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We present an analysis of the quenching of local observed and simulated galaxies, including an investigation of the dependence of quiescence on both intrinsic and environmental parameters. We apply an advanced machine learning technique utilizing random forest classification to predict when galaxies are star forming or quenched. We perform separate classification analyses for three groups of galaxies: (a) central galaxies; (b) high-mass satellites ($M_{*} > 10^{10.5}{\rm M_{\odot}}$); and (c) low-mass satellites ($M_{*} < 10^{10}{\rm M_{\odot}}$) for three cosmological hydrodynamical simulations (EAGLE, Illustris, and IllustrisTNG), and observational data from the SDSS. The simulation results are unanimous and unambiguous: quiescence in centrals and high-mass satellites is best predicted by intrinsic parameters (specifically central black hole mass), whilst it is best predicted by environmental parameters (specifically halo mass) for low-mass satellites. In observations, we find black hole mass to best predict quiescence for centrals and high mass satellites, exactly as predicted by the simulations. However, local galaxy over-density is found to be most predictive parameter for low-mass satellites. Nonetheless, both simulations and observations do agree that it is environment which quenches low mass satellites. We provide evidence which suggests that the dominance of local over-density in classifying low mass systems may be due to the high uncertainty in halo mass estimation from abundance matching, rather than it being fundamentally a more predictive parameter. Finally, we establish that the qualitative trends with environment predicted in simulations are recoverable in the observation space. This has important implications for future wide-field galaxy surveys.

We report 850 $\mu$m continuum polarization observations toward the filamentary high-mass star-forming region NGC 2264, taken as part of the B-fields In STar forming Regions Observations (BISTRO) large program on the James Clerk Maxwell Telescope (JCMT). These data reveal a well-structured non-uniform magnetic field in the NGC 2264C and 2264D regions with a prevailing orientation around 30 deg from north to east. Field strengths estimates and a virial analysis for the major clumps indicate that NGC 2264C is globally dominated by gravity while in 2264D magnetic, gravitational, and kinetic energies are roughly balanced. We present an analysis scheme that utilizes the locally resolved magnetic field structures, together with the locally measured gravitational vector field and the extracted filamentary network. From this, we infer statistical trends showing that this network consists of two main groups of filaments oriented approximately perpendicular to one another. Additionally, gravity shows one dominating converging direction that is roughly perpendicular to one of the filament orientations, which is suggestive of mass accretion along this direction. Beyond these statistical trends, we identify two types of filaments. The type-I filament is perpendicular to the magnetic field with local gravity transitioning from parallel to perpendicular to the magnetic field from the outside to the filament ridge. The type-II filament is parallel to the magnetic field and local gravity. We interpret these two types of filaments as originating from the competition between radial collapsing, driven by filament self-gravity, and the longitudinal collapsing, driven by the region's global gravity.

The Cosmic Baryon Cycle

These are incredibly exciting times for extra-galactic astrophysics; above all for studies of galaxy formation and growth of structure. New observatories and advanced simulations are revolutionising our understanding of the cycling of matter into, through, and out of galaxies. In this talk I will provide an overview of the normal matter in collapsed structures and their chemical make-up. I will review the physical properties of the multi-phase gas at the interface between the Intergalactic Medium and the galaxies themselves, the so-called Circum-Galactic Medium (CGM). Together, these results provide an increasingly accurate description of the baryon cycle which plays many crucial roles in transforming the bare pristine Universe left after the Big Bang into the rich and diverse Universe in which we live today.

• Speaker: Dr. Celine Peroux (ESO)
• Thursday 01 February 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

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Thomson scattering of cosmic microwave background (CMB) photons imprints various properties of the baryons around galaxies on the CMB. One such imprint, called patchy screening, is a direct probe of the gas density profile around galaxies. It usefully complements the information from the kinematic and thermal Sunyaev-Zel'dovich effects and does not require individual redshifts. In this paper, we derive new estimators of patchy screening called the "temperature inversion" (TI) and "signed" estimators, analogous to the gradient inversion estimator of CMB lensing. Pedagogically, we clarify the relation between these estimators and the standard patchy screening quadratic estimator (QE). The new estimators trade optimality for robustness to biases caused by the dominant CMB lensing and foreground contaminants, allowing the use of smaller angular scales. We perform a simulated analysis to realistically forecast the expected precision of patchy screening measurements from four CMB experiments, ACT, SPT, Simons Observatory (SO) and CMB-S4, cross-correlated with three galaxy samples from BOSS, unWISE and the simulated Rubin LSST Data Challenge 2 catalog. Our results give further confidence in the first detection of this effect from the ACT$\times$unWISE data in the companion paper and show patchy screening will be a powerful observable for future surveys like SO, CMB-S4 and LSST. Implementations of the patchy screening QE and the TI and signed estimators are publicly available in our LensQuEst and ThumbStack software packages, available at https://github.com/EmmanuelSchaan/LensQuEst and https://github.com/EmmanuelSchaan/ThumbStack , respectively.

Spatial variations in the cosmic electron density after reionization generate cosmic microwave background anisotropies via Thomson scattering, a process known as the ``patchy screening" effect. In this paper, we propose a new estimator for the patchy screening effect that is designed to mitigate biases from the dominant foreground signals. We use it to measure the cross-correlation between \textit{unWISE} galaxies and patchy screening, the latter measured by the Atacama Cosmology Telescope and \textit{Planck} satellite. We report the first detection of the patchy screening effect, with the statistical significance of the cross-correlation exceeding $7\sigma$. This measurement directly probes the distribution of electrons around these galaxies and provides strong evidence that gas is more extended than the underlying dark matter. By comparing our measurements to electron profiles extracted from simulations, we demonstrate the power of these observations to constrain galaxy evolution models. Requiring only the 2D positions of objects and no individual redshifts or velocity estimates, this approach is complementary to existing gas probes, such as those based on the kinetic Sunyaev-Zeldovich effect.

We present HOMERUN (Highly Optimized Multi-cloud Emission-line Ratios Using photo-ionizatioN), a new approach to modelling emission lines from photoionized gas that can simultaneously reproduce all observed line intensities from a wide range of ionization levels and with high accuracy. Our approach is based on the weighted combination of multiple single-cloud photoionization models and, contrary to previous works, the novelty of our approach consists in using the weights as free parameters of the fit and constraining them with the observed data. One of the main applications of HOMERUN is the accurate determination of gas-phase metallicities and we show that a critical point is to allow for a variation of the N/O and S/O abundance ratios which can significantly improve the quality of the fit and the accuracy of the results. Moreover, our approach provides a major improvement compared to the single-cloud, constant-pressure models commonly used in the literature. By using high-quality literature spectra of H ii regions where 10 to 20 emission lines (including several auroral lines) are detected with high signal-to-noise ratio, we show that all lines are reproduced by the model with an accuracy better than 10%. In particular, the model is able to simultaneously reproduce [O i], [O ii], [O iii], [S ii], and [S iii] emission lines which, to our knowledge, is an unprecedented result. Finally, we show that the gas metallicities estimated with our models for HII regions in the Milky Way are in agreement with the stellar metallicities than the estimates based on the Te-method. Overall, our method provides a new accurate tool to estimate the metallicity and the physical conditions of the ionized gas. It can be applied to many different science cases from HII regions to AGN and wherever there are emission lines from photoionized gas.

Positivity bounds on effective field theories

Unitarity and analyticity are some of the most fundamental properties of S-matrix/quantum field theory. I will explain why these simple properties can impose strong constraints on the Wilson coefficients/coupling constants of a Lorentz invariant effective field theory. The central tool to use is the dispersion relations, and stronger bounds can be extracted if the crossing symmetry and unitarity conditions are fully utilized. I will discuss a couple of applications of these bounds in cosmology and particle physics.

• Speaker: Shuang-Yong Zhou (USTC)
• Monday 29 January 2024, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Dr Dong-Gang Wang.

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Planetary debris at white dwarfs

White dwarfs often accrete material from their remnant planetary systems, polluting their pristine hydrogen or helium atmospheres with metals. I will talk about how we can use this phenomenon to study exoplanetary compositions: photospheric metal lines can reveal the bulk compositions of the building blocks of rocky planets, while infrared emission gives us our only glimpse of the circumstellar debris before it arrives at the stellar surface. Results from dozens of systems tell us that accretion of chondritic material is common, though not universal, while the debris disks feeding accretion are dynamically active environments. The advent of JWST is opening new windows onto those disks, while large spectroscopic surveys are delivering an order-of-magnitude increase in the number of systems to study, and I will present ongoing work on both fronts.

• Speaker: Andrew Swan (Warwick)
• Tuesday 30 January 2024, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Emily Sandford.

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Spin-eccentricity interplay in merging binary black holes

Orbital eccentricity and spin precession are precious observables to infer the formation history of binary black holes with gravitational-wave data. We present a post-Newtonian, multi-timescale analysis of the binary dynamics able to capture both precession and eccentricity over long inspirals. We show that the evolution of an eccentric binary can be reduced that of effective source on quasi-circular orbits, coupled to a post-Newtonian prescription for the secular evolution of the eccentricity. Our findings unveil an interplay between precession and eccentricity: the spins of eccentric binaries precess on shorter timescales and their nutation amplitude is altered compared to black holes on quasi-circular orbits, consequently affecting the so-called spin morphology. Even if binaries circularize by the time they enter the sensitivity window of our detectors, their spin orientations retain some memory of the past evolution on eccentric orbits, thus providing a new link between gravitational-wave detection and astrophysical formation. At the same time, we point out that residual eccentricity should be considered a source of systematics when reconstructing the past history of black-hole binaries using the spin orientations.

• Speaker: Giulia Fumagalli, Università degli Studi di Milano-Bicocca
• Friday 26 January 2024, 13:00-14:00
• Venue: Potter room/Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Daniela Cors.

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Spin-eccentricity interplay in merging binary black holes

Orbital eccentricity and spin precession are precious observables to infer the formation history of binary black holes with gravitational-wave data. We present a post-Newtonian, multi-timescale analysis of the binary dynamics able to capture both precession and eccentricity over long inspirals. We show that the evolution of an eccentric binary can be reduced that of effective source on quasi-circular orbits, coupled to a post-Newtonian prescription for the secular evolution of the eccentricity. Our findings unveil an interplay between precession and eccentricity: the spins of eccentric binaries precess on shorter timescales and their nutation amplitude is altered compared to black holes on quasi-circular orbits, consequently affecting the so-called spin morphology. Even if binaries circularize by the time they enter the sensitivity window of our detectors, their spin orientations retain some memory of the past evolution on eccentric orbits, thus providing a new link between gravitational-wave detection and astrophysical formation. At the same time, we point out that residual eccentricity should be considered a source of systematics when reconstructing the past history of black-hole binaries using the spin orientations.

• Speaker: Giulia Fumagalli, Università degli Studi di Milano-Bicocca
• Friday 26 January 2024, 13:00-14:00
• Venue: Potter room/Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Daniela Cors.

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The spectra of high-redshift ($z\gtrsim 6$) quasars contain valuable information on the progression of the Epoch of Reionization (EoR). At redshifts $z<6$, the observed Lyman-series forest shows that the intergalactic medium (IGM) is nearly ionized, while at $z>7$ the observed quasar damping wings indicate high neutral gas fractions. However, there remains a gap in neutral gas fraction constraints at $6\lesssim z \lesssim 7$ where the Lyman series forest becomes saturated but damping wings have yet to fully emerge. In this work, we use a sample of 18 quasar spectra at redshifts $6.0<z<7.1$ to close this gap. We apply neural networks to reconstruct the quasars' continuum emission around the partially absorbed Lyman $\alpha$ line to normalize their spectra, and stack these continuum-normalized spectra in three redshift bins. To increase the robustness of our results, we compare the stacks to a grid of models from two hydrodynamical simulations, ATON and CROC, and we measure the volume-averaged neutral gas fraction, $\bar{x}_{\rm HI}$, while jointly fitting for the mean quasar lifetime, $t_{\rm Q}$, for each stacked spectrum. We chronicle the evolution of neutral gas fraction using the ATON (CROC) models as follows: $\bar{x}_{\rm HI} = 0.21_{-0.07}^{+0.17}$ ($\bar{x}_{\rm HI} = 0.10_{<10^{-4}}^{+0.73}$) at $\langle z \rangle =6.10$, $\bar{x}_{\rm HI} = 0.21_{-0.07}^{+0.33}$ ($\bar{x}_{\rm HI} =0.57_{-0.47}^{+0.26}$) at $\langle z \rangle =6.46$, and $\bar{x}_{\rm HI} = 0.37_{-0.17}^{+0.17}$ ($\bar{x}_{\rm HI} =0.57_{-0.21}^{+0.26}$) at $\langle z \rangle =6.87$. At the same time we constrain the average quasar lifetime to be $t_{\rm Q} \lesssim 7\ {\rm Myr}$ across all redshift bins, in good agreement with previous studies.

Gravitational Instability in Irradiated Protoplanetary Discs

Young protoplanetary discs are expected to be massive and self-gravitating. The resulting gravitational instability (GI) causes large spiral structures in the disc and is a potential formation mechanism of giant planets via direct collapse of overdense regions of gas. The onset of GI is sensitive to thermal processes in the disc. It is, therefore, influenced by irradiation, such as that from the central star, which is expected to suppress the instability. However, this is dependent on the exact implementation of heating. In this talk I will present the results of 2D hydrodynamic simulations comparing the outcomes of two different heating prescriptions and show that a disc heated per unit area remains gravitationally unstable even under high levels of irradiation.

• Speaker: Cat Leedham
• Wednesday 24 January 2024, 13:15-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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The black hole mass-metallicity relation and insights into galaxy quenching

Understanding the quenching of star formation in galaxies remains a central question within extragalactic astrophysics. In this talk I will explore the intricate interplay between galaxy quenching, black hole mass, and ‘starvation’. Previous research has established a strong correlation between quiescence and black hole mass, while other studies have highlighted the role of ‘starvation’—the halting of gas inflows—as a driving force in quenching, based on rapid gas depletion through star formation and chemical enrichment. This investigation bridges the gap between these findings, shedding light on the underlying mechanisms. Leveraging an extensive dataset of galaxies, this study uses random forest regression and partial correlation coefficients to uncover the fundamental relationships governing stellar metallicity with respect to other galaxy characteristics. I will show that for actively star-forming galaxies, stellar metallicity exhibits a strong dependence on stellar mass. However, intriguingly, for passive galaxies, a dramatically different narrative emerges, where the primary driver of stellar metallicity is the black hole mass. This means the integrated impact of black hole feedback emerges as the crucial determinant, cutting off gas inflows and inducing a state of ‘starvation’, ultimately leading to the observed rapid rise in stellar metallicity. This, in turn, paves the way for the transition of a galaxy from star-forming to quiescent. This finding not only deepens our understanding of galaxy evolution but also offers key insights into the intricate mechanisms shaping the fate of galaxies across cosmic epochs.

• Speaker: William Baker
• Wednesday 24 January 2024, 13:15-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: .

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We construct a theory of gravity in which a propagating massive vector field arises from a quadratic curvature invariant. The Einstein-Cartan formulation and a partial suppression of torsion ensure the absence of ghost and strong-coupling problems, as we prove with nonlinear Lagrangian and Hamiltonian analysis. Augmenting General Relativity with a propagating torsion vector, our theory provides a purely gravitational origin of Einstein-Proca models and constrains their parameter space. As an outlook to phenomenology, we discuss the gravitational production of fermionic dark matter.

Multiple theories have been proposed to describe the formation of black hole seeds in the early Universe and to explain the emergence of very massive black holes observed in the first billion years after Big Bang. Models consider different seeding and accretion scenarios, which require the detection and characterisation of black holes in the first few hundred million years after Big Bang to be validated. Here we present an extensive analysis of the JWST-NIRSpec spectrum of GN-z11, an exceptionally luminous galaxy at z=10.6, revealing the detection of the [NeIV]2423 and CII*1335 transitions (typical of Active Galactic Nuclei, AGN), as well as semi-forbidden nebular lines tracing gas densities higher than 10^9 cm-3, typical of the Broad Line Region of AGN. These spectral features indicate that GN-z11 hosts an accreting black hole. The spectrum also reveals a deep and blueshifted CIV1549 absorption trough, tracing an outflow with velocity 800-1000 km/s, likely driven by the AGN. Assuming local virial relations, we derive a black hole mass of log(M_BH/Msun) = 6.2 +- 0.3, accreting at about 5 times the Eddington rate. These properties are consistent with both heavy seeds scenarios, or scenarios envisaging intermediate/light seeds experiencing episodic super-Eddington phases. Our finding naturally explains the high luminosity of GN-z11 and can also provide an explanation for its exceptionally high nitrogen abundance.

We study little red dots (LRD) detected by JADES and covered by the SMILES MIRI survey. Our sample contains 31 sources, $\sim70$% detected in the two bluest MIRI bands, 40% in redder filters. The median/quartiles redshifts are $z=6.9_{5.9}^{7.7}$ (55% spectroscopic). We analyze the rest-frame ultraviolet through near/mid-infrared spectral energy distributions of LRDs combining NIRCam and MIRI observations, using a variety of modeling techniques that include emission from stars, dust, and (un)obscured active galactic nuclei (AGN). The NIRCam$-$MIRI colors, for $\geq10$ $\mu$m, are bluer than direct pure emission from AGN tori; the spectral slope flattens in the rest-frame near-infrared, consistent with a 1.6 $\mu$m stellar bump. Both observations imply that stellar emission makes the dominant contribution at these wavelengths, expediting a stellar mass estimation: the median/quartiles are $\log \mathrm{M_\star/M_\odot}=9.4_{9.1}^{9.7}$. The number density of LRDs is $10^{-4.0\pm0.1}$ Mpc$^{-3}$, accounting for $14\pm3$% of the global population of galaxies with similar redshifts and masses. The flat ultraviolet spectral range is dominated by young stars. The rest-frame near/mid-infrared (2-4 $\mu$m) spectral slope reveals significant amounts of dust (bolometric stellar attenuation $\sim3-4$ mag) heated by strong radiation fields arising from highly embedded compact sources. Our models imply $<0.4$ kpc heating knots, containing dust-enshrouded OB stars or an AGN producing a similar radiation field, obscured by $\mathrm{A(V)}>10$ mag. We conclude that LRDs are extremely intense and compact starburst galaxies with mass-weighted ages 5-10 Myr, very efficient in producing dust, their global energy output dominated by the direct and dust-recycled emission from OB stars, with some contribution from obscured AGN in the mid-infrared.

We present observational evidence for a stellar Fundamental Metallicity Relation (FMR), a smooth relation between stellar mass, star-formation rate (SFR) and the light-weighted stellar metallicity of galaxies (analogous to the well-established gas-phase FMR). We use the flexible, non-parametric software pPXF to reconstruct simultaneously the star-formation and chemical-enrichment history of a representative sample of galaxies from the local MaNGA survey. We find that (i) the metallicity of individual galaxies increases with cosmic time and (ii) at all stellar masses, the metallicity of galaxies is progressively higher, moving from the star-burst region above the main sequence (MS) towards the passive galaxies below the MS, manifesting the stellar FMR. These findings are in qualitative agreement with theoretical expectations from IllustrisTNG, where we find a mass-weighted stellar FMR. The scatter is reduced when replacing the stellar mass $M_{*}$ with $M_{*}/R_{\rm e}$ (with $R_{\rm e}$ being the effective radius), in agreement with previous results using the velocity dispersion $\sigma_{\rm e}$, which correlates with $M_{*}/R_{\rm e}$. Our results point to starvation as the main physical process through which galaxies quench, showing that metal-poor gas accretion from the intergalactic/circumgalactic medium -- or the lack thereof -- plays an important role in galaxy evolution by simultaneously shaping both their star-formation and their metallicity evolutions, while outflows play a subordinate role. This interpretation is further supported by the additional finding of a young stellar FMR, tracing only the stellar populations formed in the last 300 Myr. This suggests a tight co-evolution of the chemical composition of both the gaseous interstellar medium and the stellar populations, where the gas-phase FMR is continuously imprinted onto the stars over cosmic times.

We apply the hierarchical probabilistic SED model BayeSN to analyse a sample of 475 SNe Ia (0.015 < z < 0.4) from Foundation, DES3YR and PS1MD to investigate the properties of dust in their host galaxies. We jointly infer the dust law $R_V$ population distributions at the SED level in high- and low-mass galaxies simultaneously with dust-independent, intrinsic differences. We find an intrinsic mass step of $-0.049\pm0.016$ mag, at a significance of 3.1$\sigma$, when allowing for a constant intrinsic, achromatic magnitude offset. We additionally apply a model allowing for time- and wavelength-dependent intrinsic differences between SNe Ia in different mass bins, finding $\sim$2$\sigma$ differences in magnitude and colour around peak and 4.5$\sigma$ differences at later times. These intrinsic differences are inferred simultaneously with a difference in population mean $R_V$ of $\sim$2$\sigma$ significance, demonstrating that both intrinsic and extrinsic differences may play a role in causing the host galaxy mass step. We also consider a model which allows the mean of the $R_V$ distribution to linearly evolve with redshift but find no evidence for any evolution - we infer the gradient of this relation $\eta_R = -0.38\pm0.70$. In addition, we discuss in brief a new, GPU-accelerated Python implementation of BayeSN suitable for application to large surveys which is publicly available and can be used for future cosmological analyses; this code can be found here: https://github.com/bayesn/bayesn.

This has solved one of the most puzzling mysteries in astronomy – why astronomers detect light from hydrogen atoms that should have been entirely blocked by the pristine gas that formed after the Big Bang.

These new observations have found small, faint objects surrounding the galaxies that show the ‘inexplicable’ hydrogen emission. In conjunction with state-of-the-art simulations of galaxies in the early Universe, the observations have shown that the chaotic merging of these neighbouring galaxies is the source of this hydrogen emission. The results are reported in the journal Nature Astronomy.

Light travels at a finite speed (300 000 km a second), which means that the further away a galaxy is, the longer it has taken the light from it to reach our Solar System. As a result, not only do observations of the most distant galaxies probe the far reaches of the Universe, but they also allow us to study the Universe as it was in the past.

To study the early Universe, astronomers require exceptionally powerful telescopes that are capable of observing very distant – and therefore very faint – galaxies. One of Webb’s key capabilities is its ability to observe these galaxies, and probe the early history of the Universe.

The earliest galaxies were sites of vigorous and active star formation, and were rich sources of a type of light emitted by hydrogen atoms called Lyman-α emission. However, during the epoch of reionisation, an immense amount of neutral hydrogen gas surrounded these stellar nurseries. Furthermore, the space between galaxies was filled by more of this neutral gas than is the case today. The gas can effectively absorb and scatter this kind of hydrogen emission, so astronomers have long predicted that the abundant Lyman-α emission released in the early Universe should not be observable today.

This theory has not always stood up to scrutiny, however, as examples of early hydrogen emission have previously been observed by astronomers. This has presented a mystery: how is it that this hydrogen emission – which should have long since been absorbed or scattered – is being observed?

“One of the most puzzling issues that previous observations presented was the detection of light from hydrogen atoms in the very early Universe, which should have been entirely blocked by the pristine neutral gas that was formed after the Big Bang,” said lead author Callum Witten from Cambridge’s Institute of Astronomy. “Many hypotheses have previously been suggested to explain the great escape of this ‘inexplicable’ emission.”

The team’s breakthrough came thanks to Webb’s combination of angular resolution and sensitivity. The observations with Webb’s NIRCam instrument were able to resolve smaller, fainter galaxies that surround the bright galaxies from which the ‘inexplicable’ hydrogen emission had been detected. In other words, the surroundings of these galaxies appear to be a much busier place than we previously thought, filled with small, faint galaxies.

These smaller galaxies were interacting and merging with one another, and Webb has revealed that galaxy mergers play an important role in explaining the mystery emission from the earliest galaxies.

“Where Hubble was seeing only a large galaxy, Webb sees a cluster of smaller interacting galaxies, and this revelation has had a huge impact on our understanding of the unexpected hydrogen emission from some of the first galaxies,” said co-author Sergio Martin-Alvarez from Stanford University.

The team then used computer simulations to explore the physical processes that might explain their results. They found that the rapid build-up of stellar mass through galaxy mergers both drove strong hydrogen emission and facilitated the escape of that radiation via channels cleared of the abundant neutral gas. So, the high merger rate of the previously unobserved smaller galaxies presented a compelling solution to the long-standing puzzle of the ‘inexplicable’ early hydrogen emission.

The team is planning follow-up observations with galaxies at various stages of merging, to continue to develop their understanding of how the hydrogen emission is ejected from these changing systems. Ultimately, this will enable them to improve our understanding of galaxy evolution.

Reference:
Callum Witten et al. ‘Deciphering Lyman-α emission deep into the epoch of reionization.’ Nature Astronomy (2024). DOI: 10.1038/s41550-023-02179-3

Adapted from an ESA press release.

A team of astronomers, led by the University of Cambridge, has used the NASA/ESA/CSA James Webb Space Telescope to reveal, for the first time, what lies in the local environment of galaxies in the very early Universe.

ESA/Webb, NASA & CSA, S. Finkelstein (UT Austin), M. Bagley (UT Austin), R. Larson (UT Austin), A. Pagan (STScI), C. Witten, M. Zooming in on three neighbouring galaxies (NIRCam image)

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Yes

On-shell techniques for the standard-model EFTs

Bypassing fields, operators and Lagrangians to target directly physical amplitudes advantageously avoids gauge and field-redefinition redundancies. Unitarity and analyticity allow for the construction of on-shell amplitudes, recursively in leg and loop numbers. After decades of developments focusing mostly on renormalisable theories, the associated on-shell amplitude techniques have recently been more extensively applied to effective field theories, including those which parameterise hypothetical heavy physics appearing beyond the standard model. The enumeration of independent operators can be substituted for that of contact-term amplitudes. In a given scattering, the possible kinematic structures can be fully characterised. Unitarity and analyticity allow for the derivation of positivity constraints, for the extraction of anomalous dimensions and of matching coefficients. This talk will cover some of these developments.

• Speaker: Gauthier Durieux (Universite Catholique de Louvain)
• Wednesday 24 January 2024, 14:00-15:00
• Venue: MR2.
• Series: Theoretical Physics Colloquium; organiser: Hannah Banks.

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