Claude-André Faucher-Giguère

APOGEE and Gaia data suggest the Milky Way’s kinematic structure evolved through three distinct phases: a disordered protogalaxy, which ‘spun up’ into a thick stellar disc, and then ‘cooled down’ to a final cold, thin stellar disc. We use a suite of FIRE-2 cosmological zoom-in simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in simulations, and we study their physical origin. In our simulations, the early protogalaxy phase occurs when the rate of cool gas ($T \le 10^4$ K) converting into stars is low, star formation is bursty, and the baryonic mass ‘sloshes’ within the host potential with respect to the centre of mass. The gas begins to spin coherently after sloshing ends, followed by the spin-up of young stars. The central potential is least concentrated just prior to gas spin-up. This second, thick disc phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disc phase occurs when the inner circumgalactic medium virializes, and is associated with steady star formation and intermediate consumption rates of cool gas converting into stars. Mergers do not appear to play a defining role in driving transitions between the three phases. The condition for thick disc formation appears to be minimal: a stable centre-of-mass motion. Thin disc formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.

Galactic outflows play a significant role in regulating galaxy evolution. Scaling relations between stellar mass ( M _⋆ ), star formation rate (SFR), and outflow properties have been extensively studied at low redshifts ( z < 1) but less so beyond z ∼ 2. We construct a joint sample of 387 galaxies from z ∼ 0–9, including 98 new Cosmic Noon galaxies from the Keck Baryonic Structure Survey and the Keck Lyman Continuum Spectroscopic Survey. Using high signal-to-noise emission lines (SNR > 50 for H α or [O iii ] λ 5007) from Keck/MOSFIRE spectra, we detect warm-ionized outflows by decomposing lines into narrow and broad components. With the joint sample, we explore the redshift evolution of outflow scaling relations. On average, outflows at Cosmic Noon have higher maximum velocities than those at low z by up to a factor of 3 for a fixed M _⋆ , SFR, or SFR surface density. They also have higher mass outflow rates for a fixed M _⋆ . Despite faster and stronger outflows, there is no evolution in the mass loading factor for a fixed M _⋆ . We find evidence for galactic fountains, as the majority of outflowing gas is recycled at a radius of ∼0.03 R _vir . We also constrain the overall outflow occurrence rate in our galaxy sample to at least 30% when taking galaxy orientation and outflow geometry into account. By analyzing the largest sample of warm-ionized outflows at Cosmic Noon to date and compiling large galaxy samples across all redshifts, we present a comprehensive analysis of galactic outflows throughout cosmic time.

Recent Atacama Large Millimeter/submillimeter Array observations reveal diffuse [C ii ] emission (“[C ii ] halos”) extending to ∼10 kpc in galaxies at 4 < z < 6. These measurements provide new insights into high-redshift galactic ecosystems and processes that drive metal enrichment on inner circumgalactic scales. To better understand the nature of [C ii ] halos, we analyze a suite of high-redshift FIRE-2 simulations at 5 ≤ z ≤ 6 in the stellar mass range of 10 ⁹ –10 ^10.5 M _⊙ . By postprocessing these simulations with three-dimensional dust radiative transfer and photoionization modeling, we generate synthetic images of [C ii ] and UV continuum emission, from which we extract one-dimensional surface brightness profiles. Our results reproduce both the galaxy-integrated and spatial distribution of [C ii ] and UV emission, capturing in particular the more extended profile of [C ii ] emission. Comparing the time evolution of [C ii ] halos with bursty star formation histories of the simulated galaxies, we find that [C ii ] emission becomes more spatially extended following the decline of star formation rate in recent starburst episodes. This implies a strong correlation between extended [C ii ] emission and bursty star formation, consistent with a key role for star formation-driven outflows in producing [C ii ] halos—though the kinematics of [C ii ]-emitting gas suggest that inflows and turbulent motions are also significant contributors. We also find a modest contribution from satellite galaxies to extended [C ii ] emission. Our framework can be readily applied to predict the observability of [C ii ] halos at higher redshifts and extended to create spatially resolved synthetic observations of other important emission lines, such as [O iii ] and H α .

Ultraluminous infrared galaxies are powered by a combination of rapid star formation and active galactic nucleus (AGN) emission, but their relative importance is not always observationally clear. We study the galactic continuum spectrum of a cosmologically simulated ∼4 × 10 ¹⁰ M _⊙ stellar mass starburst galaxy at redshift z ∼ 4.4 that refines down to resolve beyond the dust sublimation boundary of its super-Eddington-accreting ∼10 ⁷ M _⊙ supermassive black hole. We find that this system resembles the rare class of hot dust-obscured galaxy (Hot DOG), with a roughly flat (in νF _ν ) IR emission spectrum that sharply drops off at wavelengths ≲5 μ m. Our system also matches with the observational properties of many Hot DOGs, including undergoing multiple galaxy mergers and being the most massive galaxy within a dense cosmological environment. The distinctive Hot DOG spectral shape in our system is caused by AGN-heated mid-IR warm dust, predominately starburst-heated far-IR cold dust, and a steep near- to mid-IR cutoff caused by strong absorption in the dense ISM of the galactic nucleus, rather than the dust torus itself. This system is lower luminosity ( L _IR ∼ 2 × 10 ¹² L _⊙ ) than those detected by the WISE survey at similar redshifts, but will be a prime target for future far-IR surveys such as PRIMA. Our results show that Hot DOGs can naturally result as a transitional phase during rapid AGN accretion, but before significant AGN-driven outflows clear optically thin paths.

Galactic winds shape galaxy evolution; however, the outflowing gas is complex: it consists of multiple ionization phases, and its properties vary spatially. Therefore, methods that combine high-fidelity observations with state-of-the-art galactic-wind models are limited. Here we investigate methods for fitting the column density profiles derived from high-quality outflow observations with the multiphase, multiscale wind model from D. B. Fielding & G. L. Bryan. We identify three key outflow parameters: the initial hot-phase mass-loading factor ( η _M,hot,0 ), the initial cool-phase mass-loading factor ( η _M,cool,0 ), and the initial cool-cloud mass ( M _cl,0 ). We obtain good fits (reduced χ ² < 1.5) for most galaxies, with tight constraints on η _M,cool,0 and moderate constraints on the other two parameters. We find the inferred η _M,cool,0 and η _M,hot,0 are mostly of order unity, with significant scatter. The constraints on η _M,hot,0 suggest that the interaction between the cool and hot phases allow us to indirectly constrain the properties of the hot wind from cool-outflow observations. The model also predicts various radial trends. First, for all galaxies, the cool-phase outflow velocity increases rapidly between 1 and 2 r ₅₀ , then reaches a plateau, where r ₅₀ is the half-light radius of the galaxy. Second, most galaxies exhibit increasing η _M,cool and decreasing η _M,hot with radius, with a few showing the reverse trends. These results should be interpreted as effective, model-conditional constraints but are consistent with other recent multiphase simulations and observations. This highlights that the velocity–radius mapping encoded in UV absorption profiles enables recovery of outflow spatial structures from spatially integrated spectra. Our method paves the way for future broad parameter studies and guides updates of outflow simulations in future work.

The James Webb Space Telescope has unveiled an abundant population of potential active galactic nuclei (AGN) at high redshift ( z ≳ 4) known as little red dots (LRDs), which are likely hosted in relatively low-mass galaxies. However, previous theoretical models have highlighted the difficulty in continuously feeding massive black holes in the central regions of bursty, high-redshift galaxies because of repeated gas evacuation by stellar feedback. We analyze galaxies in high-redshift FIRE-2 simulations to understand whether they are capable of hosting the observed abundant population of high-redshift AGN. We use a gravitational torque-driven accretion (GTDA) model and a simple freefall accretion model to derive black hole accretion rates and construct predicted AGN bolometric luminosity functions for z = 5–7. The GTDA model and the freefall model with black holes accreting ≲1% of their central gas supply (M _BH ≳ 2 × 10 ⁵ M _⊙ black holes residing in M _⋆ ≳ 2 × 10 ⁷ M _⊙ galaxies. We show that, under simple assumptions, mock observations of such sources can reproduce key observed LRD characteristics.

Feedback from active galactic nuclei (AGN) is widely acknowledged to regulate the growth of massive galaxies, though its driving mechanisms are debated. Prevailing theories suggest that AGN-driven outflows are driven either by radiation pressure acting directly on the dusty interstellar medium (ISM) or by hot winds entraining cooler ISM gas, but the relative contribution of each mechanism remains uncertain. By combining optical emission line measurements with highly ionized UV emission lines, it is possible to constrain whether the pressure source applied to ionized clouds is primarily radiation or primarily hydrodynamic, and thus constrain the dominant driver. This study presents the first multiobject analysis of far-UV spectra from galactic-scale AGN-driven outflows in obscured quasars, based on Cosmic Origins Spectrograph observations of five low-redshift targets. By comparing narrow-line region UV emission-line ratios to theoretical models that vary the importance of the two pressure sources, we find that three out of five targets fall within the radiation-pressure-dominated regime. A fourth target exhibits intermediate emission-line ratios that suggest radiation pressure and pressure from a hot wind are both dynamically important. Finally, the lowest-luminosity object in our sample may have a dynamically important hot wind component, but nondetections prevent a clear conclusion in this case. These results suggest radiation pressure dominates circumnuclear narrow-line region cloud dynamics, but pressure from a hot wind also plays a role in some cases. This is consistent with AGN feedback scenarios mediated by radiation pressure or a short-lived hot wind phase that dissipates after initially accelerating outflows.

While galaxy rotation curves (RCs) provide one of the most powerful methods for measuring dark matter profiles in the inner regions of rotation-supported galaxies, at the dwarf scale there are factors that can complicate this analysis. Given the expectation of a universal profile in dark-matter-only simulations, the diversity of the observed RCs has become an often-discussed issue in Lambda cold dark matter cosmology on galactic scales. We analyze a suite of Feedback in Realistic Environments simulations of 10 ¹⁰ –10 ¹² M _⊙ halos with standard cold dark matter and compare the true circular velocity to RC reconstructions. We find that, for galaxies with well-ordered gaseous disks, the measured RC may deviate from the true circular velocity by at most ∼10% within the radius of the disk. However, nonequilibrium behaviors, noncircular motions, and nonthermal and nonkinetic stresses may cause much larger discrepancies, of ∼50% or more. Most RC reconstructions underestimate the true circular velocity, while some reconstructions transiently overestimate it in the central few kiloparsecs, due to dynamical phenomena. We further demonstrate that the features that contribute to these failures are not always visibly obvious in H I observations. If such dwarf galaxies are included in galaxy catalogs, they may give rise to the appearance of “artificial” RC diversity that does not reflect the true variation in underlying dark matter profiles.