Keywords

There is compelling evidence for a highly energetic Seyfert explosion (10^56–57 erg) that occurred in the Galactic center a few million years ago. The clearest indications are the X-ray/γ-ray "10 kpc bubbles" identified by the ROSAT and Fermi satellites. In an earlier paper, we suggested another manifestation of this nuclear activity, i.e., elevated Hα emission along a section of the Magellanic Stream due to a burst (or flare) of ionizing radiation from Sgr A*. We now provide further evidence for a powerful flare event: UV absorption line ratios (in particular ${\rm{C}}\,{\rm{IV}}$/${\rm{C}}\,{\rm{II}}$, Si iv/Si ii) observed by the Hubble Space Telescope reveal that some Magellanic Stream clouds toward both galactic poles are highly ionized by a source capable of producing ionization energies up to at least 50 eV. We show how these are clouds caught in a beam of bipolar, radiative "ionization cones" from a Seyfert nucleus associated with Sgr A*. In our model, the biconic axis is tilted by about 15° from the south Galactic pole with an opening angle of roughly 60°. For the Magellanic Stream at such large Galactic distances (D ≳ 75 kpc), nuclear activity is a plausible explanation for all of the observed signatures: elevated Hα emission and H ionization fraction (x_e ≳ 0.5), enhanced ${\rm{C}}\,{\rm{IV}}$/${\rm{C}}\,{\rm{II}}$ and Si iv/Si ii ratios, and high ${\rm{C}}\,{\rm{IV}}$ and Si iv column densities. Wind-driven "shock cones" are ruled out because the Fermi bubbles lose their momentum and energy to the Galactic corona long before reaching the Magellanic Stream. Our time-dependent Galactic ionization model (stellar populations, hot coronal gas, cloud–halo interaction) is too weak to explain the Magellanic Stream's ionization. Instead, the nuclear flare event must have had a radiative UV luminosity close to the Eddington limit (f_E ≈ 0.1–1). Our time-dependent Seyfert flare models adequately explain the observations and indicate that the Seyfert flare event took place T_o = 3.5 ± 1 Myr ago. The timing estimates are consistent with the mechanical timescales needed to explain the X-ray/γ-ray bubbles in leptonic jet/wind models (≈2–8 Myr).

We investigate the nature of dense gas in the 3–10 pc circumnuclear ring (CNR) in the galactic center of the Milky Way, which is a structure that may be dynamically connecting the supermassive black hole Sgr A* with the central molecular zone at the 100 pc scale, and is the closest reservoir of molecular gas to the massive stars located within the central cluster. In the first of several papers addressing open issues with the CNR, we use far-infrared (FIR) diagnostic emission lines to probe the hot and dense phase of the photodissociation region (PDR) exposed to the radiation field of the central population of massive stars. We use the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) instrument on board the Stratospheric Observatory For Infrared Astronomy airborne observatory to obtain spatially resolved maps of FIR emission lines of the region with an angular resolution approximately 4 times higher than previous published data. We complement our data with archival continuum images at 19.7, 31.5 and 37.1 μm obtained with FORCAST and 70, 100 and 160 μm archival continuum images from PACS. We use the FIFI-LS emission line flux maps from ionized ([C ii] 157.7 μm), atomic ([O i] 63.2 μm, [O i] 145.5 μm), and molecular (CO J = 14–13 186.0 μm) species for a comparison with model predictions for PDRs. We present a method that dissects emission from the low and from the high excitation phase of the PDR and that also accounts for, e.g., absorption especially in the [O i] 63.2 μm transition. We present spatially resolved maps of dust temperature, atomic hydrogen column density, and FIR flux. The derived atomic hydrogen column density map is aligned with the galactic plane and extends spatially beyond previous near-infrared and radio based A_v determinations. The atomic hydrogen column densities range from 10^22.5 to 10^23.1 cm⁻² resulting in a total enclosed mass of the order of 10^3.5 M_⊙. We derive a [O i] 63.2 μm absorption map that is aligned with the galactic plane with no or little absorption in the northern lobe of the CNR but moderate absorption in the southern lobe of the CNR, which is consistent with the picture where the illuminated front surfaces of gas clouds in the northern lobe are directly visible to us, while in the southern lobe the illuminated surfaces are hidden by the clouds within the lobe itself. Local gas densities in the CNR are generally below the Roche limit.

In the vicinity of a massive black hole, stars move on precessing Keplerian orbits. The mutual stochastic gravitational torques between the stellar orbits drive a rapid reorientation of their orbital planes, through a process called vector resonant relaxation. We derive, from first principles, the correlation of the potential fluctuations in such a system, and the statistical properties of random walks undergone by the stellar orbital orientations. We compare this new analytical approach with numerical simulations. We also provide a simple scheme to generate the random walk of a test star's orbital orientation using a stochastic equation of motion. We finally present quantitative estimations of this process for a nuclear stellar cluster such as that of the Milky Way.

We performed numerical simulations of general relativistic magnetohydrodynamics with uniform resistivity to investigate the occurrence of magnetic reconnection in a split-monopole magnetic field around a Schwarzschild black hole. We found that magnetic reconnection happens near the black hole at its equatorial plane. The magnetic reconnection has a point-like reconnection region and slow shock waves, as in the Petschek reconnection model. The magnetic reconnection rate decreases as the resistivity becomes smaller. When the global magnetic Reynolds number is 10⁴ or larger, the magnetic reconnection rate increases linearly with time from 2τ_S to ∼10τ_S (τ_S = r_S/c, r_S is the Schwarzschild radius and c is the speed of light). The linear increase of the reconnection rate agrees with the magnetic reconnection in the Rutherford regime of the tearing mode instability.

Numerical simulations of the force-free magnetodynamics (FFMD) of the electromagnetic field around a spinning black hole are useful to investigate the dynamic electromagnetic processes around a spinning black hole, such as the emergence of the Blandford–Znajek mechanism. To reveal the basic physics of magnetic fields around a black hole through the dynamic process, we use one-dimensional (1D) FFMD along the axisymmetric magnetic surface, which provides a relatively simple, sufficiently precise, and powerful tool to analyze the dynamic process around a spinning black hole. We review the analytic and numerical aspects of 1D FFMD for an arbitrary magnetic surface around a black hole. In addition, we also show some numerical simulation test results for three types of magnetic surfaces at the equatorial plane of the black hole.

In this work, we study the impact of asymmetric dark matter (ADM) on low-mass main-sequence stars in the Milky Way's nuclear star cluster, where the dark matter (DM) density is expected to be orders of magnitude above what is found near the Sun (${\rho }_{\mathrm{DM}}\gtrsim {10}^{3}\ \mathrm{GeV}\ {\mathrm{cm}}^{-3}$). Using a modified stellar evolution code and considering a DM particle (m_χ = 4 GeV) with a spin-dependent interaction cross section close to the limits allowed by direct detection, we found that the interactions of ADM with baryons in the star's core can have two separate effects on the evolution of these stars: a decrease in the hydrogen burning rate, extending the duration of the main-sequence of stars with M ∼ 1M_⊙ by a few Gyr; the suppression of the onset of convection in the core of stars with M ≲ 1.5M_⊙ and consequent quench of supply for the nuclear reactions. If we consider ρ_DM > 10³ GeV cm⁻³ (corresponding to the inner 5 pc of the Milky Way), stars lighter than the Sun will have a main-sequence life span comparable to the current age of the universe. Stars heavier than two solar masses are not sensitive to the DM particles considered here.

The merger rate of stellar-mass black hole binaries (sBHBs) inferred by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests the need for an efficient source of sBHB formation. Active galactic nucleus (AGN) disks are a promising location for the formation of these sBHBs, as well as binaries of other compact objects, because of powerful torques exerted by the gas disk. These gas torques cause orbiting compact objects to migrate toward regions in the disk where inward and outward torques cancel, known as migration traps. We simulate the migration of stellar mass black holes in an example of a model AGN disk, using an augmented N-body code that includes analytic approximations to migration torques, stochastic gravitational forces exerted by turbulent density fluctuations in the disk, and inclination and eccentricity dampening produced by passages through the gas disk, in addition to the standard gravitational forces between objects. We find that sBHBs form rapidly in our model disk as stellar-mass black holes migrate toward the migration trap. These sBHBs are likely to subsequently merge on short timescales. The process continues, leading to the build-up of a population of over-massive stellar-mass black holes. The formation of sBHBs in AGN disks could contribute significantly to the sBHB merger rate inferred by LIGO.

Numerical simulations of 1D force-free magnetodynamics (FFMD) by Koide and Imamura showed details of the energy extraction mechanism around a rapidly spinning black hole under the force-free condition in the case of a radial magnetic surface along the equatorial plane. The energy is transported like a tsunami from the ergosphere and spreads to the outside. In this paper, we perform 1D FFMD simulations with nonradial magnetic surfaces along the equatorial plane, which are more general magnetic surface configurations. Using the results of simulations and analytic solutions of the steady-state force-free magnetic field, we find that, except in the case of a radial magnetic surface at infinity, the tsunami induced at the neighborhood of the horizon damps gradually and energy flux along the equatorial plane vanishes after a long period of time or at infinity. This suggests the energy extracted from the spinning black hole through the nonradial magnetic field is transported toward the high latitude around the axis of the black hole and is converted to the kinetic energy of the jet or outflow.

We study the dynamical evolution of the stellar-mass binary black holes (BBHs) in a galactic nucleus that contains a massive black hole (MBH). For a comprehensive study of their merging events, we consider simultaneously the nonresonant and resonant relaxations of the BBHs, the binary–single encounters of the BBHs with the field stars, the Kozai–Lidov (KL) oscillation, and the close encounters between the BBHs and the central MBH, which usually lead to binaries' tidal disruptions. As the BBHs are usually heavier than the background stars, they sink to the center by mass segregation, making the KL oscillation an important effect in merging BBHs. The binary–single encounters can not only lead to softening and ionization of the BBHs but also make them harden, which increases the merging rates significantly. The mergers of BBHs are mainly contributed by galaxies containing MBHs less massive than 10⁸ ${M}_{\odot }$, and the total event rates are likely on order of 1–10 Gpc⁻³ yr⁻¹, depending on the detailed assumptions of the nucleus clusters. About 3%–10% of these BBH mergers are with eccentricity ≥0.01 when their gravitational-wave oscillating frequencies enter the LIGO band (10 Hz). Our results show that merging the BBHs within galactic nuclei can be an important source of the merging events detected by the Advanced LIGO/Virgo detectors, and they can be distinguished from BBH mergers from the galactic fields and globular clusters when enough events are accumulated.

The density and mass of the spiral galaxy NGC 3198 have been studied as functions of the distance from the center of galaxy depending on its observed rotation curve. Calculation of density and mass at different radii of the galaxy NGC 3198 show that the central inner density ρ1(r) at r < r_t (r_t is the turn-off radius) is found to be constant ρ1 = 0.387 M_o pc⁻³, while the density ρ2(r) of the outer region r > r_t is much less than the inner region, which varies from 0.0187 M_o pc⁻³ at r = 4 kpc to 0.00278 M_o pc⁻³ at r = 30 kpc. On the other hand, the inner mass m1(r) at r < r_t grows from 1.33 × 10⁹ M_o at r = 0.1 kpc to a mass of 8.539 × 10⁹ M_o at r = 4 kpc, while the outer mass m2(r) at r ≥ r_t changes from 8.539 × 10⁹ M_o at the turn-off radius r_t to 5.262 × 10¹⁰ M_o at r = 30 kpc of the galaxy. Finally, the behavior of the calculated density and mass is consistent with other works.