Talks

Using zoom-in cosmological simulations, I will discuss some of the physical and observable properties of the circumgalactic medium (CGM). I will focus on non-thermal physical processes and how they impact the galactic ecosystem. The presence of magnetic fields and feedback from relativistic cosmic rays changes the flow of gas in the CGM, affecting the efficiency of outflows driven by stars or supermassive black holes as well as the turbulence-driven mixing in the CGM. This also affects our simulations’ predictions for neutral hydrogen and metal ions, observable through 21 cm emission and quasar absorption lines. I will show how these effects of magnetic fields and cosmic rays vary across a wide range in halo mass, from dwarf galaxies to galaxy groups.

Project AMIGA (Absorption Maps In the Gas of Andromeda) provides an unprecedented view of the circumgalactic medium (CGM) of our nearest large galaxy neighbor. Using 55 sightlines obtained largely from two large HST COS programs, we map Andromeda's CGM across distances spanning from 10 to 570 kpc, nearly reaching twice its virial radius (Rvir=300 kpc). Using extensive diagnostics of different gas phases (OI/VI, SiII/III/IV, CII/CIV, FeII, AlII), this study uniquely bridges the smaller scales of the CGM with its largest scales extending into the intergalactic medium (IGM). In this talk, I will demonstrate how gas complexity and gas-phase structures significantly change with impact parameters but show little variation with azimuth relative to major/minor projected axes. Our program can differentiate components associated with the thick disk from those in the CGM, providing crucial insights for characterizing gas phases and accurately determining the total mass of Andromeda's CGM. I will place these findings in the broader context of other CGM observations, recent cosmological zoom simulations of Milky Way-mass galaxies at z~0, and how this pathfinder study may inform next-generation observations of the CGM/IGM with HWO.

We report results from observations of the CGM ionized, atomic, molecular, and condensed phases using a combination of integral field spectroscopy (MaNGA, VLT MUSE, JWST MRS), and imaging and spectroscopy from HST, VLT, Magellan. In a study of the warm and cool CGM of galaxies mapped with IFS, and a comparison of the kinematics, ionization, and metallicity of this gas with the ionized gas in star-forming regions in the galaxies, we find consistency with a co-rotation of the cool CGM with galaxy disks and hints of changes in gas ionization, potentially due to the stronger intergalactic radiation field at larger galactocentric distance. Our spatially resolved maps of gas metallicity and ionization around galaxies provide constraints on models of the metal distribution around galaxies. Our results are also consistent with higher metallicity and higher ionization parameter for gas at higher elevation angles, as expected for outflows. Our JWST studies of the composition, structure, and extinction properties of the dust grains in both the diffuse and dense ISM/CGM of galaxies at 0

Simulations with fixed spatial resolution are an excellent tool to investigate the interplay between different phases of gas in and around galaxies because they mitigate the disparity in cell sizes due to density variations in traditional mass-based refinement schemes. Additionally, the moving-mesh technique implemented in Arepo has been shown to minimize numerical mixing and instability suppression. In this talk, I will introduce a new suite of cosmological zoom simulations with 200 pc resolution covering the inner CGM of a Milky Way-mass galaxy, utilizing the full IllustrisTNG galaxy formation model. At this high resolution, we find increased turbulent velocities, many small, cool cloudlets, and a smooth and homogeneous hot phase. I will outline these results and discuss the implications for high- and intermediate-velocity cloud studies and gas mixing in the CGM.

Despite its vast extent—spanning hundreds of kiloparsecs beyond the galactic disk—the circumgalactic medium (CGM) is shaped by microphysical processes operating on much smaller scales. One key example is the coupling between cosmic rays and gas. Under the right conditions, cosmic rays can dominate the pressure support in the CGM of low-redshift L* galaxies. However, this coupling depends sensitively on AU-scale magnetic field fluctuations—well below the resolution limit of modern galaxy-scale simulations. In this talk, I will highlight recent theoretical developments in cosmic-ray transport and their implications for CGM pressure profiles. I’ll also introduce CGSM, a new subgrid model designed to represent unresolved cold gas structures in hydrodynamic simulations, and discuss its potential to bridge the gap between microphysics and galaxy evolution.

Abstract – Merging black holes, collapsing dark matter, giant supernova explosions: a tapestry of cosmic events stretching over the past 13.8 billion years have shaped our existence in a vast universe. Faced with this complexity, humanity has increasingly turned to computers to help extract a clear understanding of the cosmos and our place within it. This lecture will explore how the history of how these tools have developed, in parallel with more down-to-earth computational pursuits like weather forecasting. We will see how the resulting codes have unlocked our understanding of the universe, from galaxies and black holes to the essence of matter. And the lecture will conclude with a look at a contentious idea put forward by some philosophers and scientists – that we may already be living inside a simulation.    Bio – Andrew Pontzen is a professor of cosmology, and from January 2026 will direct Durham University’s Institute for Computational Cosmology. His research concerns how structure formed in our universe, from its opening moments to the present day. He has written for the New Scientist, BBC Sky at Night and BBC Science Focus; lectured at the Royal Institution; appeared on BBC, Amazon Prime and Discovery Channel documentaries; and contributed to BBC Radio 4 programmes including Inside Science and The Curious Cases of Rutherford & Fry. He is also the author of The Universe in a Box which dives into the role of simulations in cosmology and beyond, recently published to critical acclaim.

The circumgalactic/intergalactic medium (CGM/IGM) represents a significant baryon reservoir for sustaining star formation and provides insights into the inflows, outflows, and feedback history of galaxies.  Star-forming dwarf galaxies, with their shallow potential wells, are predicted to drive metal-enriched gas into the CGM/IGM. Therefore, a census of the CGM around dwarf galaxies can provide insights into the stellar feedback. We present highly sensitive absorption-line measurements in quasar sightlines adjacent to 91 isolated dwarf galaxies with a median stellar mass of M_star/M_sun≈8.4 from the Cosmic Ultraviolet Baryon Survey (CUBS). This survey uses HST absorption spectroscopy to access a range of ion transitions from 0.077

Active Galactic Nuclei (AGN) feedback is known to play a key role in galaxy evolution and in regulating star formation. Studying the interplay between the central AGN and the different gas phases permeating galaxies is crucial to further our understanding of this powerful mechanism. We have observed the central regions of four brightest cluster galaxies at optical wavelengths using the Keck Cosmic Web Imager. With the high-resolution integral field unit data obtained from these observations, we map the fluxes and velocities of both emission lines and stellar absorption lines. This allows for a detailed tracing of gas cooling in galaxy centres. These galaxies have extensive X-ray and radio observations, allowing us to compare the dynamics of different gas phases and to study their interactions. Nebular emission extends up to tens of kiloparsecs from the central cluster galaxies of Abell 1835, PKS 0745-191, Abell 262, and RX J0820.9+0752. With the stellar continua, we map the kinematics and ages of the stars, learning about the systems’ star formation histories. Our findings highlight the complex stellar and gas dynamics which can be induced by radio-mechanical feedback. Surprisingly, three of the four systems have substantial (~ 150 km/s) velocity differences between their central galaxy and its associated nebular gas. This shows that the central galaxy is not at rest with respect to its surrounding nebula. In PKS 0745-191 and Abell 1835, nebular gas is churned up by buoyantly rising bubbles and jets. The churned gas is also surrounded by larger scale, lower velocity dispersion nebular emission. These complex motions will affect thermally unstable cooling, the interactions between the AGN and its atmosphere and how jet energy dissipates in its surroundings. These novel results highlight the deeply complex dynamics of AGN feedback and the multiphase gas in the centre of massive galaxies.

The circum-galactic medium (CGM) is at the nexus of the gas inflows and outflows that regulate galaxy evolution. Consequently, the CGM provides an ideal laboratory for studying galaxy fueling, feedback, and interactions. In the last decade, the simultaneous availability of UV spectra from the Cosmic Origins Spectrograph, deep integral field spectrographs, and wide galaxy redshift surveys have revolutionized our ability to characterize the CGM empirically. I will review recent progress enabled by the Cosmic Ultraviolet Baryon Survey (CUBS) and MUSE Quasar Blind Emitter Survey (MUSEQuBES), which combine these data for 31 intermediate redshift quasar fields. These surveys simultaneously provide for the first studies of physical conditions and abundances of the CGM and IGM around low-mass dwarf galaxies that constrain the physical conditions and abundances of the gas while also enabling the discovery of giant rest-frame optical emission nebulae around quasar hosts. I will highlight enlightening case studies, including filamentary accretion from 100 kpc scales into the ISM of a massive quasar host confirmed by down-the-barrel inflows observed in the UV and the first studies of relative abundances in the CGM/IGM around isolated dwarf galaxies that reveal surprisingly high metallicity and low [C/O] and [N/O] ratios, suggestive of core-collapse supernova outflows with modest mass loading.