UCL Astrophysics Group


Past Seminars

Seminars 2020/2021

Term 1

21-Sep-2020: Franck Marchis (SETI), First Results with a Network of Small Digital & Smart Telescopes: Citizen Science for Astronomy

Unistellar is building the largest network of compact, easy-to-use, smart and digital telescopes called the Enhanced Vision telescope (or eVscope) with the goal of bringing back to the people the joy of exploring the universe by observing deep sky objects from their home. The SETI Institute is a scientific partner of the project which will develop the scientific applications of this network. To date, ~2000 eVscopes have been shipped and by December the network will be larger than 5,000 eVscopes from citizen astronomers located in Europe, America, Australia, Japan and other places. We are currently developing pilot programs to test the capabilities of the eVscope network to conduct meaningful scientific studies, such as asteroid occultations, transiting exoplanets, Planetary Defense, and the study of comets. First scientific results collected with citizen astronomers in Europe and North America will be presented, this includes the detections of TESS Objects of Interest (TOIs), as well as successful detection of asteroid occultations and lightcurve of main-belt asteroids and near-earth asteroids. Today 10Tb of scientific data have been already collected from hundreds of active citizen astronomers.

We will also discuss the potential of the eVscope at schools (e.g. community colleges in the US) as a tool to learn about astronomy and data processing while bringing space to the classrooms. We will also describe programs with informal education centers (national parks, museums, youth associations) that could become a tool to connect the young generation to astronomy, educate them on the importance of the dark sky and finally demystify science by allowing all of us to participate in the scientific conversation.

28-Sep-2020: Mihkel Kama (UCL), Unlocking the meaning of planetary composition

JWST and Ariel are about to revolutionise our view of the chemical composition of exoplanet atmospheres, while solar system missions are deepening our appreciation of the complex interplay of physical and chemical processes around the young Sun.  Planets build up their total budget of chemical elements as they accrete and migrate in a protoplanetary disk.  The study of the budget of chemical elements in such planet-forming disks is becoming an essential pillar of the exoplanet revolution, with progress facilitated by innovative methods, and telescopes like APEX and ALMA.  I will briefly review recent advances and open questions in disk composition studies, and outline the two complementary approaches through which we aim to advance our understanding of the origin and diversity of planets and life itself.

5-Oct-2020: Jason Sanders (UCL), Our multi-dimensional, time-evolving Galaxy

How galaxies form and evolve over cosmic time is a key question in modern astrophysics. Our current understanding is being driven by results from detailed multi-dimensional observations, both from integral field unit measurements of large samples of nearby galaxies and photometric, spectroscopic and astrometric measurements of individual stars in our own Galaxy. In the Milky Way, we have access to the rich dataset of positions, velocities, abundances and ages for millions to billions of stars. This immense data presents a significant theoretical modelling challenge but also a great opportunity of measuring the detailed dynamical evolution of a single galaxy and forming a complete picture of its history. This enterprise has been brought into focus by results from the Gaia satellite, which has revealed the intricate structure and non-equilibrium nature of our Galaxy.
I will describe recent work on characterising and modelling the evolution of the Milky Way in the era of Gaia. I will discuss new results on three key phases of the Milky Way’s history: 1. the early accretion history of our Galaxy and the impact on the young Milky Way, 2. the formation and properties of the Galactic bar using the very first panoramic view of the transverse velocities of stars in this region and 3. the quiescent late-time dynamical re-structuring of the disc through heating and migration.

12-Oct-2020: Chiara Mingarelli (UConn / Flatiron), Pulsar Timing Arrays: The Next Window to Open on the Gravitational-Wave Universe

Galaxy mergers are a standard aspect of galaxy formation and evolution, and most (likely all) large galaxies contain supermassive black holes. As part of the merging process, the supermassive black holes should in-spiral together and eventually merge, generating a background of gravitational radiation in the nanohertz to microhertz regime.  An array of precisely timed pulsars spread across the sky can form a galactic-scale gravitational wave detector in the nanohertz band. I describe the current efforts to develop and extend the pulsar timing array concept, together with the recent  NANOGrav 12.5 year "hint" of a signal, and how this can be used to constrain astrophysical phenomena at the heart of supermassive black hole mergers.

19-Oct-2020: Zama Katamzi-Joseph (SANSA), Waves in the Earth’s ionosphere

Traveling ionospheric disturbances (TIDs) are signatures of atmospheric gravity waves that commonly occur in the ionospheric F-region. In this seminar we will look at the brief history of TID studies conducted at the South African National Space Agency (SANSA) before presenting a climatology of medium scale TIDs (MSTIDs). This climatology was obtained using night-time airglow intensity measurements from an all-sky camera located at Sutherland, South Africa (32.4˚ S, 20.8˚ E; magnetic latitude 40.7˚ S). This climatology reveals that MSTIDs occur in all seasons but predominantly during the winter months, and mostly propagate in the westerly direction with speeds of 17—165 m/s. Their periods and wavelengths range are 19—70 minutes and 58—252 km, respectively. Analysis of neutral wind measurements from a co-located Fabry-Perot interferometer (FPI) showed that this favoured propagation direction was the least constricted by neutral winds.

26-Oct-2020: Keith Hawkins (UT Austin), Galactic Archaeology with Gaia and Large Spectroscopic Surveys

One of the key objectives of modern astrophysics is to understand the formation and evolution galaxies. In this regard, the Milky Way is a critical testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical, chemical, and age distributions of its stellar populations.  Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have been and can be used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.

2-Nov-2020: Ting Li (Carnegie Observatories / University of Toronto), The Southern Stellar Stream Spectroscopic Survey: Overview and Latest Science Results

The Southern Stellar Stream Spectroscopic Survey (S5) is an ongoing spectroscopic program that maps the newly discovered stellar streams with the fiber-fed AAOmega spectrograph on the Anglo-Australian Telescope (AAT). S5 is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere, providing a uniquely powerful sample for understanding the building blocks of the Milky Way's stellar halo, the progenitors and formation of stellar streams, the mass and shape of the Milky Way's halo, and ultimately the nature of dark matter. The survey started in Summer 2018 and have mapped 14 streams with ~45 nights on AAT. In this talk, I will give a brief overview of the current status of the program, highlighting the latest science results from the survey, and end the talk with the public data release plan.

16-Nov-2020: Tom Charnock (IAP), Machine learning, statistics and the relation between neural networks and physics

Based on my current research into the statistical interpretation of modern machine learning methods, I will present the connection between machine learning, in particular the use of neural networks, and scientific model building. By understanding neural networks in the form of statistical models I will show why they are so adept for tackling current physical problems, but I will also highlight their limitations in terms of comprehensibility and scientific understanding and safety. From this perspective I will suggest where these modern techniques could be useful to pursue further, but overall finish on a slightly skeptical note about where these methods can take us for science. The studies presented can be viewed in the Artificial Intelligence for Particle Physics book chapter - Bayesian neural networks (https://arxiv.org/abs/2006.01490) as well as several blog posts including https://medium.com/@tom_14692/all-deep-learning-is-statistical-model-building-fc310328f07

23-Nov-2020: Sam Thompson (Cavendish Labs), Terra Hunting Experiment: Finding another Earth with HARPS3

The Terra Hunting Experiment is a 10-year radial-velocity (RV) search designed to discover Earth-mass planets on long-period, Earth-like orbits, around our nearest Solar-like stars.  To undertake this programme, we are currently building the High Accuracy Radial-velocity Planet Searcher 3 (HARPS3), an R = 115,000 echelle spectrograph, which will be operated in a robotic mode on the refurbished 2.5m Isaac Newton Telescope.  In this talk I will discuss some of the challenges we need to meet in order to push our detection threshold to 10 cm/s - the RV semi-amplitude of an Earth-like planet.  I will also present an overview of the HARPS3 instrument and describe our strategy for the Terra Hunting Experiment.

7-Dec-2020: Marcelle Soares-Santos (University of Michigan), Cosmology in the era of multi-messenger astronomy with gravitational waves

Motivated by the exciting prospect of a new wealth of information arising from the first observations of gravitational and electromagnetic radiation from the same astrophysical phenomena, the Dark Energy Survey (DES) has established a search and discovery program for the optical transients associated with LIGO/Virgo events (DESGW). Using the Dark Energy Camera (DECam), DESGW has contributed to the discovery of the optical transient associated with the neutron star merger GW170817, and produced the first cosmological measurements using gravitational wave events as standard sirens. After three successful observing campaigns, I present, in this talk, an overview of our results and their implications for the emerging field of multi-messenger cosmology with gravitational waves and optical data.

Term 2

11-Jan-2021: Seshadri Nadathur (UCL), Unveiling the dark side with cosmic voids

New galaxy surveys will soon provide us unprecedented maps of structure in the cosmic web. This data contains a wealth of information beyond what can be extracted using traditional methods based on two-point statistics and power spectra. Using cosmic voids – vast regions deficient in galaxies – is one particularly promising new technique. I will describe how they can be used to measure gravitational effects on light propagation, the growth rate of structure, and – in particular – to improve distance measurements by a factor of 2. I will show current results from the completed BOSS and eBOSS surveys, and describe the prospects with DESI and Euclid in the next few years.

18-Jan-2021: Jo Bovy (UoT), Constraining the nature of dark matter with stellar streams in the Milky Way

Stellar tidal streams originating from disrupting globular clusters in the Milky Way’s halo hold enormous promise as probes of both the large-scale structure of the Milky Way halo’s density distribution and its small-scale structure. As such, the observed density, spatial, and kinematic structure of stellar streams can provide important new constraints on the interactions and small-scale structure of dark matter. I will discuss recent progress on modeling the dynamics of stream formation and its application to fitting observed stellar stream data from photometric, spectroscopic, and astrometric observations. I will in particular focus on how the most recent photometric and astrometric data (from Gaia) helps us map the small-scale structure of streams and how this lets us answer the fundamental question: Is dark matter cold?

25-Jan-2021: Joe Silk (IAP/Oxford), The Future of Cosmology

One of the greatest challenges in cosmology is understanding the origin of the structure of the universe. The fossil radiation from the beginning of the universe, the cosmic microwave background, has provided a unique window for probing inflation and the initial conditions from which structure evolved. Large surveys of galaxies have provided complementary information that has enabled us to reach the era of precision cosmology, with parameter determination approaching percent level accuracy.  But where do we go next? Future experiments are planned with the next generation of observatories that will increase this precision by at most an order of magnitude. However, we need to do far better if there is to be a guaranteed science return that will definitively probe our cosmic origins. I shall argue that the ultimate goal for our future strategy must be astronomy from lunar-based telescopes, and I will provide several examples of what could be accomplished within the next two decades.

1-Feb-2021: Nicole Pawellek (Cambridge), Fast and Furious - Debris Discs in the beta Pic Moving Group

Only 20% of old field stars have detectable analogues to our Asteroid and Kuiper belts which are called debris discs. This leaves open the question of what disc, if any, is present around the remaining 80%. Young moving groups allow to probe this population, since discs are expected to have been brighter early on. We consider the population of F stars in the 23 Myr-old BPMG where we find that 9/12 targets possess discs. Comparing the BPMG results to disc samples from ∼45 Myr and ∼150 Myr-old moving groups, and to discs found around field stars, we find the disc incidence rate in young moving groups is comparable to that of the BPMG and significantly higher than that of field stars. We also find that the disc luminosities in the populations considered drop by two orders of magnitude within the first 100 Myr. This is much faster than expected by collisional evolution! We attribute this depletion to embedded planets which would be around 170 M_earth to cause a depletion on the appropriate timescale. However, we cannot rule out that different birth environments of nearby young clusters result in brighter debris discs than the progenitors of field stars which likely formed in a more dense environment.

8-Feb-2021: Hannah Wakeford (Bristol), Observing Alien Atmospheres

The study of exoplanets, planets that orbit stars other than the Sun, is at the forefront of the public imagination in space and an exciting field of research. The question "how do stars and planetary systems form and evolve?"" is one of the biggest in Astronomy, and is at the root of one of the most important questions in science today: "How did we get here?". To resolve these questions we need to observe, interpret, and understand the nature of planets beyond our Solar System.

In this talk I will go through the methods, observations, and the physics behind some of the interpretations we are making about the nature of these strange new worlds. As an observer I will show some of the work we have been doing measuring the transmission spectra of giant exoplanet atmospheres looking for the tale-tale absorption of water vapor and the presence of exotic clouds.  I will discuss some of the challenges associated with the measurement of atmospheric abundances, a brief look at the role clouds can play in understanding dynamics and chemistry, and what future measurements can help further constrain our understanding of planetary atmospheres.

22-Feb-2021: Claudia Gutierrez (Turku), Hydrogen-rich supernovae: understanding their explosion and progenitor properties

Hydrogen-rich supernovae (type II Supernovae; SNe II) are produced by the final explosions of massive stars (>8-10 Msun ), which retain a significant part of their hydrogen envelopes before the explosion. These SNe show a large diversity in their transient behaviour, which is likely determined by differences, not just in explosion properties, but also in the progenitor star characteristics. In this talk, I will present the current status of SNe II studies. I will focus on new results found with a sample of SNe II occurring within low-luminosity galaxies. I will also describe the challenges in the determination of the explosion and progenitor parameters for these objects.

1-Mar-2021: Gianrico Filacchione (INAF, Rome), The characterization of comet 67P/CG nucleus by means of VIS-IR imaging spectroscopy

In this seminar a review of the main results achieved by the Visible, Infrared and Thermal Imaging Spectrometer (VIRTIS) aboard ESA’s Rosetta mission [1] are given. The instrument, operating at both visible (0.23-1 µm) and infrared (1-5 µm) wavelengths, has returned spatially-resolved data of asteroids 2867 Steins and 21 Lutetia observed during two fast flybys occurred during the mission cruise phase and of 67P/Churyumov-Gerasimenko comet, Rosetta's main scientific objective, which has been continuously explored for 26 months during its perihelion passage. Once at 67P, VIRTIS has characterized the overall nucleus surface composition which appears made by an assemblage of organic matter, refractories and ices forming a very dark (albedo 0.06) surface dust layer. Thanks to the high spatial resolution achieved by VIRTIS (up to about 2.5 m/px) exposed water ice and carbon dioxide-rich ares have been identified on the surface and correlated with local morphology. Moreover, time-resolved observations have allowed to study diurnal and seasonal evolution of the surface and nearby coma particles showing temperature and composition trends with heliocentric distance. These results demonstrate how visible and infrared imaging spectroscopy techniques are capable to infer composition and physical properties of surface’s regolith and to correlate this information with morphological and geological features on maps. By resolving diagnostic bands associated to vibrational modes, inversion methods based on radiative transfer theory allow to recognize minerals, organic materials, salts, and ices of different composition, their mixing modalities (areal, intimate, intraparticle), grain size distribution, surface roughness, medium porosity, surface temperature and thermal inertia. For these reasons, VIS-IR imaging spectrometers remain a high-gain scientific payloads proficient in maximizing the scientific return from any future asteroid and cometary mission.

8-Mar-2021: Gabriela Calistro Rivera (ESO), The multiwavelength properties of red QSOs - Evidence for dusty winds as the origin of QSO reddening

An outstanding question on the nature of quasars (QSOs) is to understand what is the origin of QSO reddening. This question is motivated by the fundamental differences found in the radio properties of red QSOs as compared to blue QSOs, positioning red QSOs as a potential key population in the context of galaxy and black hole evolution. In this seminar, I will present a systematic study of the multiwavelength (radio-to-UV) properties of SDSS quasars at 0.2<z<2.5, classified into red and control QSOs and matched in redshift and luminosity.

Capitalising on rich multi-wavelength ancillary data sets, we analysed their optical spectral properties and modelled the panchromatic spectral energy distributions (SEDs) using the SED-ftting code AGNfitter. We inferred parameters driving the physics of QSO accretion, obscuration, and outflow properties, as well as interstellar medium and star formation properties of their host galaxies, in order to identify any signature that elucidate the scales in which the reddening in QSOs is originated. We discovered a higher incidence of high-velocity wind components in nuclear emission lines in red QSOs, connected to a hot-dust emission excess detected at 2-5 um that cannot be explained by torus emission models. This finding provides indirect evidence for the scenario where the presence of dusty winds at nuclear scales are potentially the physical ingredient responsible for the optical colours in red QSOs, as well as a key parameter for the regulation of accretion material in the nucleus.

22-Mar-2021: Yashar Hezafeh (Montreal), Probing the particle nature of dark matter with strong gravitational lensing

The nature of dark matter is one of the most important outstanding questions in modern cosmology and astrophysics. Uncovering the properties of the dark matterparticle could result in significant leaps in our understanding of fundamental physics and impact numerous astrophysical models. It is well understood that the microphysics of the dark matter particle impacts its clustering properties on different scales. The most widely accepted dark matter model, cold dark matter, has had tremendous success explaining the large-scale structure of the universe. However, it has faced many challenges for its predictions of the distribution of matter on small, sub-galactic scales, with some observations seemingly favoring a warm dark matter alternative. A definitive answer to this question can only be achieved by mapping the distribution of dark matter on small scales with a purely gravitational probe. Strong gravitational lensing is the only probe capable of doing this at cosmological distances. In this talk, I will discuss how the discovery of a new population of strong gravitational lenses, a new observatory, ALMA, and new advances in analysis methods are allowing us to map the distribution of darkmatter on small scales with high precision. In the coming years, thousands of new lenses from large surveys (e.g., LSST), existing and new facilities (e.g., ALMA, JWST, TMT), and new analysis methods (machine learning) will transform this field, allowing us to understand the small-scale behavior of dark matter with unprecedented accuracy and precision, opening a new window for testing dark matter models in a previously inaccessible regime.

Term 3

10-May-2021: Ferah Munshi (Oklahoma), Can we constrain dark matter at the limits of galaxy formation?

Dwarf galaxies provide a very important and intriguing laboratory for the study of dark matter physics. I will present results from extremely high resolution, fully cosmological simulations that contain many isolated dwarf galaxies [the MARVELous dwarfs] as well as satellite dwarf galaxies [the DC Justice League]. Together, they create the largest collection (211) of high-resolution simulated dwarf galaxies to date and the first collection to resolve ultra-faint dwarf galaxies in multiple environments. With this flagship suite I can statistically characterize dwarf galaxies in order to constrain dark matter physics. I will present results answering the following key question: What testable predictions can we make to constrain dark matter at the lowest dwarf masses?

17-May-2021: Sara Ellison (Victoria), Gas and star formation in the nearby universe with the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey

Understanding the processes that regulate star formation in galaxies remains at the heart of modern extra-galactic astronomy, and debate continues as to why some galaxies exhibit powerful starbursts whilst others are largely dormant.  The key to progress in this arena lies with assembling large, diverse samples of galaxies for which we can measure all of the critical ingredients, such as the current star formation rate (SFR) and gas content.  However, it is insufficient to simply measure global galactic quantities, as we know that a given galaxy will manifest considerable internal diversity in its star formation activity.  It is therefore essential to study gas and star formation on spatially resolved scales.  The ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey combines maps of stellar properties obtained from the MaNGA (optical) integral field unit survey with ALMA CO (1-0) maps on a common kpc-scale grid, in order to tackle questions of star formation in local galaxies.  The 46 galaxies in the ALMaQUEST sample span a wide range of SFRs, including both normal star forming galaxies, as well as those both on their way to quenching, and those undergoing starbursts.  In this talk, I will present the ALMaQUEST view of understanding the interplay of gas and star formation across this broad galactic landscape.

24-May-2021: Rebecca Smethurst (Oxford), The growth of supermassive black holes in the absence of mergers and the effect on their host galaxies

The co-evolution of galaxies and their supermassive black holes (SMBHs) via mergers is a long-held paradigm that has recently been overturned. With a sample of `bulgeless’ disk-dominated galaxies Simmons, Smethurst & Lintott (2017) showed that in the absence of mergers, SMBHs can grow to 10^9 solar masses. A follow up study by Martin et al. (2018) found that in simulations, 65% of all the matter contained in SMBHs at z~0 was acquired through non-merger processes. We now present narrow band imaging of the [OIII] 5007 Å component for 12 of these `bulgeless’ disk-dominated galaxies hosting luminous, unobscured AGN with spectrally confirmed blue shifted outflows, presumed to be powered by the AGN itself. These massive galaxies’ formation histories are dominated by non-merger secular processes and thus provide a unique opportunity to study how inflows from secular processes fuel significant growth of black holes in the absence of significant mergers. We have constrained this contribution from secular processes by using the luminosity and extent of the [OIII] 5007 Å outflow in the narrow band images to measure the rate of mass loss in outflows from the central AGN of these disk-dominated galaxies. By combining these measurements with spectroscopically derived data constraining the black hole accretion rates, we have placed a limit on the total inflow rate to the centre of these galaxies. Further analysis of these systems with integral field spectroscopy techniques offers the promise of directly measuring the black hole growth that is occurring in present day systems evolving solely via secular evolution, and the impact of AGN feedback in such systems due to outflows.

Seminars 2019/2020

Term 1

30-Sep-2019: Iary Davizon (DAWN/NBI), Machine learning for skeptical astronomers

A new generation of machine learning codes is becoming ubiquitous in the literature, and seminal studies indicate they may allow us to solve long-standing astrophysical problems. However, such great expectations may not be fulfilled because of the "black box" nature of most of these algorithms, inextricable biases in their training sample, and other limitations. Sharing these concerns, we tested unsupervised "manifold learning" algorithms with a realistic mock galaxy catalog (up to z=4) derived from cosmological hydro-dynamical simulations. I will present the results we obtained by analyzing this data with self-organizing maps (SOMs), and discuss the groundbreaking applications the SOM may have in understanding galaxy formation and evolution. I will highlight general aspects of  manifold learning that can be also useful in other areas of Astrophysics, but also the open issues that still justify some skepticism.

7-Oct-2019: Nadia Blagorodnova (Radboud), Common-envelope transients: buried in the infrared

Most stars in our Universe live in binaries.  Unstable mass transfer from one star to another can lead to the formation of a shared gaseous shell where both stars orbit: the common envelope. The end of this phase is marked by the quick spiral-in of the secondary star towards its companion, leading to violent interactions between the components. The whole, or part of the binary's common envelope may get ejected, and the pair may even completely merge. This last phase has been serendipitously witnessed as astrophysical transients called luminous red novae (LRNe), allowing us to study the progenitor stars, the energetics of the outburst and the properties of the ejected material. In my talk, I will provide an overview of LRNe, their progenitor systems and their main formation scenarios, explored by recent theoretical models. Observations of these common-envelope transients show that, while the emission quickly fades in the optical bands, their infrared signatures remain bright, revealing the formation of cold dust shells reprocessing the light of the newly coalesced star.

14-Oct-2019: Jens Chluba (Manchester), Spectral distortions in CMB

Following the pioneering observations with COBE in the early 1990s, studies of the cosmic microwave background (CMB) have primarily focused on temperature and polarization anisotropies. CMB spectral distortions – tiny departures of the CMB energy spectrum from that of a perfect blackbody – provide a second, independent probe of fundamental physics, with a reach deep into the primordial Universe. The theoretical foundation of spectral distortions has seen major advances in recent years, which highlight the immense potential of this emerging field. Spectral distortions probe a fundamental property of the Universe – its thermal history – thereby providing additional insight into processes within the cosmological standard model as well as new physics beyond. Spectral distortions are an important tool for understanding inflation and the nature of dark matter. They shed new light on the physics of recombination and reionization, both prominent stages in the evolution of our Universe, and furnish critical information on baryonic feedback processes, in addition to probing primordial correlation functions at scales inaccessible to other tracers. I will provide a broad brush overview of the exciting opportunities awaiting us using CMB spectral distortions.

21-Oct-2019: Peter Laursen (Univ. of Oslo), Lyman α — a window to the distant Universe

Lyman α is the most common type of light emitted from the most common element in the Universe and is hence used to study galaxies across the entire Universe. Being sensitive to many different physical processes — such as gas temperature, dust contents, galactic outflows, and intergalactic ionization state — it is simultaneously extremely informative and notoriously hard to interpret. Most of the Lyman α emitted from galaxies is related to newly formed stars, but a significant fraction is expected to be emitted from the gas feeding the galaxies from the surrounding intergalactic medium, cooling as it plummets down the potential well of the galaxy. Whereas the former is observed regularly, detection of the latter is challenging, and as yet the evidence for its existence is at best tentative. In this seminar, I will review the wonders and pitfalls of Lyman α, and present work-in-progress analytical estimates of the amount of cooling radiation.

28-Oct-2019: Jonathan Tan (Chalmers), A Light in the Dark - Massive Star Birth Through Cosmic Time

Massive stars have played a dominant role in shaping our universe since its earliest times, but there is still no consensus on the mechanism by which they form. I review the physics that is important for massive star formation and the connection this process may have with star cluster formation. I then focus on a particular theoretical model, Turbulent Core Accretion, which assumes the initial conditions are massive, turbulent, magnetized cores of gas and dust that are reasonably close to virial equilibrium. Our group has been exploring this scenario via analytic models and numerical simulations of the physics and chemistry of the interstellar medium, ranging from the earliest pre-stellar core phase to protostellar cores being impacted by strong self-feedback. Crucially, these models can now be tested in detail with ALMA, SOFIA and other facilities, and I present the latest results from multiple projects that are zooming in to massive star birth in the darkest shadows of giant molecular clouds. Extension of this work has the potential to also determine how the full stellar initial mass function is established across different Galactic environments. Finally, I discuss an application of massive star formation theory to the early universe: how massive were the first stars and could they have been the progenitors of supermassive black holes?

4-Nov-2019: Sarah Rugheimer (Oxford), Ultraviolet, Biosignatures and Life

When we observe the first terrestrial exoplanet atmospheres, we expect to find planets around a wide range of stellar types, ultraviolet environments, and geological conditions. Since the first exoplanets available for characterization will be likely for M dwarf host stars, understanding the ultraviolet environment of these cool stars is a vital step in understanding the atmospheres of these planets. Additionally the atmospheres of these planets will not been fixed in time. Earth itself offers many possible atmospheric states of a planet. We set out to examine how an Earth-like planet at different geological epochs might look around other star types Additionally, we examine the plausibility of detecting prebiotically interesting molecules, such as HCN, NH3, CH4, and C2H6 in an early-Earth type atmosphere around stars with very different ultraviolet environments, an M dwarf and a solar analogue.

18-Nov-2019: Mickael Rigault (CNRS), Astrophysical biases of Type Ia Supernovae and the Hubble Constant

Type Ia supernova are powerful cosmological distance indicators that enable us to measure the expansion history of the Universe. Using SNe Ia distances, scientists discovered the accelerating expansion of the Universe, leading to a Nobel prize and a broad focus on understanding the underlying cause of this acceleration.  SNe Ia distances are also key to measuring the Hubble Constant, the current expansion rate of the Universe and a key cosmological parameter.  Interestingly, the SNe Ia measurements of H0 are ~5 sigma away from the those derived from CMB temperature anisotropy measurements from Planck.  This highly discussed tension could be a sign of new physics, though no simple theoretical models are able to explain it.  However, I will discuss how recent studies of SNe Ia in the nearby Universe indicate two separate populations of SNe Ia with different peak luminosities. These differences in the underlying SNe Ia population could introduce a bias in the derived H0 and be the true cause of the tension with CMB measurements.

2-Dec-2019: Luca Fossati (Graz), Planet atmospheric escape: a tool for understanding both stars and planets

Planets in short-period orbits provide a unique opportunity to directly study atmospheric escape, which is a phenomenon having a profound impact on our understanding of the observed exoplanet demographics. Among all planets known to have an escaping atmosphere, those undergoing extreme mass loss are key: their escape may be representative of young planets, at a time when atmospheric escape matters most. I will briefly review our knowledge and understanding of planet atmospheric escape and present four concrete examples of how studying this phenomenon can bring forward our knowledge of both planets and stars. I will then conclude presenting a new upcoming facility for the study of planetary upper atmospheres and escape.

Term 2

20-Jan-2020: Stephen Feeney (UCL), Clarifying the Hubble Constant Tension

Our best estimate of the Universe's current expansion rate (the Hubble constant) from the local Universe (via the Cepheid distance ladder) is in four-sigma tension with the value extrapolated from cosmic microwave background data assuming the standard cosmology. Whether this discrepancy represents physics beyond the Standard Model or deficiencies in our understanding of the data is the subject of intense debate. In this talk, I will review the community's attempts to explain and interpret the Hubble constant tension, clarifying the current picture using Bayesian probability theory, and consider the potential for independent gravitational wave observations to arbitrate the dispute.

3-Feb-2020: Megan Bedell (Flatiron), Investigating the Star-Planet Connection with Precision Spectroscopy

More than two decades after the discovery of the first exoplanet with radial velocity (RV) measurements, large quantities of data exist from dedicated planet-hunting spectrographs. These spectra play an important role not only in the discovery and characterization of planets but also as a rich source of information about bright stars in the solar neighborhood. I will summarize recent efforts to gain insight into the planet formation process via extremely precise (0.01 dex) abundance measurements of solar twin stars with and without planets. I will also discuss approaches to the data analysis challenge of extracting maximally-precise RV measurements from these spectra in pursuit of Earth-like planets.

10-Feb-2020: Chiara Circosta (UCL), Looking for observational signatures of feedback from active galactic nuclei

Supermassive black holes, residing in the centre of massive glaxies, grow through accretion of gas and become visible as active galactic nuclei (AGN). The extraordinary amount of energy injected into the interstellar medium during these accretion episodes is thought to be key in shaping the life cycle of host galaxies and regulating star formation, via the so-called AGN feedback. The impact of AGN is expected to be maximised at z~2, i.e. the peak of supermassive black hole and galaxy assembly. Despite the wide observational support that AGN drive outflows into their host galaxies, a comprehensive picture is still missing. Moreover, measuring the gas content out of which stars form is necessary to understand how AGN regulate star formation. In this talk I will describe our efforts in performing a systematic study of AGN outflows by using integral field spectroscopy as well as characterising the molecular gas reservoir of the host galaxies through an ALMA campaign. Eventually our work will create a detailed picture of how AGN affect star formation in their host galaxies.

17-Feb-2020: Konrad Kuijken (Leiden), Cosmology from gravitational lensing with the Kilo-Degree Survey

The Lambda-CDM model is a very succesful description of the universe. It predicts how large-scale structures form and grow with time, leading to the present-day population of clustered galaxies. In this talk I will describe how we are testing this model with gravitational lensing measurements of the large-scale distribution of matter, in particular from the Kilo-Degree Survey, a project to map 1300 square degrees of sky using the VLT Survey Telescope at ESO in Chile.

24-Feb-2020: Clara Sousa Silva (MIT), Finding an Alien Biosphere with Computational Chemistry

At the edge of our present scientific frontier lies the question: “Can we identify the signs of life on an exoplanet?”. Establishing whether a planet is habitable, or inhabited, relies both on the observation of an exoplanet atmosphere and, crucially, its subsequent interpretation. This interpretation requires knowledge of the spectral behavior of every significant atmospheric molecule. However, though thousands of molecular candidates can contribute towards the spectrum of an atmosphere, data exist for only a few hundred gases. Among these, only a fraction have complete spectra (e.g. ammonia, water). This deep incompleteness in the knowledge of molecular spectra presents a pressing vulnerability in the atmospheric study of planets; there exists a strong possibility of mis-assignment, false positives, and false negatives in the detection of molecules.
The work presented here combines structural organic chemistry and quantum mechanics to obtain the necessary tools for the interpretation of astrophysical spectra and, ultimately, the detection of life on an exoplanet. Whether alien life will produce familiar gases (e.g., oxygen) or exotic biosignatures (e.g., phosphine), painting a confident picture of a potential biosphere will require a holistic interpretation of an atmosphere and its molecules. In this talk Clara will describe the ongoing efforts to decipher exoplanet atmospheres through the identification of volatile molecules, in particular those that might be produced by non-Earth-like life on exoplanets.

23-Mar-2020: Rita Tojeiro (St Andrews), The galaxy-halo connection in the cosmic web

Over the last two decades, extragalactic surveys have mapped the positions of millions of galaxies over extraordinary volumes, in an effort to understand the dynamics, composition and underlying rules that govern our Universe. Galaxies and the dark matter halos in which they reside are intrinsically connected, and that relationship holds information about important processes in galaxy and structure formation: understanding it is key to unlocking the full statistical power of forthcoming redshift surveys and their cosmological analyses. In this seminar, I will consider how the galaxy-halo connection might depend on its position within the cosmic web - the familiar decomposition of large-scale structure in filaments, knots and voids. I will finish by introducing the forthcoming DESI survey, and the role DESI will play in unravelling the galaxy-halo connection within the cosmic web.

Term 3

27-Apr-2020: Tessa Baker (QMUL), Testing Gravity with Gravitational Waves

Gravitational waves (GWs) have already proved immensely powerful for constraining cosmological extensions of GR, both from data-driven and theoretical perspectives. However, GWs really come into their own when used in combination with complementary electromagnetic data. I’ll review some of the latest bounds on modified gravity from GWs, and look at how the remaining gravity parameter space can be tested with future experiments like LISA and accompanying galaxy surveys.

11-May-2020: Freeke van de Voort (Cardiff), Cosmic gas flows in the circumgalactic medium around Milky Way-like galaxies

Galaxies are intimately connected to the environments they live in. The haloes around them contain the gas reservoir from which the galaxies grow, while galactic outflows heat and enrich this 'circumgalactic medium' (CGM). In this talk, I will use cosmological, magnetohydrodynamical simulations to study the physical and observable properties of the gas around galaxies. We use a new simulation refinement technique to reach orders of magnitude higher resolution than the current state-of-the-art. Our spatially refined simulations show that the CGM has more 'cool' gas than previously thought, which strongly affects predicted observables in the CGM: The neutral hydrogen (HI) column densities are dramatically enhanced, more in line with observations. I will show how the presence of magnetic fields alters the gas flows into and out of galaxies, resulting in less mixing and higher gas fractions inside the halo. I will briefly discuss the effects on the galaxies themselves.

18-May-2020: Adam Amara (Portsmouth), Forward Modeling the Universe: Application to Cosmic Shear

Observational cosmology is going through a golden age. In particular, we are in the midst of an influx of data from on-going experiments, such as the Dark Energy Survey (DES). In the coming five years, the volume and quality of data will rapidly increase as Stage IV surveys, Euclid, LSST and WFIRST, come online. Processing this data will require new algorithms and methods to maximise our science reach and to control for systematic errors. In this talk, I will present a method that we have developed called Monte-Carlo-Control-Loops that relies heavily on forward modelling the observed data by simulating all the processes from cosmology theory to images. Given the complexities of the late-time Universe, these forward models need to capture the important properties of galaxy populations and key features imprinted on the data from the experiments themselves. By bringing together all these elements with advanced statistical methods and new machine learning algorithms, we can build a process for extracting maximal information from the new data, which will allow us to extensively test the physics of the dark sector.