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Kavli Institute for Cosmology, Cambridge

 
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Tue 12 Mar 16:00: Star Formation, Feedback, and Cosmic Evolution: A Modern Primer

Thu, 08/02/2024 - 08:03
Star Formation, Feedback, and Cosmic Evolution: A Modern Primer

The cosmic history of galaxy formation is the history of star formation writ large. While the contents of the universe are mostly invisible and interact with baryons only weakly, a wide array of physical processes affect evolution of the observable baryons. Some of the most important processes involve coupling between stellar and gaseous components, since massive stars are the primary energy source in the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium (IGM). The majority of stellar energy — including UV radiation, winds, and supernovae — is returned rapidly after a given population of stars forms, and is therefore collectively termed “star formation feedback.” Because the state of the ISM determines the star formation rate, and stellar feedback determines the ISM state, quantifying how this co-regulation works is crucial to theoretical modeling. The need to quantify feedback responses also extends to galaxy formation theory on larger scales, where galactic winds driven by feedback heat and add metals to the CGM , thereby regulating the accretion that replenishes the ISM , and where escaping stellar UV ionizes the IGM . Because the observational characterization of galaxies — both near and far — relies on emission lines and infrared continuum from gas and dust subject to photoheating and photochemistry from starlight, quantitative interpretation of observations also relies on calibration using physical models that accurately represent radiative transfer in complex environments. In this lecture, I will review current theory of the physics of feedback, showcasing results from state-of-the-art, high-resolution numerical radiation-magnetohydrodynamic simulations that directly follow multiphase ISM evolution including the effects of UV radiation, stellar winds, and supernovae. These simulations, on both scales of individual star-forming molecular clouds, and scales of galactic disks, show star formation efficiencies and rates that are consistent with detailed observations in the nearby universe, and also indicate strong sensitivity to environment. At high densities and where dust and metal abundances are high, stellar radiation does not propagate as far, and cooling rates are enhanced. As a result of the reduced effectiveness of feedback in maintaining the ISM pressure (turbulent, thermal, and magnetic), star formation rates and efficiencies are expected to increase in high-density environments. Results from suites of resolved star-forming ISM simulations have been used to calibrate new subgrid models, and incorporation of these new results in galaxy formation models may potentially significantly change predictions for star formation at high redshift.

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Mon 17 Jun 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:45
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Mon 10 Jun 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:44
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Mon 03 Jun 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:43
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Tue 28 May 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:42
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Mon 20 May 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:41
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Mon 13 May 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:39
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Tue 07 May 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:38
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Mon 29 Apr 14:00: Title to be confirmed

Wed, 07/02/2024 - 14:37
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Fri 09 Feb 13:00: Nonlinear stability of Einstein-matter models near the big bang singularity

Mon, 05/02/2024 - 16:59
Nonlinear stability of Einstein-matter models near the big bang singularity

In our recent work, we have made progress in understanding the complex mathematical landscape of the big bang singularity in general relativity. Utilizing Fuchsian partial differential equation techniques, we have established rigorous nonlinear stability results for certain Friedmann cosmological models, particularly focusing on solutions to the fully coupled Einstein-matter equations. This does not only reinforce some of the standard cosmological model’s assumptions in certain scenarios but also brings to light new critical phenomena that have yet to be fully understood. This research was conducted in collaboration with Todd Oliynyk from Monash University.

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Thu 08 Feb 16:00: Stellar populations and dust build-up in the early Universe

Mon, 05/02/2024 - 16:15
Stellar populations and dust build-up in the early Universe

We are witnessing an exciting revolution in our understanding of the first galaxies. JWST observations have already revealed galaxies in the first few hundred million years of the Universe and their detailed characterization in terms of chemical enrichment and stellar populations, revealing galaxies as metal poor as 1% of the solar value, and even potential traces of the first (Pop III ) stars at z ≈ 6.5 – 10.5. In addition to the stunning discoveries made recently by JWST , there have also been recent probes of the dust content of galaxies in the first billion years of the Universe with ALMA . This is a key and rapid transition phase for the evolution of dust, as galaxy evolutionary timescales become comparable with the formation timescales of dust. In this talk, I will attempt to provide some indications on the diversity of physical properties of the first galaxies expected by theoretical models, and on the current (limited) understanding of dust enrichment at very early cosmic epochs.

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Thu 15 Feb 16:00: Title to be confirmed

Mon, 05/02/2024 - 11:52
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Thu 08 Feb 16:00: Stellar populations and dust build-up in the early Universe

Mon, 05/02/2024 - 11:48
Stellar populations and dust build-up in the early Universe

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Tue 13 Feb 13:00: Disc evolution in young intermediate-mass stars

Mon, 05/02/2024 - 09:05
Disc evolution in young intermediate-mass stars

Planets are born from circumstellar discs and the evolution of these discs determines the final architecture of planetary systems. The stellar mass range between 1.5 and 3.5 solar masses presents a particularly interesting circumstellar disc evolution; most notably, it is dominated by the EUV /FUV photoevaporation regime on the pre-main sequence, it contains the majority of gaseous debris discs, and it also shows the highest giant planet frequency. In our recent spectroscopic VLT /X-Shooter survey (UV to nIR), combined with WISE data (nIR to mIR), we identified 135 pre-main sequence (PMS) intermediate mass stars (IMSs) in the Southern sky. This is the first unbiased sample of IMSs in the PMS , allowing a study of disc evolution. Our sample, encompassing protoplanetary and debris discs, also revealed a significant number of discs between these two evolutionary stages. We find that the IR excess evolution of IMSs differs from that seen for low-mass stars (LMSs), exemplified by samples drawn from nearby star forming regions. We observe that, in IMSs, the inner disc regions are vacated in their entirety, in contrast to the LMSs where we note a more gradual inside-out dissipation. We also investigated the presence of gas absorption features in our sample via optical high-resolution spectroscopy to identify gas-bearing debris discs. This requires detailed comparisons to spectra of nearby stars to eliminate objects with foreground cloud absorption as cause of the absorption features. In particular, we apply this effective method to one such disc, eta Tel, discarding the earlier claim of disc wind as the origin for the absorption features. Finally, we discuss our several ongoing and future surveys investigating the nature of circumstellar discs around IMSs.

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Mon 12 Feb 14:00: The life of erupting flux ropes

Fri, 02/02/2024 - 13:31
The life of erupting flux ropes

Unstable magnetic flux ropes represent the primary driver of eruptive solar flares and thus space weather. We summarize the recent developments of the 3D extensions of the Standard model of solar flares, including formation of the flux ropes, identification of the erupting mechanism(s), and processes during the eruption proper. New reconnection geometries are reviewed, both from theoretical as well as observational standpoint, as these lead to re-building of the erupting flux rope once the eruption is underway.

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Thu 08 Feb 11:30: Problems with (our) galaxy formation simulations and some new angles

Thu, 01/02/2024 - 14:26
Problems with (our) galaxy formation simulations and some new angles

Stellar feedback is a crucial component in controlling the baryon cycle in galaxies. However, it is not very clear how this can be done without assuming exotic models of stellar feedback. In this talk I will first discuss why we have not been very successful in producing realistic galaxies in our simulations, and present some attempts to solve this problem. I will also discuss how Lyman alpha profiles can potentially be used to place some constraints on cosmological simulations with cosmic ray feedback. This talk is intended to be thought-provoking rather than a collection of success stories.

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Thu 08 Feb 11:30: Title to be confirmed

Thu, 01/02/2024 - 14:11
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Fri 28 Jun 11:30: Title to be confirmed

Wed, 31/01/2024 - 16:38
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