123 research outputs found
Time-Independent Gravitational Fields in the BGK Scheme for Hydrodynamics
We incorporate a time-independent gravitational field into the BGK scheme for
numerical hydrodynamics. In the BGK scheme the gas evolves via an approximation
to the collisional Boltzmann equation, namely the Bhatnagar-Gross-Krook (BGK)
equation. Time-dependent hydrodynamical fluxes are computed from local
solutions of the BGK equation. By accounting for particle collisions, the
fundamental mechanism for generating dissipation in gas flow, a scheme based on
the BGK equation gives solutions to the Navier-Stokes equations: the fluxes
carry both advective and dissipative terms. We perform numerical experiments in
both 1D Cartesian geometries and axisymmetric cylindrical coordinates.Comment: 31 pages including 19 figures (For higher resolution figs. see
http://www.mpia-hd.mpg.de/MPIA/Projects/THEORY/slyz), Accepted for
publication in Astronomy and Astrophysics, Supplement Serie
AGN feedback using AMR cosmological simulations
Feedback processes are thought to solve some of the long-standing issues of
the numerical modelling of galaxy formation: over-cooling, low angular
momentum, massive blue galaxies, extra-galactic enrichment, etc. The accretion
of gas onto super-massive black holes in the centre of massive galaxies can
release tremendous amounts of energy to the surrounding medium. We show, with
cosmological Adaptive Mesh Refinement simulations, how the growth of black
holes is regulated by the feedback from Active Galactic Nuclei using a new dual
jet/heating mechanism. We discuss how this large amount of feedback is able to
modify the cold baryon content of galaxies, and perturb the properties of the
hot plasma in their vicinity.Comment: 4 pages, 2 figures, contribution to the Astronomical Society of the
Pacific Conference Series for the Cefal\`u meeting "Advances in computational
astrophysics: methods, tools and outcomes
Zooming in on supermassive black holes: how resolving their gas cloud host renders their accretion episodic
Born in rapidly evolving mini-halos during the first billion years of the
Universe, super- massive black holes (SMBH) feed from gas flows spanning many
orders of magnitude, from the cosmic web in which they are embedded to their
event horizon. As such, accretion onto SMBHs constitutes a formidable challenge
to tackle numerically, and currently requires the use of sub-grid models to
handle the flow on small, unresolved scales. In this paper, we study the impact
of resolution on the accretion pattern of SMBHs initially inserted at the heart
of dense galactic gas clouds, using a custom super-Lagrangian refinement scheme
to resolve the black hole (BH) gravitational zone of influence. We find that
once the self-gravitating gas cloud host is sufficiently well re- solved,
accretion onto the BH is driven by the cloud internal structure, independently
of the BH seed mass, provided dynamical friction is present during the early
stages of cloud collapse. For a pristine gas mix of hydrogen and helium, a slim
disc develops around the BH on sub-parsec scales, turning the otherwise chaotic
BH accretion duty cycle into an episodic one, with potentially important
consequences for BH feedback. In the presence of such a nuclear disc, BH mass
growth predominantly occurs when infalling dense clumps trigger disc
instabilities, fuelling intense albeit short-lived gas accretion episodes.Comment: Resubmitted to mnras after reviewer comments, 24 page
Black hole evolution: II. Spinning black holes in a supernova-driven turbulent interstellar medium
Supermassive black holes (BH) accrete gas from their surroundings and
coalesce with companions during galaxy mergers, and both processes change the
BH mass and spin. By means of high-resolution hydrodynamical simulations of
galaxies, either idealised or embedded within the cosmic web, we explore the
effects of interstellar gas dynamics and external perturbations on BH spin
evolution. All these physical quantities were evolved on-the-fly in a
self-consistent manner. We use a `maximal' model to describe the turbulence
induced by stellar feedback to highlight its impact on the angular momentum of
the gas accreted by the BH. Periods of intense star formation are followed by
phases where stellar feedback drives large-scale outflows and hot bubbles. We
find that BH accretion is synchronised with star formation, as only when gas is
cold and dense do both processes take place. During such periods, gas motion is
dominated by consistent rotation. On the other hand, when stellar feedback
becomes substantial, turbulent motion randomises gas angular momentum. However
BH accretion is strongly suppressed in that case, as cold and dense gas is
lacking. In our cosmological simulation, at very early times (z>6), the
galactic disc has not yet settled and no preferred direction exists for the
angular momentum of the accreted gas, so the BH spin remains low. As the gas
settles into a disc (6>z>3), the BH spin then rapidly reaches its maximal
value. At lower redshifts (z<3), even when galaxy mergers flip the direction of
the angular momentum of the accreted gas, causing it to counter-rotate, the BH
spin magnitude only decreases modestly and temporarily. Should this be a
typical evolution scenario for BH, it potentially has dramatic consequences
regarding their origin and assembly, as accretion on maximally spinning BH
embedded in thin Shakura-Sunyaev disc is significantly reduced.Comment: 16 pages, 13 figures, MNRAS accepte
Towards simulating star formation in turbulent high-z galaxies with mechanical supernova feedback
Feedback from supernovae is essential to understanding the self-regulation of
star formation in galaxies. However, the efficacy of the process in a
cosmological context remains unclear due to excessive radiative losses during
the shock propagation. To better understand the impact of SN explosions on the
evolution of galaxies, we perform a suite of high-resolution (12 pc), zoom-in
cosmological simulations of a Milky Way-like galaxy at z=3 with adaptive mesh
refinement. We find that SN explosions can efficiently regulate star formation,
leading to the stellar mass and metallicity consistent with the observed
mass-metallicity relation and stellar mass-halo mass relation at z~3. This is
achieved by making three important changes to the classical feedback scheme: i)
the different phases of SN blast waves are modelled directly by injecting
radial momentum expected at each stage, ii) the realistic time delay of SNe,
commencing at as early as 3 Myr, is required to disperse very dense gas before
a runaway collapse sets in at the galaxy centre via mergers of gas clumps, and
iii) a non-uniform density distribution of the ISM is taken into account below
the computational grid scale for the cell in which SN explodes. The last
condition is motivated by the fact that our simulations still do not resolve
the detailed structure of a turbulent ISM in which the fast outflows can
propagate along low-density channels. The simulated galaxy with the SN feedback
model shows strong outflows, which carry approximately ten times larger mass
than star formation rate, as well as smoothly rising circular velocity. Other
feedback models that do not meet the three conditions form too many stars,
producing a peaked rotation curve. Our results suggest that understanding the
structure of the turbulent ISM may be crucial to assess the role of SN and
other feedback processes in galaxy formation theory.Comment: 22 pages, 18 figures, Accepted for publication in MNRA
Probing Cosmic Dawn: Modelling the Assembly History, SEDs, and Dust Content of Selected Galaxies
The presence of spectroscopically confirmed Balmer breaks in galaxy spectral
energy distributions (SEDs) at provides one of the best probes of the
assembly history of the first generations of stars in our Universe. Recent
observations of the gravitationally lensed source, MACS 1149_JD1 (JD1),
indicate that significant amounts of star formation likely occurred at
redshifts as high as . The inferred stellar mass, dust mass, and
assembly history of JD1, or any other galaxy at these redshifts that exhibits a
strong Balmer break, can provide a strong test of our best theoretical models
from high-resolution cosmological simulations. In this work, we present the
results from a cosmological radiation-hydrodynamics simulation of the region
surrounding a massive Lyman-break galaxy. For two of our most massive systems,
we show that dust preferentially resides in the vicinity of the young stars
thereby increasing the strength of the measured Balmer break such that the
simulated SEDs are consistent with the photometry of JD1 and two other
systems (GN-z10-3 and GN-z9-1) that have proposed Balmer breaks at high
redshift. We find strong variations in the shape and luminosity of the SEDs of
galaxies with nearly identical stellar and halo masses, indicating the
importance of morphology, assembly history, and dust distribution in making
inferences on the properties of individual galaxies at high redshifts. Our
results stress the importance that dust may play in modulating the observable
properties of galaxies, even at the extreme redshifts of .Comment: 16 pages, 13 Figures, Accepted to MNRA
Bursty star formation feedback and cooling outflows
We study how outflows of gas launched from a central galaxy undergoing
repeated starbursts propagate through the circumgalactic medium (CGM), using
the simulation code RAMSES. We assume that the outflow from the disk can be
modelled as a rapidly moving bubble of hot gas at above
disk, then ask what happens as it moves out further into the halo around the
galaxy on scales. To do this we run 60 two-dimensional
simulations scanning over parameters of the outflow. Each of these is repeated
with and without radiative cooling, assuming a primordial gas composition to
give a lower bound on the importance of cooling. In a large fraction of
radiative-cooling cases we are able to form rapidly outflowing cool gas from in
situ cooling of the flow. We show that the amount of cool gas formed depends
strongly on the 'burstiness' of energy injection; sharper, stronger bursts
typically lead to a larger fraction of cool gas forming in the outflow. The
abundance ratio of ions in the CGM may therefore change in response to the
detailed historical pattern of star formation. For instance, outflows generated
by star formation with short, intense bursts contain up to 60 per cent of their
gas mass at temperatures ; for near-continuous star
formation the figure is 5 per cent. Further study of cosmological
simulations, and of idealised simulations with e.g., metal-cooling, magnetic
fields and/or thermal conduction, will help to understand the precise signature
of bursty outflows on observed ion abundances.Comment: 8 pages, 6 figures, accepted in MNRA
Probing for Dark Matter within Spiral Galaxy Disks
We explore the relative importance of the stellar mass density as compared to
the inner dark halo, using the observed gas kinematics throughout the disk of
the spiral galaxy NGC 4254 (Messier 99). We perform hydrodynamical simulations
of the gas flow for a sequence of gravitational potentials in which we vary the
stellar disk contribution to the total potential. This stellar portion of the
potential was derived empirically from color corrected K-band photometry
reflecting the spiral arms in the stellar mass, while the halo was modelled as
an isothermal sphere. The simulated gas density and the gas velocity field are
then compared to the observed stellar spiral arm morphology and to the H-alpha
gas kinematics. We find that this method is a powerful tool to determine the
corotation radius of the spiral pattern and that it can be used to place an
upper limit on the mass of the stellar disk. For the case of the galaxy NGC
4254 we find R_cr = 7.5 +/- 1.1 kpc, or R_cr = 2.1 R_exp(K'). We also
demonstrate that for a maximal disk the prominent spiral arms of the stellar
component over-predict the non-circular gas motions unless an axisymmetric dark
halo component contributes significantly (>~ 1/3) to the total potential inside
2.2 K-band exponential disk scale lengths.Comment: 16 pages including 9 figures, accepted for publication in the Ap
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