3,226 research outputs found
Fast cold gas in hot AGN outflows
Observations of the emission from spatially extended cold gas around bright
high-redshift QSOs reveal surprisingly large velocity widths exceeding 2000 km
s^(-1), out to projected distances as large as 30 kpc. The high velocity widths
have been interpreted as the signature of powerful AGN-driven outflows.
Naively, these findings appear in tension with hydrodynamic models in which
AGN-driven outflows are energy-driven and thus very hot with typical
temperatures T = 10^6-7 K. Using the moving-mesh code Arepo, we perform
'zoom-in' cosmological simulations of a z = 6 QSO and its environment,
following black hole growth and feedback via energy-driven outflows. In the
simulations, the QSO host galaxy is surrounded by a clumpy circum-galactic
medium pre-enriched with metals due to supernovae-driven galactic outflows. As
a result, part of the AGN-driven hot outflowing gas can cool radiatively,
leading to large amounts (> 10^9 M_sun) of cold gas comoving with the hot
bipolar outflow. This results in velocity widths of spatially extended cold gas
similar to those observed. We caution, however, that gas inflows, random
motions in the deep potential well of the QSO host galaxy and cooling of
supernovae-driven winds contribute significantly to the large velocity width of
the cold gas in the simulations, complicating the interpretation of
observational data
Smoothed particle hydrodynamics simulations of gas and dust mixtures
We present a 'two-fluid' implementation of dust in smoothed particle
hydrodynamics (SPH) in the test particle limit. The scheme is able to handle
both short and long stopping times and reproduces the short friction time
limit, which is not properly handled in other implementations. We apply novel
tests to verify its accuracy and limitations, including multi-dimensional tests
that have not been previously applied to the drag-coupled dust problem and
which are particularly relevant to self-gravitating protoplanetary discs. Our
tests demonstrate several key requirements for accurate simulations of gas-dust
mixtures. Firstly, in standard SPH particle jitter can degrade the dust
solution, even when the gas density is well reproduced. The use of integral
gradients, a Wendland kernel and a large number of neighbours can control this,
albeit at a greater computational cost. Secondly, when it is necessary to limit
the artificial viscosity we recommend using the Cullen & Dehnen (2010) switch,
since the alternative, using , can generate a large velocity
noise up to in the dust particles. Thirdly, we
find that an accurate dust density estimate requires neighbours, since,
unlike the gas, the dust particles do not feel regularization forces. This
density noise applies to all particle-based two-fluid implementations of dust,
irrespective of the hydro solver and could lead to numerically induced
fragmentation. Although our tests show accurate dusty gas simulations are
possible, care must be taken to minimize the contribution from numerical noise
Seeding high-redshift QSOs by collisional runaway in primordial star clusters
We study how runaway stellar collisions in high-redshift, metal-poor star
clusters form very massive stars (VMSs) that can directly collapse to
intermediate-mass black holes (IMBHs). We follow the evolution of a pair of
neighbouring high-redshift mini-haloes with high-resolution, cosmological
hydrodynamical zoom-in simulations using the adaptive mesh refinement code
RAMSES combined with the non-equilibrium chemistry package KROME. The first
collapsing mini-halo is assumed to enrich the central nuclear star cluster
(NSC) of the other to a critical metallicity, sufficient for Population II
(Pop. II) star formation at redshift . Using the spatial
configuration of the flattened, asymmetrical gas cloud forming in the core of
the metal enriched halo, we set the initial conditions for simulations of an
initially non-spherical star cluster with the direct summation code NBODY6
which are compared to about 2000 NBODY6 simulations of spherical star clusters
for a wide range of star cluster parameters. The final mass of the VMS that
forms depends strongly on the initial mass and initial central density of the
NSC. For the initial central densities suggested by our RAMSES simulations,
VMSs with mass > 400 M can form in clusters with stellar masses of
M, and this can increase to well over 1000 M
for more massive and denser clusters. The high probability we find for forming
a VMS in these mini-haloes at such an early cosmic time makes collisional
runaway of Pop. II star clusters a promising channel for producing large
numbers of high-redshift IMBHs that may act as the seeds of supermassive black
holes
Feedback from active galactic nuclei: Energy- versus momentum-driving
We employ hydrodynamical simulations using the moving-mesh code AREPO to
investigate the role of energy and momentum input from Active Galactic Nuclei
(AGN) in driving large-scale galactic outflows. We start by reproducing
analytic solutions for both energy- and momentum-driven outflowing shells in
simulations of a spherical isolated dark matter potential with gas in
hydrostatic equilibrium and with no radiative cooling. We confirm that for this
simplified setup, galactic outflows driven by a momentum input rate of order
L_Edd/c can establish an M_BH - sigma relation with slope and normalisation
similar to that observed. We show that momentum input at a rate of L_Edd/c is
however insufficient to drive efficient outflows once cooling and gas inflows
as predicted by cosmological simulations at resolved scales are taken into
account. We argue that observed large-scale AGN-driven outflows are instead
likely to be energy-driven and show that such outflows can reach momentum
fluxes exceeding 10 L_Edd/c within the innermost 10 kpc of the galaxy. The
outflows are highly anisotropic, with outflow rates and a velocity structure
found to be inadequately described by spherical outflow models. We verify that
the hot energy-driven outflowing gas is expected to be strongly affected by
metal-line cooling, leading to significant amounts (>10^9 M_sun) of entrained
cold gas
Recommended from our members
The environment of bright QSOs at z̃6: Star-forming galaxies and X-ray emission
We employ cosmological hydrodynamical simulations to investigate models in
which the supermassive black holes (BHs) powering luminous z ~ 6 QSOs grow from
massive seeds. We simulate at high resolution 18 fields sampling regions with
densities ranging from the mean cosmic density all the way to the highest sigma
peaks in the Millennium simulation volume. Only in the most massive halos, BHs
can grow to masses up to ~ 10^9 Msun by z ~ 6 without invoking super-Eddington
accretion. Accretion onto the most massive BHs becomes limited by thermal AGN
feedback by z ~ 9-8 with further BH growth proceeding in short Eddington
limited bursts. Our modelling suggests that current flux-limited surveys of
QSOs at high redshift preferentially detect objects at their peak luminosity
and therefore miss a substantial population of QSOs powered by similarly
massive BHs but with low accretion rates. To test whether the required host
halo masses are consistent with the observed galaxy environment of z ~ 6 QSOs,
we produce realistic rest-frame UV images of our simulated galaxies. Without
strong stellar feedback, our simulations predict numbers of bright galaxies
larger than observed by a factor ten or more. Supernova-driven galactic winds
reduce the predicted numbers to a level consistent with observations indicating
that stellar feedback was already very efficient at high redshifts. We have
further investigated the effect of thermal AGN feedback on the surrounding gas.
Our adopted AGN feedback prescription drives mostly energy-driven highly
anisotropic outflows with gas speeds of >= 1000 km/s to distances of >= 10 kpc
consistent with observations. The spatially extended thermal X-ray emission
around bright QSOs powered by these outflows can exceed by large factors the
emission expected without AGN feedback and is an important diagnostic of the
mechanism whereby AGN feedback energy couples to surrounding gas
Simulations of AGN feedback in galaxy clusters and groups: impact on gas fractions and the Lx-T scaling relation
Recently, rapid observational and theoretical progress has established that
black holes (BHs) play a decisive role in the formation and evolution of
individual galaxies as well as galaxy groups and clusters. In particular, there
is compelling evidence that BHs vigorously interact with their surroundings in
the central regions of galaxy clusters, indicating that any realistic model of
cluster formation needs to account for these processes. This is also suggested
by the failure of previous generations of hydrodynamical simulations without BH
physics to simultaneously account for the paucity of strong cooling flows in
clusters, the slope and amplitude of the observed cluster scaling relations,
and the high-luminosity cut-off of central cluster galaxies. Here we use
high-resolution cosmological simulations of a large cluster and group sample to
study how BHs affect their host systems. We focus on two specific properties,
the halo gas fraction and the X-ray luminosity-temperature scaling relation,
both of which are notoriously difficult to reproduce in self-consistent
hydrodynamical simulations. We show that BH feedback can solve both of these
issues, bringing them in excellent agreement with observations, without
alluding to the `cooling only' solution that produces unphysically bright
central galaxies. By comparing a large sample of simulated AGN-heated clusters
with observations, our new simulation technique should make it possible to
reliably calibrate observational biases in cluster surveys, thereby enabling
various high-precision cosmological studies of the dark matter and dark energy
content of the universe.Comment: 4 pages, 2 figures, minor revisions, ApJL in pres
Resolving flows around black holes: the impact of gas angular momentum
Cosmological simulations almost invariably estimate the accretion of gas on to supermassive black holes using a Bondi-Hoyle-like prescription. Doing so ignores the effects of the angular momentum of the gas, which may prevent or significantly delay accreting material falling directly on to the black hole. We outline a black hole accretion rate prescription using a modified Bondi-Hoyle formulation that takes into account the angular momentum of the surrounding gas. Meaningful implementation of this modified Bondi-Hoyle formulation is only possible when the inner vorticity distribution is well resolved, which we achieve through the use of a super-Lagrangian refinement technique around black holes within our simulations. We then investigate the effects on black hole growth by performing simulations of isolated as well as merging disc galaxies using the moving-mesh code AREPO. We find that the gas angular momentum barrier can play an important role in limiting the growth of black holes, leading also to a several Gyr delay between the starburst and the quasar phase in major merger remnants. We stress, however, that the magnitude of this effect is highly sensitive to the thermodynamical state of the accreting gas and to the nature of the black hole feedback present.MC is supported by the Science and Technology Facilities Council (STFC). DS acknowledges support by the STFC and the ERC Starting Grant 638707 ‘Black holes and their host galaxies: co-evolution across cosmic time’. This work was performed on the following: the COSMOS Shared Memory system at DAMTP, University of Cambridge operated on behalf of the STFC DiRAC HPC Facility – this equipment is funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1; DiRAC Darwin Supercomputer hosted by the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council; DiRAC Complexity system, operated by the University of Leicester IT Services. This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1; COSMA Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility. This equipment was funded by a BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/K00087X/1, DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure
Resolving flows around black holes: Numerical technique and applications
Black holes are believed to be one of the key ingredients of galaxy formation models, but it has been notoriously challenging to simulate them due to the very complex physics and large dynamical range of spatial scales involved. Here we address a significant shortcoming of a Bondi-Hoyle-like prescription commonly invoked to estimate black hole accretion in cosmological hydrodynamic simulations of galaxy formation, namely that the Bondi-Hoyle radius is frequently unresolved. We describe and implement a novel super-Lagrangian refinement scheme to increase, adaptively and 'on the fly', the mass and spatial resolution in targeted regions around the accreting black holes at limited computational cost. While our refinement scheme is generically applicable and flexible, for the purpose of this paper we select the smallest resolvable scales to match black holes' instantaneous Bondi radii, thus effectively resolving Bondi-Hoyle-like accretion in full galaxy formation simulations. This permits us to not only estimate gas properties close to the Bondi radius much more accurately, but also allows us to improve black hole accretion and feedback implementations. We thus devise a more generic feedback model where accretion and feedback depend on the geometry of the local gas distribution and where mass, energy and momentum loading are followed simultaneously. We present a series of tests of our refinement and feedback methods and apply them to models of isolated disc galaxies. Our simulations demonstrate that resolving gas properties in the vicinity of black holes is necessary to follow black hole accretion and feedback with a higher level of realism and that doing so allows us to incorporate important physical processes so far neglected in cosmological simulations.MC is supported by the Science and Technology Facilities Council (STFC). This work was performed on the following: the COSMOS Shared Memory system at DAMTP, University of Cambridge operated on behalf of the STFC DiRAC HPC Facility – this equipment is funded by BIS National E-infrastructure capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1; DiRAC Darwin Supercomputer hosted by the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council; DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure
Stellar spiral structures in triaxial dark matter haloes
We employ very high resolution simulations of isolated Milky Way-like galaxies to study the effect of triaxial dark matter haloes on exponential stellar discs. Non-adiabatic halo shape changes can trigger two-armed grand-design spiral structures which extend all the way to the edge of the disc. Their pattern speed coincides with the inner Lindblad resonance indicating that they are kinematic density waves which can persist up to several Gyr. In dynamically cold discs, grand-design spirals are swing amplified and after a few Gyr can lead to the formation of (multi-armed) transient recurrent spirals. Stellar discs misaligned to the principal planes of the host triaxial halo develop characteristic integral shaped warps, but otherwise exhibit very similar spiral structures as aligned discs. For the grand-design spirals in our simulations, their strength dependence with radius is determined by the torque on the disc, suggesting that by studying grand-design spirals without bars it may be possible to set constraints on the tidal field and host dark matter halo shape.SH is supported by the CSC Cambridge Scholarship, jointly funded by the China Scholarship Council and by the Cambridge Overseas Trust. DS acknowledges support by the STFC and ERC Starting Grant 638707 ‘Black holes and their host galaxies: co-evolution across cosmic time’. This work was performed on DiRAC Darwin Supercomputer hosted by the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council; DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1; COSMA Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility. This equipment was funded by a BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/K00087X/1, DiRAC Operations grant ST/K003267/1 and Durham University. DiRAC is part of the National E-Infrastructure
- …