2021 Spring Schedule

Title: AGN Warm Absorbers: The Rosetta Stone of AGN phenomenology?

Abstract: Warm Absorbers are blue shifted absorption features in the AGN X-ray spectra, the result of outflows photoionized by the AGN continuum. They span a wide range of ionization parameter, a feature that allows one to estimate their density profiles along the observer's line of sight. These are found to be quite shallow and extending over many decades in radius, inconsistent with radiation or thermally driven outflows, but consistent with magnetohydrodynamic ones launched across the entire accretion disk domain. It is shown that the corresponding wind mass flux increases with radius, a fact that affects crucially the dynamics of accretion. It is concluded that under these conditions, the global accretion rate is the parameter that determines the luminosity ratio of the Big Blue Bump to that of the hard X-ray emission, and as such determine the AGN structure in the black hole vicinity along with a host of phenomenology details. It is further proposed that similar structures are present in radio loud AGN too and they are crucial in the determination of their global phenomenology, usually referred to as "the Blazar Sequence".

Host: Apostolos Mastichiadis

Title: Why the day is 24 hours long

Abstract: Geologic data show that the number of days per month decreased over the last ~1,300 Myr, while the length of day increased. In contrast, the number of days per month increased between ~2,800 Myr and ~1,300 Myr, while the length of day was roughly constant, at ~19 hours, consistent with a balance between the Lunar tidal torque and the resonant solar thermal atmospheric torque. The data also show that the angular momentum of the Earth-Moon system (not including the orbital angular momentum of Earth) increased by several percent over the epoch when the day length was constant. We use the data to infer the mean surface temperature T as a function of time; a resonant atmospheric period of 19.5 hours corresponds to T~50 C. Similarly high temperatures are reached for plausible atmospheric pressures and compositions, despite the lower Solar flux at that epoch.

Host: Maria Petropoulou

Title: Dwarf Galaxies and the Black-Hole Scaling Relations

Abstract: The masses of supermassive black holes correlate tightly with properties of their host galaxies. One theory of these scaling relations ascribes them to feedback from black hole accretion, while a completely different picture argues that they arise purely statistically as galaxies and their central black holes are assembled by repeated mergers. Recent observations extend one of these scalings (the M - sigma relation) to dwarf galaxies. I will argue that this allows a clear decision between the two alternative theories of the scaling relations.

Host: Kalliopi Dasyra

Title: A polarimetric view of the Edge of M87's Black Hole

Abstract: The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole, has just revealed a new view of the massive object at the centre of the M87 galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic plasma jets from its core. In her presentation, Dr. Mościbrodzka will talk about these new observational results and their theoretical interpretation.

Host: Maria Petropoulou

Title: Understanding the formation of the Milky Way in a cosmological context: how unique is our Galaxy?

Abstract: Over the past few years, great advances have been made in our understanding of the formation of Milky Way, thanks to a wealth of data from Gaia and numerous other surveys. Whilst piecing together the past assembly events of our Galaxy, we also want to understand how this history fits within the wider picture of galaxy formation. Is the formation history of Milky Way typical for galaxy for its mass or, on the contrary, is somewhat odd and unrepresentative?
I will address this question with a new suite of zoomed cosmological hydrodynamical simulations, called ARTEMIS, which follow the formation of 45 Milky Way-mass galaxies in a Lambda CDM cosmology. The large number of simulated 'Milky Way analogues', each with its own formation history, allows us to put our own Galaxy within the proper cosmological context. Specifically, I will discuss the structure of present-day stellar haloes and of the population of surviving satellite galaxies of the simulated Milky Way analogues. Further, I will compare these properties with available observational data for the Milky Way and galaxies of similar mass.
I will also address previous claims of tensions between the properties of the Milky Way and those of galaxies simulated in a cosmological context. Such tensions refer to the metallicity of the Milky Way's stellar halo, or to the shape of surface brightness profile, but also to the luminosity function and radial distribution of its satellite galaxies. I will show that, after accounting for the specifics of various observational surveys, the properties of the Milky Way fit comfortably with the mean trends and scatter expected in a Lambda CDM cosmology.

Host: Despina Hatzidimitriou

Title: Results from the Solar Orbiter Mission

Abstract: Solar Orbiter, launched on 10 February 2020, is a space mission of international collaboration between ESA and NASA. It is exploring the linkage between the Sun and the heliosphere and has started to collect unique data at solar distances down to 0.49 AU. By ultimately approaching as close as 0.28 AU, Solar Orbiter will view the Sun with very high spatial resolution and combine this with in-situ measurements of the surrounding heliosphere. Over the course of the mission, the highly elliptical orbit will get progressively more inclined to the ecliptic plane. Thanks to this new perspective, Solar Orbiter will deliver images and comprehensive data of the unexplored Sun’s polar regions and the side of the Sun not visible from Earth. This talk will provide an overview of the Solar Orbiter mission and highlight its first science results.

Host: Ioannis Daglis