17 research outputs found
Resolution criteria to avoid artificial clumping in Lagrangian hydrodynamic simulations with a multi-phase interstellar medium
Large-scale cosmological galaxy formation simulations typically prevent gas
in the interstellar medium (ISM) from cooling below K. This has
been motivated by the inability to resolve the Jeans mass in molecular gas
(>>) which would result in undesired artificial
clumping. We show that the classical Jeans criteria derived for Newtonian
gravity are not applicable in the simulated ISM if the spacing of resolution
elements representing the dense ISM is below the gravitational force softening
length and gravity is therefore softened and not Newtonian. We re-derive the
Jeans criteria for softened gravity in Lagrangian codes and use them to analyse
gravitational instabilities at and below the hydrodynamical resolution limit
for simulations with adaptive and constant gravitational softening lengths. In
addition, we define criteria for which a numerical runaway collapse of dense
gas clumps can occur caused by over-smoothing of the hydrodynamical properties
relative to the gravitational force resolution. This effect is illustrated
using simulations of isolated disk galaxies with the smoothed particle
hydrodynamics code Swift. We also demonstrate how to avoid the formation of
artificial clumps in gas and stars by adjusting the gravitational and
hydrodynamical force resolutions.Comment: 24 pages, 15 figures, accepted for publication in MNRAS, smaller
updates to match published versio
The interplay between AGN feedback and precipitation of the intracluster medium in simulations of galaxy groups and clusters
Using high-resolution hydrodynamical simulations of galaxy clusters, we study
the interaction between the brightest cluster galaxy, its supermassive black
hole (BH) and the intracluster medium (ICM). We create initial conditions for
which the ICM is in hydrostatic equilibrium within the gravitational potential
from the galaxy and an NFW dark matter halo. Two free parameters associated
with the thermodynamic profiles determine the cluster gas fraction and the
central temperature, where the latter can be used to create cool-core or
non-cool-core systems. Our simulations include radiative cooling, star
formation, BH accretion, and stellar and active galactic nucleus (AGN)
feedback. Even though the energy of AGN feedback is injected thermally and
isotropically, it leads to anisotropic outflows and buoyantly rising bubbles.
We find that the BH accretion rate (BHAR) is highly variable and only
correlates strongly with the star formation rate (SFR) and the ICM when it is
averaged over more than . We generally find good agreement with the
theoretical precipitation framework. In haloes, AGN
feedback quenches the central galaxy and converts cool-core systems into
non-cool-core systems. In contrast, higher-mass, cool-core clusters evolve
cyclically. Episodes of high BHAR raise the entropy of the ICM out to the
radius where the ratio of the cooling time and the local dynamical time , thus suppressing condensation and, after a delay, the
BHAR. The corresponding reduction in AGN feedback allows the ICM to cool and
become unstable to precipitation, thus initiating a new episode of high SFR and
BHAR.Comment: 22 pages, 15 figures (including appendix); submitted to MNRAS;
Supplementary material is available on Youtube at
https://youtu.be/HQhc_mytj0A and online on a single website page at
https://home.strw.leidenuniv.nl/~nobels/supplementary_idealised_cluster_paper.ph
Winds versus jets: a comparison between black hole feedback modes in simulations of idealized galaxy groups and clusters
Using the SWIFT simulation code, we compare the effects of different forms of active galactic nuclei (AGNs) feedback in idealized galaxy groups and clusters. We first present a physically motivated model of black hole (BH) spin evolution and a numerical implementation of thermal isotropic feedback (representing the effects of energy-driven winds) and collimated kinetic jets that they launch at different accretion rates. We find that kinetic jet feedback is more efficient at quenching star formation in the brightest cluster galaxies (BCGs) than thermal isotropic feedback, while simultaneously yielding cooler cores in the intracluster medium (ICM). A hybrid model with both types of AGN feedback yields moderate star formation rates, while having the coolest cores. We then consider a simplified implementation of AGN feedback by fixing the feedback efficiencies and the jet direction, finding that the same general conclusions hold. We vary the feedback energetics (the kick velocity and the heating temperature), the fixed efficiencies and the type of energy (kinetic versus thermal) in both the isotropic and the jet case. The isotropic case is largely insensitive to these variations. On the other hand, jet feedback must be kinetic in order to be efficient at quenching. We also find that it is much more sensitive to the choice of energy per feedback event (the jet velocity), as well as the efficiency. The former indicates that jet velocities need to be carefully chosen in cosmological simulations, while the latter motivates the use of BH spin evolution models
Tests of subgrid models for star formation using simulations of isolated disk galaxies
We use smoothed-particle hydrodynamics simulations of isolated Milky Way-mass
disk galaxies that include cold, interstellar gas to test subgrid prescriptions
for star formation (SF). Our fiducial model combines a Schmidt law with a
gravitational instability criterion, but we also test density thresholds and
temperature ceilings. While SF histories are insensitive to the prescription
for SF, the Kennicutt-Schmidt (KS) relations between SF rate and gas surface
density can discriminate between models. We show that our fiducial model, with
an SF efficiency per free-fall time of 1 per cent, agrees with
spatially-resolved and azimuthally-averaged observed KS relations for neutral,
atomic and molecular gas. Density thresholds do not perform as well. While
temperature ceilings selecting cold, molecular gas can match the data for
galaxies with solar metallicity, they are unsuitable for very low-metallicity
gas and hence for cosmological simulations. We argue that SF criteria should be
applied at the resolution limit rather than at a fixed physical scale, which
means that we should aim for numerical convergence of observables rather than
of the properties of gas labelled as star-forming. Our fiducial model yields
good convergence when the mass resolution is varied by nearly 4 orders of
magnitude, with the exception of the spatially-resolved molecular KS relation
at low surface densities. For the gravitational instability criterion, we
quantify the impact on the KS relations of gravitational softening, the SF
efficiency, and the strength of supernova feedback, as well as of observable
parameters such as the inclusion of ionized gas, the averaging scale, and the
metallicity.Comment: Submitted to MNRAS, 23 pages, 20 figure
A thermal-kinetic subgrid model for supernova feedback in simulations of galaxy formation
We present a subgrid model for supernova feedback designed for simulations of
galaxy formation. The model uses thermal and kinetic channels of energy
injection, which are built upon the stochastic kinetic and thermal models for
stellar feedback used in the OWLS and EAGLE simulations, respectively. In the
thermal channel, the energy is distributed statistically isotropically and
injected stochastically in large amounts per event, which minimizes spurious
radiative energy losses. In the kinetic channel, we inject the energy in small
portions by kicking gas particles in pairs in opposite directions. The
implementation of kinetic feedback is designed to conserve energy, linear
momentum and angular momentum, and is statistically isotropic. To test and
validate the model, we run simulations of isolated Milky Way-mass and dwarf
galaxies, in which the gas is allowed to cool down to 10 K. Using the thermal
and kinetic channels together, we obtain smooth star formation histories and
powerful galactic winds with realistic mass loading factors. Furthermore, the
model produces spatially resolved star formation rates and velocity dispersions
that are in agreement with observations. We vary the numerical resolution by
several orders of magnitude and find excellent convergence of the global star
formation rates and the mass loading of galactic winds. We show that large
thermal-energy injections generate a hot phase of the interstellar medium (ISM)
and modulate the star formation by ejecting gas from the disc, while the
low-energy kicks increase the turbulent velocity dispersion in the neutral ISM,
which in turn helps suppress star formation.Comment: 22 pages, 17 figures (including appendix); submitted to MNRA
Tests of subgrid models for star formation using simulations of isolated disk galaxies
We use smoothed-particle hydrodynamics simulations of isolated Milky Way-mass disk galaxies that include cold, interstellar gas to test subgrid prescriptions for star formation (SF). Our fiducial model combines a Schmidt law with a gravitational instability criterion, but we also test density thresholds and temperature ceilings. While SF histories are insensitive to the prescription for SF, the Kennicutt-Schmidt (KS) relations between SF rate and gas surface density can discriminate between models. We show that our fiducial model, with an SF efficiency per free-fall time of 1 per cent, agrees with spatially-resolved and azimuthally-averaged observed KS relations for neutral, atomic and molecular gas. Density thresholds do not perform as well. While temperature ceilings selecting cold, molecular gas can match the data for galaxies with solar metallicity, they are unsuitable for very low-metallicity gas and hence for cosmological simulations. We argue that SF criteria should be applied at the resolution limit rather than at a fixed physical scale, which means that we should aim for numerical convergence of observables rather than of the properties of gas labelled as star-forming. Our fiducial model yields good convergence when the mass resolution is varied by nearly 4 orders of magnitude, with the exception of the spatially-resolved molecular KS relation at low surface densities. For the gravitational instability criterion, we quantify the impact on the KS relations of gravitational softening, the SF efficiency, and the strength of supernova feedback, as well as of observable parameters such as the inclusion of ionized gas, the averaging scale, and the metallicity
The impact of stochastic modeling on the predictive power of galaxy formation simulations
All modern galaxy formation models employ stochastic elements in their
sub-grid prescriptions to discretise continuous equations across the time
domain. In this paper, we investigate how the stochastic nature of these
models, notably star formation, black hole accretion, and their associated
feedback, that act on small ( kpc) scales, can back-react on macroscopic
galaxy properties (e.g. stellar mass and size) across long ( Gyr)
timescales. We find that the scatter in scaling relations predicted by the
EAGLE model implemented in the SWIFT code can be significantly impacted by
random variability between re-simulations of the same object, even when
galaxies are resolved by tens of thousands of particles. We then illustrate how
re-simulations of the same object can be used to better understand the
underlying model, by showing how correlations between galaxy stellar mass and
black hole mass disappear at the highest black hole masses (
M), indicating that the feedback cycle may be interrupted by external
processes. We find that although properties that are collected cumulatively
over many objects are relatively robust against random variability (e.g. the
median of a scaling relation), the properties of individual galaxies (such as
galaxy stellar mass) can vary by up to 25\%, even far into the well-resolved
regime, driven by bursty physics (black hole feedback) and mergers between
galaxies. We suggest that studies of individual objects within cosmological
simulations be treated with caution, and that any studies aiming to closely
investigate such objects must account for random variability within their
results.Comment: Accepted for publication in MNRA
Hydrodynamic simulations of the Disk of Gas Around Supermassive black holes (HDGAS) -I; Molecular Gas Dynamics
We present hydrodynamic simulations of the interstellar medium (ISM) within
the circumnuclear disk (CND) of a typical AGN-dominated galaxy influenced by
mechanical feedback from an active galactic nucleus(AGN). The simulations are
coupled with the CHIMES non-equilibrium chemistry network to treat the
radiative-cooling and AGN-heating. A focus is placed on the central 100 pc
scale where AGN outflows are coupled to the ISM and constrained by
observational Seyfert-2 galaxies. AGN-feedback models are implemented with
different wind-velocity and mass-loading factors. We post-process the
simulation snapshots with a radiative-transfer code to obtain the molecular
emission lines. We find that the inclusion of an AGN promotes the formation of
CO in clumpy and dense regions surrounding supermassive-blackholes (SMBH). The
CO(1-0) intensity maps (6 Myr) in the CND seem to match well with
observations of NGC 1068 with a best match for a model with 5000
wind-velocity and a high mass-loading factor. We attempt to discern between
competing explanations for the apparent counter-rotating gas disk in the NGC
1068 through an analysis of kinematic maps of the CO line emission. We suggest
that mechanical AGN-feedback could explain the alignment-stability of
position-angle across the different CND radii around the SMBH through momentum
and energy loading of the wind. It is the wind-velocity that drives the disk
out of alignment on a 100 pc scale for a long period of time. The
position-velocity diagrams are in broad agreement with the predicted Keplerian
rotation-curve in the model without-AGN, but the AGN models exhibit a larger
degree of scatter, in better agreement with NGC 1068 observations.Comment: 16 pages, 13 figures. Accepted for publication in MNRA
Hydrodynamic simulations of the disc of gas around supermassive black holes (HDGAS) – I. Molecular gas dynamics
We present hydrodynamic simulations of the interstellar medium (ISM) within the circumnuclear disc (CND) of a typical active galactic nucleus (AGN)-dominated galaxy influenced by mechanical feedback from an AGN. The simulations are coupled with the CHIMES non-equilibrium chemistry network to treat the radiative-cooling and AGN-heating. A focus is placed on the central 100 pc scale where AGN outflows are coupled to the ISM and constrained by observational Seyfert-2 galaxies. AGN-feedback models are implemented with different wind-velocity and mass-loading factors. We post-process the simulation snapshots with a radiative-transfer code to obtain the molecular emission lines. We find that the inclusion of an AGN promotes the formation of CO in clumpy and dense regions surrounding supermassive black holes (SMBHs). The CO(1-0) intensity maps (<6 Myr) in the CND seem to match well with observations of NGC 1068 with a best match for a model with 5000 km s-1 wind-velocity and a high mass-loading factor. We attempt to discern between competing explanations for the apparent counter-rotating gas disc in the NGC 1068 through an analysis of kinematic maps of the CO line emission. We suggest that mechanical AGN-feedback could explain the alignment-stability of position-angle across the different CND radii around the SMBH through momentum and energy loading of the wind. It is the wind-velocity that drives the disc out of alignment on a 100 pc scale for a long period of time. The position-velocity diagrams are in broad agreement with the predicted Keplerian rotation-curve in the model without AGN, but the AGN models exhibit a larger degree of scatter, in better agreement with NGC 1068 observations
The FLAMINGO project: cosmological hydrodynamical simulations for large-scale structure and galaxy cluster surveys
We introduce the Virgo Consortium's FLAMINGO suite of hydrodynamical
simulations for cosmology and galaxy cluster physics. To ensure the simulations
are sufficiently realistic for studies of large-scale structure, the subgrid
prescriptions for stellar and AGN feedback are calibrated to the observed
low-redshift galaxy stellar mass function and cluster gas fractions. The
calibration is performed using machine learning, separately for three
resolutions. This approach enables specification of the model by the
observables to which they are calibrated. The calibration accounts for a number
of potential observational biases and for random errors in the observed stellar
masses. The two most demanding simulations have box sizes of 1.0 and 2.8 Gpc
and baryonic particle masses of and ,
respectively. For the latter resolution the suite includes 12 model variations
in a 1 Gpc box. There are 8 variations at fixed cosmology, including shifts in
the stellar mass function and/or the cluster gas fractions to which we
calibrate, and two alternative implementations of AGN feedback (thermal or
jets). The remaining 4 variations use the unmodified calibration data but
different cosmologies, including different neutrino masses. The 2.8 Gpc
simulation follows particles, making it the largest ever
hydrodynamical simulation run to . Lightcone output is produced on-the-fly
for up to 8 different observers. We investigate numerical convergence, show
that the simulations reproduce the calibration data, and compare with a number
of galaxy, cluster, and large-scale structure observations, finding very good
agreement with the data for converged predictions. Finally, by comparing
hydrodynamical and `dark-matter-only' simulations, we confirm that baryonic
effects can suppress the halo mass function and the matter power spectrum by up
to per cent.Comment: 44 pages, 23 figures. Accepted for publication in MNRAS. V3 includes
changes made in published version: jet simulations were redone to fix a bug,
but the differences are nearly invisible. For visualizations, see the
FLAMINGO website at https://flamingo.strw.leidenuniv.nl