The multiphase circumgalactic medium traced by low metal ions in EAGLE zoom simulations

We explore the circumgalactic metal content traced by commonly observed low ion absorbers, including C II, SiII, SiIII, SiIV, and MgII. We use a set of cosmological hydrodynamical zoom simulations run with the EAGLE model and including a non-equilibrium ionization and cooling module that follows 136 ions. The simulations of z ≈ 0.2 L* (M200= 1011.7- 1012.3M⊙) haloes hosting star-forming galaxies and group-sized (M200= 1012.7- 1013.3M⊙) haloes hosting mainly passive galaxies reproduce key trends observed by the COS-Halos survey - low ion column densities show 1) little dependence on galaxy-specific star formation rate, 2) a patchy covering fraction indicative of 104K clumps with a small volume filling factor, and 3) a declining covering fraction as impact parameter increases from 20-160kpc. Simulated Si II, Si III, Si IV, CII, and C III column densities show good (mostly within 0.3 dex) agreement with observations, while MgII is under-predicted. Low ions trace a significant metal reservoir, ≈108M⊙, residing primarily at 10-100kpc from star-forming and passive central galaxies. These clumps preferentially flow inwards and most will accrete onto the central galaxy within the next several Gyr, while a small fraction are entrained in strong outflows. A multiphase structure describes the inner CGM ( 0.5R200) tracing virial temperature gas around L* galaxies. Our simulations support previous ionization models indicating that cloud covering factors decline while densities and pressures show little decline with increasing impact parameter (typically < 0.3 dex from 40 to 160 kpc). We find the cool clumps have lower pressures than the ambient medium they are embedded in, and discuss that numerical effects within the hydrodynamic solver likely play a role. © 2018 The Author(s)

Pressure balance in the multiphase ISM of cosmologically simulated disc galaxies

Pressure balance plays a central role in models of the interstellar medium (ISM), but whether and how pressure balance is realized in a realistic multiphase ISM is not yet well understood. We address this question by using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disc galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. We analyse how gravity determines the vertical pressure profile as well as how the total ISM pressure is partitioned between different phases and components (thermal, dispersion/turbulence, and bulk flows). We show that, on average and consistent with previous more idealized simulations, the total ISM pressure balances the weight of the overlying gas. Deviations from vertical pressure balance increase with increasing galactocentric radius and with decreasing averaging scale. The different phases are in rough total pressure equilibrium with one another, but with large deviations from thermal pressure equilibrium owing to kinetic support in the cold and warm phases, which dominate the total pressure near the mid-plane. Bulk flows (e.g. inflows and fountains) are important at a few disc scale heights, while thermal pressure from hot gas dominates at larger heights. Overall, the total mid-plane pressure is well-predicted by the weight of the disc gas and we show that it also scales linearly with the star formation rate surface density (ςSFR). These results support the notion that the Kennicutt-Schmidt relation arises because ςSFR and the gas surface density (ςg) are connected via the ISM mid-plane pressure

Reproducing the CO-to-H2conversion factor in cosmological simulations of Milky-Way-mass galaxies

We present models of CO(1-0) emission from Milky-Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which, in our models, sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H2 abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of ~3 pc in cold gas (T &lt; 300 K) for both CO and H2, is able to reproduce both the observed CO(1-0) luminosity and inferred CO-to-H2 conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model, which controls the amount of radiation that penetrates giant molecular clouds

Different Flavors of Nonadiabatic Molecular Dynamics

The Born‐Oppenheimer approximation constitutes a cornerstone of our understanding of molecules and their reactivity, partly because it introduces a somewhat simplified representation of the molecular wavefunction. However, when a molecule absorbs light containing enough energy to trigger an electronic transition, the simplistic nature of the molecular wavefunction offered by the Born‐Oppenheimer approximation breaks down as a result of the now non‐negligible coupling between nuclear and electronic motion, often coined nonadiabatic couplings. Hence, the description of nonadiabatic processes implies a change in our representation of the molecular wavefunction, leading eventually to the design of new theoretical tools to describe the fate of an electronically‐excited molecule. This Overview focuses on this quantity—the total molecular wavefunction—and the different approaches proposed to describe theoretically this complicated object in non‐Born‐Oppenheimer conditions, namely the Born‐Huang and Exact‐Factorization representations. The way each representation depicts the appearance of nonadiabatic effects is then revealed by using a model of a coupled proton–electron transfer reaction. Applying approximations to the formally exact equations of motion obtained within each representation leads to the derivation, or proposition, of different strategies to simulate the nonadiabatic dynamics of molecules. Approaches like quantum dynamics with fixed and time‐dependent grids, traveling basis functions, or mixed quantum/classical like surface hopping, Ehrenfest dynamics, or coupled‐trajectory schemes are described in this Overview

TDDFT and quantum-classical dynamics: A universal tool describing the dynamics of matter

Time-dependent density functional theory (TDDFT) is currently the most efficient approach allowing to describe electronic dynamics in complex systems, from isolated molecules to the condensed phase. TDDFT has been employed to investigate an extremely wide range of time-dependent phenomena, as spin dynamics in solids, charge and energy transport in nanoscale devices, and photoinduced exciton transfer in molecular aggregates. It is therefore nearly impossible to give a general account of all developments and applications of TDDFT in material science, as well as in physics and chemistry. A large variety of aspects are covered throughout these volumes. In the present chapter, we will limit our presentation to the description of TDDFT developments and applications in the field of quantum molecular dynamics simulations in combination with trajectory-based approaches for the study of nonadiabatic excited-state phenomena. We will present different quantum-classical strategies used to describe the coupled dynamics of electrons and nuclei underlying nonadiabatic processes. In addition, we will give an account of the most recent applications with the aim of illustrating the nature of the problems that can be addressed with the help of these approaches. The potential, as well as the limitations, of the presented methods is discussed, along with possible avenues for future developments in TDDFT and nonadiabatic dynamics

TDDFT and Quantum-Classical Dynamics: A Universal Tool Describing the Dynamics of Matter

Time-dependent density functional theory (TDDFT) is currently the most efficient approach allowing to describe electronic dynamics in complex systems, from isolated molecules to the condensed phase. TDDFT has been employed to investigate an extremely wide range of time-dependent phenomena, as spin dynamics in solids, charge and energy transport in nanoscale devices, and photoinduced exciton transfer in molecular aggregates. It is therefore nearly impossible to give a general account of all developments and applications of TDDFT in material science, as well as in physics and chemistry. A large variety of aspects are covered throughout these volumes. In the present chapter, we will limit our presentation to the description of TDDFT developments and applications in the field of quantum molecular dynamics simulations in combination with trajectory-based approaches for the study of nonadiabatic excited-state phenomena. We will present different quantum-classical strategies used to describe the coupled dynamics of electrons and nuclei underlying nonadiabatic processes. In addition, we will give an account of the most recent applications with the aim of illustrating the nature of the problems that can be addressed with the help of these approaches. The potential, as well as the limitations, of the presented methods is discussed, along with possible avenues for future developments in TDDFT and nonadiabatic dynamics