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arXiv:2309.15984v2 Announce Type: replace Abstract: This work compares cosmological matching conditions used in approximating generic pre-inflationary phases of the universe. We show that the joining conditions for primordial scalar perturbations assumed by Contaldi et al. are inconsistent with the physically motivated Israel junction conditions, however; performing general relativistic matching with the aforementioned constraints results in unrealistic primordial power spectra. Eliminating the need for ambiguous matching, we look at an alternative semi-analytic model for producing the primordial power spectrum allowing for finite duration cosmological phase transitions.

arXiv:2308.05795v2 Announce Type: replace Abstract: Large-scale outflows driven by supermassive black holes are thought to play a fundamental role in suppressing star formation in massive galaxies. However, direct observational evidence for this hypothesis is still lacking, particularly in the young universe where star formation quenching is remarkably rapid, thus requiring effective removal of gas as opposed to slow gas heating. While outflows of ionized gas are commonly detected in massive distant galaxies, the amount of ejected mass is too small to be able to suppress star formation. Gas ejection is expected to be more efficient in the neutral and molecular phases, but at high redshift these have only been observed in starbursts and quasars. Here we report JWST spectroscopy of a massive galaxy experiencing rapid quenching at redshift z=2.445. We detect a weak outflow of ionized gas and a powerful outflow of neutral gas, with a mass outflow rate that is sufficient to quench the star formation. Neither X-ray or radio activity are detected; however, the presence of a supermassive black hole is suggested by the properties of the ionized gas emission lines. We thus conclude that supermassive black holes are able to rapidly suppress star formation in massive galaxies by efficiently ejecting neutral gas.

arXiv:2405.07903v1 Announce Type: new Abstract: We present a coherent, re-usable python framework which further builds on the cosmological emulator code CosmoPower. In the current era of high-precision cosmology, we require high-accuracy calculations of cosmological observables with Einstein-Boltzmann codes. For detailed statistical analyses, such codes often incur high costs in terms of computing power, making parameter space exploration costly, especially for beyond-$\Lambda$CDM analyses. Machine learning-enabled emulators of Einstein-Boltzmann codes have emerged as a solution to this problem and have become a common way to perform fast cosmological analyses. To enable generation, sharing and use of emulators for inference, we define standards for robustly describing, packaging and distributing them, and present software for easily performing these tasks in an automated and replicable manner. We provide examples and guidelines for generating your own sufficiently accurate emulators and wrappers for using them in popular cosmological inference codes. We demonstrate our framework by presenting a suite of high-accuracy emulators for the CAMB code's calculations of CMB $C_\ell$, $P(k)$, background evolution, and derived parameter quantities. We show that these emulators are accurate enough for both $\Lambda$CDM analysis and a set of single- and two-parameter extension models (including $N_{\rm eff}$, $\sum m_{\nu}$ and $w_0 w_a$ cosmologies) with stage-IV observatories, recovering the original high-accuracy Einstein-Boltzmann spectra to tolerances well within the cosmic variance uncertainties across the full range of parameters considered. We also use our emulators to recover cosmological parameters in a simulated cosmic-variance limited experiment, finding results well within $0.1 \sigma$ of the input cosmology, while requiring typically $\lesssim1/50$ of the evaluation time than for the full Einstein-Boltzmann computation.

Title to be confirmed

Abstract not available

• Speaker: Georges Maynet (Geneva)
• Thursday 13 June 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

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arXiv:2309.05659v2 Announce Type: replace Abstract: We present tomographic measurements of structure growth using cross-correlations of Atacama Cosmology Telescope (ACT) DR6 and Planck CMB lensing maps with the unWISE Blue and Green galaxy samples, which span the redshift ranges $0.2 \lesssim z \lesssim 1.1$ and $0.3 \lesssim z \lesssim 1.8$, respectively. We improve on prior unWISE cross-correlations not just by making use of the new, high-precision ACT DR6 lensing maps, but also by including additional spectroscopic data for redshift calibration and by analysing our measurements with a more flexible theoretical model. An extensive suite of systematic and null tests within a blind analysis framework ensures that our results are robust. We determine the amplitude of matter fluctuations at low redshifts ($z\simeq 0.2-1.6$), finding $S_8 \equiv \sigma_8 (\Omega_m / 0.3)^{0.5} = 0.813 \pm 0.021$ using the ACT cross-correlation alone and $S_8 = 0.810 \pm 0.015$ with a combination of Planck and ACT cross-correlations; these measurements are fully consistent with the predictions from primary CMB measurements assuming standard structure growth. The addition of Baryon Acoustic Oscillation data breaks the degeneracy between $\sigma_8$ and $\Omega_m$, allowing us to measure $\sigma_8 = 0.813 \pm 0.020$ from the cross-correlation of unWISE with ACT and $\sigma_8 = 0.813\pm 0.015$ from the combination of cross-correlations with ACT and Planck. These results also agree with the expectations from primary CMB extrapolations in $\Lambda$CDM cosmology; the consistency of $\sigma_8$ derived from our two redshift samples at $z \sim 0.6$ and $1.1$ provides a further check of our cosmological model. Our results suggest that structure formation on linear scales is well described by $\Lambda$CDM even down to low redshifts $z\lesssim 1$.

arXiv:2405.06013v1 Announce Type: new Abstract: Type Ia supernovae (SNe Ia) are standarizable candles whose observed light curves can be used to infer their distances, which can in turn be used in cosmological analyses. As the quantity of observed SNe Ia grows with current and upcoming surveys, increasingly scalable analyses are necessary to take full advantage of these new datasets for precise estimation of cosmological parameters. Bayesian inference methods enable fitting SN Ia light curves with robust uncertainty quantification, but traditional posterior sampling using Markov Chain Monte Carlo (MCMC) is computationally expensive. We present an implementation of variational inference (VI) to accelerate the fitting of SN Ia light curves using the BayeSN hierarchical Bayesian model for time-varying SN Ia spectral energy distributions (SEDs). We demonstrate and evaluate its performance on both simulated light curves and data from the Foundation Supernova Survey with two different forms of surrogate posterior -- a multivariate normal and a custom multivariate zero-lower-truncated normal distribution -- and compare them with the Laplace Approximation and full MCMC analysis. To validate of our variational approximation, we calculate the pareto-smoothed importance sampling (PSIS) diagnostic, and perform variational simulation-based calibration (VSBC). The VI approximation achieves similar results to MCMC but with an order-of-magnitude speedup for the inference of the photometric distance moduli. Overall, we show that VI is a promising method for scalable parameter inference that enables analysis of larger datasets for precision cosmology.

Title to be confirmed

Abstract not available

• Speaker: Suhail Dhawan (KICC)
• Friday 12 July 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Title to be confirmed

Abstract not available

• Speaker: Mark Krumholz (ANU)
• Friday 06 September 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Prototyping a Sparse-Aperture, Segmented, Parabolic Primary Mirror Telescope for SUPERSHARP

The motivation for my research comes from the SUPERSHARP mission concept for large, unfolding, lightweight space telescopes which take advantage of unfolding segmented optics and a sparse aperture primary mirror to generate powerful observations while maintaining limited cost, mass, and volume requirements. The original motivation for the SUPERSHARP design comes from the ongoing search for life in the universe, but the technology has wider applications in both space and Earth observation. Prototyping of the optical system is integral to ensuring technological readiness of key aspects of the telescope design – in particular, the active control and maintenance of optics alignment. In this talk, I will present the work I have done designing and building a prototype of a sparse-aperture, segmented, parabolic primary mirror telescope using two mirror segments. I will also outline the immediate improvements and next steps required for the prototype to more accurately model an effective imaging system.

• Speaker: Zoe Horvath
• Wednesday 15 May 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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arXiv:2405.03744v2 Announce Type: replace Abstract: Feedback is the key physical mechanism regulating galaxy formation. Stars in galaxies form when baryons radiatively cool down and fall into gravitational wells. Eventually, star formation quenches as gas is depleted and/or perturbed by feedback processes, no longer being able to collapse and condense. For massive galaxies, astronomers identify feedback from accreting supermassive black holes (active galactic nuclei, AGN) as the main agent responsible for quenching. We report the first spatially resolved spectroscopic observations of a massive, completely quiescent galaxy at $z=3.714$ (Jekyll) and its neighborhood. Jekyll is part of a galaxy pair with a compact, dusty, massive star-forming companion (Hyde). We find large amounts of ionized and neutral gas in the intergalactic medium around the pair, yet Jekyll has remained quiescent for more than 500~Myr. The emitting gas is consistent with AGN photoionization, but no AGN is observed in Jekyll. We find that, in contrast to standard scenarios, AGN in satellite galaxies can be critical contributors for keeping massive galaxies quiescent in the early Universe. After the accelerated formation and quenching of the massive central galaxy, tidally induced gas stripping additionally contributes to the star-formation regulation on subsequent satellite galaxy generations.

arXiv:2405.05772v1 Announce Type: new Abstract: We report the detection of continuum excess in the rest-frame UV between 3000 {\AA} and 3550 {\AA} in the JWST/NIRSpec spectrum of GN-z11, a galaxy hosting an active galactic nucleus (AGN) at z = 10.603. The shape of the continuum excess resembles a Balmer continuum but has a break around 3546 {\AA} in the rest frame, which is 100 {\AA} bluewards to the Balmer limit at 3646 {\AA}. A Balmer continuum model alone cannot fit the spectrum, implying a different origin for the continuum excess. The absence of the Balmer jump indicates an electron temperature of $\sim 3\times 10^4$ K, which is significantly higher than the temperature of $T_{e}({\rm O^{2+}}) \approx 1.3\times 10^{4}$ K inferred from [OIII]$\lambda 4363$. The temperature difference must result from mixing of different ionized regions: the Balmer emission mainly arises from dense and hot clouds in the Broad Line Region, close to the accreting black hole, whereas the forbidden lines originate from less dense and colder gas in the host galaxy (although these ionized regions are kinematically similar in GN-z11 due to its small BH mass). We propose a potential explanation for the observed continuum excess to come from a complex of FeII emission, which shows a characteristic jump bluewards to the Balmer limit as previously seen in the spectra of many lower-redshift quasars. Through comparisons with Cloudy models, we show an Fe abundance or an overall metallicity above $\sim 1/3$ solar is likely needed. Besides the FeII emission, part of the small blue bump might also be associated with an OIII Bowen fluorescent line, a line often enhanced in dense AGN-ionized gas. Finally, the spectrum provides further evidence against Wolf-Rayet or massive stars dominating the nebular emission in GN-z11.

arXiv:2405.05598v1 Announce Type: new Abstract: Optimal extraction of cosmological information from observations of the Cosmic Microwave Back- ground critically relies on our ability to accurately undo the distortions caused by weak gravitational lensing. In this work, we demonstrate the use of denoising diffusion models in performing Bayesian lensing reconstruction. We show that score-based generative models can produce accurate, uncor- related samples from the CMB lensing convergence map posterior, given noisy CMB observations. To validate our approach, we compare the samples of our model to those obtained using established Hamiltonian Monte Carlo methods, which assume a Gaussian lensing potential. We then go beyond this assumption of Gaussianity, and train and validate our model on non-Gaussian lensing data, obtained by ray-tracing N-body simulations. We demonstrate that in this case, samples from our model have accurate non-Gaussian statistics beyond the power spectrum. The method provides an avenue towards more efficient and accurate lensing reconstruction, that does not rely on an approx- imate analytic description of the posterior probability. The reconstructed lensing maps can be used as an unbiased tracer of the matter distribution, and to improve delensing of the CMB, resulting in more precise cosmological parameter inference.

Non-smooth horizons in Kerr black hole mergers

Dynamical black holes are known to develop non-smooth structures on their horizon. We begin by reviewing a classification of all generic non-smooth structures that may appear on black hole horizons in four-dimensional spacetimes. Introducing a time function, we describe how two of these features – namely creases and caustics – evolve, and in particular discuss processes known as ‘perestroikas’, where the non-smooth structure on a horizon cross-section changes qualitatively. We then study the merger of two Kerr black holes in the extreme mass ratio limit, and focus on the creases and caustics that are present on the horizon. We explain how our results differ from an older analysis of the same system by Emparan et al., and show that these novel results are consistent with the properties of creases expected generically. This talk is based on work done with Harvey Reall and Robie Hennigar.

• Speaker: Maxime Gadioux, DAMTP, University of Cambridge
• Friday 17 May 2024, 13:00-14:00
• Venue: Potter room/Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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For decades, cosmologists have been fighting over the Hubble constant, a number that represents the expansion rate of the universe – it may have finally been pinned down

Surprising recent measurements hint that the universe isn’t expanding in the way we had thought, and it could be explained by still-theoretical dark radiation

Attractor reconstruction of active stellar light curves

Stellar activity is notoriously difficult to model, being neither periodic nor purely stochastic. In light curves, the interplay between the stellar rotation period and the birth and death of spots and faculae gives rise to quasi-periodic modulation over time scales of hours to weeks. Despite the complexity of this interplay, the resulting light curves bear strong qualitative resemblance to systems known to exhibit low-dimensional dynamical chaos, such as the Rössler attractor.

In the 1980s and 1990s, a suite of techniques for nonlinear dynamical analysis, called attractor reconstruction, evolved to study exactly this type of system. Attractor reconstruction works by embedding a 1-dimensional time series, such as stellar light curve, in a higher-dimensional phase space capable of capturing its full dynamical behavior: too low a dimensionality, and the system’s trajectory will self-intersect and tangle, which we know to be physically unrealistic given the non-periodicity of the observed signal. This technique has been used successfully to model the historical sunspot record and the light curves of variable stars (both simulated and observed) and to recover important features of their underlying dynamics, including their dimensionality and the time scales over which they can be meaningfully forecast into the future. Here, I discuss the application of attractor reconstruction to the light curve of the Sun over Solar cycles 23-25, as observed by the Solar and Heliospheric Observatory.

• Speaker: Emily Sandford (Cavendish)
• Tuesday 14 May 2024, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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Convection, waves and mixing in stars: insights and challenges from numerical simulations

The lifecycle of stars is profoundly shaped by the internal transport and mixing of chemical elements. Within most stars, regions of convective motions and stable stratification coexist, separated by so-called convective boundaries. While convective regions are very efficient at mixing chemical species, stably stratified regions mostly inhibit the vertical transport of elements. Current models suggest that mixing beyond convective regions is needed to explain observed stellar properties. Waves, excited by convection at convective boundaries, could play a crucial role by providing additional mixing in the vertical direction.

In this talk, I will highlight results from recent numerical simulations with the MUSIC code on the study of waves, convection, and mixing in stars. I will discuss challenges in measuring vertical mixing and transport by waves in numerical simulations, and present some prospects for improving our understanding of mixing mechanisms through numerical experiments.

• Speaker: Thomas Guillet (Exeter)
• Monday 13 May 2024, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.

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arXiv:2405.04598v1 Announce Type: new Abstract: The complex physics governing nebular emission in galaxies, particularly in the early universe, often defy simple low-dimensional models. This has proven to be a significant barrier in understanding the (often diverse) ionizing sources powering this emission. We present Cue, a highly flexible tool for interpreting nebular emission across a wide range of abundances and ionizing conditions of galaxies at different redshifts. Unlike typical nebular models used to interpret extragalactic nebular emission, our model does not require a specific ionizing spectrum as a source, instead approximating the ionizing spectrum with a 4-part piece-wise power-law. We train a neural net emulator based on the CLOUDY photoionization modeling code and make self-consistent nebular continuum and line emission predictions. Along with the flexible ionizing spectra, we allow freedom in [O/H], [N/O], [C/O], gas density, and total ionizing photon budget. This flexibility allows us to either marginalize over or directly measure the incident ionizing radiation, thereby directly interrogating the source of the ionizing photons in distant galaxies via their nebular emission. Our emulator demonstrates a high accuracy, with $\sim$1% uncertainty in predicting the nebular continuum and $\sim$5% uncertainty in the emission lines. Mock tests suggest Cue is well-calibrated and produces useful constraints on the ionizing spectra when $S/N (\mathrm{H}_\alpha) \gtrsim 10$, and furthermore capable of distinguishing between the ionizing spectra predicted by single and binary stellar models. The compute efficiency of neural networks facilitates future applications of Cue for rapid modeling of the nebular emission in large samples and Monte Carlo sampling techniques.

Title to be confirmed

Abstract not available

• Speaker: Sergio Martin Alvarez (Stanford)
• Friday 14 June 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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arXiv:2405.03744v1 Announce Type: new Abstract: Feedback is the key physical mechanism regulating galaxy formation. Stars in galaxies form when baryons radiatively cool down and fall into gravitational wells. Eventually, star formation quenches as gas is depleted and/or perturbed by feedback processes, no longer being able to collapse and condense. For massive galaxies, astronomers identify feedback from accreting supermassive black holes (active galactic nuclei, AGN) as the main agent responsible for quenching. We report the first spatially resolved spectroscopic observations of a massive, completely quiescent galaxy at $z=3.714$ (Jekyll) and its neighborhood. Jekyll is part of a galaxy pair with a compact, dusty, massive star-forming companion (Hyde). We find large amounts of ionized and neutral gas in the intergalactic medium around the pair, yet Jekyll has remained quiescent for more than 500~Myr. The emitting gas is consistent with AGN photoionization, but no AGN is observed in Jekyll. We find that, in contrast to standard scenarios, AGN in satellite galaxies can be critical contributors for keeping massive galaxies quiescent in the early Universe. After the accelerated formation and quenching of the massive central galaxy, tidally induced gas stripping additionally contributes to the star-formation regulation on subsequent satellite galaxy generations.