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arXiv:2407.08026v2 Announce Type: replace Abstract: Highly-multiplexed, robotic, fiber-fed spectroscopic surveys are observing tens of millions of stars and galaxies. For many systems, accurate positioning relies on imaging the fibers in the focal plane and feeding that information back to the robotic positioners to correct their positions. Inhomogeneities and turbulence in the air between the focal plane and the imaging camera can affect the measured positions of fibers, limiting the accuracy with which fibers can be placed on targets. For the Dark Energy Spectroscopic Instrument, we dramatically reduced the effect of turbulence on measurements of positioner locations in the focal plane by taking advantage of stationary positioners and the correlation function of the turbulence. We were able to reduce positioning errors from 7.3 microns to 3.5 microns, speeding the survey by 1.6% under typical conditions.

The importance of being wide: The key role of wide binaries, from GW sources to Pluto

Dense stellar clusters are known to be hotbeds of collisional stellar dynamics, driving a diverse array of phenomena, from gravitational-wave sources and X-ray binaries to blue stragglers. While the Galactic field is typically considered too sparse for such interactions, this view overlooks the significant role of ultra-wide binaries and triples (separations > 1000 AU). These systems possess large cross-sections for encounters, leading to surprisingly high interaction rates even in low-density environments. Cumulative effects from repeated flybys can trigger strong interactions between system components, effectively enabling collisional dynamics and associated phenomena within the field.

Shifting focus to smaller scales, wide binary Kuiper Belt objects can also undergo mergers and collisions. However, instead of direct collisions, these events are driven by secular evolution within hierarchical triple systems. This mechanism offers a compelling explanation for the formation of objects like Arrokoth and the Pluto-Charon system.

• Speaker: Hagai Perets
• Thursday 20 February 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Mor Rozner.

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Unimodular JT gravity and de Sitter quantum cosmology

In this talk, I will show how a gauge-theoretic approach to Jackiw–Teitelboim (JT) gravity naturally yields a two-dimensional Henneaux–Teitelboim (HT) unimodular theory, applicable to both flat and curved spacetimes. Under a mini-superspace reduction, the Wheeler–DeWitt equation becomes a Schrödinger-like equation admitting a consistent, unitary quantum description. The resulting wavefunction describes a quantum distribution for the scale factor, illuminating cosmic expansion and contraction, and allowing topology change at a=0.

• Speaker: Farbod Rassouli, University of Nottingham
• Friday 07 February 2025, 13:00-14:00
• Venue: Potter room / https://cam-ac-uk.zoom.us/j/87235967698.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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Title to be confirmed

Abstract not available

• Speaker: Piyush Sharda (Leiden)
• Friday 04 April 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Exclusive: Cern’s next director general Mark Thomson says AI is paving the way for huge advances in particle physics

Advanced artificial intelligence is to revolutionise fundamental physics and could open a window on to the fate of the universe, according to Cern’s next director general.

Prof Mark Thomson, the British physicist who will assume leadership of Cern on 1 January 2026, says machine learning is paving the way for advances in particle physics that promise to be comparable to the AI-powered prediction of protein structures that earned Google DeepMind scientists a Nobel prize in October.

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Towards Habitable Worlds: Detailed Characterization of the Nearest Planetary Systems

Over the past thirty years, astronomers have made extraordinary progress in detecting planets around other stars. We now know that stars commonly host planets with a wider range of physical properties and system architectures than exist in our own solar system, and that planets likely outnumber stars in our galactic neighborhood. Now, planet detection and characterization technologies have advanced to the point that it should be possible to search for signs of life in the atmospheres of Earth-like exoplanets around Sun-like stars within a few decades. These observations will give us our first glimpse at how common—or rare—life is in the universe. However, before we can carry out these observations and understand the implications for the abundance of life outside the Solar system, we must first find the nearest habitable planets to observe, learn their detailed properties, and refine our understanding of habitability. In this talk, I will describe my group’s work to fill in these knowledge gaps by developing new tools and methods to detect and characterize exoplanets. First, I will show how cutting-edge machine learning methods could help reveal the closest potentially habitable planets to Earth—ideal for biosignature searches in the 2040s. Next, I will show how we can learn about extrasolar geochemistry by studying planetary accretion onto white dwarf stars—allowing us to see whether geological processes important for habitability on Earth take place in other systems. And finally, I will describe our work to understand what happens to planets when stars run out of nuclear fuel and find out whether life can continue in a system after the host star’s death.

• Speaker: Prof. Andrew Vanderburg, Massachusetts Institute of Technology
• Wednesday 12 February 2025, 10:00-11:00
• Venue: Hoyle Committee Room - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Mariona Badenas Agusti.

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Towards Habitable Worlds: Detailed Characterization of the Nearest Planetary Systems

Over the past thirty years, astronomers have made extraordinary progress in detecting planets around other stars. We now know that stars commonly host planets with a wider range of physical properties and system architectures than exist in our own solar system, and that planets likely outnumber stars in our galactic neighborhood. Now, planet detection and characterization technologies have advanced to the point that it should be possible to search for signs of life in the atmospheres of Earth-like exoplanets around Sun-like stars within a few decades. These observations will give us our first glimpse at how common—or rare—life is in the universe. However, before we can carry out these observations and understand the implications for the abundance of life outside the Solar system, we must first find the nearest habitable planets to observe, learn their detailed properties, and refine our understanding of habitability. In this talk, I will describe my group’s work to fill in these knowledge gaps by developing new tools and methods to detect and characterize exoplanets. First, I will show how cutting-edge machine learning methods could help reveal the closest potentially habitable planets to Earth—ideal for biosignature searches in the 2040s. Next, I will show how we can learn about extrasolar geochemistry by studying planetary accretion onto white dwarf stars—allowing us to see whether geological processes important for habitability on Earth take place in other systems. And finally, I will describe our work to understand what happens to planets when stars run out of nuclear fuel and find out whether life can continue in a system after the host star’s death.

• Speaker: Andrew Vanderburg, Massachusetts Institute of Technology
• Wednesday 12 February 2025, 10:00-11:00
• Venue: Hoyle Committee Room - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Mariona Badenas Agusti.

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Title to be confirmed

Abstract not available

• Speaker: William Baker (Dark)
• Friday 02 May 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Unimodular JT gravity and de Sitter quantum cosmology

In this talk, I will show how a gauge-theoretic approach to Jackiw–Teitelboim (JT) gravity naturally yields a two-dimensional Henneaux–Teitelboim (HT) unimodular theory, applicable to both flat and curved spacetimes. Under a mini-superspace reduction, the Wheeler–DeWitt equation becomes a Schrödinger-like equation admitting a consistent, unitary quantum description. The resulting wavefunction describes a quantum distribution for the scale factor, illuminating cosmic expansion and contraction, and allowing topology change at a=0.

• Speaker: Farbod Rassouli, University of Nottingham
• Friday 07 February 2025, 13:00-14:00
• Venue: Venue to be confirmed.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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What makes a planet “giant”?

The formation history of giant planets, both within and beyond our solar system, remains an open question. I propose a new pathway for giant planet formation, where runaway gas accretion begins only when a planet reaches about 100 Earth masses and occurs only after a few million years (Myrs). This suggests that the transition to a gas giant—defined as a planet primarily composed of hydrogen and helium—occurs at around Saturn’s mass. The delay in runaway gas accretion to later stages and higher planetary masses results from an intermediate phase of efficient heavy-element accretion. This process generates enough energy to hinder rapid gas accretion. Consequently, Saturn may never have undergone runaway gas accretion, classifying it as a “failed giant planet.” This proposed transition to a gas giant planet above Saturn’s mass naturally explains the distinct bulk metallicities and internal structures of Jupiter and Saturn, as well as the unique characteristics of Uranus and Neptune. In the context of giant exoplanets, postponing runaway gas accretion to planets exceeding Saturn’s mass explains the transitions in the mass-radius (M-R) relations of observed exoplanets, the relatively low occurrence of gas giants, and the high metallicity of intermediate-mass exoplanets.

• Speaker: Ravit Helled
• Thursday 27 February 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Matthew Grayling.

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arXiv:2501.17239v1 Announce Type: new Abstract: Galaxies form and evolve via a multitude of complex physics. In this work, we investigate the role of cosmic ray (CR) feedback in galaxy evolution and reionisation, by examining its impact on the escape of ionising radiation from galaxies. For this purpose, we present two Sphinx cosmological radiation-magneto-hydrodynamics simulations, allowing for the first time a study of the impact of CR feedback on thousands of resolved galaxies during the Epoch of Reionisation (EoR). The simulations differ in their feedback prescriptions: one adopts a calibrated strong supernova (SN) feedback, while the other simulation reduces the strength of SN feedback and includes CR feedback instead. We show that both comparably regulate star formation, reasonably match observations of high-redshift UV luminosity functions, and produce a similar amount of hydrogen ionising photons. In contrast to the model with strong SN feedback, the model with CRs lead to incomplete reionisation, which is in strong disagreement with observational estimates of the reionisation history. This is due to CR feedback shaping the ISM differently, filling with gas the low density cavities otherwise carved by SN explosions. As a result, this reduces the escape of ionising photons, at any halo mass, and primarily in the close vicinity of the stars. Our study indicates that CR feedback regulates galaxy growth during the EoR, but negatively affects reionisation, a tension which paves the way for further exploration and refinement of existing galaxy formation and feedback models. Such improvements are crucial in order to capture and understand the process of reionisation and the underlying evolution of galaxies through cosmic time.

Star-forming Galaxies at Cosmic Noon and Beyond

Lookback studies over the past two decades have assembled a fairly complete census of galaxies over 85% of cosmic time and established that the bulk of stars, which today reside in massive ellipticals and spirals, formed rapidy at redshift z~1-3, “cosmic noon”. An exciting new era has now begun, with dramatic advances driven by facilities such as JWST , the Very Large Telescope, ALMA and NOEMA . I will summarize the state-of-the-art and discuss recent key insights in our understanding of galaxy evolution at and beyond cosmic noon, with a focus on the assembly and transformations of galactic structure and kinematics. I will highlight emerging frontiers for science with the Extremely Large Telescope and other upcoming transformative capabilities towards the end of the decade.

• Speaker: Natascha M. Foerster Schreiber
• Thursday 06 February 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Jan Scholtz.

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arXiv:2501.16983v1 Announce Type: new Abstract: While cosmological simulations of galaxy formation have reached maturity, able to reproduce many fundamental galaxy and halo properties, no consensus has yet been reached on the impact of `baryonic feedback' on the non-linear matter power spectrum. This severely limits the precision of (and potentially biases) small-scale cosmological constraints obtained from weak lensing and galaxy surveys. Recent observational evidence indicates that `baryonic feedback' may be more extreme than commonly assumed in current cosmological hydrodynamical simulations. In this paper, we therefore explore a range of empirical AGN feedback models, within the FABLE simulation suite, with different parameterizations as a function of cosmic time, host halo properties, and/or spatial location where feedback energy is thermalized. We demonstrate that an AGN radio-mode feedback acting in a larger population of black holes, with jets thermalizing at relatively large cluster-centric distances, as exemplified by our XFABLE model, is in good agreement with the latest weak lensing + kSZ constraints across all k-scales. Furthermore, XFABLE maintains good agreement with the galaxy stellar mass function, gas fraction measurements, and all key galaxy group and cluster properties, including scaling relations and ICM radial profiles. Our work highlights the pressing need to model black hole accretion and feedback physics with a greater level of realism, including relativistic, magnetized jets in full cosmological simulations. Finally, we discuss how a range of complementary observational probes in the near future will enable us to constrain AGN feedback models, and therefore reduce `baryonic feedback' modelling uncertainty for the upcoming era of large cosmological surveys.

arXiv:2406.05060v4 Announce Type: replace Abstract: The JWST Advanced Deep Extragalactic Survey (JADES) is a multi-cycle JWST program that has taken among the deepest near-/mid-infrared images to date (down to $\sim$30 ABmag) over $\sim$25 arcmin$^2$ in the GOODS-S field in two sets of observations with one year of separation. This presented the first opportunity to systematically search for transients, mostly supernovae (SNe), out to $z$$>$2. We found 79 SNe: 38 at $z$$<$2, 23 at 2$<$$z$$<$3, 8 at 3$<$$z$$<$4, 7 at 4$<$$z$$<$5, and 3 with undetermined redshifts, where the redshifts are predominantly based on spectroscopic or highly reliable JADES photometric redshifts of the host galaxies. At this depth, the detection rate is $\sim$1-2 per arcmin$^2$ per year, demonstrating the power of JWST as a supernova discovery machine. We also conducted multi-band follow-up NIRCam observations of a subset of the SNe to better constrain their light curves and classify their types. Here, we present the survey, sample, search parameters, spectral energy distributions (SEDs), light curves, and classifications. Even at $z$$\geq$2, the NIRCam data quality is high enough to allow SN classification via multi-epoch light-curve fitting with confidence. The multi-epoch SN sample includes a Type Ia SN at $z_{\mathrm{spec}}$$=$2.90, Type IIP SN at $z_{\mathrm{spec}}$$=$3.61, and a Type Ic-BL SN at $z_{\mathrm{spec}}$$=$2.83. We also found that two $z$$\sim$16 galaxy candidates from the first imaging epoch were actually transients that faded in the second epoch, illustrating the possibility that moderate/high-redshift SNe could mimic high-redshift dropout galaxies.

arXiv:2501.16555v1 Announce Type: new Abstract: The 2-point correlation function of the galaxy spatial distribution is a major cosmological observable that enables constraints on the dynamics and geometry of the Universe. The Euclid mission aims at performing an extensive spectroscopic survey of approximately 20--30 million H$\alpha$-emitting galaxies up to about redshift two. This ambitious project seeks to elucidate the nature of dark energy by mapping the 3-dimensional clustering of galaxies over a significant portion of the sky. This paper presents the methodology and software developed for estimating the 3-dimensional 2-point correlation function within the Euclid Science Ground Segment. The software is designed to overcome the significant challenges posed by the large and complex Euclid data set, which involves millions of galaxies. Key challenges include efficient pair counting, managing computational resources, and ensuring the accuracy of the correlation function estimation. The software leverages advanced algorithms, including kd-tree, octree, and linked-list data partitioning strategies, to optimise the pair-counting process. The implementation also includes parallel processing capabilities using shared-memory open multi-processing to further enhance performance and reduce computation times. Extensive validation and performance testing of the software are presented. The results indicate that the software is robust and can reliably estimate the 2-point correlation function, which is essential for deriving cosmological parameters with high precision. Furthermore, the paper discusses the expected performance of the software during different stages of the Euclid Wide Survey observations and forecasts how the precision of the correlation function measurements will improve over the mission's timeline, highlighting the software's capability to handle large data sets efficiently.

Uncovering the stellar impact on planetary systems through population demographics

Host stars play a huge role in shaping the planetary systems they host, both through their evolution and through the influence they have over planet formation. While the exact processes through which stars influence their planetary systems remain unclear, these processes are all expected to imprint signatures on the overall population of planets that exist throughout the galaxy. The all sky coverage of the TESS mission provides enables us to study these populations and uncover the influence of the star. In this talk I will discuss how I use the TESS Full-Frame-Image light curves to measure the occurrence rates of different planets and planetary systems, and what these occurrence rates can teach us about the impact of the host star on planet formation and evolution. In particular I will present the results from studying two populations. The first is the population of giant planets around low-mass stars, through which we can probe an extreme of giant planet formation. The second is the population of giant planets around post-main sequence stars. By studying this population we can better understand the impact of the early stages of post-main sequence stellar evolution on close-in planets, including strong tidal interactions and rapid orbital decay.

• Speaker: Ed Bryant (UCL)
• Tuesday 04 February 2025, 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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Title to be confirmed

Abstract not available

• Speaker: Alkistis Pourtsidou (University of Edinburgh)
• Monday 10 March 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Louis Legrand.

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arXiv:2501.16311v1 Announce Type: new Abstract: The Time Domain Extragalactic Survey (TiDES) conducted on the 4-metre Multi-Object Spectroscopic Telescope (4MOST) will perform spectroscopic follow-up of extragalactic transients discovered in the era of the NSF-DOE Vera C. Rubin Observatory. TiDES will conduct a 5-year survey, covering ${>}14\,000\,\mathrm{square\, degrees}$, and use around 250 000 fibre hours to address three main science goals: (i) spectroscopic observations of ${>}30 000$ live transients, (ii) comprehensive follow-up of ${>}200 000$ host galaxies to obtain redshift measurements, and (iii) repeat spectroscopic observations of Active Galactic Nuclei to enable reverberation mapping studies. The live spectra from TiDES will be used to reveal the diversity and astrophysics of both normal and exotic supernovae across the luminosity-timescale plane. The extensive host-galaxy redshift campaign will allow exploitation of the larger sample of supernovae and improve photometric classification, providing the largest-ever sample of spec-confirmed type Ia supernovae, capable of a sub-2 per cent measurement of the equation-of-state of dark energy. Finally, the TiDES reverberation mapping experiment of 700--1\,000 AGN will complement the SN Ia sample and extend the Hubble diagram to $z\sim2.5$

arXiv:2405.18589v2 Announce Type: replace Abstract: Gravitationally lensed type Ia supernovae (glSNe Ia) are unique astronomical tools that can be used to study cosmological parameters, distributions of dark matter, the astrophysics of the supernovae, and the intervening lensing galaxies themselves. A small number of highly magnified glSNe Ia have been discovered by ground-based telescopes such as the Zwicky Transient Facility (ZTF), but simulations predict that a fainter population may also exist. We present a systematic search for glSNe Ia in the ZTF archive of alerts distributed from June 1 2019 to September 1 2022. Using the AMPEL platform, we developed a pipeline that distinguishes candidate glSNe Ia from other variable sources. Initial cuts were applied to the ZTF alert photometry before forced photometry was obtained for the remaining candidates. Additional cuts were applied to refine the candidates based on their light curve colours, lens galaxy colours, and the resulting parameters from fits to the SALT2 SN Ia template. The candidates were also cross-matched with the DESI spectroscopic catalogue. Seven transients were identified that had an associated galaxy DESI redshift, which we present as glSN Ia candidates. Although superluminous supernovae (SLSNe) cannot be fully rejected as contaminants, two events, ZTF19abpjicm and ZTF22aahmovu, are significantly different from typical SLSNe and their light curves can be modelled as two-image glSN Ia systems. From this two-image modelling, we estimate time delays of 22 $\pm$ 3 and 34 $\pm$ 1 days for the two events, respectively, which suggests that we have uncovered a population of glSNe Ia with longer time delays. The pipeline is currently being applied to the live ZTF alert stream to identify and follow-up future candidates while active, and it could be the foundation for glSNe Ia searches in future surveys, such as the Rubin Observatory Legacy Survey of Space and Time.

The WEAVE-TwiLight Survey: Implementing and Testing a New Low Field Density Observing Mode

Bright exoplanet host stars provide highly precise stellar and planetary parameters, as well as chemical abundances. However, modern multi-object spectroscopic surveys often neglect stars brighter than 11 visual magnitudes due to their relatively low on-sky number density, resulting in significant observational overhead. The WEAVE -TwiLight Survey (WTLS) will address this gap by employing a groundbreaking observing mode, that allows for observations of low-density/bright star fields, without compromising survey efficiency. With an input catalogue derived primarily from the northern PLATO long-duration phase field, WEAVE -TwiLight will result in a highly homogeneous spectral dataset, characterizing approximately 6,000 future PLATO targets, including 68 confirmed planet hosts. In this talk, I will present the progress made in implementing the new observing mode, alongside preliminary results from test observations obtained in late summer 2024, using WEAVE ’s high-resolution setup. Full-scale science operations for WEAVE -TwiLight are expected to begin in Q2 of 2025.

• Speaker: Thomas Hajnik / IoA
• Wednesday 05 February 2025, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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