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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.

• Speaker: David Yallup / IoA
• Wednesday 04 June 2025, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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Title TBC

Abstract TBC

• Speaker: Dr. Weiyang Wang (University of Chinese Academy of Sciences)
• Tuesday 21 October 2025, 11:15-12:00
• Venue: TBC.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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arXiv:2502.03085v3 Announce Type: replace Abstract: The shape of the Ly-$\alpha$ transmission in the near zone of the redshift $z=5.9896$ quasar ULAS J0148$+$0600 (hereafter J0148) is consistent with a damping wing arising from an extended neutral hydrogen island in the diffuse intergalactic medium (IGM). Here we use simulations of late-ending reionisation from Sherwood-Relics to assess the expected incidence of quasars with Ly-$\alpha$ and Ly-$\beta$ absorption similar to the observed J0148 spectrum. We find a late end to reionisation at $z=5.3$ is a necessary requirement for reproducing a Ly-$\alpha$ damping wing consistent with J0148. This occurs in $\sim3$ per cent of our simulated spectra for an IGM neutral fraction $\langle x_{\rm HI}\rangle=0.14$ at $z=6$. However, using standard assumptions for the ionising photon output of J0148, the a priori probability of drawing a simulated quasar spectrum with a Ly-$\alpha$ damping wing profile \emph{and} Ly-$\alpha$ near zone size that simultaneously match J0148 is low, $p<10^{-2}$. This may indicate that the ionising emission from J0148 is variable on timescales $t<10^{5}\rm\,yr$, or alternatively that the Ly-$\alpha$ transmission in the J0148 near zone is impacted by the transverse proximity effect from nearby star-forming galaxies or undetected quasars. We also predict the IGM temperature should be $T\sim 4\times 10^{4}\rm\,K$ within a few proper Mpc of the Ly-$\alpha$ near zone edge due to recent H$\,\rm \scriptstyle I$ and He$\,\rm \scriptstyle II$ photo-heating. Evidence for enhanced thermal broadening in the Ly-$\alpha$ absorption near the damping wing edge would provide further evidence that the final stages of reionisation are occurring at $z<6$.

arXiv:2505.22567v2 Announce Type: replace Abstract: The recent discovery of a large number of massive black holes within the first two billion years after the Big Bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04, hosted in a galaxy with very low dynamical mass. The weakness of the [OIII]5007 emission line relative to the narrow Hbeta emission indicates an extremely low chemical enrichment, less than 0.01 solar. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in rare cases. Models invoking "primordial black holes" (i.e. putative black holes formed shortly after the Big Bang) may potentially explain the low chemical enrichment associated with this black hole.

Title TBC

Abstract TBC

• Speaker: Prof. Howard Reader
• Tuesday 01 July 2025, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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arXiv:2409.02172v2 Announce Type: replace Abstract: Dwarf galaxies have historically posed challenges to the cold dark matter (CDM) model and, while many of the so-called 'dwarf galaxy problems' have been mitigated by incorporating baryonic processes, the observed diversity of dwarf galaxy rotation curves remains a contentious topic. Meanwhile, the growing observational samples of active galactic nuclei (AGN) in dwarf galaxies have prompted a paradigm shift in our understanding of dwarf galaxy evolution, traditionally thought to be regulated by stellar feedback. In this study, we explore the potential role of AGN feedback in shaping dark matter distributions and increasing the diversity of dwarf galaxy rotation curves, using a new suite of cosmological zoom-in simulations of dwarf galaxies with the FIRE-3 model. Our findings indicate that the presence of active black holes (BHs) in dwarf galaxies can lead to diverse outcomes, ranging from cuspier to more core-like profiles. This variability arises from the dual role of BHs in providing additional feedback and regulating the extent of stellar feedback. Consistent with previous research, we find that AGN feedback is most impactful when cosmic ray (CR) modelling is included, with CRs from any source significantly influencing dark matter profiles. Overall, our results highlight that the interplay between stellar feedback, BHs, and CRs produces a broad spectrum of dark matter density profiles, which align with observed correlations between rotation curve shapes and baryonic dominance. This underscores the importance of including the full range of baryonic processes in dwarf galaxy simulations to address the persistent 'small-scale challenges' to the CDM paradigm.

Rapid accretion and state changes in strongly magnetised disks

Accretion disks power many of the universe’s most luminous phenomena, acting as intermediaries that enable matter to shed angular momentum and accrete onto stars or compact objects. While angular momentum transport in disks has been extensively studied, especially in the context of magneto-rotational turbulence, significant challenges remain. These include reconciling simulation results with observed accretion rates and understanding state transitions in cataclysmic variables, x-ray binaries, and quasars.

In this talk, I explore how strongly magnetised disks — where azimuthal magnetic fields dominate, with energies exceeding the plasma’s thermal energy — may help resolve these issues. Interest in this regime is motivated by recent “hyper-refined” cosmological simulations, in which such a disk forms self-consistently around a black hole and supports super-Eddington accretion rates. Using local shearing-box simulations, we identify two distinct turbulent states: the previously known “high-β” state with modest accretion stresses (α << 1) and weak magnetic fields, and a new “low-β” state with strong, self-sustaining azimuthal magnetic fields, supersonic turbulence, and rapid accretion (α ≈ 1). The transition between these states is abrupt and occurs when sufficiently strong azimuthal fields are present, allowing the system to sustain a Parker-instability-driven dynamo. While many aspects of this behaviour remain uncertain, it offers a promising pathway to reconcile simulations and observations, with interesting implications for quasars and other rapidly accreting systems.

• Speaker: Jonathan Squire [Otago, New Zealand]
• Thursday 05 June 2025, 12:00-13:00
• Venue: MR12 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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Prebiotic Chemistry, Exoplanets and Stellar Flaring

Nitroprusside is an important prebiotic molecule, thought to contribute to reaction pathways that lead to the production of amino acid chains (Mariani et al. [2018]). Nitroprusside can be made from Ferrocyanide photochemically. It has been found that the timescales for this reaction on Early Earth would have been between an order of days to months , making this route of abiotic production very useful in further prebiotic reaction networks and an important factor to consider when discussing the viability of life to evolve on a planet (Rimmer et al. [2021]). Here we investigate this reaction with a focus on constant and time varied radiation, meaning experimental runs involving the sample being subjected to a constant flux of UV light and runs with UV flux changing over time. FlareLab makes use of a broad band UV-Vis Laser Driven Light Source (LDLS), to experimentally simulate stellar irradiation and stellar flaring activity. The reasoning behind investigating flares is based on recent findings that have shown that M-dwarves are prone to flaring (G¨unther et al. [2020]). Flaring for M-dwarves is also shown to be the best way to get enough UV to an exoplanet’s surface for good yield of photochemical products (Ranjan et al. [2017]). With M-dwarves seen as the best stars to look at to detect small rocky planets, it is important to consider how flaring could effect the production of Nitroprusside and if there’s a discrepancy between assuming a constant irradiation of the surface or taking into account flaring.

We show that FlareLab can be used as a means of detecting the production of Nitroprusside in-situ during the irradiation period. We also compare the constant flux and variable flux regimes, and discuss the implications of these findings.

• Speaker: Lukas Rossmanith
• Tuesday 03 June 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: David Buscher.

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Title TBC

Abstract TBC

• Speaker: Prof. Natasha Hurley-Walker (Curtin University)
• Thursday 12 June 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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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.

• Speaker: Annie Cheng / Stanford University
• Wednesday 04 June 2025, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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arXiv:2502.03085v2 Announce Type: replace Abstract: The shape of the Ly-$\alpha$ transmission in the near zone of the redshift $z=5.9896$ quasar ULAS J0148$+$0600 (hereafter J0148) is consistent with a damping wing arising from an extended neutral hydrogen island in the diffuse intergalactic medium (IGM). Here we use simulations of late-ending reionisation from Sherwood-Relics to assess the expected incidence of quasars with Ly-$\alpha$ and Ly-$\beta$ absorption similar to the observed J0148 spectrum. We find a late end to reionisation at $z=5.3$ is a necessary requirement for reproducing a Ly-$\alpha$ damping wing consistent with J0148. This occurs in $\sim3$ per cent of our simulated spectra for an IGM neutral fraction $\langle x_{\rm HI}\rangle=0.14$ at $z=6$. However, using standard assumptions for the ionising photon output of J0148, the a priori probability of drawing a simulated quasar spectrum with a Ly-$\alpha$ damping wing profile and Ly-$\alpha$ near zone size that simultaneously match J0148 is very low, $p<10^{-3}$. We speculate this is because the ionising emission from J0148 is variable on timescales $t<10^{5}\rm\,yr$, or alternatively that the Ly-$\alpha$ transmission in the J0148 near zone is impacted by the transverse proximity effect from nearby star-forming galaxies or undetected quasars. We also predict the IGM temperature should be $T\sim 4\times 10^{4}\rm\,K$ within a few proper Mpc of the Ly-$\alpha$ near zone edge due to recent HI and HeII photo-heating. Evidence for enhanced thermal broadening in the Ly-$\alpha$ absorption near the damping wing edge would provide further evidence that the final stages of reionisation are occurring at $z<6$.

Gravitational Wave Signatures of Dark Matter in Neutron Star Mergers

Binary neutron star mergers provide insights into strong-field gravity and the properties of ultra-dense nuclear matter. These events offer the potential to search for signatures of physics beyond the standard model, including dark matter. We present the first numerical-relativity simulations of binary neutron star mergers admixed with dark matter, based on constraint-solved initial data. Modeling dark matter as a non-interacting fermionic gas, we investigate the impact of varying dark matter fractions and particle masses on the merger dynamics, ejecta mass, post-merger remnant properties, and the emitted gravitational waves. Our simulations suggest that the dark matter morphology – a dense core or a diluted halo – may alter the merger outcome. Scenarios with a dark matter core tend to exhibit a higher probability of prompt collapse, while those with a dark matter halo develop a common envelope, embedding the whole binary. Furthermore, gravitational wave signals from mergers with dark matter halo configurations exhibit significant deviations from standard models when the tidal deformability is calculated in a two-fluid framework neglecting the dilute and extended nature of the halo. This highlights the need for refined models in calculating the tidal deformability when considering mergers with extended dark matter structures. These initial results provide a basis for further exploration of dark matter’s role in binary neutron star mergers and their associated gravitational wave emission and can serve as a benchmark for future observations from advanced detectors and multi-messenger astrophysics.

• Speaker: Violetta Sagun, University of Southampton
• Friday 30 May 2025, 13:00-14:00
• Venue: MR9/Zoom https://cam-ac-uk.zoom.us/j/87235967698.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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Rapid accretion and state changes in strongly magnetised disks

Accretion disks power many of the universe’s most luminous phenomena, acting as intermediaries that enable matter to shed angular momentum and accrete onto stars or compact objects. While angular momentum transport in disks has been extensively studied, especially in the context of magneto-rotational turbulence, significant challenges remain. These include reconciling simulation results with observed accretion rates and understanding state transitions in cataclysmic variables, x-ray binaries, and quasars.

In this talk, I explore how strongly magnetised disks — where azimuthal magnetic fields dominate, with energies exceeding the plasma’s thermal energy — may help resolve these issues. Interest in this regime is motivated by recent “hyper-refined” cosmological simulations, in which such a disk forms self-consistently around a black hole and supports super-Eddington accretion rates. Using local shearing-box simulations, we identify two distinct turbulent states: the previously known “high-β” state with modest accretion stresses (α << 1) and weak magnetic fields, and a new “low-β” state with strong, self-sustaining azimuthal magnetic fields, supersonic turbulence, and rapid accretion (α ≈ 1). The transition between these states is abrupt and occurs when sufficiently strong azimuthal fields are present, allowing the system to sustain a Parker-instability-driven dynamo. While many aspects of this behaviour remain uncertain, it offers a promising pathway to reconcile simulations and observations, with interesting implications for quasars and other rapidly accreting systems.

• Speaker: Jonathan Squire [Otago, New Zealand]
• Thursday 05 June 2025, 14:00-15:00
• Venue: MR12 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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Title to be confirmed

Abstract not available

• Speaker: Irene Shivaei (CAB, Madrid)
• Friday 18 July 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Free floating planets and their possible origins

In recent years, free floating planets, i.e. those planets not found to be in a planetary system and with no observable companions, have begun to be found in microlensing and direct imaging surveys. Observations have shown that they have a wide variety of masses, ranging from terrestrial-like to giant planets. Microlensing surveys predict that there could be on order tens of free floating planets per star in the Milky Way. How these planets form and arrive on their observed trajectories remains a very open and intriguing question.

Whilst there are many mechanisms for forming free floating planets, e.g. ejections from planet-planet interactions or gravitational collapse of gas within molecular clouds, very few models have predicted the properties of free floating planets on a global scale. In this talk I will present the outcomes of state-of-the-art circumbinary planet formation models, that naturally produce a large abundance free floating planets per system. I will show the resulting mass and velocity distributions arising from the models, which will then be extended to include stellar populations of both single and binary stars, taking into binary fractions, and separations. The population distributions show clear observable features that can be investigated by future missions such as Roman, where evidence of these features will directly point to the specific formation pathways of specific planets, as well as informing on the processes of the planet forming environment in which they originated.

• Speaker: Gavin Coleman [Queen Mary University London]
• Monday 16 June 2025, 14:00-15:00
• Venue: Venue to be confirmed.
• Series: DAMTP Astrophysics Seminars; organiser: Thomas Jannaud.

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Title to be confirmed

Abstract not available

• Speaker: Jonathan Squire [Otago, New Zealand]
• Thursday 05 June 2025, 14:00-15:00
• Venue: MR12 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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arXiv:2505.18258v1 Announce Type: new Abstract: The impact of feedback from galaxy formation on cosmological probes is typically quantified in terms of the suppression of the matter power spectrum in hydrodynamical compared to gravity-only simulations. In this paper, we instead study how baryonic feedback impacts halo assembly histories and thereby imprints on cosmological observables. We investigate the sensitivity of the thermal Sunyaev-Zel'dovich effect (tSZ) power spectrum, X-ray number counts, weak lensing and kinetic Sunyaev-Zel'dovich (kSZ) stacked profiles to halo populations as a function of mass and redshift. We then study the imprint of different feedback implementations in the FLAMINGO suite of cosmological simulations on the assembly histories of these halo populations, as a function of radial scale. We find that kSZ profiles target lower-mass halos ($M_{\rm 200m}\sim 10^{13.1}\,\mathrm{M}_\odot$) compared to all other probes considered ($M_{200\mathrm{m}}\sim 10^{15}\,\mathrm{M}_\odot$). Feedback is inefficient in high-mass clusters with $\sim 10^{15} \, \mathrm{M}_\odot$ at $z=0$, but was more efficient at earlier times in the same population, with a $\sim 5$-$10\%$ effect on mass at $22$). These findings are tied together by noting that, regardless of redshift, feedback most efficiently redistributes baryons when halos reach a mass of $M_{\rm 200m} \simeq {10^{12.8}}\,\mathrm{M}_{\odot}$ and ceases to have any significant effect by the time $M_{\rm 200m} \simeq {10^{15}}\,\mathrm{M}_{\odot}$. We put forward strategies for minimizing sensitivity of lensing analyses to baryonic feedback, and for exploring baryonic resolutions to the unexpectedly low tSZ power in cosmic microwave background observations.

arXiv:2411.14846v2 Announce Type: replace Abstract: Evidence has emerged for a stochastic signal correlated among 67 pulsars within the 15-year pulsar-timing data set compiled by the NANOGrav collaboration. Similar signals have been found in data from the European, Indian, Parkes, and Chinese PTAs. This signal has been interpreted as indicative of the presence of a nanohertz stochastic gravitational wave background. To explore the internal consistency of this result we investigate how the recovered signal strength changes as we remove the pulsars one by one from the data set. We calculate the signal strength using the (noise-marginalized) optimal statistic, a frequentist metric designed to measure correlated excess power in the residuals of the arrival times of the radio pulses. We identify several features emerging from this analysis that were initially unexpected. The significance of these features, however, can only be assessed by comparing the real data to synthetic data sets. After conducting identical analyses on simulated data sets, we do not find anything inconsistent with the presence of a stochastic gravitational wave background in the NANOGrav 15-year data. The methodologies developed here can offer additional tools for application to future, more sensitive data sets. While this analysis provides an internal consistency check of the NANOGrav results, it does not eliminate the necessity for additional investigations that could identify potential systematics or uncover unmodeled physical phenomena in the data.

arXiv:2410.14404v2 Announce Type: replace Abstract: According to CMB measurements, baryonic matter constitutes about $5\%$ of the mass-energy density of the universe. A significant population of these baryons, for a long time referred to as `missing', resides in a low density, warm-hot intergalactic medium (WHIM) outside galaxy clusters, tracing the ``cosmic web'', a network of large scale dark matter filaments. Various studies have detected this inter-cluster gas, both by stacking and by observing individual filaments in compact, massive systems. In this paper, we study short filaments (< 10 Mpc) connecting massive clusters ($M_{500} \approx 3\times 10^{14} M_{\odot}$) detected by the Atacama Cosmology Telescope (ACT) using the scattering of CMB light off the ionised gas, a phenomenon known as the thermal Sunyaev-Zeldovich (tSZ) effect. The first part of this work is a search for suitable candidates for high resolution follow-up tSZ observations. We identify four cluster pairs with an intercluster signal above the noise floor (S/N $>$ 2), including two with a tentative $>2\sigma$ statistical significance for an intercluster bridge from the ACT data alone. In the second part of this work, starting from the same cluster sample, we directly stack on ${\sim}100$ cluster pairs and observe an excess SZ signal between the stacked clusters of $y=(7.2^{+2.3}_{-2.5})\times 10^{-7}$ with a significance of $3.3\sigma$. It is the first tSZ measurement of hot gas between clusters in this range of masses at moderate redshift ($\langle z\rangle\approx 0.5$). We compare this to the signal from simulated cluster pairs with similar redshifts and separations in the THE300 and MAGNETICUM Pathfinder cosmological simulations and find broad consistency. Additionally, we show that our measurement is consistent with scaling relations between filament parameters and mass of the embedded halos identified in simulations.

arXiv:2505.17188v1 Announce Type: new Abstract: Ly$\alpha$ emitters (LAEs) have now been discovered out to redshift $z=13$, and are valuable probes of the reionization history at redshifts beyond the reach of other currently available tracers. Most inferences of the neutral hydrogen fraction from LAE observations rely on one-point and two-point statistics like the luminosity function and the power spectrum. We present here an analysis of the bispectrum of high-redshift LAEs and demonstrate its sensitivity to the Epoch of Reionization. We use the Sherwood-Relics suite of cosmological hydrodynamical simulations post-processed with the GPU-based radiative transfer code ATON-HE to generate realistic LAE mock catalogues for a wide range of reionization models, varying the ionization history and the source populations, including contributions of AGN to hydrogen reionization. We demonstrate that the bispectrum provides greater sensitivity than the power spectrum to both the timing and spatial morphology of reionization. Using reduced-$\chi^2$ analysis we further show that the bispectrum also responds more strongly to variations in source population and AGN contribution, apparently more efficiently capturing morphological features missed by two-point statistics. The redshift evolution of the bispectrum reflects the increased clustering of ionizing sources at earlier epochs. The sensitivity of the bispectrum to peculiar velocities underscores the importance of velocity corrections in comparisons to observations. Our findings demonstrate that the LAE bispectrum is a powerful higher-order statistic for probing reionization through current and future LAE surveys using telescopes such as Subaru and JWST.