Talks

I will present new constraints on the halo masses and gas fractions of DESI galaxy groups via cross-correlations with the ACT DR6 CMB lensing map. This lensing-based calibration addresses a key uncertainty in interpreting kSZ measurements: the underlying halo mass distribution and allows us to estimate the amount by which baryons have been redistributed relative to the dark matter. Our results indicate that while baryons trace dark matter on large scales, the gas is significantly more extended, with cumulative gas fractions falling well below predictions from hydrodynamical simulations like TNG300. These discrepancies, seen at 4σ significance or higher, point to strong feedback processes in the real Universe. I will also highlight the excellent agreement between our lensing-based gas fraction measurements and recent results from X-rays, and discuss the implications for modeling feedback, galaxy formation, and baryon cycling in halos.

The circumgalactic medium (CGM) represents both a significant reservoir of baryons around galaxies as well as the region through which gas flows on to and out of galactic disks providing fuel for continued star formation. It is, however, challenging to study due to the low densities of gas in the CGM. Previous UV absorption studies have shown that the CGM is ubiquitous around star-forming galaxies. Project AMIGA has shown that the Andromeda Galaxy (M31), specifically, has an extensive CGM, which has further been confirmed by recent results from Fast Radio Bursts. Here, we present two complementary approaches to further characterize the CGM of M31. First, using archival rotation measure (RM) measurements of background radio point sources projected within the virial radius of M31, we present evidence of the existence of a magneto-ionized plasma extending out to $\gtrsim$100 kpc from M31. Second, using HI observations from the Green Bank Telescope (GBT) and MeerKAT, we show evidence of infalling gas being disrupted by the hot CGM at similar distances. Both observations confirm the presence of an extended, hot, ionized, and magnetized CGM around M31.

Observing the cycling of baryons in and out of galaxies, which largely takes places in the circumgalactic medium (CGM), is key to understanding how galaxies grow and evolve. This is especially true for dwarf galaxies, whose shallow potential wells produce even more effective feedback than more massive haloes, and whose cold virial temperatures imply the possibility of a CGM rich in cold accretion, in which gas efficiently inflows and settles, perhaps explaining the degree to which nearly all isolated dwarf galaxies are actively star forming. Understanding how baryons cycle in and out of dwarf galaxies is thus essential for understanding how these galaxies connect to their large scale environment, and is now tractable with recent and upcoming state-of-the-art instrumentation. I will present sub-kiloparsec scale resolution integral field spectroscopy of emission lines mapping cool ionized gas inside and close to the optical extent of a star forming, low mass (M*~10^8 Msun) galaxy out to 10 kpc. This high spatial and spectral resolution data will be combined with the large scale gas distribution of diffuse, ionized gas on scales up to 1 degree (~200 kpc) with the one thousand lens, narrowband upgrade of the Dragonfly telephoto array concept. I will show results of the spatial distribution, kinematics, and ionization properties of gas in the galaxy itself and its inner and outer CGM and additionally provide insight into the degree of cospatiality of neutral to ionized extragalactic hydrogen in the outskirts of this low mass galaxy, connecting galactic processes such as star formation and feedback to those occurring in the CGM.

The upcoming COLIBRE project promises to provide a generational leap in the capabilities of cosmological, hydrodynamical simulations of galaxy formation. The simulations model the evolution of cold gas down to temperatures of 10 K, alongside the formation and evolution of dust, in large cosmological volumes, and incorporate new prescriptions for cooling, chemical enrichment, and feedback associated with star formation and black hole growth. COLIBRE’s flagship simulations have been run in much larger cosmological volumes, at a given resolution, than its predecessor (EAGLE), producing commensurately larger galaxy populations to study. In my talk I will present a census of baryons in the circumgalactic medium (CGM) for the flagship COLIBRE simulations, as a function of halo mass and gas phase, and present some initial comparisons with available observational data. I will discuss how the properties of the CGM are influenced by COLIBRE’s new prescriptions for feedback from star formation and AGN, and compare the importance of these feedback channels for different halo mass ranges. In turn, I will demonstrate how the properties of the CGM relate to the cold atomic and molecular gas reservoirs of galaxies, and how the effects of feedback on the CGM play a crucial role in future star formation activity and quenching. I will end by exploring why diversity exists in the properties of galaxies and their CGM in haloes of the same mass, by showing that galaxy-CGM ecosystems with different properties exhibit markedly different histories in terms of mass assembly, mergers, and feedback.

The ultra-diffuse nature of the Intergalactic Medium (IGM) makes it inherently difficult to resolve at high spatial resolution in simulations. Typical cosmological simulations are resolution limited by large box sizes (L > 50 Mpc) used to capture accurate statistical properties of large-scale structure, while higher-resolution zoom-in simulations rarely focus on the IGM. Thus, few simulations to date resolve IGM gas at sub-kpc scales, leaving potentially important scales for gas physics unresolved. We present semi-idealized simulations of cosmic sheet collapse at higher resolutions than previously explored to study the substructure and characteristics of IGM gas. We introduce a small 1D density perturbation to the initial conditions, allowing us to use small box volumes (L = 4-8 cMpc) to resolve IGM gas at kpc scales. We confirm previous work suggesting the IGM is inherently multiphase due to cooling-based instabilities causing fragmentation. We explore how IGM multiphase fragmentation manifests with both changing resolution and sheet-virial mass/temperature. With increasing resolution, we observe enhanced neutral hydrogen column densities through the cosmic sheet. Similar to subgrid-feedback physics, the unknown effects of resolution-limited cooling instabilities represent a fundamental limitation in our understanding of diffuse baryons in the universe. We aim to quantify this phenomenon and its broader implications for Lyman limit system statistics and the Lyman-alpha forest.

I will present work on measuring thermal and kinetic Sunyaev-Zeldovich signals alongside x-ray fluxes from clusters and groups of galaxies identified either in the DESI Legacy Survey or selected from eROISTA x-ray observations. I will show joint inference of matter and gas density as well as temperature and x-ray emissivity to better understand the structure within these objects and the feedback processes which are relevant to the modelling of cosmological observables. Among other aspects, I will address claims of discrepancy between the feedback observed in x-ray cross-correlations with cosmic shear and inferred from kSZ observations.

As cosmic microwave background photons travel through the Universe a small fraction of them interact with the intervening cosmic gas and thereby imprint the properties of this gas on our CMB observations. In this talk I will describe how data from the Atacama Cosmology Telescope can be used to isolate the signals arising from hot gas throughout the Universe and how the data can be used to measure both the integrated electron pressure and temperature of galaxy clusters. I will discuss how upcoming CMB experiments, such as the Simons Observatory, will allow us to improve our characterisation of the properties and evolution of cosmic gas.

Galaxies and their surroundings imprint shadows on the cosmic microwave background. These shadows contain tantalizing information about galaxy formation: from the baryon density, temperature, pressure, velocities, to the dark matter potential and its time-evolution due to accretion. In this talk, I will review the unique opportunities and challenges in learning about galaxy formation and cosmology from CMB observables.

Information about the late-time Universe is imprinted on the small-scale CMB as photons travel to us from the surface of last scattering. Several processes are at play and small-scale fluctuations are very rich and non-Gaussian in nature. I will review some recent and exciting results that use the Sunyaev-Zel'dovich (SZ) effects and gravitational lensing to paint a full picture of the visible and dark matter in and around DESI galaxies. I will discuss how a combination of measurements can probe velocity fields at cosmological distances and inform us on galaxy energetics. I will also show recent measurements of weak lensing of the CMB and its cross-correlation with DESI, and how they can help us interpret intriguing discrepancies in cosmological parameters between the high and low redshift Universe.

I will present evidence from both simulations and observations that the inner CGM (≲0.3 Rvir) of ≲L* galaxies departs significantly from the conventional paradigm of cool clouds embedded in a volume-filling hot phase. Instead, these regions are characterized by a supersonically turbulent medium in which kinetic energy dominates over thermal energy, with gas temperatures 10^4 –10^5 K and wide lognormal density distributions. I will show that UV absorption features observed at redshifts z ≲ 1, as well as DLAs at z ≳ 2, support this turbulence-dominated CGM framework. I will also discuss the broader implications of the transition from kinetic to thermal energy dominance for models of galaxy accretion, feedback processes, and the evolution from thick to thin star-forming disks.

The hot circumgalactic medium (CGM), a reservoir of missing baryons, metals, and energy, plays a key role in our understanding of galaxy evolution. However, extraordinary observational challenges make the hot CGM one of the least understood components of galaxies. Studying the hot CGM was not the objective of current X-ray or mm facilities during the design phase. However, as an excellent byproduct, observing the hot CGM has emerged as a promising field over the last two decades, coming at the forefront of priority science goals for the current and upcoming decades. I will discuss three snippets of our recent efforts to detect and characterize the hot CGM: 1) X-raying the Milky Way: Investigating thermal, chemical, and kinematic anomalies; 2) Is CGM detectable? Conducting deep searches in individual external galaxies using X-ray; and 3) Test for self-similarity: stacking many galaxies in mm (Sunyaev-Zeldovich Effect). I will highlight how our findings provide insights into the impact of galactic feedback on the hot CGM, establish our confidence in leveraging current telescopes to inform theoretical simulations, and set a benchmark for designing experiments with next-generation X-ray and mm facilities.