Talks, Seminars, Events

Abstract
Fast radio bursts (FRBs), bright millisecond duration radio transients, are quickly becoming a subject of intense interest in time-domain astronomy. FRBs have the exciting potential to be used as cosmological probes of both matter and fundamental parameters, but such studies require large populations. Advances in FRB detection using current and next-generation radio telescopes will enable the growth of the population in the next few years from 30 to hundreds. Real-time discovery and follow-up, and new studies of the FRB population will provide us with some of the greatest insights in the coming years. I will discuss many observational aspects of the FRB population, including polarisation, searches for multi-wavelength emission, localisation, and repeating FRBs. I will also discuss how our response to these events can inform next generation surveys and pave the way for the enormous number of FRB discoveries expected in the SKA era.

Abstract
The Galactic bulge is of importance to understand the formation of the Milky Way, as a massive and old stellar component. Studying the bulge is, however, hampered by the crowd of stars from various components, as well as extremely high and variable foreground extinction. The presence of two red clumps (RCs) observed in the color-magnitude diagram of the Milky Way bulge provides a great opportunity to study the structure and formation of the bulge. This double RC is widely accepted as evidence for a giant X-shaped structure originated from the bar instability. We suggested, however, a drastically different interpretation based on the multiple stellar population phenomenon as is observed in globular clusters (GCs), where the bright RC is from He and Na enhanced second-generation stars, while the faint RC is representing first-generation stars with normal abundances. Because these two scenarios imply very different pictures of the formation of the bulge, understanding the origin of the double RC is of crucial importance. As supporting evidence for multiple population scenario, here we report our discovery that the stars in the two RCs show a significant difference in CN-band strength. We also found that the red giant branch stars in the outer bulge are divided into two groups according to Na abundance in the [Na/Fe] - [Fe/H] plane. Since these chemical patterns and characteristics are only explained by stars originated in GCs, this is evidence that the double RC is due to the multiple population phenomenon. Furthermore, this result indicates that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way.

Abstract
Planets are common and mysterious astrophysical objects. Giant planets are key planets to investigate because they have a critical role in shaping the architecture of young planetary systems and their composition provides information on the physical and chemical properties of proto-planetary disks, the birth places of planets.
Gas giants are thought to have heavy-element cores in their deep interiors, and the division into a core and hydrogen-helium envelope is applied in both formation and internal structure models.
However, recent giant planet formation and evolution simulations show that this is an over-simplification. First, I will present updated formation and evolution models that follow the heavy-element distribution imply that giant planets are inhomogeneous and are expected to have dilute/fuzzy cores. I will then present updated structure models of Jupiter that fit the recent measurements of the Juno mission and discuss the importance of these results for our understanding of Jupiter's origin, and for the characterization of giant exoplanets.

Abstract
The first stars and first galaxies formed a few hundred million years after the Big Bang. Their emergence transformed the universe: the first heavy elements changed the gas physics and high energy photons reionized their surroundings. Hence, understanding this early era is at the frontier of modern astrophysics and cosmology. It can be well probed with ancient ultra-faint dwarf galaxies that orbit the Milky Way today. Two ultra-faint dwarf galaxies have been particularly interesting in this regard. Reticulum II is the first known "r-process galaxy". A prolific nucleosynthesis event must have gone off in this system very early on so that subsequent stars formed from gas enriched in large amounts of the very heaviest elements. Calculations for the elemental yield of a neutron star merger match the observed chemical abundances of Reticulum II's stars, thus solving a 60 year old puzzle about the astrophysical site of the rapid (r-) process. Tucana II was recently confirmed to be an extremely metal-poor galaxy ([Fe/H]~-3) with member stars up to 8 half light radii away from the center region. Other systems also contain stars in their outskirts, suggesting that such extended "halos" may not be uncommon among the tinyest dwarf galaxies, possibly being a signature of the very first merger events between galaxies at the earliest times.

Abstract
The Large and Small Magellanic Clouds (LMC/SMC), as two of the closest and most massive satellites of the Milky Way, have significant effects on the local Universe including the distribution of ultra-faint satellites and the orbits of tidal streams. Ongoing survey efforts ith the Dark Energy Camera have revealed a wealth of low-surface-brightness stellar substructures in the periphery of the Magellanic Clouds; characterising these structures will provide significant insight into the currently poorly-constrained masses and interaction history of the Clouds. In order to elucidate the properties of the structures, we have used 2df+AAOmega at the Anglo Australian Telescope to instigate a large-scale spectroscopic follow-up of stars across the Magellanic Cloud periphery. We are able to detect the kinematic signature of the Clouds up to projected distances of 23 degrees from the centre of the LMC. Combining our spectroscopically derived radial velocities with Gaia DR2 astrometry provides the first 3D kinematics for these regions. Our initial set of measurements, along a large substructure to the North of the LMC, reveal velocities near the extremity of the substructure and are significantly different from those expected from an extrapolation of the LMC rotation curve. Our ultimate aim is to use these 3D kinematics to assess dynamical models of the Magellanic Clouds; this will shed new light on the origin of the substructures and the evolution of the Magellanic/Milky Way system.

Abstract
Most of what we know about star and planet formation has been secured from spatial 2D observations of the local Galactic neighborhood, collected over the last 70 years. During this time we have established a series of ground truths developed around a poorly understood structure called Gould’s Belt. In this framework, we use Orion as the template for massive star formation and Taurus for low-mass star formation, but we do not know if these two clouds are related, nor why different clouds have different star formation yields. In this talk I will report the 3-D structure of all local cloud complexes (d < 2kpc), using accurate distances. We find a narrow and coherent 2.7 kpc arrangement of dense gas in the Solar neighborhood that contains many of the clouds thought to be associated with the Gould Belt. This finding is inconsistent with the notion that these clouds are part of a ring, disputing the Gould Belt model. The new structure comprises the majority of nearby star-forming regions, has an aspect ratio of about 1:20, and contains about 3 million solar masses of gas. Remarkably, the new structure appears to be undulating and its 3-D distribution is well described by a damped sinusoidal wave on the plane of the Milky Way, with an average period of about 2 kpc and a maximum amplitude of about 160 pc. Our results represent a first step in the revision of the local gas distribution and offer a new, broader context to studies on the transformation of molecular gas into stars.

Abstract
Large photometric and spectroscopic surveys, together with large cosmological simulations, have shown that galaxy properties are controlled by both galaxy secular evolution and galaxy environment. The latter is typically parameterized in terms of local galaxy density, dark matter mass of galaxy groups/clusters, and galaxy hierarchy (centrals vs. satellites). Galaxy environment acts on galaxies in two main different ways: i) it can prolong the star-formation history and sustain the mass assembly history of centrals, the most massive galaxies in groups/clusters. ii) It also removes gas and stars from satellites with the result of progressively switching off their star formation activity. In this talk I will present the observational trends between satellites stellar properties and environment that we have derived as a function of galaxy stellar mass, group/cluster mass and phase space, and discuss the physical mechanisms responsible for them. I will also address how the current and future telescope, either ground- or space-based, can foster our understanding of environmental effects in the local and high-redshift Universe.

Abstract
Large area catalogs of galaxy clusters constructed from ROSAT All Sky Survey provide the base for our knowledge on the population of clusters thanks to the long-term efforts on their follow-up. The advent of large area photometric surveys superseding in depth previous all-sky data allows us to revisit the construction of X-ray cluster catalogs, extending it to lower cluster masses and to higher redshifts. We perform a wavelet detection of X-ray sources and make extensive simulations of the detection of clusters in the RASS data. We conduct photometric and spectroscopic identification of our sample using SDSS public data and own programs within SDSS, extending RASS cluster catalogs to a redshift of 0.6. We show that there is no obvious separation of sources on galaxy clusters and AGN, based on distribution of systems on their richness. We conclude that optical cluster richness has to be included in the modelling of cluster selection function. Optical identification results in a substantial change in the expectations for detectability of galaxy clusters at X-ray. We show that cluster X-ray shape influences the X-ray cluster detection and report the results of the full modelling of the cluster selection function. We discuss the redshift evolution of the high end of the X-ray Luminosity function and provide a comparison of our sample with the currently discussed parameters of the LCDM model.

Abstract
Interacting galaxies and mergers are key events in galaxy evolution, and are often associated with boosts of the star formation activity, in particular in the form of massive clusters. Observations still struggle to identify the main driver(s) of these starbursts and it is still not clear whether they are triggered by an increased mass of dense gas, and/or an increased star formation efficiencies. Knowing which process dominates in which physical conditions is key to extrapolate our understanding of star formation to the early Universe. In this talk, I will use simulations of interacting galaxies, at low and high redshift, to pin down the underlying physics of starbursts. I will show that complex combinations of physical processes are playing different roles along the evolution of a merger, but also across a given galaxy, and account for a diversity of efficiencies. I will demonstrate that, despite having comparable outcomes in the star formation rates, these different physical mechanisms leave different signatures, for instance in the CO emission and the formation of massive clusters.

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At high accretion rates, the outward force of radiation pressure generated by energy released by infalling matter can exceed the inward pull of gravity. Such super-Eddington accretion flows occur in many systems, such as the inner regions of quasars and luminous AGN, ultra-luminous X-ray sources (ULXs), and tidal disruption events. Understanding such flows is important not only for interpreting the spectra and variability of these sources, but also to predict the rate of growth of black holes in the early universe, and to quantify energy and momentum feedback into the medium surrounding the black hole, a process likely to be important in galaxy formation. New results from a study of the magnetohydrodynamics of luminous accretion flows, in which radiation pressure dominates, will be presented. Our results reveal new physical effects, such as turbulent transport of radiation energy, that require extension of standard thin-disk models. We discuss the implications of our results for the astrophysics of accreting black holes.

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We are currently witnessing a golden age in Galactic halo science. Largely thanks to the Gaia mission, we now have phase-space plus chemical information for significant numbers of halo stars, globular clusters and satellite dwarf galaxies in the Milky Way. These halo populations can be used to uncover the assembly history of the Galaxy, probe the nature of dark matter, and scrutinise cosmological models. In this talk I will describe recent efforts to address these fundamental questions using a combination of new observational data and state-of-the-art cosmological simulations. In particular, I will discuss the total mass and mass profile of the Milky Way, the last major accretion event that shaped the Galaxy's history, and the (surviving and destroyed) dwarf satellite luminosity function.

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Understanding the first steps in the formation of stars and protoplanetary disks is a great unsolved problem of modern astrophysics. Observationally, the key to constraining theoretical models lies in high-resolution studies of the youngest protostars. \;
I will present the state of the art regarding observations and modeling of the youngest protostellar disks, observed less than 0.1 Myrs after the onset of protostellar formation. \;
In a first part, I will show how our millimeter dust continuum interferometric data suggests that most (>\;75%) protostellar disks observed less than 0.1 Myrs after the onset of protostellar formation are only found at very small radii <\;60 au, which favors magnetized models for protostellar disk formation. I will also present our ALMA observations of a very young solar-type protostar suggesting a disk is currently forming in counter-rotation with respect to the protostellar core rotation, and discuss potential scenarii to understand this oddity. I will show our SMA and ALMA observations of the magnetic field topology in a sample of young protostars and compare all observed protostellar properties to the typical outcome of models for protostellar formation. I will argue that our observations of small disks, counter-rotating disks and organized magnetic fields in the youngest star-forming cores question the established paradigm of disk formation as a simple consequence of angular momentum conservation during the main accretion phase: they instead highlight the need to investigate magnetized models in order to unveil the mechanisms responsible for protostellar disk properties.
In a second part, I will show both our observations and models of the dust continuum emission call for significant grain growth (grains larger than 10 microns and up to submillimeter sizes) at radii 100-1000 au in the protostellar envelopes observed less than tenth of a million years after the onset of collapse. I will describe new avenues to explore dust pristine properties and describe better in the future, for example, the initial conditions for the formation of planetesimals.

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KoCo Signature Speaker