Talks, Seminars, Events

Abstract
Giant molecular clouds (GMCs) form the communication channel between the galactic environment and the sub-cloud physics of star formation and stellar feedback. Recent observations reveal galaxy-scale trends in GMC properties (e.g. surface densities, velocity dispersions, and turbulent pressures), demanding a systematic and predictive theory of the GMC lifecycle across the parameter space of observable star-forming environments. In this contribution, I will combine a simple analytic theory for the influence of galactic dynamics on GMC lifetimes/evolution with a statistical sample of ~60,000 numerically-simulated GMCs across three isolated disc galaxies in the moving-mesh code Arepo. The analytic predictions depend on just three sets of galactic-scale observables: the rotation curve, surface densities and velocity dispersions of the host galaxy. The numerical simulations incorporate the features required to accurately model the influence of stellar feedback on the ISM, including stochastic star formation, mechanical supernova feedback, pre-supernova feedback from HII regions, and ISM chemistry. Using this combined analytic/numerical framework, I will show (1) that stellar feedback truncates the GMC lifetime across a wide range of galactic environments, and (2) that the lifetimes and properties of GMCs are predicted to follow clear galactic-dynamical trends driven by gravitational instability, galactic shearing, radial orbital perturbations, and cloud-cloud collisions. These results reveal that the complex interplay between galactic dynamics and stellar feedback plays a crucial role in setting the course of GMC evolution, and therefore in determining the star formation rates of the galaxies they live in.

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KoCo Signature Speaker

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Cosmic reionization is one of the last major milestones in the evolution of the Universe. It has only taken one billion years for the bulk of the hydrogen in the Universe to be fully ionized by the radiation produced by early galaxies and quasars. While significant progress has been made in the recent years, completing the census of these ionizing sources is still a major challenge on both the observational and theoretical sides.
In this talk, I will discuss how radiation hydrodynamical simulations can be used to study the properties of the first galaxies and AGN with a particular focus on their relative role in reionizing the Universe.

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We explore the stellar mass density and colour profiles of 118 low redshift, massive, quiescent, central galaxies in order to attempt to find hints of the minor merger activity postulated to be the driver behind the size growth of MASSIVE galaxies since high redshift. We use deep imaging data (down to ~26 mag) in 5 bands from the Subaru Hyper Suprime Cam survey combined with Voronoi binning to maximise signal to noise, and perform SED fitting to construct spatially well-resolved colour and stellar mass density radial profiles, stacking these profiles to utilise the full statistical power of the sample. We find slowly decreasing colour profiles, and an expected smooth, declining stellar mass density profile in the central regions of our sample (~3 Re), however excesses of stellar mass density in the outskirts in the form of bumps in the profile. By visually inspecting the data, we morphologically split the sample, finding that almost 45% of the sample display signs of minor merger activity, 45% display a diffuse stellar envelope, possibly indicative of previous minor merger activity and the rest display no activity. We find that the excesses of stellar mass in the outskirts are driven by those galaxies with clear signs of minor merger activity or a diffuse stellar halo, and that this material only contributes to a few % of the total stellar mass. We also apply similar techniques to the data as in previous studies, finding that these techniques smooth out the signatures of these interactions.

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1. Supermassive black holes (SMBHs) form predominantly by accreting gas from an accretion disc, which is efficient only at radii < 0.001 pc. As this is well inside the region where the SMBHs gravity dominates, it remains unclear how material to be accreted can lose its angular momentum to reach these scales. Here, I will detail a possible solution to this problem, which is based on the cancellation of angular momenta of different inflows onto the innermost neighbourhood of the SMBH . 2. The recent by fly-bys of ‘Oumuamua and Borisov suggest a large space density of such interstellar asteroids and comets. I investigate the possibility of capturing such objects into the Solar system via a fly-by of Jupiter or Saturn, presenting analytical arguments and estimates as well as results of various numerical simulations. The most likely captures occur for an incoming speed of around 0.6 km/s and populate orbits akin to those occupied by long-period comets. We estimate that the Solar system may contain around 10000 captured Oumuamua-like asteroids and 100 captured comets.

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The presence of a gas volume-density gradient inside star-forming regions allow them to raise their star formation rate (SFR) compared to what they would experience if their gas was of uniform density. I define the “magnification factor 𝝵” as the ratio between the SFR of a centrally-concentrated clump and the SFR that this clump would experience should its gas be uniformly distributed. I show that magnification factors higher than 10 are achieved by power-law gas density profiles with logarithmic slopes steeper than -3. Such steep density profiles describe well the densest regions of the nearby molecular clouds MonR2 and NGC 6334. Clumps with a high magnification factor form stars much faster than expected based on the mean free-fall time of their gas. Therefore, not only does a gas density gradient inflate the clump SFR, it also inflates the star formation efficiency per free-fall time that we measure. The diversity in measured star formation efficiencies per free-fall time thus partly reflects the diversity in star-forming region structures. I provide a method to quantify the contribution of the gas density gradient of a clump to its SFR, thus allowing one to estimate its magnification factor.

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Blocked for talk related to ROSE/IAU OAE.

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