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Institute of Astronomy

 
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Institute of Astronomy(IoA) Colloquia usually held in the Hoyle Building Lecture Theatre on Thursdays during term time at 4:00pm (after afternoon tea).
Cambridge Astrophysics Joint Colloquia
Updated: 37 min 46 sec ago

Thu 14 Mar 16:00: How Did Cassiopeia A Explode?

Mon, 26/02/2024 - 16:45
How Did Cassiopeia A Explode?

Cassiopeia A is the best-observed core-collapse supernova remnant in our galaxy. Analyses of the 1 Million second Chandra Very Large Project X-ray observation and the data from infrared spectroscopy by Spitzer lead to a “complete” (within the limitations of the data quality) assessment of the elemental composition of the explosion ejecta, comprising both the reverse shocked X-ray emitting plasma and the photoionized unshocked ejecta emitting primarily in the infrared. This is the first time such a detailed census of supernova ejecta has ever been accomplished. More recently, Cassiopeia A has been observed with the James Webb Space Telescope. A first look suggests that these data corroborate and extend our previous analysis. Hard X-ray observations by NUSTAR reveal the mass and location of the radioactive nucleus 44Ti and optical imaging reveals a natal kick imparted to the compact central object (presumed to be a neutron star), anti-correlated with the 44Ti location, as expected. However, X-ray imaging reveals almost “pure” Fe knots on the east limb, presumably the ashes of alpha rich freeze out, which do not correlate so well. All these observables carry information about processes at the core of the supernova and allow us (and others) to speculate about the nature of the explosion, in ways that complement conclusions drawn from the prompt observations of supernovae.

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Thu 07 Mar 16:00: Eddington lecture 2024: The Dawn of Galaxy-scale Gravitational Wave Astronomy

Mon, 26/02/2024 - 13:02
Eddington lecture 2024: The Dawn of Galaxy-scale Gravitational Wave Astronomy

For more than 15 years, NANO Grav and other pulsar-timing array collaborations have been carefully monitoring networks of pulsars across the Milky Way. The goal was to find a tell-tale correlation signature amid the data from all those pulsars that would signal the presence of an all-sky background of nanohertz-frequency gravitational waves, washing through the Galaxy. At the end of June 2023, the global pulsar-timing array community finally announced its evidence for this gravitational-wave background, along with a series of studies that interpreted this signal as either originating from a population of supermassive black-hole binary systems, or as relics from cosmological processes in the very early Universe. I will describe the journey up to this point (including the integral role that the IoA played), what led to the ultimate breakthrough, how this affects our knowledge of supermassive black holes and the early Universe, and what lies next for gravitational-wave astronomy at light-year wavelengths.

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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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