Abstract
Dust-obscured galaxies (DOGs), which are observationally characterized as faint in the optical and bright in the infrared, are the final stage of galaxy mergers and are essential objects in ...the evolution of galaxies and active galactic nuclei (AGNs). However, the relationship between the torus-scale gas dynamics around AGNs and the DOGs’ lifetime remains unclear. We obtained the evolution of the spectral energy distributions (SEDs) of a galaxy merger system with AGN feedback from postprocessed pseudo-observations based on an
N
-body/smoothed particle hydrodynamics (SPH) simulation. We focused on a late-stage merger of two identical galaxies with a supermassive black hole (SMBH) of 10
8
M
⊙
. We found that the infrared luminosity of the system reaches ultra- and hyperluminous infrared galaxy classes (10
12
and 10
13
L
⊙
, respectively). The DOG phase corresponds to a state in which the AGNs are buried in dense gas and dust, with the infrared luminosity exceeding 3.3 × 10
12
L
⊙
. We also identified subcategories of DOGs, namely bump and power-law DOGs, from the SEDs and their evolution. The bump DOGs tend to evolve to power-law DOGs over several Myrs. We found that contribution from the hot dust around the nucleus in the infrared radiation is essential for identifying the system as a power-law DOG; the gas and dust are distributed nonspherically around the nucleus, therefore, the observed properties of DOGs depend on the viewing angle. In our model, the lifetime of merger-driven DOGs is less than 4 Myr, suggesting that the observed DOG phase is a brief aspect of galaxy mergers.
In this study, we investigate the line emissions from cold molecular gas based on our previous "radiation-driven fountain model," which reliably explains the spectral energy distribution of the ...nearest type 2 Seyfert galaxy, the Circinus galaxy. Using a snapshot of the best-fit radiation-hydrodynamic model for the central pc, in which non-equilibrium X-ray-dominated region chemistry is solved, we conduct post-processed non-local thermodynamic equilibrium radiation transfer simulations for the CO lines. We obtain a spectral line energy distribution with a peak around , and its distribution suggests that the lines are not thermalized. However, for a given line of sight, the optical depth distribution is highly non-uniform between and . The CO-to-H2 conversion factor ( ), which can be directly obtained from the results and is not a constant, depends strongly on the integrated intensity and differs from the fiducial value for local objects. exhibits a large dispersion of more than one order of magnitude, reflecting the non-uniform internal structure of a "torus." In addition, we found that the physical conditions differ between grid cells on a scale of a few parsecs along the observed lines of sight; therefore, a specific observed line ratio does not necessarily represent a single physical state of the interstellar medium.
'Galactic shocks' (Fujimoto; Roberts) are investigated using full three- dimensional hydrodynamic simulations that take into account the self-gravity of the ISM, radiative cooling, and star formation ...followed by energy feedback from supernovae. This is an essential improvement over previous numerical models, in which two-dimensional isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks are unstable and transient, and they shift to a globally quasi-steady, inhomogeneous pattern due to the nonlinear development of instabilities in the disk. The spiral patterns consist of many giant molecular cloud-like dense condensations, but those local structures are not steady, and they evolve into irregular spurs in the interarm regions. Energy feedback from supernovae does not destroy the quasi- steady spiral arms; rather, it mainly contributes to the vertical motion and the structures of the ISM. The results and methods presented here are a starting point for a more consistent treatment of the ISM in spiral galaxies, in which the effects of magnetic fields, radiative transfer, chemistry, and dynamical evolution of a stellar disk are taken into account.
We study the obscuring structure of circumnuclear disks (CNDs) by considering supernova (SN) feedbacks from nuclear starburst and the effect of anisotropic radiative pressure from active galactic ...nuclei (AGNs). We suppose that the mass accretion onto a central supermassive black hole (SMBH) is triggered by SN-driven turbulence within CNDs, and we explore how the structures of CNDs depend on the BH mass (MBH) and AGN luminosity (LAGN). We find that the obscuring fraction (fobs) peaks at ∼10% of the Eddington luminosity (LEdd), and its maximal value is fobs ∼ 0.6 for less massive SMBHs (e.g., MBH < 108 M ). This is because the scale height of CNDs is determined by the SN-driven accretion for a smaller LAGN, while the dusty molecular gas in CNDs is blown away by the radiation pressure from AGNs beyond the critical luminosity. On the other hand, for massive SMBHs (e.g., MBH > 108 M ), fobs is always smaller than 0.2, and it is almost independent of LAGN because the scale height of CNDs is mainly controlled by the maximal star formation efficiency ( ) in CNDs. In comparison with the obscuring fractions suggested from the mid-infrared observations of nearby AGNs, the SN plus radiative feedback model with reproduces the observations for MBH = 108 M well. We also find that the intense starburst or the existence of dust-free absorbers inside CNDs are necessary for explaining X-ray observations.
Abstract
We investigated dusty and dust-free gas dynamics for a radiation-driven sub-parsec-scale outflow in an active galactic nucleus (AGN) associated with a supermassive black hole 10
7
M
⊙
and ...bolometric luminosity 10
44
erg s
−1
based on the two-dimensional radiation-hydrodynamic simulations. A radiation-driven “lotus-like” multi-shell outflow is launched from the inner part (
r
≲ 0.04 pc) of the geometrically thin disk, and it repeatedly and steadily produces shocks as mass accretion continues through the disk to the center. The shape of the dust sublimation radius is not spherical and depends on the angle (
θ
) from the disk plane, reflecting the nonspherical radiation field and nonuniform dust-free gas. Moreover, we found that the sublimation radius of
θ
∼ 20°–60° varies on a timescale of several years. The “inflow-induced outflow” contributes to the obscuration of the nucleus in the sub-parsec region. The column density of the dust-free gas is
N
H
≳ 10
22
cm
−2
for
r
≲ 0.04 pc. Gases near the disk plane (
θ
≲ 30°) can be the origin of the Compton-thick component, which was suggested by the recent X-ray observations of AGNs. The dusty outflow from the sub-parsec region can be also a source of material for the radiation-driven fountain for a larger scale.
As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path ...from submicron sized icy dust monomers to Earth-sized bodies outside the "snow line," located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier" does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1-1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10-100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with 10 times the mass of the Earth. The growth timescale depends on the turbulent strength of the circumnuclear disk and the black hole mass MBH, and it is comparable to the AGN's lifetime (∼108 yr) for low mass (MBH ∼ 106M ) SMBHs.
Abstract
We investigate how magnetically driven outflows are powered by a rotating, weakly magnetized accretion flow onto a supermassive black hole using axisymmetric magnetohydrodynamic simulations. ...Our proposed model focuses on the accretion dynamics on an intermediate scale between the Schwarzschild radius and the galactic scale, which is ∼1–100 pc. We demonstrate that a rotating disk formed on a parsec-scale acquires poloidal magnetic fields via accretion, and this produces an asymmetric bipolar outflow at some point. The formation of the outflow was found to follow the growth of strongly magnetized regions around disk surfaces (magnetic bubbles). The bipolar outflow grew continuously inside the expanding bubbles. We theoretically derived the growth condition of the magnetic bubbles for our model that corresponds to a necessary condition for outflow growth. We found that the north–south asymmetrical structure of the bipolar outflow originates from the complex motions excited by accreting flows around the outer edge of the disk. The bipolar outflow comprises multiple mini-outflows and downflows (failed outflows). The mini-outflows emanate from the magnetic concentrations (magnetic patches). The magnetic patches exhibit inward drifting motions, thereby making the outflows unsteady. We demonstrate that the inward drift can be modeled using a simple magnetic patch model that considers magnetic angular momentum extraction. This study could be helpful for understanding how asymmetric and nonsteady outflows with complex substructures are produced around supermassive black holes without the help of strong radiation from accretion disks or entrainment by radio jets such as molecular outflows in radio-quiet active galactic nuclei, e.g., NGC 1377.
We investigate the properties of the ionized gas irradiated by an active galactic nucleus (AGN) based on our "radiation-driven fountain" model for the nearest type-2 Seyfert galaxy, the Circinus ...galaxy. We conducted "quasi-three-dimensional" spectral analysis using the spectral synthesis code Cloudy and obtained the surface brightness distributions of lines, such as H , Hβ, O iii, N ii, and S ii for the central 16 pc region. The ionized regions observed based on these lines show a conical morphology around the rotation axis, even if we do not phenomenologically postulate the presence of an optically thick "torus." This region also shows non-uniform internal structures, reflecting the inhomogeneous structure of fountain flows. Using ionization diagnostic diagrams, we investigated the spectral properties of the ionized gas. The diagrams based on the line ratios of N ii/H and S ii/H show that most regions of the cone have the same properties as those in the narrow line regions (NLRs) in AGNs, whereas using O i/H , the central 10 pc regions are rather LINER-like. The gas density, temperature, and ionizing parameter in regions identified as "NLR" are typically n ∼ 300-1500 cm−3, T ∼ (1-3) × 104 K, and U ∼ 0.01, respectively. The morphology and O iii intensity are similar to the base of the observed O iii cone in the Circinus galaxy, implying some physical connections with the origin of the ∼100 pc scale NLR.
Abstract
In this study, we examine photoionization outflows during the late stages of galaxy mergers, with a specific focus on the relation between the observed velocity of outflowing gas and the ...apparent effects of dust extinction. We used the
N
-body/smoothed particle hydrodynamics code ASURA for galaxy merger simulations. These simulations concentrated on identical galaxy mergers featuring supermassive black holes of 10
8
M
⊙
and gas fractions of 30% and 10%. From the simulation data, we derived velocity and velocity dispersion diagrams for the active galactic nuclei (AGN)-driven ionized outflowing gas. Our findings show that high-velocity outflows with velocity dispersions of 500 km s
−1
or greater can be observed in the late stages of galactic mergers. Particularly, in buried AGNs, both the luminosity-weighted outflow velocity and velocity dispersion increase owing to the apparent effects of dust extinction. Owing to these effects, velocity–velocity dispersion diagrams display a noticeable blue-shifted tilt in models with higher gas fractions. Crucially, this tilt is not influenced by the AGN luminosity but emerges from the observational impacts of dust extinction. Our results imply that the observed high-velocity O
iii
λ
5007 outflow exceeding 1000 km s
−1
in buried AGNs may be linked to the dust extinction that occurs during the late stages of gas-rich galaxy mergers.
We present the results of our ALMA Cycle 4 high-spatial-resolution (0 04-0 07) observations, at HCN J = 3-2 and HCO+ J = 3-2 lines, of the nucleus of NGC 1068, the nearby prototypical type 2 active ...galactic nucleus (AGN). Our previous ALMA observations identified the compact emission of these lines at the putative location of the torus around a mass-accreting supermassive black hole. We now report that we have detected the rotation of this compact emission, with the eastern and western sides being redshifted and blueshifted, respectively. Unlike the previously reported CO J = 6-5 emission, both the morphological and dynamical alignments of the HCN J = 3-2 and HCO+ J = 3-2 emission are roughly along the east-west direction (i.e., the expected torus direction), suggesting that these molecular lines are better probes of a rotating dense molecular gas component in the torus. The western part of the torus exhibits larger velocity dispersion and stronger emission in the HCN J = 3-2 and HCO+ J = 3-2 lines than the eastern part, revealing a highly inhomogeneous molecular torus. The dense molecular gas in the torus and that of the host galaxy at 0 5-2 0 from the AGN along the torus direction are found to be counter-rotating, suggesting an external process happened in the past at the NGC 1068 nucleus.
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