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Abstract Among the many different pieces of physics that go into simulations of the circumgalactic medium (CGM), the metagalactic ultraviolet background (UVB) plays a significant role in determining the ionization state of different metal species. However, the UVB is uncertain, with multiple models having been developed by various research groups over the past several decades. In this work, we examine how different UVB models influence the ionic column densities of CGM absorbers. We use these UVB models to infer ion number densities in the Figuring Out Gas and Galaxies In Enzo (FOGGIE) galaxy simulations atz= 2.5 and use the Synthetic Absorption Line Surveyor Application package to identify absorbers. Absorbers are then matched across UVB models based on their line-of-sight position so that their column densities can be compared. From our analysis, we find that changing the UVB model produces significant changes in ionization, specifically at lower gas densities and higher temperatures where photoionization dominates over collisional ionization. We also find that the scatter of column density differences between models tends to increase with increasing ionization energy, with the exception of Hi, which has the highest scatter of all species we examined. 

Abstract Cosmological simulations are a powerful tool to study galaxy evolution as they can span a substantial fraction of the cosmic time. In this research note, we use the Figuring Out Gas and Galaxies In Enzo simulations—cosmological hydrodynamic simulation of Milky Way-like galaxies—to measure the evolution of the radius of the galaxy disk. Additionally, we analyze the simulations along three different lines of sight. Lastly, we show that the disk size increases over time regardless of angle of projection. 

Abstract The circumgalactic medium (CGM) is often assumed to exist in or near hydrostatic equilibrium, with the regulation of accretion and the effects of feedback treated as perturbations to a stable balance between gravity and thermal pressure. We investigate global hydrostatic equilibrium in the CGM using four highly resolvedL^*galaxies from the Figuring Out Gas & Galaxies in Enzo (FOGGIE) project. The FOGGIE simulations were specifically targeted at fine spatial and mass resolution in the CGM (Δx≲ 1 kpch⁻¹andM≃ 200M_⊙). We develop a new analysis framework that calculates the forces provided by thermal pressure gradients, turbulent pressure gradients, ram pressure gradients of large-scale radial bulk flows, centrifugal rotation, and gravity acting on the gas in the CGM. Thermal and turbulent pressure gradients vary strongly on scales of ≲5 kpc throughout the CGM. Thermal pressure gradients provide the main supporting force only beyond ∼0.25R₂₀₀, or ∼50 kpc atz= 0. Within ∼0.25R₂₀₀, turbulent pressure gradients and rotational support provide stronger forces than thermal pressure. More generally, we find that global equilibrium models are neither appropriate nor predictive for the small scales probed by absorption line observations of the CGM. Local conditions generally cannot be derived by assuming a global equilibrium, but an emergent global equilibrium balancing radially inward and outward forces is obtained when averaging over the nonequilibrium local conditions on large scales in space and time. Approximate hydrostatic equilibrium holds only at large distances from galaxies, even when averaging out small-scale variations. 

Abstract The classical definition of the virial temperature of a galaxy halo excludes a fundamental contribution to the energy partition of the halo: the kinetic energy of nonthermal gas motions. Using simulations of low-redshift, ∼ L * galaxies from the Figuring Out Gas & Galaxies In Enzo (FOGGIE) project that are optimized to resolve low-density gas, we show that the kinetic energy of nonthermal motions is roughly equal to the energy of thermal motions. The simulated FOGGIE halos have ∼2× lower bulk temperatures than expected from a classical virial equilibrium, owing to significant nonthermal kinetic energy that is formally excluded from the definition of T vir . We explicitly derive a modified virial temperature including nonthermal gas motions that provides a more accurate description of gas temperatures for simulated halos in virial equilibrium. Strong bursts of stellar feedback drive the simulated FOGGIE halos out of virial equilibrium, but the halo gas cannot be accurately described by the standard virial temperature even when in virial equilibrium. Compared to the standard virial temperature, the cooler modified virial temperature implies other effects on halo gas: (i) the thermal gas pressure is lower, (ii) radiative cooling is more efficient, (iii) O vi absorbing gas that traces the virial temperature may be prevalent in halos of a higher mass than expected, (iv) gas mass estimates from X-ray surface brightness profiles may be incorrect, and (v) turbulent motions make an important contribution to the energy balance of a galaxy halo. 

Abstract We present the KODIAQ-Z survey aimed to characterize the cool, photoionized gas at 2.2 ≲z≲ 3.6 in 202 Hi-selected absorbers with 14.6 ≤ $\log N_{\mathrm{H}\mathrm{I}}$< 20 that probe the interface between galaxies and the intergalactic medium (IGM). We find that gas with $14.6\le \log N_{\mathrm{H}\mathrm{I}}<20$at 2.2 ≲z≲ 3.6 can be metal-rich (−1.6 ≲ [X/H] ≲ − 0.2) as seen in damped Lyαabsorbers (DLAs); it can also be very metal-poor ([X/H] < − 2.4) or even pristine ([X/H] < − 3.8), which is not observed in DLAs but is common in the IGM. For $16<\log N_{\mathrm{H}\mathrm{I}}<20$absorbers, the frequency of pristine absorbers is about 1%–10%, while for $14.6\le \log N_{\mathrm{H}\mathrm{I}}\le 16$absorbers it is 10%–20%, similar to the diffuse IGM. Supersolar gas is extremely rare (<1%) at these redshifts. The factor of several thousand spread from the lowest to highest metallicities and large metallicity variations (a factor of a few to >100) between absorbers separated by less than Δv< 500 km s⁻¹imply that the metals are poorly mixed in $14.6\le \log N_{\mathrm{H}\mathrm{I}}<20$gas. We show that these photoionized absorbers contribute to about 14% of the cosmic baryons and 45% of the cosmic metals at 2.2 ≲z≲ 3.6. We find that the mean metallicity increases withN_Hi, consistent with what is found inz< 1 gas. The metallicity of gas in this column density regime has increased by a factor ∼8 from 2.2 ≲z≲ 3.6 toz< 1, but the contribution of the $14.6\le \log N_{\mathrm{H}\mathrm{I}}<19$absorbers to the total metal budget of the universe atz< 1 is a quarter of that at 2.2 ≲z≲ 3.6. We show that FOGGIE cosmological zoom-in simulations have a similar evolution of [X/H] withN_Hi, which is not observed in lower-resolution simulations. In these simulations, very metal-poor absorbers with [X/H] < − 2.4 atz∼ 2–3 are tracers of inflows, while higher-metallicity absorbers are a mixture of inflows and outflows. 

ABSTRACT The propagation and evolution of cold galactic winds in galactic haloes is crucial to galaxy formation models. However, modelling of this process in hydrodynamic simulations of galaxy formation is oversimplified owing to a lack of numerical resolution and often neglects critical physical processes such as hydrodynamic instabilities and thermal conduction. We propose an analytic model, Physically Evolved Winds, that calculates the evolution of individual clouds moving supersonically through a uniform ambient medium. Our model reproduces predictions from very high resolution cloud-crushing simulations that include isotropic thermal conduction over a wide range of physical conditions. We discuss the implementation of this model into cosmological hydrodynamic simulations of galaxy formation as a subgrid prescription to model galactic winds more robustly both physically and numerically. 

ABSTRACT Many phenomenologically successful cosmological simulations employ kinetic winds to model galactic outflows. Yet systematic studies of how variations in kinetic wind scalings might alter observable galaxy properties are rare. Here we employ gadget-3 simulations to study how the baryon cycle, stellar mass function, and other galaxy and CGM predictions vary as a function of the assumed outflow speed and the scaling of the mass-loading factor with velocity dispersion. We design our fiducial model to reproduce the measured wind properties at 25 per cent of the virial radius from the Feedback In Realistic Environments simulations. We find that a strong dependence of η ∼ σ5 in low-mass haloes with $$\sigma \lt 106\mathrm{\, km\, s^{-1}}$$ is required to match the faint end of the stellar mass functions at $$z$$ > 1. In addition, faster winds significantly reduce wind recycling and heat more halo gas. Both effects result in less stellar mass growth in massive haloes and impact high ionization absorption in halo gas. We cannot simultaneously match the stellar content at $$z$$ = 2 and 0 within a single model, suggesting that an additional feedback source such as active galactic nucleus might be required in massive galaxies at lower redshifts, but the amount needed depends strongly on assumptions regarding the outflow properties. We run a 50 $$\mathrm{Mpc}\, h^{-1}$$, 2 × 5763 simulation with our fiducial parameters and show that it matches a range of star-forming galaxy properties at $$z$$ ∼ 0–2.