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Kavli Institute for Cosmology, Cambridge

 
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Fri 24 Sep 11:30: The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

Thu, 22/07/2021 - 16:54
The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

ALMA observations of z>6 galaxies have revealed abnormally high [OIII]/[CII] ratios and [CII] deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionisation parameter, gas density, or photodissociation region (PDR) covering fraction. In order to elucidate the underlying physics that drives this high-redshift phenomenon, I will introduce SPHINX20 , a new state-of-the-art, cosmological radiation-hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization and discuss the physics that drives high [OIII]/[CII] ratios.

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Fri 24 Sep 11:30: The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

Thu, 22/07/2021 - 16:50
The Nature of High [OIII]/[CII] Galaxies in the Epoch of Reionization: Low Carbon Abundance and a Top-Heavy IMF?

ALMA observations of $z>6$ galaxies have revealed abnormally high [OIII]$$/[CII]${\rm 158\mu m}$ ratios and [CII]$$ deficits compared to local galaxies. The origin of this behaviour is unknown. Numerous solutions have been proposed including differences in C and O abundance ratios, observational bias, and differences in ISM properties, including ionisation parameter, gas density, or photodissociation region (PDR) covering fraction. In order to elucidate the underlying physics that drives this high-redshift phenomenon, we employ {\small SPHINX $^{20}$}, a state-of-the-art, cosmological radiation-hydrodynamics simulation, that resolves detailed ISM properties of thousands of galaxies in the epoch of reionization. We find that the observed $z>6$ [OIII]${\rm 88\mu m}$-SFR and [CII]$$-SFR relations can only be reproduced when the C/O abundance ratio is $\sim8\times$ lower than Solar and the total metal production is $\sim5.7\times$ higher than that of a Kroupa IMF . This implies that high-redshift galaxies are potentially primarily enriched by low-metallicity core-collapse supernovae with a more top-heavy IMF . As AGB stars and type-Ia supernova begin to contribute to the galaxy metallicity, both the [CII]${\rm 158\mu m}$-SFR and [CII]$$ luminosity functions are predicted to converge to observed values at $z\sim4.5$. While we demonstrate that ionisation parameter, LyC escape fraction, ISM gas density, and CMB attenuation all drive galaxies towards higher [OIII]${\rm 88\mu m}$/[CII]$$, observed values at $z>6$ can only be reproduced with substantially lower C/O abundances compared to Solar. The combination of [CII]${\rm 158\mu m}$ and [OIII]$$ can be used to predict the values of ionisation parameter, ISM gas density, and LyC escape fraction and we provide estimates of these quantities for nine observed $z>6$ galaxies. Finally, we demonstrate that [OI]${\rm 63\mu m}$ can be used as a replacement for [CII]$$ in high-redshift galaxies where [CII]${\rm 158\mu m}$ is unobserved and argue that more observation time should be used to target [OI]$_{\rm 63\mu m}$ at $z>6$.

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Fri 24 Sep 11:30: Title to be confirmed

Thu, 22/07/2021 - 12:52
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Fri 24 Sep 11:30: Title to be confirmed

Thu, 22/07/2021 - 10:03
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Fri 18 Jun 13:00: Calm after the storm: black hole ringdown beyond linear perturbation theory.

Wed, 16/06/2021 - 12:27
Calm after the storm: black hole ringdown beyond linear perturbation theory.

Recent analyses have shown that the gravitational-wave signal emitted by merging black holes is surprisingly simple in its final stages. It has long been known that the very end stage of a black hole merger—the late ringdown—is well-described by black hole perturbation theory. The latter, however, has been found to be in excellent agreement with numerical simulations even at much earlier times, immediately following the merger. In this talk, I will describe efforts to understand this phenomenon by characterizing the size and nature of backreaction effects in black-hole ringdowns. Our analysis, which we validate against numerical relativity simulations, is based on a novel perturbation theory incorporating a bilinear form between quasinormal modes. From a toy-model system (an asymptotically anti-de Sitter black hole with a ringing scalar field), to gravitational perturbations of Schwarzschild, the lessons we learn can help explain the simplicity of ringdown signals. I will also discuss ongoing efforts to extend these studies to more realistic (Kerr) black holes.

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Thu 17 Jun 16:00: New results on galaxy clusters from the eROSITA early data release

Mon, 14/06/2021 - 21:46
New results on galaxy clusters from the eROSITA early data release

The eROSITA X-ray telescope is the soft X-ray instrument on the Spectrum-Roentgen-Gamma (SRG) mission, launched successfully in 2019. Before the start of its four-year all-sky survey, eROSITA observed a number of calibration and performance verification targets. The German eROSITA team will shortly be releasing its data in an early data release. I will report on a number of results from these data focusing on galaxy clusters. These observations include a 140 square-degree survey of the sky containing a number of clusters and superclusters, a detailed mosaic of a galaxy cluster pair, and deep observations of relaxed and merging clusters. With its wide field of view and well-characterised background eROSITA is perfect for detecting clusters and other faint structures which would be hard to find using other X-ray telescopes. These results provide a number of insights about cluster populations and astrophysical processes.

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Wed 16 Jun 14:00: Results of the 2020 IoA Annual Staff Survey

Mon, 14/06/2021 - 14:56
Results of the 2020 IoA Annual Staff Survey

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Wed 16 Jun 16:00: Neutron stars as gravitational-wave sources: dense matter and stellar mass

Thu, 10/06/2021 - 16:04
Neutron stars as gravitational-wave sources: dense matter and stellar mass

Astronomical observations of neutron stars inform our understanding of matter at the highest densities. Already, we have used the gravitational-wave data of GW170817 - the first signal from merging neutron stars – to constrain the equation of state of dense matter in neutron stars. The heavy neutron-star merger GW190425 indicated that the gravitational-wave population may include heavier stars not previously observed in galactic double neutron star binaries. For distant sources, the distribution of masses in neutron-star mergers will be a key observable in the coming years of gravitational-wave astronomy. In this talk, I will discuss methods being used to explore matter and mass properties for LIGO /Virgo neutron stars. I will discuss how these results fit with other neutron-star observations, outline prospects of learning about matter in the current Advanced-detector era, and extrapolate to the potential of next-generation gravitational-wave observatories to map the phase diagram of dense neutron-rich matter and the endpoints of stellar evolution.

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Fri 11 Jun 13:00: Oscillations in the Stochastic Gravitational Wave Background

Wed, 09/06/2021 - 16:15
Oscillations in the Stochastic Gravitational Wave Background

Observational constraints and prospects for detection of features, i.e. oscillations in the primordial power spectrum, have so far concentrated on the CMB and Large Scale Structure surveys. After a broad introduction, I will show how different features in the primordial spectrum lead to characteristic oscillatory patterns in the cosmological stochastic gravitational wave background. Probing these features could, for instance, establish the existence of heavy particles beyond the reach of terrestrial experiments, and even test the inflationary paradigm or point to alternatives to it. Further, high energy embeddings of inflation often lead to departures from the single-field slow-roll paradigm, resulting in features in the primordial scalar power spectrum. This provides a clear target for gravitational wave observatories as well as a challenge for developing dedicated data analysis techniques to look for this unique insight into the physics of the early universe.

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Mon 14 Jun 15:00: Love and Naturalness

Tue, 08/06/2021 - 17:37
Love and Naturalness

Tidal deformability coefficients (“Love numbers”) are key parameters of compact objects, which can be probed with gravitational wave measurements. Also they are Wilson coefficients of the effective field theory (EFT) of inspiraling binaries. Remarkably, black hole Love numbers vanish identically in general relativity in four dimensions, which represents a major naturalness problem in the EFT context. In my talk, I will present a new hidden SL(2,R)xU(1) symmetry of general relativity (“Love symmetry”), which elegantly resolves the naturalness paradox. In particular, I will demonstrate that all known properties of black hole tidal Love numbers follow from the SL(2,R) representation theory. Finally, I will discuss implications of the Love symmetry for gravitational wave science and its relation to near-horizon isometries of extremal black holes.

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Wed 09 Jun 13:30: The Invention of Dark Matter and Dark Energy.

Tue, 08/06/2021 - 10:43
The Invention of Dark Matter and Dark Energy.

High quality photometry and spectroscopy of disk galaxies has shown that the mass of a galaxy is the only parameter that determines the dark matter attributed to a galaxy. Together with other results on elliptical galaxies we look carefully at the fundamentals that led to the idea that Dark Matter was needed to account for the motions of stars and gas in galaxies. It turns out that the problem is with the use of Newtonian dynamics to explain motions in galaxies. Einstein showed that the field of the gravitating object must not be neglected and that assumptions of homogeneity, isotropy and spherical symmetry are the root cause. Gravitational field lines are affected by other masses in the same way that light is affected. This changes the way that gravitational forces act upon material orbiting in the disk of a galaxy and are proportional to 1/r as observed rather than 1/r^2 predicted by Newton. This error led to the invention of Dark Matter. Gravitational field interaction also reduces gravitational forces outside an object, reducing the forces between galaxies and giving effects that could be mistaken for Dark Energy. Dark Matter and Dark Energy were each introduced as concepts in order to make specific astronomical observations fit in with the then understanding of the physics involved. We demonstrate that by replacing Newtonian dynamics which essentially forces symmetries on objects with Einstein’s general relativistic dynamics is very likely to make the need for either Dark Matter or Dark Energy to go away.

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Wed 16 Jun 14:00: EDI Committee

Mon, 07/06/2021 - 10:39
EDI Committee

Abstract not available

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Wed 09 Jun 14:15: Celestial Holography and w(1+infinity)

Sat, 05/06/2021 - 10:15
Celestial Holography and w(1+infinity)

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Mon 14 Jun 16:00: Tides in the high-eccentricity migration of hot Jupiters: Triggering diffusive growth by nonlinear mode interactions PLEASE NOTE THE DIFFERENT START TIME OF **4 PM**!

Wed, 02/06/2021 - 23:41
Tides in the high-eccentricity migration of hot Jupiters: Triggering diffusive growth by nonlinear mode interactions

High eccentricity migration is a possible formation channel for hot Jupiters. However, in order for it to be consistent with the observed population of planets, tides must circularize the orbits in an efficient way. A potential mechanism for such rapid circularization is the diffusive growth of the tidally driven planetary f-mode. Such growth occurs if the f-mode’s phase at pericenter varies chaotically from one pericenter passage to the next. Previous studies focused on the variation of the orbital period due to tidal back-reaction on the orbit as the source of chaos. Here we show that nonlinear mode interactions can also be an important source. Specifically, we show that nonlinear interactions between a parent f-mode and daughter f-/p-modes induce an energy-dependent shift in the oscillation frequency of the parent. This frequency shift varies randomly from orbit to orbit because the parent’s energy varies. As a result, the parent’s phase at pericenter varies randomly, which we find can trigger it to grow diffusively. We show that the phase shift induced by nonlinear mode interactions in fact dominates the shift induced by tidal back-reaction and significantly lowers the one-kick energy threshold for diffusive growth by about a factor of 5 compared to the linear theory’s prediction. Nonlinear interactions could thus enhance the formation rate of hot Jupiters through the high-eccentricity migration channel and potentially mitigate the discrepancy between the observed and predicted occurrence rates for close-in gas giants as compared to those further from the star.

PLEASE NOTE THE DIFFERENT START TIME OF **4 PM**!

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Mon 07 Jun 13:00: SDSS: 20 years of cosmological results

Tue, 01/06/2021 - 14:12
SDSS: 20 years of cosmological results

I will present the cosmological implications from the final cosmological measurements of clustering using galaxies, quasars, and Lyα forests from the completed Sloan Digital Sky Survey (SDSS) lineage of experiments in large-scale structure. These experiments, composed of data from SDSS , SDSS-II, BOSS , and eBOSS, offer independent measurements of baryon acoustic oscillation (BAO) measurements of angular-diameter distances and Hubble distances relative to the sound horizon, rd, from eight different samples and six measurements of the growth rate parameter, fσ8, from redshift-space distortions (RSD). This composite sample is the most constraining of its kind and allows us to perform a comprehensive assessment of the cosmological model after two decades of dedicated spectroscopic observation. I will discuss the constraints on dark energy, massive neutrinos, and the Hubble tension. Additionally, galaxy clustering can be used to constrain primordial non-Gaussianity (PNG) through the scale dependent halo bias. I will present constraints on PNG from the SDSS quasar sample.

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Wed 02 Jun 13:30: The fascinating cluster merger Abell 2146

Tue, 01/06/2021 - 09:38
The fascinating cluster merger Abell 2146

Abstract not available

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