This week's PhD colloquia are highlighted.
| Date | Time | Title/Abstract | Speaker | Affil. |
|---|---|---|---|---|
| 03/09 Thursday GM4.13 | Building astrochemical models - Open source software, surrogate models and interpretable machine learning for astrochemistry Present-day astrochemistry relies on observations, laboratory studies, theory, and computational models to understand the formation and destruction of molecules across diverse astrophysical environments. My thesis and talk focus on computational astrochemistry, specifically astrochemical models that track the abundances of molecules over time. The first part of the talk introduces our core astrochemical simulation framework, UCLCHEM (uclchem.github.io), which has been upgraded and expanded into a well-maintained, open-source code for the community. In the second part, I present a strong use case of UCLCHEM: creating and analysing templates of chemical signatures for regions in the Central Molecular Zone of our Milky Way, in support of the ACES collaboration. In the final part, I discuss modern methodology development for astrochemistry, including explainable machine learning methods for interpreting simulated molecular abundance ratios, surrogate machine learning models for accelerating three-dimensional simulations of star-forming regions, and differentiable astrochemical codes that will enable the next generation of computational studies. Together, these developments push astrochemical modelling toward faster, more interpretable, and more physically integrated simulations of the interstellar medium. | Gijs Vermariën | STRW | |
| 02/10 Friday BW.0.17 | 15:00 | Massive galaxies in the first billion years Sometime within the first billion years of cosmic history, the first galaxies emerged. How quickly these galaxies could assemble their stars, metals, dust, and structure remains a fundamental question in galaxy evolution. Thanks to the advent of JWST, as well as increasingly rich ALMA datasets, it is now possible to investigate the properties of these early galaxies in unprecedented detail. In this thesis, we use complementary observations from these facilities to build a multi-wavelength view of massive star-forming galaxies at z~6-8 (lookback times of over 12.8 billion years), focusing on the REBELS-IFU sample. Using high-resolution ALMA and JWST observations of the z=7.31 galaxy REBELS-25, we show that a rotationally supported gas disc was already in place only ~700 million years after the Big Bang, while dust obscuration produces a very different appearance at rest-UV and optical wavelengths. Across the wider REBELS-IFU sample, JWST/NIRSpec spectroscopy reveals substantial chemical enrichment and a mass-metallicity relation already in place in the first Gyr, while Ly-alpha damping wings and [CII] emission provide complementary insight into the massive neutral gas reservoirs of these systems. Together, these results reveal galaxies that were already massive, chemically enriched, dusty, and dynamically cold at very early cosmic times, while highlighting the importance of multi-wavelength observations for building a complete picture of early galaxy evolution. | Lucie Rowland | STRW |
| 05/10 Monday DM.1.15 | 14:00 | Uncovering the Hidden Universe: Luminous Type-2 QSOs at Cosmic Noon and Beyond How much supermassive black hole growth is hidden by dust, and what can obscured quasars reveal about the evolution of galaxies? Although luminous unobscured quasars have been extensively studied, their obscured counterparts remain poorly explored beyond redshift two. Identifying this missing population is essential for understanding whether obscuration primarily reflects viewing angle or a distinct evolutionary phase. In this colloquium, I will present a systematic search for luminous Type-2 quasars at cosmic noon and beyond, combining optical and infrared selection with spectroscopic observations from Keck and Gemini. This effort has identified 53 new Type-2 quasars, including 40 at redshifts above two, and suggests that luminous obscured quasars are at least as common as their unobscured counterparts at comparable luminosities. Their spectral energy distributions reveal powerful dust emission and a possible contribution from scattered nuclear light, while their diverse emission-line profiles challenge a simple orientation-only interpretation. I will discuss potential connections to the little red dots discovered by JWST and an extension of our search to a Type-2 quasar at redshift 6.9. Moving beyond the central black hole, I will present Keck/KCWI observations of giant Lyα nebulae and ALMA observations of molecular gas and companion galaxies, probing how these quasars interact with their surroundings. Together, these observations connect hidden black hole growth to gas reservoirs and galaxy environments. I will conclude with a brief outlook on developing AI tools for astronomical spectroscopy and scientific discovery. | Ben Wang | STRW |
| 20/10 BW.0.18 | 11:00 | Cradle to Climate: Tracing the Composition and Dynamics of Extrasolar Atmospheres Over the past decades, a growing number of giant exoplanets have been directly imaged next to their host stars. Characterising their atmospheres in detail and how these planets even came to be, however, remains challenging. Free-floating brown dwarfs, which cool through the same range of temperatures, serve as valuable analogues to better understand the processes that shape extrasolar atmospheres as a whole. In my thesis, I use high-resolution spectroscopy from the ground and space-based observations with JWST to study the atmospheres of an isolated brown dwarf, the nearest brown-dwarf binary, and a planetary- mass companion. I will show how atmospheric retrieval models are used to infer elemental and isotopic compositions, thereby helping to inform the object's formation environments. All targets are affected by dynamical processes, showing signs that vertical mixing and patchy cloud-covers alter their chemistry and spectral appearance. In addition, I introduce a new software package for computing the molecular and atomic opacities needed for this kind of analysis. Together, these analyses demonstrate how the chemical abundances in their atmospheres can be used to trace the origin and atmospheric dynamics of both brown dwarfs and giant exoplanets. | Sam de Regt | MPIA (formerly STRW) |
| 03/12 Thursday BW.0.18 | 15:00 | Patrick Dorval | STRW |
For questions and/or suggestions concerning the colloquium series. Please contact Andrew Sellek (e-mail ).