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Register a new userThe role of bars on the dynamical-friction driven inspiral of massive perturbers
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In this paper, we explore the impact of a galactic bar on the inspiral time-scale of a massive perturber (MP) within a Milky Way-like galaxy. We integrate the orbit of MPs in a multi-component galaxy model via a semi-analytical approach including an accurate treatment for dynamical friction generalized to rotationally supported backgrounds. We compare the MP evolution in a galaxy featuring a Milky Way-like rotating bar to the evolution within an analogous axisymmetric galaxy without the bar. We find that the bar presence may significantly affect the inspiral, sometimes making it shorter by a factor of a few, sometimes hindering it for a Hubble time, implying that dynamical friction alone is greatly insufficient to fully characterize the orbital decay. The effect of the bar is more prominent for initially in-plane, prograde MPs, especially those crossing the bar co-rotation radius or outer Lindblad resonance. In the barred galaxy, we find the sinking of the most massive MPs (>~10^7.5 Msun) approaching the galaxy from large separations (>~8 kpc) to be most efficiently hampered. Neglecting the effect of global torques associated to the non-symmetric mass distribution is thus not advisable even within our idealized, smooth Milky Way model, and it should be avoided when dealing with more complex and realistic galaxy systems. This has important implications for the orbital decay of massive black holes in late-type spirals, the natural candidate sources to be detected with the Laser Interferometer Space Antenna (LISA).
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Galaxies are self-gravitating structures composed by several components encompassing spherical, axial and triaxial symmetry. Although real systems feature heterogeneous components whose properties are intimately connected, semi-analytical approaches often exploit the linearity of the Poissons equation to represent the potential and mass distribution of a multi-component galaxy as the sum of the individual components. In this work, we expand the semi-analytical framework developed in Bonetti et al. (2020) by including both a detailed implementation of the gravitational potential of exponential disc (modelled with a ${rm sech}^2$ and an exponential vertical profile) and an accurate prescription for the dynamical friction experienced by massive perturbers in composite galaxy models featuring rotating disc structures. Such improvements allow us to evolve arbitrary orbits either within or outside the galactic disc plane. We validate the results obtained by our numerical model against public semi-analytical codes as well as full N-body simulations, finding that our model is in excellent agreement to the codes it is compared with. The ability to reproduce the relevant physical processes responsible for the evolution of massive perturber orbits and its computational efficiency make our framework perfectly suited for large parameter-space exploration studies.
Following a wave-mechanical treatment we calculate the drag force exerted by an infinite homogeneous background of stars on a perturber as this makes its way through the system. We recover Chandrasekhars classical dynamical friction (DF) law with a modified Coulomb logarithm. We take into account a range of models that encompasses all plausible density distributions for satellite galaxies by considering the DF exerted on a Plummer sphere and a perturber having a Hernquist profile. It is shown that the shape of the perturber affects only the exact form of the Coulomb logarithm. The latter converges on small scales, because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm this way earlier results based on the impulse approximation of small angle scatterings.
We present a comparative analysis of the properties of AGN emitting at radio and X-ray wavelengths. The study is performed on 907 X-ray AGN and 100 radio AGN selected on the CDFS and UDS fields and makes use of new and ancillary data available to the VANDELS collaboration. Our results indicate that the mass of the host galaxy is a fundamental quantity which determines the level of AGN activity at the various wavelengths. Indeed large stellar masses are found to be connected with AGN radio emission, as virtually all radio-active AGN reside within galaxies of M*>10^{10} Msun. Large stellar masses also seem to favour AGN activity in the X-ray, even though X-ray AGN present a mass distribution which is more spread out and with a non-negligible tail at M*<10^{9} Msun. Stellar mass alone is also observed to play a fundamental role in simultaneous radio and X-ray emission: the percentage of AGN active at both wavelengths increases from around 1% of all X-ray AGN residing within hosts of M*<10^{11} Msun to about 13% in more massive galaxies. In the case of radio-selected AGN, such a percentage moves from about 15% to about 45% (but up to 80% in the deepest fields). Neither cosmic epoch, nor radio luminosity, X-ray luminosity, Eddington ratio or star-formation rate of the hosts are found to be connected to an enhanced probability for joint radio+X-ray emission of AGN origin. Furthermore, only a loose relation is observed between X-ray and radio luminosity in those AGN which are simultaneously active at both frequencies.
In many astrophysical situations, as in the coalescence of supermassive black hole pairs at gas rich galactic nuclei, the dynamical friction experienced by an object is a combination of its own wake as well as the wakes of its companions. Using a semi-analytic approach, we investigate the composite wake due to, and the resulting drag forces on, double perturbers that are placed at the opposite sides of the orbital center and move on a circular orbit in a uniform gaseous medium. The circular orbit makes the wake of each perturber asymmetric, creating an overdense tail at the trailing side. The tail not only drags the perturber backward but it also exerts a positive torque on the companion. For equal-mass perturbers, the positive torque created by the companion wake is, on average, a fraction ~40-50% of the negative torque created by its own wake, but this fraction may be even larger for perturbers moving subsonically. This suggests that the orbital decay of a perturber in a double system, especially in the subsonic regime, can take considerably longer than in isolation. We provide the fitting formulae for the forces due to the companion wake and discuss our results in light of recent numerical simulations for mergers of binary black holes.
We investigate the abundance of Super-Massive Black Hole (SMBH) seeds in primordial galaxy halos. We explore the assumption that dark matter halos outgrowing a critical halo mass M_c have some probability p of having spawned a SMBH seed. Current observations of local, intermediate-mass galaxies constrain these parameters: For $M_c=10^{11}M_odot$, all halos must be seeded, but when adopting smaller M_c masses the seeding can be much less efficient. The constraints also put lower limits on the number density of black holes in the local and high-redshift Universe. Reproducing z~6 quasar space densities depends on their typical halo mass, which can be constrained by counting nearby Lyman Break Galaxies and Lyman Alpha Emitters. For both observables, our simulations demonstrate that single-field predictions are too diverse to make definitive statements, in agreement with mixed claims in the literature. If quasars are not limited to the most massive host halos, they may represent a tiny fraction (~10^-5) of the SMBH population. Finally, we produce a wide range of predictions for gravitational events from SMBH mergers. We define a new diagnostic diagram for LISA to measure both SMBH space density and the typical delay between halo merger and black hole merger. While previous works have explored specific scenarios, our results hold independent of the seed mechanism, seed mass, obscuration, fueling methods and duty cycle.
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