Unless otherwise noted, the Astrophysics Seminars take place on Monday, usually every other week, at 1:30pm in Aula Caldirola.
November 2024
November 4th
Speaker: Davide Bianchi (Università degli Studi di Milano)
Title: Measuring our expanding Universe: one year of DESI observations
Abstract: In its first year of observations the Dark Energy Spectroscopic Instrument (DESI) has built the largest map of galaxy redshifts to date, spanning the nearby universe to 11 billion light years. Accurate measurements of how galaxies distribute and cluster within this map allow us to trace the expansion history of the Universe to unprecedented precision and understand what role dark energy played in such process. In this talk I will present the first set of DESI cosmological results, coming from measurements of the baryonic acoustic oscillations (BAO) characteristic scale. (In the unlikely event that the results from “full shape” analysis will be released by the date of the talk I will include them in the presentation).
November 18th –> moved to November 25th
Speaker: Marion Villenave (Università degli Studi di Milano)
Title: Observational constraints on dust concentration and planetesimal formation in protoplanetary disks
Abstract: To form giant planets during protoplanetary disk lifetime, small micron sized particles must grow rapidly to larger grains. A full understanding of that process requires a detailed characterization of the radial and vertical structure of the gas-rich disks associated with young pre-main sequence stars. This is because the level of dust concentration controls grain growth efficiency and planetesimal formation. Multi-wavelengths observations of protoplanetary disks, for example in the millimeter and near-infrared, allow to probe very different grain sizes that are differently affected by evolutionary mechanisms. Here, I will discuss observational constraints on dust accumulation in early and evolved disks, with a particular focus on recent constraints on vertical dust concentration using multi-wavelength observations from various instruments, such as ALMA, JWST, HST… The modeling of multi-wavelength observations of inclined disks allows to identify high density regions, favorable for grain growth and planet formation, and to study the efficiency of planet formation in protoplanetary disks.
December 2024
December 2nd
Speaker: Mattia Sormani (Università degli Studi dell’Insubria)
Title: The formation of nuclear rings in barred galaxies
Abstract: I will give an introduction to our theory for the formation of nuclear rings, which are remarkable gaseous structures commonly found at the centre of barred galaxies including our own Milky Way. The rotating external bar potential excites strong density waves near the inner Lindblad Resonance. The density waves remove angular momentum from the gas disc and transport the gas inwards. The accumulation of gas at the inner edge of the gap is the nuclear ring. The process has many similarities with the opening of gaps in protoplanetary disks driven by embedded planets & with the opening of gaps in planetary rings driven by satellites.
December 16th
Speaker: Michele Moresco (Alma Mater Studiorum – Università di Bologna)
Title: Cosmology beyond standard probes: exploring new paths to constrain the expansion history of the Universe
Abstract: The measurement of the local expansion rate of the Universe, the Hubble constant, has recently triggered an important scientific debate. It has been found that this quantity, when measured with local probes, shows a 4-sigma tension with the one extrapolated from the analysis of the high-redshift Universe with Cosmic Microwave Background. This issue, often referred to as the ‘Hubble tension’, is now one of the hot topics in physics. Exploring new and complementary approaches to measuring how the Universe (and the structures therein) have evolved has, therefore, become fundamentally important. This will increase the accuracy of the measurements and keep systematic effects under control. In this talk, I will discuss different approaches and methods that can be pursued to obtain independent constraints on the Universe’s expansion rate, recently reviewed in Moresco et al. (2022). I will present how cosmic chronometers, the ages of the oldest star in the Universe, and gravitational waves can be exploited as cosmological probes and how they can provide fundamental information in modern cosmology in the future.
January 2025
January 20th
Speaker: Simone Paradiso INAF-OAS (Bologna)
Title: Advancing cosmological data analysis: reducing biases and including model choice as a source of uncertainty
Abstract: In contemporary cosmology, precise data analysis is essential for understanding the universe’s most profound mysteries. Upcoming surveys will provide unprecedented high-precision data, offering new insights into “dark” aspects of the cosmological model, such as the nature and evolution of Dark Matter and Dark Energy. Yet, tensions within the standard ΛCDM model—such as discrepancies in the Universe’s expansion rate (Hubble tension) and the amplitude of clustering—underscore the need for innovative statistical techniques.This talk introduces two novel methods to address these challenges. Bayesian Model Averaging (BMA) provides a principled framework to incorporate model uncertainty as an additional source of error while enabling data-driven model selection. We demonstrate its application to the Hubble tension, exploring Early Dark Energy as an alternative to ΛCDM and analyzing the most prominent extensions of the standard model using the latest CMB, LSS and SNIa datasets. The second focus is on cosmological parameter biases in LSS analyses, particularly the projection effects arising when extending analyses to small-scale features. These biases stem from nuisance parameter priors, which can feedback into and distort linear-regime cosmological parameters. This talk presents an innovative solution: a reparameterization of nuisance parameters using non-linear transformations via Generalized Additive Models (GAMs), effectively mitigating projection effects. By leveraging well-established techniques from data-intensive fields, this talk demonstrates how these approaches unlock the full potential of cosmological datasets, advancing the cosmological model and overcoming longstanding limitations in standard analyses.
February 2025
February 3rd
Speaker: Cristina Baglio INAF-OAB (Osservatorio Astronomico di Brera – Merate)
Title: Low mass X-ray binaries: an optical/NIR polarimetric view
Abstract: Low mass X-ray binaries are binary systems hosting a compact object (a stellar mass black hole or a neutron star) which accretes mass from a low-mass companion star through an accretion disc. These systems are perfect laboratories to study accretion mechanisms, and how these are coupled with ejections in the form of jets and/or winds. Optical and near-infrared (NIR) observations are of great importance, since at these frequencies, the companion star, accretion disc, the jet, the hot spot, the accretion disc wind and the hot accretion flow could be detected.
Carefully modeling the broad band spectral energy distribution of these systems can reveal sources of emission such as the disc and jet, but sometimes this is insufficient to disentangle all the components involved in the emission. Spectroscopic observations are an important tool in order to unambiguously detect the presence of discs or winds, thanks to the observation of specific features in the spectrum, like double-peaked emission lines and P-Cygni profiles. A very powerful tool is also offered by polarimetric observations, which, especially if combined with photometry or spectroscopy, can help to unveil unambiguously which physical processes are at play in the system, and can give important information about the geometry of the source.
Synchrotron radiation produced by collimated jets, Thomson scattering with free electrons in the accretion disc, scattering of the accretion disc’s radiation in the hot accretion flow located close to the inner radius of the disc are typically the main processes capable of producing linear polarization in X-ray binaries.
In this seminar I will present a review of the most recent and significant polarimetric measurements of these sources at optical/NIR frequencies. I will first focus on the polarization measurements aimed at detecting the emission of jets, showing how these can be crucial to probe the accretion/ejection coupling scenario proposed for these sources. I will also review how polarimetric observations can give important information on the geometry of the systems, and I will show how the contribution of jets to the emission of X-ray binaries can be constrained thanks to polarimetric observations.
February 10th
Speaker: Rebecca Nealon (University of Warwick)
Title: Warped discs as a pathway for planet formation
Abstract: The last decade of observations of protoplanetary discs have shown a wealth of substructure including rings, gaps and spiral arms. Perhaps most intriguingly these observations also revealed the importance of the 3D structure of discs, where some discs are observed to have orientations that change as a function of radius or may also be broken. These so called ‘warped discs’ have challenged our understanding of disc evolution, and recent work has shown that these discs form a significant fraction of the disc population. In this talk I will discuss the connection between warped discs and the onset of planet formation, a major open question in planetary science.
February 17th
Speaker: Avishek Basu (University of Manchester)
Title: Long-term subtle emission changes and the interplay with Timing Noise in Pulsar Magnetosphere
Abstract: The average pulse profile of a pulse serves as its fingerprint and is unique to every pulsar. A wealth of information about the pulsar magnetosphere is derived from its profile shape. For the majority of the pulsars, the average shape remains invariant over time. However, for very few sources variations in the profile shape have been observed. Sometimes these changes are correlated with the changes in the rotation of the pulsar, which gives insight into the dynamic nature of the magnetosphere. The MeerKAT monitoring data accrued over the last few years on ~500 pulsars under the MeerTIME’s Thousand Pulsar Array (TPA) programme, has enabled the detection of subtle emission changes in seven sources not yet known to exhibit long-term profile evolution. These variations are theorised to originate from the magnetospheric state changes with associated current density variations. Precession could play a role, and emission heights could be affected. In this talk, I will briefly describe the techniques adopted to measure the subtle changes in the emission state of the pulsar both in the total intensity and polarization domain and discuss how they are connected to the rotational evolution of the neutron stars. Further, I will elucidate how these measurements have been used to derive the limits on the various measurable parameters such as changes in the emission height, impact parameter of the line of sight, and changes in the magnetospheric charge density.
February 24th AT 1:00 PM (instead of 1:30 PM)
Speaker: Nicoletta Krachmalnicoff (SISSA)
Title: The Present and Future of CMB Polarization Observations
Abstract:
Following the significant successes of the Planck space missions, we have entered a new era of Cosmic Microwave Background (CMB) observations, with experiments now focusing on precise measurements of the polarized signal. These measurements have the potential to shed light on many open questions in fundamental physics. A primary goal of current and future CMB experiments is the detection of the signature of primordial gravitational waves in the so-called B-mode polarization signal, which would provide unprecedented insight into the physics of the Inflationary epoch. In this talk, I will briefly introduce the physics of CMB polarization, review the current status of observations, and discuss how we are preparing to meet the numerous challenges posed by these measurements. I will highlight the importance of both ground-based and space-based observations, present preliminary maps from the Simons Observatory, and outline ongoing studies aimed at finalizing the design of the future LiteBIRD space mission.
March 2025
March 10th
Speaker: Heshou Zhang (Italian National Institute for astrophysics)
Title: A magnetized halo of the Milky Way from inner Galaxy outflows
Abstract: The large-scale structures such as Fermi Bubbles and eROSITA Bubbles provide a unique opportunity to study our Milky Way. However, the nature and origin of these large structures are still under debate. In this talk, I will present the identification of several kpc-scale magnetised structures based on their polarized radio emission and their gamma-ray counterparts, which can be interpreted as the radiation of relativistic electrons in the Galactic magnetic halo. These non-thermal structures extend far above and below the Galactic plane and are spatially coincident with the thermal X-ray emission from the eROSITA Bubbles. Multi-wavelength spectral energy distribution analyses have revealed that both thermal and non-thermal extended structures can be explained by Galactic outflows driven by the active star-forming regions located at 3 − 5 kpc from the Galactic Centre (Zhang, Ponti et al. 2024 Nature Astronomy, https://ui.adsabs.harvard.edu/abs/2024NatAs…8.1416Z/abstract ). These results reveal how X-ray-emitting and magnetised halos of spiral galaxies can be related to intense star formation activities.
March 24th
Speaker: Pasquale Serpico
Title: Primordial black hole formation from self-resonant preheating?
Abstract: TBD
April 2025
April 7th at 15:00 different time
Speaker: Luca Matra (Trinity College Dublin)
Title: Exocomets in the latest stages of planet formation
Abstract: Evidence for exocomets in extrasolar planetary systems has rapidly increased over the past decade, giving rise to the budding field of exocometary science. Exocomets are detected through the gas and dust they release as they collide and grind down within their natal belts (also known as debris disks), or as they sublimate once scattered inwards to the regions closest to their host star. Most detections are in young, 10 to a few 100 Myr-old systems that are undergoing the final stages of terrestrial planet formation. This opens the exciting possibility to study exocomets at the epoch of volatile delivery to the inner regions of planetary systems. In this talk, I will start by introducing exocometary belts and show some of the latest observational results on their structure and link with planets. I will then present the different lines of evidence for their exocometary nature from UV to mm wavelengths, both in the innermost and outer regions of planetary systems. Finally, I will focus on how detection of gas allows us to estimate abundances in young exocomets, and allow detail comparison with younger protoplanetary disks and Solar System comets – including the latest results from the ARKS ALMA Large Program.
April 28th online seminar
Speaker: Alessia Ritacco (CNRS Grenoble)
Title: TBD
Abstract: TBD
May 2025
May 12th
Speaker: Andrea Lapi (SISSA Trieste)
Title: Little Ado About Everything: ηCDM, a stochastic cosmology from structure formation
Abstract: I will present the ηCDM framework, a new cosmological model aimed to cure some drawbacks of the standard ΛCDM scenario, such as the origin of the accelerated expansion at late times, the cosmic tensions, and the violation of the cosmological principle due to the progressive development of inhomogeneous/anisotropic conditions in the late Universe via structure formation. To this purpose, the model adopts a statistical perspective envisaging a stochastic evolution of large-scale patches in the Universe, which is meant to render our ignorance on the complex processes leading to the formation of the cosmic web. The stochasticity among different patches is technically described via the diverse realizations of a multiplicative noise term (‘a little ado’) in the cosmological equations, and the overall background evolution of the Universe is then operationally defined as an average over the patch ensemble. I will highlight that the ensemble-averaged evolution in ηCDM can be described in terms of a spatially flat cosmology and of an ‘emergent’ dark energy with a time-dependent equation of state, able to originate the cosmic acceleration with the right timing and to solve the coincidence problem. I will showcase the extremely good performances of the ηCDM model in confronting the most recent supernova type-Ia, baryon acoustic oscillations and structure growth rate datasets. I will stress that ηCDM is able to alleviate simultaneously both the H0 and the fσ8 tensions. Finally, I will propose that the Linders’ diagnostic test may constitute a helpful tool to better distinguish ηCDM from the standard scenario in the near future via upcoming galaxy surveys at intermediate redshifts such as those being conducted by the Euclid mission.
May 26th
Speaker: Ylva Gotberg (Institute of Science and Technology Austria)
Title: Stars stripped in binaries: from theory to observation
Abstract: A third of all massive stars are predicted to lose their hydrogen-rich envelope through mass transfer or common envelope ejection initiated by a binary companion star. As a result, the hot and compact helium core is exposed. These “stripped stars” are the direct progenitors of hydrogen-poor supernovae and merging binary neutron stars, but they are also so hot that they should boost the ionizing output from bursty star-forming galaxies. Despite their importance, stripped stars remained, until recently, observationally unconfirmed since their predicted existence over half a century ago. We found the first set of stripped stars by combining ultraviolet and optical photometry with follow-up spectroscopy in the Magellanic Clouds. By fitting their spectra with a new grid of models, we could measure stellar properties and thus confirm that the predictions from binary evolution models are broadly consistent with observed stripped stars.
June 9th
Speaker: Alessia Franchini (Università degli Studi di Milano)
Title: Gravitational wave sources in gaseous environments
Abstract: In this talk I will present our study of gravitational wave sources in gaseous environment. I will discuss the evolution of massive black hole binaries (MBHBs) and extreme mass ratio inspirals (EMRIs). Understanding the interaction of MBHBs with their gaseous environment is crucial since at sub-parsec scales the binary is too wide for gravitational wave emission to take over and to drive the two black holes to merge. The presence of a gaseous disc around sub-pc MBHBs can further trigger electromagnetic emission that can be detected by optical surveys (ZTF, VRO), therefore revealing the presence of hidden binaries. In this talk I will present our recent results on the evolution and emission signatures of MBHBs embedded in circumbinary discs. The second interesting class of gravitational wave sources are EMRIs, formed by small compact objects in-spiralling towards the massive black hole at the centre of galaxies. These EMRIs are impossible to detect before their in-spiral phase in absence of a gaseous environment. I will discuss how we can potentially detect EMRIs through soft X-ray flares in the light curves of tidal disruption events known as quasi periodic eruptions (QPEs).