components of over 30 galaxies both across their central regions and in their halos.

A international group of researchers have already gathered behind this project and is set upon using these data to answer a number of key question in extra-galactic astronomy. An enthusiastic and proactive Ph.D. student will have the opportunity join such a collaboration to study how the Planetary Nebulae (PNe) population of the Fornax3D galaxies varies across objects of different kinds and as a function of their position in the intra-cluster medium. Studying the whole PNe population of external galaxies is very important and is only possible with integral-field spectroscopy. For instance, the number of PNe and their peak luminosity should relate to rather unconstrained factors in stellar evolution such as stellar-mass loss yields, and how these varies in galactic environments that are significantly different from that of our own Milky Way is still not understood.

All needed MUSE and ancillary data will be at hand at the beginning of this project.


Cosmic Chemical Enrichment from the First Stars

Chiaki Kobayashi, Fiorenzo Vincenzo

The student will study cosmic chemical enrichment using computational simulations. Our simulation code self-consistently includes relevant physics of atomic matter - hydrodynamics, star formation, chemical enrichment from Type II and Ia supernovae, 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 blackhole-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-redshifts galaxies (and quasar absorption line systems).