UCL Astrophysics Group


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.