We host a mix of virtual and hybrid seminars every other Thursday from 1:30-2:30pm.  For information on how to join them, or for any other queries regarding the seminars, please contact Amery Gration.

2023/24 Seminars

14 September: Boquan Chen (Australian National University) - hybrid

What we learn about the Milky Way from reproducing the dual-[α/Fe] distribution in its disk and the unexpected  [α/Fe]-rise among its oldest stars
This talk synthesizes two pivotal studies on the Milky Way’s chemical evolution. The first employs a multi-zone galactic chemical evolution (GCE) model with updated nucleosynthesis yields and a range of physical processes. The model effectively reproduces observed [Fe/H] and [α/Fe] distributions at different Galactic coordinates (R, |z|). It negates the need for secondary infall to account for the chemical spread and [α/Fe]-gap, identifying the rapid evolution of [α/Fe] after Type Ia supernovae onset as key. The second study examines the proto-Milky Way epoch, based on APOGEE and H3 survey data, to investigate the rise in [α/Fe] during this period. A GCE model with five free parameters reveals that the rise in [α/Fe] is primarily due to a significant inflow of fresh gas, not changes in star formation efficiency (SFE). It identifies this [α/Fe]-rise as a signature of cold mode accretion preceding disk formation. Combining these studies offers a holistic view of the Galaxy's chemical history. The first study's multi-zone model provides a better understanding of how individual processes contribute to the Galaxy's chemical landscape in the [Fe/H]-[α/Fe] plane and identifies a potentially prevalent pathway to the two [α/Fe]-sequences in the disk. The second study highlights the importance of gas accretion during the proto-Galaxy phase and identifies the [α/Fe]-rise as a chemical marker for cold mode accretion. Together, they open new research avenues by reconciling observed chemical properties with theoretical predictions. The integration of these findings provides a coherent, validated model for future galactic chemical evolution research, enriching our understanding and facilitating more precise predictions for upcoming observational campaigns targeting the Galaxy or beyond.

21 September: Renuka Pechetti (Liverpool John Moores University) - hybrid

A search for IMBHs in nearby stripped nuclei
Massive black holes in stripped nuclei with masses above 107 M in the Virgo and Fornax clusters have been discovered, but so far the expected population of black holes have not been found in the more abundant stripped nuclei expected at lower masses. The Local Group's most massive clusters are the perfect place to hunt for the intermediate mass black holes (IMBHs). IMBHs, mainly cannot be detected robustly as they also imitate the presence of a collection of stellar mass black holes. These measurements are needed because they are a very important link in understanding the formation mechanisms of black hole progenitors and formation of black holes in low-mass clusters. I will talk about the possibility of IMBHs or stellar mass black holes in nearby massive (~106 M) clusters with evidence for a robust IMBH detection of ~100,000 M in the most massive cluster of M31. In addition, I will also present the analysis of dark mass at the center of Omega Centauri, the most massive cluster of Milky Way.

26 October: Tomás Ruiz Lara (University of Granada) - online

Star formation histories and galaxy evolution in the Universe, from our Galaxy to galaxies inhabiting voids
In this talk I will review the two main project I am currently working on: the study of the formation and evolution of our Galaxy from Colour-Magnitude Diagram reconstruction of Gaia data and the CAVITY project, a legacy project targeting galaxies in cosmic voids with the goal of unveiling whether void galaxies form differently than those in more dense environments like clusters or filaments. In both projects the recovery of star formation histories plays a pivotal role. However, drastically different techniques are applied in each case. For the Milky Way, as commented, SFHs will be obtained via CMD fitting. In the case of void galaxies, located beyond the local vicinity and for which individual stars cannot be resolved, we rely on integrated information, and thus, full spectral fitting techniques will be used. Along this talk, apart from commenting on the main results we are obtaining I will pay especial attention to the different methodologies to obtain SFH in the Universe.

9 November: Francesca Fragkoudi (Durham University) - hybrid

Barred galaxies in ΛCDM: Uncovering the formation history and dark matter content of Milky Way-type galaxies
The advent of high resolution hydrodynamical cosmological simulations allows us to now study the dynamics of barred spiral galaxies, such as our own Milky Way, within the full ΛCDM cosmological context. I will present what we have learned about the formation history of our galaxy and its inner structures by comparing the chemo-dynamical properties of its stellar populations to both isolated/tailored and cosmological simulations. In particular, I will discuss what cosmological simulations are revealing about the Galaxy's merger history, its subsequent dynamical evolution, and the almost entirely in-situ nature of its bulge. I will also present results on the dynamics of barred galaxies in cosmological simulations, in particular the interaction through dynamical friction of the bar and the dark matter halo, which sheds light on the amount of dark matter in massive spiral galaxies. I will discuss these results in light of recent claims in the literature of a tension between observed fast bars and ΛCDM, and within the context of galaxy formation and evolution in general.

16 November: Zahra Sharbaf (Instituto de Astrofísica de Canarias) - hybrid

What drives the variance of galaxy spectra?
We present a study aimed at understanding the physical phenomena underlying the formation and evolution of galaxies following a multivariate analysis of spectroscopic data as the main source of information, such as principal component analysis (PCA). Rather than fitting our observations to population synthesis models, we use a data-driven approach. First, we apply PCA to a homogeneous sample of galaxy spectra from SDSS in the velocity dispersion range of 100-150km/s, including all three types of galaxies concerning nebular emission, namely star-forming, AGN, and quiescent according to the standard BPT classification, to assess the properties of the spectra in a model-independent way. In PCA, information is directly related to the variance of the spectral elements and defines the so-called principal components (PCs) as eigenvectors produced by the decorrelation of the covariance matrix. Population synthesis is only used a posteriori to give physical meaning to the trends found from PCA. We restrict the analysis to the first three PCs and find that PCA segregates the three types with the highest variance mapping SSP-equivalent age, along with an inextricable degeneracy with metallicity, even when all three components are included. The result is consistent in different spectral windows, but the variance is maximal in the blue wavelength range, roughly around the 4000A break.

As a continuation of our study, we use simulated data (from EAGLE and Illustris-TNG) to understand the higher variance PCs extracted from the SDSS spectra in the three classes. The synthetic data is also split concerning nebular emission using subgrid parameters related to star formation and AGN activity. As a result of preliminary analysis of the EAGLE simulated spectra, we were consistent with PCA analyses of SDSS data, and some differences could indicate stellar population properties and how they affect structure formation in galaxies. Based on this study, we aim to address these questions: What is the physical interpretation of the first few PCs? What drives the variance of galaxy spectra? How well does subgrid physics map this variance? Is it possible to fully disentangle the well-known degeneracies between observables?

23 November: Terese Hansen (Stockholm University)

Hunting for Gold (or, more realistically, uranium)
Heavy elements like silver, gold, and uranium are produced via the rapid neutron-capture (r-)process. This process only occurs in rare explosive events in the Universe like supernovae (SNe) and neutron star mergers (NSMs), making it highly challenging for astronomers to gather direct observations of the element creation. Likewise, it is difficult for nuclear physicists to recreate and study the nuclear process in the laboratory. These obstacles are why we today, six decades after the theoretical prediction of the r-process, still do not know how or where in the Universe gold and silver are made. However, in 2017, the R-Process Alliance (RPA) initiated a successful new search to uncover bright metal-poor halo stars enriched with r-process elements. These stars are invaluable laboratories for studying the r-process as the gas from which these stars formed was polluted by at most a few enrichment events --- perhaps even a single explosion. The RPA has collected spectra of ~2000 stars and discovered over 70 new highly r-process enhanced stars. I will report on RPA efforts over the past six years and outline plans for the future, including ways this stellar sample will help constrain the astrophysical site(s) of the r-process. I will also discuss recent detections of r-process-enhanced stars in Milky Way satellites and streams, where these stars have been detected in several systems, allowing us to study the environment where the r-process occurred.

7 December: Jennifer Fiske (MSSL)

Probing galactic chemical evolution with the nuclear stellar disc
The nuclear disc is a kinematically cold stellar feature at the centre of our Galaxy. It forms together with a bar and is fuelled by gas from the bar tips flowing in along the bar edges. Its shielded position and low velocity dispersion makes it a unique laboratory to study gas flows in the central Galaxy. Unfortunately, especially its chemical composition still remains understudied. I will show how taking into account not only its dynamics, but also its chemical compositions helps to answer open questions in chemical evolution and beyond, like the effect of a multi phase (i.e. hot and cold) ISM on elemental enrichment and the central angular momentum balance. Using a suite of full chemical evolution models of a nuclear disk embedded in a full galactic simulation using our newly developed RAMICES II code, I show how the formation history of the nuclear disc results in specific radial abundance profiles for different tracer elements. This allows us to identify the key gas parameters influencing these abundance profiles, such as the fraction of cold gas from stellar death events like supernovae and neutron star mergers as well as the general loss to the CGM. Unfortunately, piercing through the Galactic disk is difficult due to strong dust obscuration. To solve this problem, I propose a series of measurements feasible with current methods to test those predictions.

1 February: Nicolas Martin (Strasbourg astronomical observatory)

Exploring the build-up of the Milky Way with the most metal-poor stars and the Pristine survey
I will present how, thanks to the metallicity-sensitive, narrow-band photometric survey Pristine, we are able to probe the assembly of the Milky Way and its halo. The most metal-poor stars are also likely the oldest stars of the Galaxy and, while extremely rare, they are crucial fossils of the Milky Way’s build-up phase. I will show why the Pristine survey is so efficient at finding those stars and what they have started to tell us about the early assembly phase of the Milky Way and the nature of some of the most intriguing stellar structures it accreted onto its stellar halo at later times.



Past seminars