47 research outputs found
The origin of fast molecular outflows in quasars: molecule formation in AGN-driven galactic winds
We explore the origin of fast molecular outflows that have been observed in
Active Galactic Nuclei (AGN). Previous numerical studies have shown that it is
difficult to create such an outflow by accelerating existing molecular clouds
in the host galaxy, as the clouds will be destroyed before they can reach the
high velocities that are observed. In this work, we consider an alternative
scenario where molecules form in-situ within the AGN outflow. We present a
series of hydro-chemical simulations of an isotropic AGN wind interacting with
a uniform medium. We follow the time-dependent chemistry of 157 species,
including 20 molecules, to determine whether molecules can form rapidly enough
to produce the observed molecular outflows. We find H outflow rates up to
140 M yr, which is sensitive to density, AGN luminosity, and
metallicity. We compute emission and absorption lines of CO, OH and warm (a few
hundred K) H from the simulations in post-processing. The CO-derived
outflow rates and OH absorption strengths at solar metallicity agree with
observations, although the maximum line of sight velocities from the model CO
spectra are a factor 2 lower than is observed. We derive a CO (1-0) to
H conversion factor of = 0.13 M (K km
s pc), 6 times lower than is commonly assumed in observations
of such systems. We find strong emission from the mid-infrared lines of H.
The mass of H traced by this infrared emission is within a few per cent of
the total H mass. This H emission may be observable by JWST.Comment: 30 pages, 21 figures (including appendices), resubmitted to MNRAS
following referee's report. Some results have changed from the previous
version, in particular for warm H2 emission (see Figs. 5 and 13
Radiative cooling of swept up gas in AGN-driven galactic winds and its implications for molecular outflows
We recently used hydro-chemical simulations to demonstrate that molecular
outflows observed in luminous quasars can be explained by molecule formation
within the AGN wind. However, these simulations cover a limited parameter
space, due to their computational cost. We have therefore developed an analytic
model to follow cooling in the shocked ISM layer of an AGN wind. We explore
different ambient densities (), density profile
slopes (), AGN luminosities (), and metallicities (). The swept up gas
mostly cools within ~1 Myr. Based on our previous simulations, we predict that
this gas would produce observable molecular outflows. The instantaneous
momentum boost initially increases as the outflow decelerates. However, it
reaches a maximum of 20, due to work done against the gravitational
potential. The predicted time-averaged observational estimate of the molecular
outflow momentum boost reaches a maximum of , partly due to our
assumed molecular fraction, 0.2, but also because the instantaneous and
observational, time-averaged definitions are not equivalent. Thus recent
observational estimates of order unity momentum boosts do not necessarily rule
out energy-driven outflows. Finally, we find that dust grains are likely to
re-form by accretion of metals after the shocked ISM layer has cooled, assuming
that a small fraction of dust grains swept up after this layer has cooled are
able to mix into the cool phase, and assuming that grain growth remains
efficient in the presence of the strong AGN radiation field. This would enable
rapid molecule formation, as assumed in our models.Comment: 22 pages, 16 figures (including appendices). Accepted for publication
in MNRA
Simulations of Jet Heating in Galaxy Clusters: Successes and Challenges
We study how jets driven by active galactic nuclei influence the cooling flow
in Perseus-like galaxy cluster cores with idealised, non-relativistic,
hydrodynamical simulations performed with the Eulerian code ATHENA using
high-resolution Godunov methods with low numerical diffusion. We use novel
analysis methods to measure the cooling rate, the heating rate associated to
multiple mechanisms, and the power associated with adiabatic
compression/expansion. A significant reduction of the cooling rate and cooling
flow within 20 kpc from the centre can be achieved with kinetic jets. However,
at larger scales and away from the jet axis, the system relaxes to a cooling
flow configuration. Jet feedback is anisotropic and is mostly distributed along
the jet axis, where the cooling rate is reduced and a significant fraction of
the jet power is converted into kinetic power of heated outflowing gas. Away
from the jet axis weak shock heating represents the dominant heating source.
Turbulent heating is significant only near the cluster centre, but it becomes
inefficient at 50 kpc scales where it only represents a few percent of the
total heating rate. Several details of the simulations depend on the choice
made for the hydro solver, a consequence of the difficulty of achieving proper
numerical convergence for this problem: current physics implementations and
resolutions do not properly capture multi-phase gas that develops as a
consequence of thermal instability. These processes happen at the grid scale
and leave numerical solutions sensitive to the properties of the chosen hydro
solver.Comment: Accepted for publication on MNRA
Key Physical Processes in the Circumgalactic Medium
Spurred by rich, multi-wavelength observations and enabled by new
simulations, ranging from cosmological to sub-pc scales, the last decade has
seen major theoretical progress in our understanding of the circumgalactic
medium. We review key physical processes in the CGM. Our conclusions include:
(1) The properties of the CGM depend on a competition between gravity-driven
infall and gas cooling. When cooling is slow relative to free fall, the gas is
hot (roughly virial temperature) whereas the gas is cold (T~10^4 K) when
cooling is rapid. (2) Gas inflows and outflows play crucial roles, as does the
cosmological environment. Large-scale structure collimates cold streams and
provides angular momentum. Satellite galaxies contribute to the CGM through
winds and gas stripping. (3) In multiphase gas, the hot and cold phases
continuously exchange mass, energy and momentum. The interaction between
turbulent mixing and radiative cooling is critical. A broad spectrum of cold
gas structures, going down to sub-pc scales, arises from fragmentation,
coagulation, and condensation onto gas clouds. (4) Magnetic fields, thermal
conduction and cosmic rays can substantially modify how the cold and hot phases
interact, although microphysical uncertainties are presently large. Key open
questions for future work include the mutual interplay between small-scale
structure and large-scale dynamics, and how the CGM affects the evolution of
galaxies.Comment: 69 pages, 13 figures. Accepted for publication in Annual Review of
Astronomy and Astrophysics. Authors' draft. Edited version will appear in the
next volum
Stellar and Quasar Feedback in Concert: Effects on AGN Accretion, Obscuration, and Outflows
We study the interaction of feedback from active galactic nuclei (AGN) and a
multi-phase interstellar medium (ISM), in simulations including explicit
stellar feedback, multi-phase cooling, accretion-disk winds, and Compton
heating. We examine radii ~0.1-100 pc around a black hole (BH), where the
accretion rate onto the BH is determined and where AGN-powered winds and
radiation couple to the ISM. We conclude: (1) The BH accretion rate is
determined by exchange of angular momentum between gas and stars in
gravitational instabilities. This produces accretion rates ~0.03-1 Msun/yr,
sufficient to power luminous AGN. (2) The gas disk in the galactic nucleus
undergoes an initial burst of star formation followed by several Myrs where
stellar feedback suppresses the star formation rate (SFR). (3) AGN winds
injected at small radii with momentum fluxes ~L/c couple efficiently to the ISM
and have dramatic effects on ISM properties within ~100 pc. AGN winds suppress
the nuclear SFR by factors ~10-30 and BH accretion rate by factors ~3-30. They
increase the outflow rate from the nucleus by factors ~10, consistent with
observational evidence for galaxy-scale AGN-driven outflows. (4) With AGN
feedback, the predicted column density distribution to the BH is consistent
with observations. Absent AGN feedback, the BH is isotropically obscured and
there are not enough optically-thin sightlines to explain Type-I AGN. A
'torus-like' geometry arises self-consistently as AGN feedback evacuates gas in
polar regions.Comment: 17 pages, 12 figures, MNRAS accepted (revised to match published
version
The formation of massive, quiescent galaxies at cosmic noon
The cosmic noon (z~1.5-3) marked a period of vigorous star formation for most
galaxies. However, about a third of the more massive galaxies at those times
were quiescent in the sense that their observed stellar populations are
inconsistent with rapid star formation. The reduced star formation activity is
often attributed to gaseous outflows driven by feedback from supermassive black
holes, but the impact of black hole feedback on galaxies in the young Universe
is not yet definitively established. We analyze the origin of quiescent
galaxies with the help of ultra-high resolution, cosmological simulations that
include feedback from stars but do not model the uncertain consequences of
black hole feedback. We show that dark matter halos with specific accretion
rates below ~0.25-0.4 per Gyr preferentially host galaxies with reduced star
formation rates and red broad-band colors. The fraction of such halos in large
dark matter only simulations matches the observed fraction of massive quiescent
galaxies (~10^10-10^11 Msun). This strongly suggests that halo accretion rate
is the key parameter determining which massive galaxies at z~1.5-3 become
quiescent. Empirical models that connect galaxy and halo evolution, such as
halo occupation distribution or abundance matching models, assume a tight link
between galaxy properties and the masses of their parent halos. These models
will benefit from adding the specific accretion rate of halos as a second model
parameter.Comment: 5 pages, 5 figures, to appear in MNRAS Letter