Astrophysics

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.

Despite a growing body of observational and theoretical work, the connection between galactic-scale feedback processes, the underlying distribution of gas in the circumgalactic medium (CGM), and host galaxy properties remains uncertain. Focusing on the latter two points, we present new results on the spatial structure and kinematics of Ly$\alpha$ and several far-UV metallic ions in the CGM of Keck Baryonic Structure Survey (KBSS) galaxies using rest-frame far-UV spectra of foreground/background galaxy pairs with angular separations $\le 30$ arcsec. Medium resolution ($R\simeq 1500$) Keck/KCWI and Keck/LRIS spectra of 736 background galaxies with $\langle z_{\rm bg}\rangle=2.58 \pm 0.38$ probe sightlines through 1033 foreground galaxies ($\langle z_{\rm fg}\rangle=2.03 \pm 0.36$) at projected distances $8\leq D_{\rm tran}/\mathrm{kpc}\leq250$. For each ion, we measure rest-frame equivalent widths ($W_{\lambda}$) as a function of $D_{\rm tran}$; we observe higher ionization species (C IV) decrease less rapidly and extend to larger $D_{\rm tran}$ compared to low ions (O I, C II, Si II). Splitting the pair sample into subsets based on foreground galaxy properties, we find $W_\lambda(\text{C IV})$ exhibits a strong dependence on stellar mass ($M_*$) and a weaker dependence on star formation rate. Similarly, $W_\lambda(\text{Ly}\alpha)$ increases with $M_*$, albeit with more scatter. In 2D, we map the excess Ly$\alpha$ and C IV absorption as functions of line-of-sight velocity and $D_{\rm tran}$ and fit the observed Ly$\alpha$ map with a simple two-component model. Combining the 1D and 2D trends, we discuss the improved constraints these results place on CGM gas-phase kinematics in the context of previous studies at $z\sim 2$.

The intergalactic medium (IGM) represents the dominant reservoir of baryons at high red- shift, traces the architecture of the cosmic web dominated by dark matter, and fuels on-going galaxy evolution. Using a purpose-built instrument, with nod-and-shuffle and dual-field subtraction, we have detected, for the first time, an emission Lyman α forest (ELAF). The emission forest is highly extended, shows filamentary morphology with filaments connecting galaxies, exhibits statistics like the absorption Lyman α forest, displays spectra resembling the absorption forest, and is correlated with galaxy-traced overdensities consistent with bias like dark matter. We conclude that the ELAF may provide a new tool for tracing a significant fraction of the cosmic web of baryons and dark matter. Finally, I will present status of the STABLE Cosmic Web Imager (SCWI) program, a Brinson Exploration Hub balloon experiment, focused on emission from the Circum-QSO, the Circum-Galactic Medium, and the cosmic web. SCWI offers the opportunity to image the cosmic web in the local universe for the first time, and compare its properties to those at high redshift.

The circumgalactic medium (CGM) serves as the interface between galaxies and their cosmic environment, hosting the baryon cycle across a wide range of temperatures, densities, and energy scales. With its unprecedented sensitivity and spectral coverage, JWST is revolutionizing our view of this cycle by enabling direct detection of warm molecular hydrogen via mid-infrared rotational lines. We present a detailed analysis of the multi-phase molecular gas in the brightest cluster galaxy (BCG) of the cool-core cluster MACS1931-26 (z = 0.35), combining new **JWST/MIRI** and archival **ALMA** observations. This BCG hosts a powerful radio-loud AGN, elevated star formation, and one of the largest known H₂ reservoirs at this redshift. We trace cold molecular gas (10–100 K) using multiple CO and [CI] lines, finding highly excited gas in the ISM, similar to local LIRGs, while the CGM appears much less excited, pointing to distinct excitation sources. Our JWST data reveal warm H₂ (100–1000 K) spatially coincident with the CO-emitting gas and exhibiting comparable kinematics. Intriguingly, the CGM shows a higher H₂ excitation temperature than the ISM, suggesting the presence of more energetic heating mechanisms, including shocks and AGN-driven X-ray emission. This highlights the CGM as a key site of feedback-regulated gas transformation. Moreover, we will discuss our plans to use upcoming JWST Cycle 4 **NIRCam + MIRI** spectroscopy (2–28 μm) to perform comprehensive radiative transfer and shock modeling, aiming to constrain heating sources and baryon cycle in the CGM and ISM. This pilot study lays the groundwork for a broader framework to trace baryon cycling in cool-core BCGs, leveraging the synergy of JWST and cold gas tracers as a transformative tool for CGM studies.

In massive galaxies a significant fraction by mass of the circumgalactic medium is expected to be in the hot, X-ray emitting phase. Little is known about this gas since it is faint and is outshone by the Milky Way's own hot circumgalactic medium, and X-ray observatories with CCDs are unable to distinguish the emission lines of the former from the latter. Future observatories with X-ray IFUs would be able to measure key emission lines of the hot CGM, and use them to map the velocity structure of this phase. In this talk, I will show predictions from galaxies from a range of cosmological simulations for the velocity field of the hot CGM, showing signatures of rotation, inflows, and outflows from AGN and SNe feedback. Crucially, the velocity structure depends on the feedback model used, so that future observations may be used to constrain models used in cosmological simulations.

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.