How supernova explosions power galactic winds Creasey, Peter; Theuns, Tom; Bower, Richard G
Monthly notices of the Royal Astronomical Society,
03/2013, Letnik:
429, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Feedback from supernovae is an essential aspect of galaxy formation. In order to improve subgrid models of feedback, we perform a series of numerical experiments to investigate how supernova ...explosions shape the interstellar medium (ISM) in a disc galaxy and power a galactic wind. We use the flash hydrodynamic code to model a simplified ISM, including gravity, hydrodynamics, radiative cooling above 104 K and star formation that reproduces the Kennicutt-Schmidt relation. By simulating a small patch of the ISM in a tall box perpendicular to the disc, we obtain subparsec resolution allowing us to resolve individual supernova events. The hot interiors of supernova explosions combine into larger bubbles that sweep-up the initially hydrostatic ISM into a dense, warm cloudy medium, enveloped by a much hotter and tenuous medium, all phases in near pressure equilibrium. The unbound hot phase develops into an outflow with wind speed increasing with distance as it accelerates from the disc. We follow the launch region of the galactic wind, where hot gas entrains and ablates warm ISM clouds leading to significantly increased mass loading of the flow, although we do not follow this material as it interacts with the galactic halo.
We run a large grid of simulations in which we vary gas surface density, gas fraction and star formation rate in order to investigate the dependencies of the mass loading,
. In the cases with the most effective outflows, we observe β = 4; however, in other cases we find β < 1. We find that outflows are more efficient in discs with lower surface densities or gas fractions. A simple model in which the warm cloudy medium is the barrier that limits the expansion of the blast wave reproduces the scaling of outflow properties with disc parameters at high star formation rates. We extend the scaling relations derived from an ISM patch to infer an effective mass loading for a galaxy with an exponential disc, finding that the mass loading depends on circular velocity as β∝V
− α
d with α 2.5 for a model which fits the Tully-Fisher relation. Such a scaling is often assumed in phenomenological models of galactic winds in order to reproduce the flat faint end slope of the mass function. Our normalization is in approximate agreement with observed estimates of the mass loading for the Milky Way. The scaling we find sets the investigation of galaxy winds on a new footing, providing a physically motivated subgrid description of winds that can be implemented in cosmological hydrodynamic simulations and phenomenological models.
The three phases of galaxy formation Clauwens, Bart; Schaye, Joop; Franx, Marijn ...
Monthly notices of the Royal Astronomical Society,
08/2018, Letnik:
478, Številka:
3
Journal Article
ABSTRACT
The role of galactic wind recycling represents one of the largest unknowns in galaxy evolution, as any contribution of recycling to galaxy growth is largely degenerate with the inflow rates ...of first-time infalling material, and the rates with which outflowing gas and metals are driven from galaxies. We present measurements of the efficiency of wind recycling from the eagle cosmological simulation project, leveraging the statistical power of large-volume simulations that reproduce a realistic galaxy population. We study wind recycling at the halo scale, i.e. gas that has been ejected beyond the halo virial radius, and at the galaxy scale, i.e. gas that has been ejected from the interstellar medium to at least $\approx 10 \, {{\ \rm per\ cent}}$ of the virial radius. Galaxy-scale wind recycling is generally inefficient, with a characteristic return time-scale that is comparable to or longer than a Hubble time, and with an efficiency that clearly peaks at the characteristic halo mass of $M_{200} = 10^{12} \, \mathrm{M_\odot }$. Correspondingly, the majority of gas being accreted on to galaxies in eagle is infalling for the first time. Recycling is more efficient at the halo scale, with values that differ by orders of magnitude from those assumed by semi-analytical galaxy formation models. Differences in the efficiency of wind recycling with other hydrodynamical simulations are currently difficult to assess, but are likely smaller. We find that cumulative first-time gas accretion rates at the virial radius are reduced relative to the expectation from dark matter accretion for haloes with mass $M_{200} \lt 10^{12} \, \mathrm{M_\odot }$, indicating efficient preventative feedback on halo scales.
Galactic outflow rates in the EAGLE simulations Mitchell, Peter D; Schaye, Joop; Bower, Richard G ...
Monthly notices of the Royal Astronomical Society,
2020, Letnik:
494, Številka:
3
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We present measurements of galactic outflow rates from the eagle suite of cosmological simulations. We find that gas is removed from the interstellar medium (ISM) of central galaxies with a ...dimensionless mass loading factor that scales approximately with circular velocity as $V_{\mathrm{c}}^{-3/2}$ in the low-mass regime where stellar feedback dominates. Feedback from active galactic nuclei causes an upturn in the mass loading for halo masses ${\gt}10^{12} \, \mathrm{M_\odot }$. We find that more gas outflows through the halo virial radius than is removed from the ISM of galaxies, particularly at low redshifts, implying substantial mass loading within the circumgalactic medium. Outflow velocities span a wide range at a given halo mass/redshift, and on average increase positively with redshift and halo mass up to $M_{200} \sim 10^{12} \, \mathrm{M_\odot }$. Outflows exhibit a bimodal flow pattern on circumgalactic scales, aligned with the galactic minor axis. We present a number of like-for-like comparisons to outflow rates from other recent cosmological hydrodynamical simulations, and show that comparing the propagation of galactic winds as a function of radius reveals substantial discrepancies between different models. Relative to some other simulations, eagle favours a scenario for stellar feedback where agreement with the galaxy stellar mass function is achieved by removing smaller amounts of gas from the ISM, but with galactic winds that then propagate and entrain ambient gas out to larger radii.
ABSTRACT
Smoothed particle hydrodynamics (SPH) is a ubiquitous numerical method for solving the fluid equations, and is prized for its conservation properties, natural adaptivity, and simplicity. We ...introduce the Sphenix SPH scheme, which was designed with three key goals in mind: to work well with sub-grid physics modules that inject energy, be highly computationally efficient (both in terms of compute and memory), and to be Lagrangian. sphenix uses a Density-Energy equation of motion, along with a variable artificial viscosity and conduction, including limiters designed to work with common sub-grid models of galaxy formation. In particular, we present and test a novel limiter that prevents conduction across shocks, preventing spurious radiative losses in feedback events. Sphenix is shown to solve many difficult test problems for traditional SPH, including fluid mixing and vorticity conservation, and it is shown to produce convergent behaviour in all tests where this is appropriate. Crucially, we use the same parameters within sphenix for the various switches throughout, to demonstrate the performance of the scheme as it would be used in production simulations. sphenix is the new default scheme in the swift cosmological simulation code and is available open source.
Galaxies fall into two clearly distinct types: 'blue-sequence' galaxies which are rapidly forming young stars, and 'red-sequence' galaxies in which star formation has almost completely ceased. Most ...galaxies more massive than ... follow the red sequence, while less massive central galaxies lie on the blue sequence. We show that these sequences are created by a competition between star formation-driven outflows and gas accretion on to the supermassive black hole at the galaxy's centre. We develop a simple analytic model for this interaction. In galaxies less massive than ..., young stars and supernovae drive a high-entropy outflow which is more buoyant than any tenuous corona. The outflow balances the rate of gas inflow, preventing high gas densities building up in the central regions. More massive galaxies, however, are surrounded by an increasingly hot corona. Above a halo mass of ..., the outflow ceases to be buoyant and star formation is unable to prevent the build-up of gas in the central regions. This triggers a strongly non-linear response from the black hole. Its accretion rate rises rapidly, heating the galaxy's corona, disrupting the incoming supply of cool gas and starving the galaxy of the fuel for star formation. The host galaxy makes a transition to the red sequence, and further growth predominantly occurs through galaxy mergers. We show that the analytic model provides a good description of galaxy evolution in the EAGLE hydrodynamic simulations. So long as star formation-driven outflows are present, the transition mass scale is almost independent of subgrid parameter choice. (ProQuest: ... denotes formulae/symbols omitted.)
The rapid growth phase of supermassive black holes McAlpine, Stuart; Bower, Richard G; Rosario, David J ...
Monthly notices of the Royal Astronomical Society,
12/2018, Letnik:
481, Številka:
3
Journal Article
ABSTRACT
We study the impact of numerical parameters on the properties of cold dark matter haloes formed in collisionless cosmological simulations. We quantify convergence in the median spherically ...averaged circular velocity profiles for haloes of widely varying particle number, as well as in the statistics of their structural scaling relations and mass functions. In agreement with prior work focused on single haloes, our results suggest that cosmological simulations yield robust halo properties for a wide range of gravitational softening parameters, ϵ, provided: (1) ϵ is not larger than a ‘convergence radius’, rconv, which is dictated by two-body relaxation and determined by particle number, and (2) a sufficient number of time-steps are taken to accurately resolve particle orbits with short dynamical times. Provided these conditions are met, median circular velocity profiles converge to within ≈10 per cent for radii beyond which the local two-body relaxation time-scale exceeds the Hubble time by a factor $\kappa \equiv t_{\rm relax}/t_{\rm H}\rm{\,\, \buildrel\gt \over \sim \,\,}0.177$, with better convergence attained for higher κ. We provide analytic estimates of rconv that build on previous attempts in two ways: first, by highlighting its explicit (but weak) softening-dependence and, second, by providing a simpler criterion in which rconv is determined entirely by the mean inter-particle spacing, l, for example better than 10 per cent convergence in circular velocity for $r\rm{\,\, \buildrel\gt \over \sim \,\,}0.05\, l$. We show how these analytic criteria can be used to assess convergence in structural scaling relations for dark matter haloes as a function of their mass or maximum circular speed.
We introduce a series of 20 cosmological hydrodynamical simulations of L* (M
200 = 1011.7–1012.3 M⊙) and group-sized (M
200 = 1012.7–1013.3 M⊙) haloes run with the model used for the eagle project, ...which additionally includes a non-equilibrium ionization and cooling module that follows 136 ions. The simulations reproduce the observed correlation, revealed by COS-Halos at z ∼ 0.2, between
${{\rm O} \small{VI}}$
column density at impact parameters b < 150 kpc and the specific star formation rate (sSFR ≡ SFR/M
*) of the central galaxy at z ∼ 0.2. We find that the column density of circumgalactic
${O \small {VI}}$
is maximal in the haloes associated with L* galaxies, because their virial temperatures are close to the temperature at which the ionization fraction of
${O \small {VI}}$
peaks (T ∼ 105.5 K). The higher virial temperature of group haloes (>106 K) promotes oxygen to higher ionization states, suppressing the
${O \small {VI}}$
column density. The observed
$N_{\rm O\,\small {VI}}$
–sSFR correlation therefore does not imply a causal link, but reflects the changing characteristic ionization state of oxygen as halo mass is increased. In spite of the mass dependence of the oxygen ionization state, the most abundant circumgalactic oxygen ion in both L* and group haloes is
${O\,{\small VII}}$
;
${O \small {VI}}$
accounts for only 0.1 per cent of the oxygen in group haloes and 0.9–1.3 per cent with L* haloes. Nonetheless, the metals traced by
${O \small {VI}}$
absorbers represent a fossil record of the feedback history of galaxies over a Hubble time; their characteristic epoch of ejection corresponds to z > 1 and much of the ejected metal mass resides beyond the virial radius of galaxies. For both L* and group galaxies, more of the oxygen produced and released by stars in the circumgalactic medium (within twice the virial radius) than in the stars and interstellar medium of the galaxy.
The observed high covering fractions of neutral hydrogen (
${\rm H\,\small {\rm I}}$
) with column densities above ∼1017 cm−2 around Lyman-Break Galaxies (LBGs) and bright quasars at redshifts ...z ∼ 2–3 has been identified as a challenge for simulations of galaxy formation. We use the Evolution and Assembly of Galaxies and their Environment (EAGLE) cosmological, hydrodynamical simulation, which has been shown to reproduce a wide range of galaxy properties and for which the subgrid feedback was calibrated without considering gas properties, to study the distribution of
${\rm H\,\small {\rm I}}$
around high-redshift galaxies. We predict the covering fractions of strong
${\rm H\,\small {\rm I}}$
absorbers (
${N_{\rm H\,\small {I}}}\gtrsim 10^{17} \,{\rm cm^{-2}}$
) inside haloes to increase rapidly with redshift but to depend only weakly on halo mass. For massive (M
200 ≳ 1012M⊙) haloes, the covering fraction profiles are nearly scale-invariant and we provide fitting functions that reproduce the simulation results. While efficient feedback is required to increase the
${\rm H\,\small {\rm I}}$
covering fractions to the high observed values, the distribution of strong absorbers in and around haloes of a fixed mass is insensitive to factor of 2 variations in the strength of the stellar feedback. In contrast, at fixed stellar mass the predicted
${\rm H\,\small {\rm I}}$
distribution is highly sensitive to the feedback efficiency. The fiducial EAGLE simulation reproduces both the observed global column density distribution function of
${\rm H\,\small {\rm I}}$
and the observed radial covering fraction profiles of strong
${\rm H\,\small {\rm I}}$
absorbers around LBGs and bright quasars.
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