188 research outputs found
Multi-phase Nature of a Radiation-Driven Fountain with Nuclear Starburst in a Low-mass Active Galactic Nucleus
The structures and dynamics of molecular, atomic, and ionized gases are
studied around a low-luminosity active galactic nucleus (AGN) with a small
() black hole using 3D radiation hydrodynamic
simulations. We studied, for the first time, the non-equilibrium chemistry for
the X-ray dominated region in the "radiation-driven fountain" (Wada 2012) with
supernova feedback. A double hollow cone structure is naturally formed without
postulating a thick "torus" around a central source. The cone is occupied with
an inhomogeneous, diffuse ionized gas and surrounded by a geometrically thick
() atomic gas. Dense molecular gases are distributed near the
equatorial plane, and energy feedback from supernovae enhances their scale
height. Molecular hydrogen exists in a hot phase ( > 1000 K) as well as in a
cold ( cm) phase. The velocity dispersion of
H in the vertical direction is comparable to the rotational velocity, which
is consistent with near infrared observations of nearby Seyfert galaxies. Using
3D radiation transfer calculations for the dust emission, we find polar
emission in the mid-infrared band (12), which is associated with bipolar
outflows, as suggested in recent interferometric observations of nearby AGNs.
If the viewing angle for the nucleus is larger than 75 deg, the spectral energy
distribution (~ 2 -- 60 ) of this model is consistent with that of the
Circinus galaxy. The multi-phase interstellar medium observed in
optical/infrared and X-ray observations is also discussed.Comment: 9 pages, 5 figures. Accepted for ApJL. A movie file for Fig.5b can be
downloaded from http://astrophysics.jp/Circinus
The life cycle of starbursting circumnuclear gas discs
High-resolution observations from the sub-mm to the optical wavelength regime
resolve the central few 100pc region of nearby galaxies in great detail. They
reveal a large diversity of features: thick gas and stellar discs, nuclear
starbursts, in- and outflows, central activity, jet interaction, etc.
Concentrating on the role circumnuclear discs play in the life cycles of
galactic nuclei, we employ 3D adaptive mesh refinement hydrodynamical
simulations with the RAMSES code to self-consistently trace the evolution from
a quasi-stable gas disc, undergoing gravitational (Toomre) instability, the
formation of clumps and stars and the disc's subsequent, partial dispersal via
stellar feedback. Our approach builds upon the observational finding that many
nearby Seyfert galaxies have undergone intense nuclear starbursts in their
recent past and in many nearby sources star formation is concentrated in a
handful of clumps on a few 100pc distant from the galactic centre. We show that
such observations can be understood as the result of gravitational
instabilities in dense circumnuclear discs. By comparing these simulations to
available integral field unit observations of a sample of nearby galactic
nuclei, we find consistent gas and stellar masses, kinematics, star formation
and outflow properties. Important ingredients in the simulations are the
self-consistent treatment of star formation and the dynamical evolution of the
stellar distribution as well as the modelling of a delay time distribution for
the supernova feedback. The knowledge of the resulting simulated density
structure and kinematics on pc scale is vital for understanding inflow and
feedback processes towards galactic scales.Comment: accepted by MNRA
Radiation-driven Fountain and Origin of Torus around Active Galactic Nuclei
We propose a plausible mechanism to explain the formation of the so-called
"obscuring tori" around active galactic nuclei (AGNs) based on
three-dimensional hydrodynamic simulations including radiative feedback from
the central source. The X-ray heating and radiation pressure on the gas are
explicitly calculated using a ray-tracing method. This radiation feedback
drives a "fountain", that is, a vertical circulation of gas in the central a
few to tens parsecs. Interaction between the non-steady outflows and inflows
causes the formation of a geometrically thick torus with internal turbulent
motion. As a result, the AGN is obscured for a wide range of solid angles. In a
quasi-steady state, the opening angles for the column density toward a black
hole < 10^23 cm^-2 are approximately +-30 deg and +-50 deg for AGNs with 10%
and 1% Eddington luminosity, respectively. Mass inflows through the torus
coexist with the outflow and internal turbulent motion, and the average mass
accretion rate to the central parsec region is 2x10^-4 ~ 10^-3, M_sun/yr this
is about ten times smaller than accretion rate required to maintain the AGN
luminosity. This implies that relatively luminous AGN activity is intrinsically
intermittent or that there are other mechanisms, such as stellar energy
feedback, that enhance the mass accretion to the center.Comment: 12 pages, 9 figures, accepted publication in Ap
A Review of the Theory of Galactic Winds Driven by Stellar Feedback
Galactic winds from star-forming galaxies are crucial to the process of
galaxy formation and evolution, regulating star formation, shaping the stellar
mass function and the mass-metallicity relation, and enriching the
intergalactic medium with metals. Galactic winds associated with stellar
feedback may be driven by overlapping supernova explosions, radiation pressure
of starlight on dust grains, and cosmic rays. Galactic winds are multiphase,
the growing observations of emission and absorption of cold molecular, cool
atomic, ionized warm and hot outflowing gas in a large number of galaxies have
not been completely understood. In this review article, I summarize the
possible mechanisms associated with stars to launch galactic winds, and review
the multidimensional hydrodynamic, radiation hydrodynamic and
magnetohydrodynamic simulations of winds based on various algorithms. I also
briefly discuss the theoretical challenges and possible future research
directions.Comment: 47 pages, 7 figures. Accepted for publication in the special issue of
Galaxie
Dynamics and formation of obscuring tori in AGNs
We considered the evolution of a self-gravitating clumpy torus in the
gravitational field of the central mass of an active galactic nucleus (AGN) in
the framework of the N-body problem. The initial conditions take into account
winds with different opening angles. Results of our N-body simulations show
that the clouds moving on orbits with a spread in inclinations and
eccentricities form a toroidal region. The velocity of the clouds at the inner
boundary of the torus is lower than in a disk model that can explain the
observed rotation curves. We discuss the scenario of torus formation related
with the beginning of the AGN stage.Comment: 9 pages, 4 figures, Proceeding of the conference "Quasars at All
Cosmic Epochs", held in Padova (Italy), April 2-7, 2017. Published in
"Frontiers in Astronomy and Space Science" (available at
https://doi.org/10.3389/fspas.2017.00060
HI observations of the nearest starburst galaxy NGC 253 with the SKA precursor KAT-7
We present HI observations of the Sculptor Group starburst spiral galaxy NGC
253, obtained with the Karoo Array Telescope (KAT-7). KAT-7 is a pathfinder for
the SKA precursor MeerKAT, under construction. The short baselines and low
system temperature of the telescope make it very sensitive to large scale, low
surface brightness emission. The KAT-7 observations detected 33% more flux than
previous VLA observations, mainly in the outer parts and in the halo for a
total HI mass of M. HI can be found at
large distances perpendicular to the plane out to projected distances of ~9-10
kpc away from the nucleus and ~13-14 kpc at the edge of the disk. A novel
technique, based on interactive profile fitting, was used to separate the main
disk gas from the anomalous (halo) gas. The rotation curve (RC) derived for the
HI disk confirms that it is declining in the outer parts, as seen in previous
optical Fabry-Perot measurements. As for the anomalous component, its RC has a
very shallow gradient in the inner parts and turns over at the same radius as
the disk, kinematically lagging by ~100 km/sec. The kinematics of the observed
extra planar gas is compatible with an outflow due to the central starburst and
galactic fountains in the outer parts. However, the gas kinematics shows no
evidence for inflow. Analysis of the near-IR WISE data, shows clearly that the
star formation rate (SFR) is compatible with the starburst nature of NGC 253.Comment: 18 pages, 20 figures, 8 Tables. Accepted for publication to MNRA
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
Cool outflows in galaxies and their implications
Neutral-atomic and molecular outflows are a common occurrence in galaxies,
near and far. They operate over the full extent of their galaxy hosts, from the
innermost regions of galactic nuclei to the outermost reaches of galaxy halos.
They carry a substantial amount of material that would otherwise have been used
to form new stars. These cool outflows may have a profound impact on the
evolution of their host galaxies and environments. This article provides an
overview of the basic physics of cool outflows, a comprehensive assessment of
the observational techniques and diagnostic tools used to characterize them, a
detailed description of the best-studied cases, and a more general discussion
of the statistical properties of these outflows in the local and distant
universe. The remaining outstanding issues that have not yet been resolved are
summarized at the end of the review to inspire new research directions.STFC and ER
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