This is the webpage for the seminar series of the Astrophysics Group at University of Bath. Seminars are open to staff and students. Theory group seminars can be found at http://people.bath.ac.uk/srjc20/theory_seminars.html, Nanoscience seminars here.
When and where: Wednesdays at 14:15. Room during second semester: 1W2.03 (not the same as first semester!)
Organiser: Hendrik van Eerten (firstname.lastname@example.org)
Next upcoming seminar
Mathew Page (Mullard Space Science Laboratory, UCL)
Other forthcoming seminars
Seminars Spring 2019 (second academic semester)
Andrew Levan (University of Warwick)
Seminars Fall 2018
Rob Crain (Liverpool John Moores University)
Calibrated simulations of the formation and evolution of galaxies, their globular clusters and the circumgalactic medium
I will briefly introduce the EAGLE project, a campaign of large cosmological hydrodynamical simulations that has ushered in the era of realistic galaxy population models. I will discuss novel new extensions to EAGLE, including the addition of a self-consistently varying stellar initial mass function, and a sub-grid model for the formation and evolution of globular cluster populations. I will showcase results recently obtained using this extended family of EAGLE simulations, focussing on the origin of the bimodality of alpha-element abundances in the Galactic disc, the use of globular cluster properties to infer the assembly history of the Galaxy, and the coupled role of supermassive black holes and the circumgalactic medium in quenching galaxies.
Tim Naylor (University of Exeter)
Stellar variability through the planet-forming process
Young stars accrete material from the discs which surround them, which results in photometric variability from both the variation of accretion rate, and from geometric effects as the accretion structures rotate in and out of our line-of-sight. Although the periodic part of this variability has been used to obtain the rotation rate of many young stars, the a-periodic variability remains little studied and poorly understood. This is despite its potential to yield information about the inner astronomical unit of young-star discs, the equivalent of the region where the terrestrial planets are thought to have formed.
I will discuss how structure functions can be used to characterise the periodic variability of young stars, clearly separating it into a short-term component related to the rotation of the star, and a longer term component presumably related to changes in accretion rate. This part of the variability extends beyond 10 years, which links to a survey we have carried out of very long-term variability using Gaia and the photographic Schmidt surveys. On timescales of a hundred thousand years we see large (5 magnitude) outbursts which imply waves of material moving through the terrestrial zone as the planets are forming, which has implications for the planet-forming process.
Richard Ellis (University College London)
Early Galaxies and Cosmic Reionisation: Progress and Challenges
The birth of galaxies represents the last unexplored frontier of cosmic history and it is commonly believed such early systems led to the transformation of neutral gas in the intergalactic medium into its presentfully-ionised state. Some progress has been made in charting the demographics of early galaxies into the era when reionisation is thought to occur, but little is known about their nature of their stellar populations, the possible role of active nuclei and whether galaxies are capable of generating sufficient ionizing radiation. Spectroscopy holds the key to addressing these questions, targeting both individual sources at high redshift as well as carefully-chosen analogs at intermediate redshift. I will describe the recent progress and challenges as we anticipate the launch of JWST and the arrival of next-generation large telescopes.
Noelia Noël (University of Surrey)
Dissecting the Small Magellanic Cloud
A “Grand Challenge” in modern astrophysics is to understand how galaxies form and evolve. Our Local Group of galaxies, i.e., the galaxies around Andromeda and the Milky Way, are the best laboratories we have at hand. In particular, the "Magellanic Clouds”, our two nearest Irregular dwarf galaxies, provide the best workplace to study galaxy formation. The Magellanic Clouds are currently interacting with one another while orbiting around the Milky Way. Their close proximity allows us to resolve their individual stars, providing a unique ‘rosetta stone’ for understanding galactic encounters and mergers. In this talk I will show how to use stars to perform "galactic archaeology", unpicking the fossil record of a galaxy’s past by forensically dissecting its stellar content.
Diana Worrall (University of Bristol)
Hard-working radio galaxies: morphologies and interactions
Radio galaxies of intermediate power dominate radio-power injection in the Universe as a whole, making them the best candidates for interactions that provide the heat that regulates the growth of galaxies and other large structures. The population spans a wide range of radio morphology and environmental richness, and sources of all ages are amenable to study. I will describe structures and interactions, with emphasis on radio galaxies with deep high-resolution Chandra X-ray data. As compared with low-power radio sources, there is evidence that the physics changes, and the work done in driving shocks can exceed that in evacuating cavities. Ongoing local interactions are common.
Nicholas Devereux (Embry-Riddle Aeronautical University, USA)
Giant Broad-Line Regions in Low Luminosity active galactic nuclei.
The nearby lenticular galaxy NGC 3998 hosts the best example of an active galactic nucleus (AGN) fueled by chaotic cold accretion (CCA) whereby 10^4 K gas condenses out of a 10^6 K circumgalactic halo and falls toward the central supermassive black hole. Photoionization modelling of visible spectra obtained with STIS aboard the Hubble Space Telescope indicate that the central UV—X-ray source of NGC 3998 has ionized a large spherical volume (~ 7pc in radius) of low density (~10^4 cm-3) gas of such extraordinarily low metallicity (~ 1/100 solar) as to dictate a circumgalactic origin. The gas pressure gradient in the H+ region of the best fitting photoionization model is several orders of magnitude smaller than required for hydrostatic equilibrium. Thus, an inflow at the free-fall velocity is inevitable, consistent with the distinctly triangular shape observed for the broad Balmer emission lines. In general, CCA can explain the low duty-cycle observed for AGN and, more specifically for NGC 3998, redirection of the jets powering the larger scale radio lobes. NGC 3998 is just one of several nearby, non-reverberating, low-luminosity AGN associated with an unusually large broad-line region. Other nearby examples will be discussed including M81, NGC 3227, NGC 3516, NGC 4051 and NGC 4203.
Barbara Ryden (Ohio State University, USA)
Red Galaxies, Blue Galaxies, and the Green Valley
Since the mid 1920s, it has been known that luminous galaxies fall into two main classes. Edwin Hubble initially classified galaxies as "late" (with a complex, irregular appearance) or "early" (with a smooth, featureless appearance). In the 21st century, it is more common to classify galaxies by the color of their stellar population; "blue" galaxies have their light dominated by hot, luminous, short-lived stars, while "red" galaxies are dominated by cooler, older red giant stars. Generally speaking, late galaxies (in Hubble's sense) are blue, while early galaxies are red. However, there also exists an intriguing population of galaxies with intermediate colors and morphologies, known as "green valley" galaxies. In my talk, I will address the nature of these unusually colored galaxies. One of the oddities of green valley galaxies is their high probability of hosting active galactic nuclei (AGN). By comparing the properties of the stellar population in a green valley galaxy with the properties of its central AGN, clues can be derived about the much-debated role of AGN feedback on a galaxy's star formation rate.
Jack Baggaley (University of Canterbury, New Zealand)
The flow of Interstellar Dust into the Solar System
Estimates of the spatial density of interstellar dust grains can be gained from light extinction methods and far IR absorption and emission spectra can be employed to investigate surface composition of the grains. Spacecraft have gained crude directions of influx direction of grains into the Solar System while the Stardust mission has been able to capture such grains and after Earth-return perform laboratory examinations. Such material is significant because of their evolution in pre-solar epochs. For accurate source identification of inflowing grains, detailed dynamical properties are needed. We have been operating a radar system that is able to delineate Earth-impacting dust trajectories and hence heliocentric orbits to better map their possible sources. I will give an outline of our radar system and discuss how the dynamical properties compare with those of space missions.
Seminars Spring 2018
Aprajita Verma (University of Oxford)
Rise of a Giant: The Extremely Large Telescope
With a primary mirror of 39.3m, ESO's Extremely Large Telescope (ELT) will be the "World's Biggest Eye on the Sky" operating in the visible to infrared wavelength range. The ELT will collect 13 times more light than the largest visible-infrared telescopes today and the telescope's fully adaptive design will deliver images 16 times sharper than the Hubble Space Telescope. The sensitivity and spatial resolution afforded by the ELT will revolutionise our understanding of the Universe addressing numerous scientific questions from the nature of extra-solar planets to detecting first-light in the early Universe. The ELT has now begun its construction phase and I will review the status of the ELT project including the design, the first light and second generation instruments along with prime science cases. I will also describe the UK's ELT instrument programme. The UK is one of the major contributors to the ELT science case and studies for ELT instruments. One of these studies (HARMONI) has been selected by ESO to be the ELT's first-light spectrograph and is being designed and built by a European consortium led by Prof N. Thatte (University of Oxford).
Martin Krause (University of Hertfordshire)
Black-hole jets: constraining the central engines
The realisation that highly energetic radio lobes that easily surpass their host galaxies in size were powered by regions in the nuclei of galaxies comparable in size to the solar system was an early milestone in establishing that black holes are real astrophysical objects. Modelling the radio emission still provides interesting new constraints on this central engine, because quantities like energy, mass, lepton and particle number, momentum and magnetic flux are conserved during transport through the supersonic beam and accumulated in the radio lobes.
From a sample of powerful jet sources, we find that the jet power is limited approximately by the Eddington luminosity, linking to both, accretion and spin-energy extraction. No evidence for dynamically important protons is found thus the jet plasmas might be dominated by electron-positron pairs.
Complementary evidence comes from Galactic black holes: We have observed the microquasar V404 in its 2015 outburst at MeV energies and for the first time clearly detected the formation of a pair plasma outflow, probably forming the basis of the later observed radio jet. The enigmatic Galaxy-wide annihilation signal could plausibly be related to a jet outburst of the Milky Way’s supermassive black hole, Sgr A*, or a number of microquasars.
We can further constrain the central engines from high-resolution radio maps: For a complete sample of powerful radio jets we find that the majority is precessing. This likely points to a new population of tightly bound, spinning supermassive black hole binaries.
Christine Done (Durham University)
Black holes: Einstein's gravity and rocket science!
I will review how black holes went from a speculative extension of Einstein's gravity to a mainstream observational science via the development of rockets and X-ray astronomy at the start of the space age. I will show how we now use the X-rays from accreting black holes in our own galaxy to test General Relativity in the strong field limit, with observational evidence for the event horizon, last stable circular orbit and, most recently, Lense-Thirring precession as the origin of the strong, low frequency quasi-periodic oscillations seen in these systems. Not only does this solve the 25 year mystery of the nature of these signals, it also addresses more recent controversies over the nature and geometry of the accretion flow in this state.
Stephen Wilkins (University of Sussex)
Exploring the Epoch of Reionisation
Thanks to the Hubble Space Telescope, ALMA, and other facilities along with advances in numerical simulations significant progress in understanding this period of the Universe’s history has been made in the last few years. In this talk I’ll outline some of this recent progress and discuss prospects for the future, and in particular what we expect to learn from JWST
Seminars Fall 2017
Mark Hannam (Cardiff University)
Decoding gravitational waves to measure black holes and neutron stars
Gravitational waves are beginning to teach us about black holes and neutron stars throughout the universe. Information about the sources (the properties of the individual objects, the binary, and its location) are encoded in the signal, and can be extracted by comparison to theoretical models. I will discuss what we can (and cannot) measure from the signals we have measured so far, and what we hope to find in the future.
Patricia Schady (Max Planck Institute for Extraterrestrial Physics, Germany)
The environment of explosive stellar transients - clues to the progenitors of gamma-ray bursts, supernovae and super-luminous supernovae
Time-domain astronomy has experienced a reemergence with the implementation of high-cadence, wide field surveys, which has led to the identification of a new class of superluminous supernova (SLSN), and a dramatic increase in the samples of different transient types. Nevertheless, there remain large discrepancies in our understanding of the progenitors and environmental dependencies that give rise to different supernova (SN) types. In this talk I present state-of-the-art, high spatial resolution spectral data of the very nearby environments of a range of transient stellar explosions. At the spatial resolution of these data, the H-alpha equivalent width (EW) can be used to trace the age of the stellar population at the transient location, and any environmental dependencies can be explored. Combined with the properties of the stellar explosion, these host galaxy data provide new insights on the role that progenitor initial mass and environmental factors have on a star’s final demise.
Seminars Spring 2017
Natascha Förster Schreiber (Max Planck Institute for Extraterrestrial Physics, Germany)
Galaxy Evolution at the Peak Epoch of Cosmic Star Formation: Witnessing In-situ the Growth and Transformations of Young Galaxies
Eight to eleven billion years ago, galaxies were undergoing their most rapid mass assembly phase, forming stars at prodigious rates 10 to 20 times faster than observed today in the Milky Way and other nearby galaxies. While the statistical census of surveys measuring the global properties of faint distant galaxy populations and the fossil record from stars in present-day galaxies have enabled us to pin down when galaxies formed, spatially- and spectrally-resolved in-situ observations of individual galaxies are required to understand how. I will present key results from detailed mapping of the internal structure and motions of stars and gas in young galaxies, enabled by sensitive state-of-the-art instrumentation at large ground-based telescopes and in space. I will discuss implications for our understanding of the physical processes that drive the lifecycle of galaxies at early times, and will highlight exciting prospects in the upcoming decade from the next generation of instruments and telescopes.
Francesco Shankar (University of Southampton)
Selection bias in dynamically-measured super-massive black hole samples and its consequences
It has been claimed for decades that almost all galaxies in the local Universe host at their centre a super-massive black hole the mass of which appears to be tightly correlated with the stellar mass and the random motion ("velocity dispersion", sigma) of the stars of the host galaxy.
In this talk I will first highlight that significant biases affect these black hole-galaxy correlations. I will specifically show that the majority of black hole hosts have significantly higher velocity dispersions than local unbiased galaxies of similar stellar mass. Through aimed Monte-Carlo simulations and residual analysis I will then illustrate how from such biased data sets we can still infer important clues on the intrinsic correlations between black hole mass and host galaxy properties.
The Monte Carlo simulations indicate that selection effects artificially increase the normalization of the intrinsic scaling relations by factors from 3 to 50, and also strongly favour velocity dispersion as more ``fundamental'' than galaxy stellar mass or galaxy size. I will then move on discussing the main implications of these findings, in particular the comparisons with scaling relations in active galaxies, the implications for black hole radiative efficiencies, feedback from active black holes, and gravitational waves.
Ralph Wijers (University of Amsterdam)
The latest news on radio transients
Radio transients at low frequencies have been declared dead numerous times in the past few years. Searches for them have indeed not been easy, but ultimately are proving that Mother Nature is not a faithful reader of obituaries. In this talk I will present a selective update on efforts to find radio transients, and on what news we learn of Nature from studying the ones we have found.
Bob Fosbury (Emeritus, previously at European Southern Observatory, Germany)
Colours from earths
Planets comparable in size to that of the Earth are beginning to be discovered within the habitable zones around stars beyond the Solar System. How are we going to study such tiny, distant objects? In this talk, I will discuss the use of planetary transits to investigate the nature of their atmospheres. We have been using a local analogue of a transiting planet, namely Lunar eclipses, to develop the methods that will be needed to enable the next generation of large ground- and space-based to attempt this task on real exo-planets. Measuring the colours transmitted by their atmospheres will be key to detecting the presence of life.
Duncan Forgan (University of St Andrews)
The Disc Instability Theory of Planet Formation
Abstract: Planets are assembled from the protostellar discs that surround young stars. Precisely how this happens remains a source of debate. The most commonly accepted theory of planet formation is referred to as core accretion (CA). In this model, the interstellar dust grains in the disc grow via collisions, scaling many orders of magnitude in size to form rocky protoplanets. Protoplanets with masses above a critical value (around 10 Earth masses) can then capture a large gaseous atmosphere from the disc, becoming gas and ice giants. Bodies which fail to reach this critical value do not capture a gaseous atmosphere, but instead undergo a series of giant impacts to form the terrestrial planets.
There is a competing model for the formation of substellar bodies - referred to as disc instability or gravitational instability theory (GI). In this model, the protostellar disc becomes gravitationally unstable and fragments into bound objects on relatively short timescales. The latest ALMA observations have shown us the first glimpses of discs potentially prone to fragmentation, and in one exciting example, a disc fragmenting to form stellar mass bodies.
Is GI a viable planet formation mechanism? I will show that GI struggles to form terrestrial planets, but that we cannot ignore the possibility that the exoplanet population may be dominated by CA objects, with a non-negligible number of interlopers formed via GI. To make this argument I will present a range of work, from high resolution hydrodynamic simulations of fragmenting discs which show the physical and chemical properties of disc fragments, to state-of-the-art population synthesis models. These theoretical tools are providing observers with the means to place strong constraints on the frequency of disc fragmentation, and the number of bodies orbiting stars that are formed via GI.
David Tsang (University of Maryland)
Resonant Shattering Flares As Electromagnetic Counterparts to Gravitational Wave Mergers
With the recent detections of gravitational waves, the Advanced LIGO experiment has established the era of gravitational-wave astronomy. While these detections have been of massive black hole binaries (BH-BH), electromagnetic counterparts are only expected to occur for neutron star-neutron star (NS-NS) or NS-BH mergers. I will discuss a promising electromagnetic counterpart to NS mergers, Resonant Shattering Flares (RSFs), which occur when the tidal frequency of the binary inspiral matches NS core-crust interface mode frequency, leading to strong mode-excitation, crust shattering and isotropic flares with predicted luminosities of 10^47-10^49 erg/s occurring seconds before the merger.
Resonant Shattering Flares are prompt, bright, and isotropic, allowing detection from well beyond the LIGO-horizon and may be the most likely source for detectable electromagnetic counterparts to GW mergers. Coincident timing of the delay between the RSF and the gravitational-wave chirp, will not only confirm the RSF model, but also allow precise asteroseismology of the NS interface-mode, constraining the NS equation of state and nuclear physics properties near nuclear saturation.
Samaya Nissanke (Radboud University, Netherlands) cancelled
Kunal Mooley (University of Oxford)
The Search for Radio Afterglows of Gravitational Wave Sources
The recent direct detections of gravitational waves (GWs) from extragalactic binary black hole mergers by the aLIGO has opened a new window into the Universe. The impending increase in the aLIGO sensitivity together with the addition of the Advanced VIRGO (AdV) detector indicate that the mergers of compact binary systems containing neutron stars will be detected in the coming years. Maximizing the science returns from aLIGO detections will require the identification and detailed study of their electromagnetic (EM) counterparts. While distinct signatures of compact binary mergers are expected in the X-rays, optical and infrared, their timescales of evolution are extremely short (hours~day). However, with the ability to capture dust-obscured phenomena and the interactions between fast outflows and the surrounding media, radio afterglows offer unique discovery opportunities and strong diagnostics for the merger events. At the same time, poor localization of aLIGO sources and the slow evolution (months~years) of radio afterglows present challenges to the search for the radio counterparts of GW sources. In this talk I will introduce the ongoing efforts for finding EM counterparts of GW sources, especially at radio wavelengths; give a description of the expected EM signatures; and argue that radio observations are just as powerful as the optical, if not more, for detecting the counterparts of GW sources. I will end by briefly describing also the prospects of finding the radio counterparts of GW sources, such as binary SMBH mergers, relevant for space-based interferometers and pulsar timing arrays.
Seminars Fall 2016
David Rosario (Durham University)
Star-formation and nuclear activity in galaxies: A perspective in the era of the Herschel Space Telescope
Abstract: The far-infrared Herschel Space Observatory has opened our eyes to the cold dusty Universe. Far-IR wavelengths provide arguably the best tracers for star-formation in active galactic nuclei (AGN), since luminous nuclear activity is rather inefficient at keeping dust cold. I will report on studies that bring together the very best modern multi-wavelength survey datasets, from the X-rays to the optical to the far-IR, aimed towards developing a coherent view of the growth of supermassive black holes (in AGN) and the growth of stellar content in galaxies (through star-formation). These studies build on the newest advances in our knowledge of galaxy evolution across most of the Universe's history. I will demonstrate that a positive relationship between star-formation and AGN activity is now clearly seen to z > 2. However, the nature of this relationship supports weak or stochastic co-evolution, driven more by the smooth increase of gas content in normal galaxies over time rather than a dominant role of short, intense episodes, such as star-bursts or mergers. This has important implications for the connections between galaxies and the black holes that reside at their hearts.
Sebastian Hönig (University of Southampton)
Tori, disks, and winds — the AGN dust emission at high angular resolution
Abstract: Mass accretion onto supermassive black holes occurs on scales beyond the diffraction limit of any single optical/infrared (IR) telescope. Thanks to the resolution power of the VLT Interferometer, we are now tapping into the outer accretion structure of active galactic nuclei (AGN) — commonly referred to as the “dusty torus”. Several surprising results are challenging our current paradigm: While the bulk of the mid-IR emission originates from perpendicular where models would put the torus, the IR emission as a whole appears to be made of two components. In this talk I will give a basic introduction to IR interferometry and discuss what our recent results tell us about AGN unification and the physical processes that regulate accretion and feedback.
Serguei Komissarov (University of Leeds)
The Crab Nebula
The Crab Nebula, one of the most iconic astronomical objects, has played and still plays a very important role in the development of modern astrophysics. The nebula was created by one of the historic supernovae almost two thousand years ago, but it is constantly invigorated by a powerful relativistic magnetised wind from the Crab pulsar. The inner part of the Crab nebula showcases a very dynamic picture of the wind interaction with the nebula. The Crab’s famous jet, torus, wisps and few bright knots result from the interaction and show evidence of relativistic motion. Dynamics of relativistic plasma, properties of relativistic shock waves, magnetic reconnection, mechanism of non-thermal particle acceleration, the Crab Nebula is a unique space laboratory to study these and other topics so important in many other phenomena of relativistic astrophysics. In my talk, I will focus on some of the recent advances in the astrophysics of the Crab Nebula, describe what we have learned from these and what still remains poorly understood.
Kim-Vy Tran (Texas A&M University, USA)
From the Fourge to the Fire -- Galaxy Evolution Over 12 Billion Years
Abstract: ZFOURGE and ZFIRE are sensitive surveys that track how galaxies assemble over the past 12 billion years. ZFOURGE identifies and measures cosmological distances to approximately 70,000 objects using observations at near-infrared wavelengths from the Magellan Telescope and Hubble Space Telescope. ZFIRE selects galaxies from ZFOURGE for spectroscopic follow-up with the Keck Observatory to measure how baryons cycle between stars, galactic winds, and the Inter-Stellar Medium (ISM). Here I highlight our results that include mapping how galaxies are distributed in the distant universe, taking a census of galaxies' spectral properties over cosmic time, and determining if a galaxy's evolution depends (or not) on its neighbors.
Anne-Marie Weijmans (University of St Andrews)
Mapping Nearby Galaxies at APO: an overview and first results of the MaNGA galaxy survey
Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) is a galaxy survey using integral-field spectroscopy to map the stars and gas in 10,000 nearby galaxies. With integral-field spectroscopy we observe the velocities of stars and gas in galaxies, as well as their chemical composition. These observations are used to reveal the formation history of the galaxies as well as their mass distributions (including their dark matter content). As galactic archeologists, we use integral-field spectroscopy to piece together the ways that galaxies formed, evolved and interacted.
Observations for this survey started in 2014, and last summer MaNGA had its first public data release. In this talk I will give an overview of how we designed the MaNGA survey, discussing its instrumentation, observing strategy and science goals. I will also summarise the first results of MaNGA, and give an outlook on what we are working on next.
1.2.2016 Kate Rowlands (University of St Andrews)
"Post-starburst galaxies: Pathways to the red sequence?"
9.11.2015 Rob Spence (University of Sheffield)
"Searching for extended outflows in Ultraluminous Infrared Galaxies (ULIRGs)"