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Institute of Astronomy

 
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IoA Seminars usually held on Wednesday lunchtime.
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Tue 24 Jun 13:15: Earth, a Cosmic Spectacle

Tue, 24/06/2025 - 13:28
Earth, a Cosmic Spectacle

Louise Beer, IoA Artist in Residence, will share a presentation that considers the philosophical impacts of dark skies, and how having access to them can help us to understand better loss and grief, our individual connection to the deep time history of Earth and the Universe, and the cosmic significance of the climate crisis. Louise will share her 2024 British Council-funded project, Earth, a Cosmic Spectacle which was developed in collaboration with astronomer Dr Ian Griffin and Tūhura Otago Museum in Aotearoa New Zealand. In this project, the artist invited astronomers, biologists, and geologists to gaze into the dark skies of New Zealand and anonymously write a letter exploring how their knowledge of Earth’s long and gradual development, starting from the dawn of the Universe, shapes their understanding of the cosmic significance of the climate crisis.

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Wed 18 Jun 13:15: Streams: A New Frontier in Constraining Dark Matter Halo Populations

Mon, 16/06/2025 - 11:35
Streams: A New Frontier in Constraining Dark Matter Halo Populations

Tidal streams—remnants of disrupted stellar systems—are powerful tracers of galactic gravitational potentials. While streams in the Milky Way have yielded insights into its dark matter halo thanks to full 6D stellar data, applying this method to external galaxies is more difficult due to the lack of kinematics and projection effects. Individually, photometric-only streams offer limited constraints, but their collective signal can be statistically powerful.

In this talk, we present a novel hierarchical Bayesian framework that uses purely photometric data to constrain the population-level properties of dark matter halos. To achieve this, we constructed STRRINGS , a catalog of long and curved streams around nearby galaxies. Our results show that even without kinematic information, an ensemble of just 50 well-characterized streams can reliably distinguish between oblate, spherical, and prolate halos. This highlights that even purely photometric datasets, when analyzed in aggregate, can yield robust insights into dark matter distributions.

This breakthrough arrives at a critical moment, as upcoming surveys from Euclid and LSST are set to deliver an unprecedented volume of high-quality stream observations. Our approach represents a paradigm shift in how we constrain dark matter properties, ultimately refining our understanding of the universe’s fundamental structure.

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Wed 18 Jun 13:40: Impact of extragalactic point sources on the foregrounds and 21-cm observations

Thu, 12/06/2025 - 13:42
Impact of extragalactic point sources on the foregrounds and 21-cm observations

The contribution of resolved and unresolved extragalactic point sources to the low-frequency sky spectrum is a potentially non-negligible part of the astrophysical foregrounds for cosmic dawn 21-cm experiments. The clustering of such point sources on the sky, combined with the frequency dependence of the antenna beam, can also make this contribution chromatic. By combining low-frequency measurements of the luminosity function and the angular correlation function of extragalactic point sources, we develop a model for the contribution of these sources to the low-frequency sky spectrum. Using this model, we find that the contribution of sources with flux density >10^-6 Jy to the sky-averaged spectrum is smooth and of the order of a few kelvins at 50–200 MHz. We combine this model with measurements of the galactic foreground spectrum and weigh the resultant sky by the beam directivity of the conical log-spiral antenna planned as part of the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) project. We find that the contribution of point sources to the resultant spectrum is ∼ 0.4 per cent of the total foregrounds, but still larger by at least an order of magnitude than the standard predictions for the cosmological 21-cm signal. As a result, not accounting for the point-source contribution leads to a systematic bias in 21-cm signal recovery. We show, however, that in the REACH case, this reconstruction bias can be removed by modelling the point-source contribution as a power law with a running spectral index. We make our code publicly available as a python package labelled epspy.

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Wed 11 Jun 13:15: Neurodiversity and Communication Styles

Mon, 02/06/2025 - 14:54
Neurodiversity and Communication Styles

Dr Maria Dias, Neurodiversity Adviser at the Accessibility and Disability Resource Centre (ADRC) and St Catharine’s College, will explore how people with different neurotypes communicate in unique ways, and why understanding these differences is important for creating more inclusive and supportive environments. Whether you’re neurodivergent yourself, work with neurodivergent people, or just want to learn more, this talk is for you. There will be time for questions and open discussion at the end.

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Wed 04 Jun 13:40: GPU Accelerated Sampling and Model Comparison

Fri, 30/05/2025 - 11:43
GPU Accelerated Sampling and Model Comparison

This talk introduces a natively vectorized implementation of the Nested Sampling algorithm, enabling deployment of the entire inference process onto GPUs for massive acceleration. I will start by reviewing the benefits, and necessity, of the paradigm shift towards vectorized compute in the physical sciences. After a brief review of the how (and why) of Bayesian inference in Astronomy and Cosmology, I will then explore the nuances and challenges of taking some of the widely used inference algorithms within this community, in particular nested sampling, to the GPU accelerated frontier. Lastly I’ll present some practical benefit that this speedup can bring and comment on how this technical development can help push the boundaries of what we can achieve in the physical sciences.

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Wed 04 Jun 13:15: Geometric mixing models as a tool for investigating the ice shell of Europa

Thu, 29/05/2025 - 10:19
Geometric mixing models as a tool for investigating the ice shell of Europa

The presence of liquid water is vital to the understanding of a planetary body’s climate, geological history, and habitability. The use of ice-penetrating radar as a probe for subsurface hydrology has been demonstrated across Earth and nearby planetary bodies. Radar sounding has uncovered hundreds of subglacial lakes across the Antarctic and Greenland ice sheets, while a recent mission to Mars (MARSIS) found anomalously bright reflectances suggesting the presence of a subglacial lake at the South Polar Layered Deposits. The recently launched Europa Clipper is similarly equipped with an ice-penetrating radar instrument, REASON , which will search for evidence of liquid water on Europa as an indicator of habitability.

However, the uniqueness of reflectivity as an identifier for subglacial water bodies has recently been called into question: conductive sediments and brine inclusions in ice have been proposed as alternate hypotheses for the origin of water-like radar signals at Mars and the Devon ice cap. Conventional approaches to studying the effective permittivity of such mixtures assume an isotropic distribution; here we apply geometric mixing models to account for realistic, anisotropic brine geometries. We demonstrate how geometric mixing models can provide more exact constraints on the presence and geometric distribution of liquid water in Europa’s ice shell. We further discuss the detectability of the eutectic zone in the ice shell and its implications for its thermal structure.

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Wed 28 May 13:15: Inward (or outward?) migration of massive planets in protoplanetary discs

Mon, 26/05/2025 - 09:46
Inward (or outward?) migration of massive planets in protoplanetary discs

According to the classical picture, type II migration is a slow, inward motion of the planet that either follows the disc viscous evolution (disc-dominated regime) or is much slower than that (planet-dominated regime). However, over the last decade, this picture of type II migration has significantly evolved, suggesting faster migration in the disc-dominated regime and even outward migration in the planet-dominated regime. In this talk, I will present recent results exploring the planet-dominated regime via live-planet, long-term simulations of planet migration. These show the existence of a correlation between the “gap-depth parameter” K and the direction of planet migration: planets migrate outward or inward depending on whether K is above or below a critical threshold Klim. This also implies the existence of “stalling radius” where migration halts. Using these results, I will introduce a toy model that allows to predict that massive planets accumulate in a band near the stalling radius (typically between 1–10 au), offering an explanation for the observed distribution of Jupiter-like exoplanets while challenging classical models of hot Jupiter formation.

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Wed 28 May 13:40: Shamrock: SPH and more, from a laptop to Exascale.

Mon, 26/05/2025 - 09:46
Shamrock: SPH and more, from a laptop to Exascale.

We introduce Shamrock, a performance-portable framework written in C++17, targeting CPU and GPUs from any vendors using the SYCL programming standard, designed for numerical astrophysics across a wide range of hardware, from laptops to Exascale systems. Astrophysical schemes often share a common structure: a combination of neighbor searching and the numerical scheme itself. Shamrock embraces such abstractions to provide a common framework for multiple hydrodynamical schemes, namely finite elements, finite volume (with adaptive mesh refinement), and Smoothed Particle Hydrodynamics. To achieve this, at its core, Shamrock features a highly optimized, parallel tree algorithm with negligible construction overhead. This tree structure is coupled with a domain decomposition strategy that enables near-linear weak scalability across multiple GPUs. Shamrock achieves 92% weak scaling efficiency on 1024 AMD M I250x GPUs in large-scale Smoothed Particle Hydrodynamics (SPH) simulations. This corresponds to processing billions of particles per second, with tens of millions of particles handled per GPU , allowing us to perform the first SPH simulations above the billion-particle mark for protoplanetary discs.

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Mon 19 May 13:15: Impact of XRB Stochasticity on 21-cm Observables from CD-EoR

Mon, 19/05/2025 - 12:46
Impact of XRB Stochasticity on 21-cm Observables from CD-EoR

Abstract: High Mass X-ray Binaries (HMXBs) are thought to be one of the key contributors to the X-ray background during the Cosmic Dawn (CD) and Epoch of Reionization (EoR). However, in traditional semi-numerical simulations of the CD-EoR, the LX-SFR relation is assumed to be fixed across cosmic time, which may not be accurate, especially for low star-forming regions. To mitigate this problem, we model the total luminosity in a stochastic manner and implement it in our 21-cm simulation from the CD-EoR to see its imprints on the 21-cm signal statistics like the Power Spectrum and 21-cm brightness temperature maps. We find the effects of XRB stochasticity in the small-scale PS (k > 0.9) and in the 21-cm maps that may have the potential for detection via the lunar based observations.

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Wed 21 May 13:40: XRISM observations of the X-ray pulsar Hercules X-1

Sun, 18/05/2025 - 15:31
XRISM observations of the X-ray pulsar Hercules X-1

Hercules X-1 is one of the most complex X-ray binary systems. It is known to harbor a strongly magnetized, highly accreting neutron star. Thanks to the high inclination of the binary, a warped accretion disk precessing every 35 days, and the neutron star rotating every 1.27 seconds, the system exhibits a very broad range of timing and spectral phenomena. These include X-ray pulsations, eclipses, absorption dips, cyclotron lines, accretion disk winds and emission lines observed from various parts of the accretion flow. The unique properties of Her X-1 allow us to reveal the physics of accretion in X-ray binaries through means that are impossible in other systems. In September 2024, we carried out a large observational campaign on Hercules X-1 led by the new XRISM observatory. With over 200 ks of XRISM exposure time and a spectral resolution of better than 5 eV in the Fe K energy band (R>1000 – more than 10 times better than previous instruments), we are for the first time able to resolve and separate the various evolving emission and absorption components spectrally, and in time. In this talk, I will present the first results from this campaign, and the novel insights it provides into our understanding of accretion disk wind physics, and of X-ray pulsar accretion.

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Wed 14 May 13:15: Noise from Above: Determining the Impact of Starlink's Unintended Electromagnetic Radiation on REACH

Fri, 09/05/2025 - 16:22
Noise from Above: Determining the Impact of Starlink's Unintended Electromagnetic Radiation on REACH

21-cm cosmology experiments have opened new frontiers in our quest to explore the early universe. However, the rapid expansion of satellite constellations in Low Earth Orbit (LEO) poses a significant threat. SpaceX’s Starlink is particularly concerning due to unintended electromagnetic radiation (UEMR) generated by its hardware and onboard electronic subsystems, as reported by observatories such as the Low-Frequency Array (LOFAR). These emissions could contaminate observations of the faint 21-cm signal, already buried beneath foreground emissions and radio frequency interference (RFI). The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) is a low-frequency radio telescope based in the Karoo radio reserve, South Africa, designed to detect the global 21-cm signal from Cosmic Dawn. In this talk, I will present my ongoing work assessing the extent to which Starlink impacts REACH . My approach combines orbital trajectory simulations using Two-Line Element (TLE) catalogues with geometric constraints to identify Starlink flyovers within REACH ’s field of view. These are cross-referenced with power spectral density (PSD) measurements to search for correlations indicating UEMR , including Doppler shift analysis. I conclude by outlining plans to automate this process and how this work contributes to broader efforts to safeguard radio astronomy from satellite interference.

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Wed 14 May 13:40: Gravitational Phase-Space Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power-Spectrum

Thu, 08/05/2025 - 16:41
Gravitational Phase-Space Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power-Spectrum

The matter power spectrum is one of the fundamental quantities in the study of large-scale structure in cosmology. In this talk, I will describe its small-scale asymptotic limit, and give a theoretical argument to the effect that, for cold dark matter, P(k) has a universal asymptotic scaling with the wave-number k, for k >> k_nl, viz. P(k) ~ k^(-3). I will explain how gravitational collapse drives a turbulent phase-space flow of the quadratic Casimir invariant, where the linear and non-linear time scales are balanced, and how this balance dictates the k dependence of the power spectrum. The coldness of the dark-matter distribution function — its non-vanishing only on a 3-dimensional sub-manifold of phase-space — underpins the analysis. I will show Vlasov-Poisson simulations that support the theory, and if time permits, also describe a stationary-phase technique for deriving an equivalent result. 

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Wed 07 May 13:15: The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

Mon, 05/05/2025 - 16:34
The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

The Great Oxidation Event (GOE) was a 200 Myr transition circa 2.4 billion years ago that converted the Earth’s anoxic atmosphere to one where molecular oxygen (O2) was abundant. This rise in O2 is thought to have substantially throttled hydrogen (H) escape and the associated water (H2O) loss. In this study we use WACCM6 , a three-dimensional Earth System Model to simulate Earth’s atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O2 concentrations based on atmospheric estimations from the GOE onward, ranging between 0.1 PAL to 150 PAL , where PAL is the present atmospheric level of 21 % by volume. O2 indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O2 leads to decreased O3 densities, reducing local temperatures by up to 5 K, which increases H2O freeze-drying. For the considered scenarios, the maximum difference in the total H mixing ratio at the homopause and calculated diffusion-limited escape rates is a factor of 3.2 and 4.7, respectively, with the prescribed CH4 mixing ratio setting a minimum diffusion escape rate of ≈ 2 × 10^10 mol H/yr. These numerical predictions support geological evidence that the majority of Earth’s hydrogen escape occurred prior to the GOE .

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Wed 07 May 13:40: On the Road to the Radius Valley

Mon, 05/05/2025 - 16:33
On the Road to the Radius Valley

Twenty years after their initial discovery, the nature of super-Earths and sub-Neptunes remains largely unknown. In this talk, I will discuss recent work addressing their interior compositions and formation pathways. In particular, I will show how the detection of young transiting exoplanets may provide a route to revealing their interior compositions.

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Tue 06 May 13:15: The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

Fri, 02/05/2025 - 10:55
The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event

The Great Oxidation Event (GOE) was a 200 Myr transition circa 2.4 billion years ago that converted the Earth’s anoxic atmosphere to one where molecular oxygen (O2) was abundant. This rise in O2 is thought to have substantially throttled hydrogen (H) escape and the associated water (H2O) loss. In this study we use WACCM6 , a three-dimensional Earth System Model to simulate Earth’s atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O2 concentrations based on atmospheric estimations from the GOE onward, ranging between 0.1 PAL to 150 PAL , where PAL is the present atmospheric level of 21 % by volume. O2 indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O2 leads to decreased O3 densities, reducing local temperatures by up to 5 K, which increases H2O freeze-drying. For the considered scenarios, the maximum difference in the total H mixing ratio at the homopause and calculated diffusion-limited escape rates is a factor of 3.2 and 4.7, respectively, with the prescribed CH4 mixing ratio setting a minimum diffusion escape rate of ≈ 2 × 10^10 mol H/yr. These numerical predictions support geological evidence that the majority of Earth’s hydrogen escape occurred prior to the GOE .

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Wed 30 Apr 13:15: Rebirth of the Ancients: Globular Clusters in Their Renaissance Era

Fri, 25/04/2025 - 13:57
Rebirth of the Ancients: Globular Clusters in Their Renaissance Era

Globular clusters (GCs) are multi-faceted cosmic tracers, currently experiencing a renaissance and resurgence of interest in both their formation mechanisms and their role in global galaxy evolution. In the context of galaxy formation and evolution, the importance of GCs in the earliest stages of galaxy assembly continues to grow. Observations of massive bound clusters at the epoch of reionization and extreme nitrogen enhancement at 𝑧 = 11 are two recent examples from JWST that hint at the significance of GCs in the growth of the earliest galaxies. Meanwhile, the discovery that 50% of ancient, in-situ Milky Way stars are nitrogen-enhanced directly connects GCs to the field of Galactic Archaeology. The local population of Milky Way globular clusters represents a unique set of objects for which age, chemical abundances, and dynamical properties can all be determined with exceptionally high precision (on the order of 1–5%), providing critical insights into galaxy formation and evolution across cosmic time. Over the course of this talk, I will discuss several ways in which we have advanced our understanding of GC formation and gained insight into rare nucleosynthetic sites in early dwarf galaxies. I will also present dynamical results that show how the Milky Way GC population serves as a sensitive probe of our Galaxy’s growth, and I will share some very recent findings that use GC ages to project the Milky Way back in time—placing it in context with star-forming galaxies at 𝑧 = 3.

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Tue 15 Apr 11:00: Growing pains: the dining habits of stars, planets and black holes

Mon, 14/04/2025 - 12:24
Growing pains: the dining habits of stars, planets and black holes

To make planets, stars and supermassive black holes, one must rapidly accrete material onto central objects. But the tiniest tangential motion combined with angular momentum conservation sends material into orbit, rather than accreting. Since work at the IoA in the 1970s we have understood that Nature solves the angular momentum problem by forming accretion discs, but the angular momentum transport mechanism remains unclear. The past 10 years have given us spectacular resolved observations of discs around both young and old stars, bringing fresh clues. In this talk I’ll explain how pairing 3D simulations with observations helps us solve the problem of accretion, revealing how stars and planets form, black holes grow and how accretion powers tidal disruption events.

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Tue 15 Apr 11:00: Growing pains: the dining habits of stars, planets and black holes

Wed, 09/04/2025 - 15:30
Growing pains: the dining habits of stars, planets and black holes

To make planets, stars and supermassive black holes, one must rapidly accrete material onto central objects. But the tiniest tangential motion combined with angular momentum conservation sends material into orbit, rather than accreting. Since work at the IoA in the 1970s we have understood that Nature solves the angular momentum problem by forming accretion discs, but the angular momentum transport mechanism remains unclear. The past 10 years have given us spectacular resolved observations of discs around both young and old stars, bringing fresh clues. In this talk I’ll explain how pairing 3D simulations with observations helps us solve the problem of accretion, revealing how stars and planets form, black holes grow and how accretion powers tidal disruption events.

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Wed 19 Mar 13:40: Perturber-disc interaction: Can we see the unseen?

Mon, 17/03/2025 - 10:52
Perturber-disc interaction: Can we see the unseen?

Protoplanetary discs are the place in which planets form and evolve, and the reservoir from which protostar accrete material. The presence of a perturber whether a planet or a stellar companion and its interaction with the parental disc play a crucial role in shaping the dynamics and evolution of the system, generating substructures such as gaps, rings and asymmetries routinely observed with different tracers in discs (large mm dust and gas with ALMA , small micrometric dust with VLT ).

However, characterizing these systems remains challenging: the only two planetary companions unambiguously detected, PDS70b and c, lie in a wide and open cavity of the same object, and once the perturber is a star, massive and wide enough to be detected, characterize the binary orbit is challenging due to the timescales at play.

In this talk, I will show new results to address and mitigate these issues. Firstly, I will show new results from the astrometry and the hydrodynamical models of GG Tau A, a multiple stellar system where the orbits of the stars is still not fully constrained. Then, I will discuss how the advent of the Extremely Large Telescope (ELT), with its first light imager MICADO /MORFEO, will revolutionize the field, providing high angular resolution images that will allow us to detect embedded protoplanets and small-scales substructures.

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Wed 19 Mar 13:15: Constraining mixed dark matter models with the high-redshift Lyman-α forest

Fri, 14/03/2025 - 16:47
Constraining mixed dark matter models with the high-redshift Lyman-α forest

In the standard cosmological model, cold dark matter gives rise to small-scale structure problems which Warm Dark Matter (WDM) in the keV range may address. Previous studies have narrowed the mass range for WDM , but further testing is increasingly challenging due to resolution limits of current spectrographs. Given the lack of success in laboratory dark matter searches, an intriguing alternative has recently re-emerged—a mixed dark matter scenario that combines cold and warm components, known as Cold+Warm Dark Matter (CWDM). In this framework, structure formation is influenced by the free-streaming of the warm component at the small scales probed by the Lyman-α forest, requiring hydrodynamical simulations to account for non-linear structure evolution and gas dynamics.

This presentation focuses on updated constraints on CWDM derived from high resolution and high-redshift (z=4.2−5.0) spectra from UVES and HIRES spectrographs. We use the 1D flux power spectrum to compare simulated Lyman-α forest to observational data in a Bayesian inference framework. The grid of simulations spans a high-dimensional parameter space, which we efficiently sample by implementing a neural network emulator at the likelihood level.

Beyond 1D clustering statistics, these allowed mixed dark matter models can accommodate small-scale deviations in the high-k regime of the matter power spectrum, potentially alleviating the S8 tension.

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