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Register a new userSuperbubbles in the Multiphase ISM and the Loading of Galactic Winds
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We use numerical simulations to analyze the evolution and properties of superbubbles (SBs), driven by multiple supernovae (SNe), that propagate into the two-phase (warm/cold), cloudy interstellar medium (ISM). We consider a range of mean background densities n_avg=0.1-10 cm^{-3} and intervals between SNe dt_sn=0.01-1 Myr, and follow each SB until the radius reaches (1-2)H, where H is the characteristic ISM disk thickness. Except for embedded dense clouds, each SB is hot until a time t_sf,m when the shocked warm gas at the outer front cools and forms an overdense shell. Subsequently, diffuse gas in the SB interior remains at T_h 10^6-10^7K with expansion velocity v_h~10^2-10^3km/s (both highest for low dt_sn). At late times, the warm shell gas velocities are several 10s to ~100km/s. While shell velocities are too low to escape from a massive galaxy, they are high enough to remove substantial mass from dwarfs. Dense clouds are also accelerated, reaching a few to 10s of km/s. We measure the mass in hot gas per SN, M_h/N_SN, and the total radial momentum of the bubble per SN, p_b/N_SN. After t_sf,m, M_h/N_SN 10-100M_sun (highest for low n_avg), while p_b/N_SN 0.7-3x10^5M_sun km/s (highest for high dt_sn). If galactic winds in massive galaxies are loaded by the hot gas in SBs, we conclude that the mass-loss rates would generally be lower than star formation rates. Only if the SN cadence is much higher than typical in galactic disks, as may occur for nuclear starbursts, SBs can break out while hot and expel up to 10 times the mass locked up in stars. The momentum injection values, p_b/N_SN, are consistent with requirements to control star formation rates in galaxies at observed levels.
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CO(J=1-0) line emission is a widely used observational tracer of molecular gas, rendering essential the X_CO factor, which is applied to convert CO luminosity to H_2 mass. We use numerical simulations to study how X_CO depends on numerical resolution, non-steady-state chemistry, physical environment, and observational beam size. Our study employs 3D magnetohydrodynamics (MHD) simulations of galactic disks with solar neighborhood conditions, where star formation and the three-phase interstellar medium (ISM) are self-consistently regulated by gravity and stellar feedback. Synthetic CO maps are obtained by post-processing the MHD simulations with chemistry and radiation transfer. We find that CO is only an approximate tracer of H_2. On parsec scales, W_CO is more fundamentally a measure of mass-weighted volume density, rather than H_2 column density. Nevertheless, $langle X_mathrm{CO} rangle=0.7-1.0times10^{20}~mathrm{cm^{-2}K^{-1}km^{-1}s}$ consistent with observations, insensitive to the evolutionary ISM state or radiation field strength if steady-state chemistry is assumed. Due to non-steady-state chemistry, younger molecular clouds have slightly lower X_CO and flatter profiles of X_CO versus extinction than older ones. The CO-dark H_2 fraction is 26-79 %, anti-correlated with the average extinction. As the observational beam size increases from 1 pc to 100 pc, X_CO increases by a factor of ~ 2. Under solar neighborhood conditions, X_CO in molecular clouds is converged at a numerical resolution of 2 pc. However, the total CO abundance and luminosity are not converged even at the numerical resolution of 1 pc. Our simulations successfully reproduce the observed variations of X_CO on parsec scales, as well as the dependence of X_CO on extinction and the CO excitation temperature.
The ISM, powered by SNe, is turbulent and permeated by a magnetic field (with a mean and a turbulent component). It constitutes a frothy medium that is mostly out of equilibrium and is ram pressure dominated on most of the temperature ranges, except for T< 200 K and T> 1E6 K, where magnetic and thermal pressures dominate, respectively. Such lack of equilibrium is also imposed by the feedback of the radiative processes into the ISM flow. Many models of the ISM or isolated phenomena, such as bubbles, superbubbles, clouds evolution, etc., take for granted that the flow is in the so-called collisional ionization equilibrium (CIE). However, recombination time scales of most of the ions below 1E6 K are longer than the cooling time scale. This implies that the recombination lags behind and the plasma is overionized while it cools. As a consequence cooling deviates from CIE. This has severe implications on the evolution of the ISM flow and its ionization structure. Here, besides reviewing several models of the ISM, including bubbles and superbubbles, the validity of the CIE approximation is discussed, and a presentation of recent developments in modeling the ISM by taking into account the time-dependent ionization structure of the flow in a full-blown numerical 3D high resolution simulation is presented.
Massive, galaxy-scale outflows are known to be ubiquitous in major mergers of disk galaxies in the local universe. In this paper, we explore the multiphase structure and power sources of galactic winds in six ultraluminous infrared galaxies (ULIRGs) at z < 0.06 using deep integral field spectroscopy with the Gemini Multi-Object Spectrograph (GMOS) on Gemini North. We probe the neutral, ionized, and dusty gas phases using Na I D, strong emission lines ([O I], Halpha, and [N II]), and continuum colors, respectively. We separate outflow motions from those due to rotation and tidal perturbations, and find that all of the galaxies in our sample host high-velocity flows on kiloparsec scales. The properties of these outflows are consistent with multiphase (ionized, neutral, and dusty) collimated bipolar winds emerging along the minor axis of the nuclear disk to scales of 1-2 kpc. In two cases, these collimated winds take the form of bipolar superbubbles, identified by clear kinematic signatures. Less collimated (but still high-velocity) flows are also present on scales up to 5 kpc in most systems. The three galaxies in our sample with obscured QSOs host higher velocity outflows than those in the three galaxies with no evidence for an AGN. The peak outflow velocity in each of the QSOs is in the range 1450-3350 km/s, and the highest velocities (2000-3000 km/s) are seen only in ionized gas. The outflow energy and momentum in the QSOs are difficult to produce from a starburst alone, but are consistent with the QSO contributing significantly to the driving of the flow. Finally, when all gas phases are accounted for, the outflows are massive enough to provide negative feedback to star formation.
Galaxy evolution and star formation are two multi-scale problems tightly linked to each other. To understand the interstellar cycle, which triggers galaxy evolution, it is necessary to describe simultaneously the large-scale evolution widely induced by the feedback processes and the details of the gas dynamics that controls the star formation process through gravitational collapse. We perform a set of three-dimensional high-resolution numerical simulations of a turbulent, self-gravitating and magnetized interstellar medium within a $1 mathrm{kpc}$ stratified box with supernova feedback correlated with star-forming regions. In particular, we focus on the role played by the magnetic field and the feedback on the galactic vertical structure, the star formation rate (SFR) and the flow dynamics. For this purpose we vary their respective intensities. We extract properties of the dense clouds arising from the turbulent motions and compute power spectra of various quantities. Using a distribution of supernovae sufficiently correlated with the dense gas, we find that supernova explosions can reproduce the observed SFR, particularly if the magnetic field is on the order of a few $mu G$. The vertical structure, which results from a dynamical and an energy equilibrium is well reproduced by a simple analytical model, which allows us to estimate the coupling between the gas and the supernovae. We found the coupling to be rather low and on the order of 1.5$%$. Strong magnetic fields may help to increase this coupling by a factor of about 2-3. To characterize the flow we compute the power spectra of various quantities in 3D but also in 2D in order to account for the stratification of the galactic disc.
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 using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disk galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. We analyze 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 midplane. Bulk flows (e.g., inflows and fountains) are important at a few disk scale heights, while thermal pressure from hot gas dominates at larger heights. Overall, the total midplane pressure is well-predicted by the weight of the disk gas, and we show that it also scales linearly with the star formation rate surface density (Sigma_SFR). These results support the notion that the Kennicutt-Schmidt relation arises because Sigma_SFR and the gas surface density (Sigma_g) are connected via the ISM midplane pressure.
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