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Sergei Nayakshin

Publications and source records attributed to Sergei Nayakshin.

At least 91 records · Page 5Linked to original sources
arXiv2010

The Accretion Disc Particle Method for Simulations of Black Hole Feeding and Feedback

Black holes grow by accreting matter from their surroundings. However, angular momentum provides an efficient natural barrier to accretion and so only the lowest angular momentum material will be available to feed the black holes. The standard sub-grid model for black hole accretion in galaxy formation simulations - based on the Bondi-Hoyle method - does not account for the angular momentum of accreting material, and so it is unclear how representative the black hole accretion rate estimated in this way is likely to be. In this paper we introduce a new sub-grid model for black hole accretion that naturally accounts for the angular momentum of accreting material. Both the black hole and its accretion disc are modelled as a composite accretion disc particle. Gas particles are captured by the accretion disc particle if and only if their orbits bring them within its accretion radius R_acc, at which point their mass is added to the accretion disc and feeds the black hole on a viscous timescale t_visc. The resulting black hole accretion rate (dM/dt)_BH powers the accretion luminosity L_acc ~ (dM/dt)_BH, which drives black hole feedback. Using a series of controlled numerical experiments, we demonstrate that our new accretion disc particle method is more physically self-consistent than the Bondi-Hoyle method. We also discuss the physical implications of the accretion disc particle method for systems with a high degree of rotational support, and we argue that the M_BH-sigma relation in these systems should be offset from the relation for classical bulges and ellipticals, as appears to be observed.

astro-ph.CO↗
arXiv2010

Rotation of the Solar System planets and the origin of the Moon in the context of the tidal downsizing hypothesis

It has been proposed recently that the first step in the formation of both rocky and gas giant planets is dust sedimentation into a solid core inside a gas clump (giant planet embryo). The clumps are then assumed to migrate closer to the star where their metal poor envelopes are sheared away by the tidal forces or by an irradiation-driven mass loss. We consider the implications of this hypothesis for natal rotation rates of both terrestrial and gas giant planets. It is found that both types of planets may rotate near their break up angular frequencies at birth. The direction of the spin should coincide with that of the parent disc and the star, except in cases of embryos that had close interactions or mergers with other embryos in the past. Furthermore, the large repository of specific angular momentum at birth also allows formation of close binary rocky planets inside the same embryos. We compare these predictions with rotation rates of planets in the Solar System and also question whether the Earth-Moon pair could have been formed within the same giant planet embryo.

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arXiv2010

Grain sedimentation inside giant planet embryos

In the context of massive fragmenting protoplanetary discs, Boss (1998) suggested that grains can grow and sediment inside giant planet embryos formed at R ~ 5 AU away from the star. Several authors since then criticised the suggestion. Convection may prevent grain sedimentation, and the embryos cannot even form so close to the parent star as cooling is too inefficient at these distances. Here we reconsider the grain sedimentation process suggested by Boss (1998) but inside an embryo formed, as expected in the light of the cooling constraints, at R ~ 100 AU. Such embryos are much less dense and are also cooler. We make analytical estimates of the process and also perform simple spherically symmetric radiation hydrodynamics simulations to test these ideas. We find that convection in our models does not become important before a somewhat massive (~ an Earth mass, this is clarified in a followup paper) solid core is built. Turbulent mixing slows down dust sedimentation but is overwhelmed by grain sedimentation when the latter grow to a centimetres size. The minimum time required for dust sedimentation to occur is a few thousand years, and is a strong function of the embryo's mass, dust content and opacity. An approximate analytical criterion is given to delineate conditions in which a giant embryo contracts and heats up faster than dust can sediment. As Boss et al (2002), we argue that core formation through grain sedimentation inside the giant planet embryos may yield an unexplored route to form giant gas and giant ice planets. The present model also stands at the basis of paper III, where we study the possibility of forming terrestrial planet cores by tidal disruption and photoevaporation of the planetary envelope.

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arXiv2010

Formation of terrestrial planet cores inside giant planet embryos

Giant planet embryos are believed to be spawned by gravitational instability in massive extended (R ~ 100 AU) protostellar discs. In a recent paper we have shown that dust can sediment inside the embryos, as argued earlier by Boss (1998) in a slightly different model. Here we study numerically the next stage of this process -- the formation of a solid core. If conditions are conducive to solid core formation, the centre of the gas cloud goes through the following sequence of phases: (i) becomes grain (and metal) rich; (ii) forms a terrestrial mass solid core via a rapid collapse driven by self-gravity of the grains; (iii) starts to accrete a gaseous atmosphere when the solid core reaches mass of a few to 10 Earth masses. This sequence of events may build either terrestrial planet cores or metal rich giant planets inside the larger gas reservoir of the giant planet embryo. In a companion Letter we argue that tidal and irradiation effects from the parent star should disrupt the outer metal poor layers of the embryo, releasing nearly "ready to use" planets. We propose this as an alternative way to build planets.

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arXiv2010

Formation of planets by tidal downsizing of giant planet embryos

We hypothesise that planets are made by tidal downsizing of migrating giant planet embryos. The proposed scheme for planet formation consists of these steps: (i) a massive young protoplanetary disc fragments at R ~ several tens to hundreds of AU on gaseous clumps with masses of a few Jupiter masses; (ii) the clumps cool and contract, and simultaneously migrate closer in to the parent star; (iii) as earlier suggested by Boss (1998), dust sediments inside the gas clumps to form terrestrial mass solid cores; (iv) if the solid core becomes more massive than ~ 10 Earth masses, a massive gas atmosphere collapses onto the solid core; (v) when the gas clumps reach the inner few AU from the star, tidal shear and evaporation due to stellar irradiation peel off the outer metal-poor envelope of the clump. If tidal disruption occurs quickly, while the system is still in stage (iii), a terrestrial planet core is left. If it happens later, in stage (iv), a metal rich gas giant planet with a solid core emerges from the envelope.

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arXiv2010

Feedback from Nuclear Star Clusters and SMBHs

The observed super-massive black hole (SMBH) mass -- galaxy velocity dispersion ($M_{\rm cmo} - σ$) correlation, and the similar correlation for nuclear star clusters, may be established when winds/outflows from the CMO ("central massive object") drive gas out of the potential wells of classical bulges. Timescales of growth for these objects may explain why smaller bulges appear to host preferentially NCs while larger ones contain SMBHs only. Despite much recent progress, feedback processes in bulge/galaxy formation are far from being understood. Our numerical simulations show that that understanding how the CMO feeds is as important a piece of the puzzle as understanding how its feedback affects its host galaxy.

astro-ph.CO↗
arXiv2009

Kelvin-Helmholtz instabilities with Godunov SPH

Numerical simulations for the non-linear development of Kelvin-Helmholtz instability in two different density layers have been performed with the particle-based method (Godunov SPH) developed by Inutsuka (2002). The Godunov SPH can describe the Kelvin-Helmholtz instability even with a high density contrast, while the standard SPH shows the absence of the instability across a density gradient (Agertz et al. 2007). The interaction of a dense blob with a hot ambient medium has been performed also. The Godunov SPH describes the formation and evolution of the fingers due to the combinations of Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholtz instabilities. The blob test result coincides well with the results of the grid-based codes. An inaccurate handling of a density gradient in the standard SPH has been pointed out as the direct reason of the absence of the instabilities. An unphysical force happens at the density gradient even in a pressure equilibrium, and repulses particles from the initial density discontinuity. Therefore, the initial perturbation damps, and a gap forms at the discontinuity. The unphysical force has been studied in terms of the consistency of a numerical scheme. Contrary to the standard SPH, the momentum equation of the Godunov SPH doesnt use the particle approximation, and has been derived from the kernel convolution or a new Lagrangian function. The new Lagrangian function used in the Godunov SPH is more analogous to the real Lagrangian function for continuum. The momentum equation of the Godunov SPH has much better linear consistency, so the unphysical force is greatly reduced compared to the standard SPH in a high density contrast.

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arXiv2009

Simulations of momentum feedback by black hole winds

The observed super-massive black hole (SMBH) mass -- galaxy velocity dispersion ($M_{\rm bh} - σ$) correlation may be established when winds/outflows from the SMBH drive gas out of the potential wells of classical bulges. Here we present numerical simulations of this process in a static isothermal potential. Simple spherically symmetric models of SMBH feedback at the Eddington luminosity can successfully explain the $M_{\rm bh} - σ$ and nuclear cluster mass $M_{\rm NC}-σ$ correlations, as well as why larger bulges host SMBHs while smaller ones host nuclear star clusters. However these models do not specify how SMBHs feed on infalling gas whilst simultaneously producing feedback that drives gas out of the galaxy. More complex models with rotation and/or anisotropic feedback allow SMBHs to feed via a disc or regions not exposed to SMBH winds, but in these more realistic cases it is not clear why a robust $M_{\rm bh} - σ$ relation should be established. In fact, some of the model predictions contradict observations. For example, an isotropic SMBH wind impacting on a disc (rather than a shell) of aspect ratio $H/R \ll 1$ requires the SMBH mass to be larger by a factor $\sim R/H$, which is opposite to what is observed. We conclude that understanding how a SMBH feeds is as important a piece of the puzzle as understanding how its feedback affects its host galaxy. Finally, we note that in aspherical cases the SMBH outflows induce differential motions in the bulge. This may pump turbulence that is known to hinder star formation in star forming regions. SMBH feedback thus may not only drive gas out of the bulge but also reduce the fraction of gas turned into stars.

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arXiv2009

Competitive feedback in galaxy formation

It is now well established that many galaxies have nuclear star clusters (NCs) whose total masses correlate with the velocity dispersion (sigma) of the galaxy spheroid in a very similar way to the well--known supermassive black hole (SMBH) M - sigma relation. Previous theoretical work suggested that both correlations can be explained by a momentum feedback argument. Observations further show that most known NCs have masses < 10^8 Msun, while SMBHs frequently have masses > 10^8 Msun, which remained unexplained in previous work. We suggest here that this changeover reflects a competition between the SMBH and nuclear clusters in the feedback they produce. When one of the massive objects reaches its limiting M-sigma value, it drives the gas away and hence cuts off its own mass and also the mass of the ``competitor''. The latter is then underweight with respect to the expected M-sigma mass (abridged).

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arXiv2009

Dynamic Monte Carlo radiation transfer in SPH. Radiation pressure force implementation

We present a new framework for radiation hydrodynamics simulations. Gas dynamics is modelled by the Smoothed Particle Hydrodynamics (SPH) method, whereas radiation transfer is simulated via a time-dependent Monte-Carlo approach that traces photon packets. As a first step in the development of the method, in this paper we consider the momentum transfer between radiation field and gas, which is important for systems where radiation pressure is high. There is no fundamental limitations on the number of radiation sources, geometry or the optical depth of the problems that can be studied with the method. However, as expected for any Monte-Carlo transfer scheme, stochastic noise presents a serious limitation. We present a number of tests that show that the errors of the method can be estimated accurately by considering Poisson noise fluctuations in the number of photon packets that SPH particles interact with per dynamical time. It is found that for a reasonable accuracy the momentum carried by photon packets must be much smaller than a typical momentum of SPH particles. We discuss numerical limitations of the code, and future steps that can be taken to improve performance and applicability of the method.

astro-ph.IM↗
arXiv2009

Simulations of the formation of stellar discs in the Galactic centre via cloud-cloud collisions

Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical simulations, we here consider whether the observed population of young stars could have originated from a large angle collision of two massive gaseous clouds at R approx. 1 parsec from Sgr A*. In all the simulations performed, the post-collision gas flow forms an inner, nearly circular gaseous disc and one or two eccentric outer filaments, consistent with the observations. Furthermore, the radial stellar mass distribution is always very steep, Sigma proportional to R^-2, again consistent with the observations. All of our simulations produce discs that are warped by between 30 to 60 degrees, in accordance with the most recent observations. The 3D velocity structure of the stellar distribution is sensitive to initial conditions (e.g., the impact parameter of the clouds) and gas cooling details. For example, the runs in which the inner disc is fed intermittently with material possessing fluctuating angular momentum result in multiple stellar discs with different orbital orientations, contradicting the observed data. In all the cases the amount of gas accreted by our inner boundary condition is large, enough to allow Sgr A* to radiate near its Eddington limit over approx. 10^5 years. This suggests that a refined model would have physically larger clouds (or a cloud and a disc such as the circumnuclear disc) colliding at a distance of a few parsecs rather than 1 parsec as in our simulations.

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arXiv2008

Simulations of the formation of a gaseous disc and young stars near Sgr A* via cloud-cloud collisions

Young massive stars in the central parsec of our Galaxy are best explained by star formation within at least one, and possibly two, massive self-gravitating gaseous discs. With help of numerical simulations, we here consider whether the observed population of young stars could have originated from a large angle collision of two massive gaseous clouds at R ~ 1 pc from Sgr A*. In all the simulations performed, the post-collision gas flow forms an inner nearly circular gaseous disc and one or two eccentric outer filaments, consistent with the observations. Furthermore, the radial stellar mass distribution is always very steep, sigma proportional to R^-2, again consistent with the data. The 3D velocity structure of the stellar distribution is however sensitive to initial conditions (e.g., the impact parameter of the clouds) and gas cooling details. In all the cases the amount of gas accreted by our inner boundary condition is large, enough to allow Sgr A* to radiate near its Eddington limit during ~ 10^5 years. This suggests that a refined model would have physically larger clouds (or a cloud and a disc such as the CND) colliding at a distance of a few pc rather than 1 pc as in our simulations.

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arXiv2007

Variable accretion and emission from the stellar winds in the Galactic centre

We present numerical simulations of stellar wind dynamics in the central parsec of the Galactic centre, studying in particular the accretion of gas on to Sgr A*, the super-massive black hole. Unlike our previous work, here we use state-of-the-art observational data on orbits and wind properties of individual wind-producing stars. Since wind velocities were revised upwards and non-zero eccentricities were considered, our new simulations show fewer clumps of cold gas and no conspicuous disc-like structure. The accretion rate is dominated by a few close `slow wind stars' v_w < 750 km/s, and is consistent with the Bondi estimate, but variable on time-scales of tens to hundreds of years. This variability is due to the stochastic in-fall of cold clumps of gas, as in earlier simulations, and to the eccentric orbits of stars. The present models fail to explain the high luminosity of Sgr A* a few hundred years ago implied by Integral observations, but we argue that the accretion of a cold clump with a small impact parameter could have caused it. Finally, we show the possibility of constraining the total mass-loss rate of the `slow wind stars' using near infra-red observations of gas in the central few arcseconds.

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arXiv2007

Forced accretion in stochastically fed AGN and quasars

Steady state accretion discs larger than ~ 0.01-0.1 pc are known to be gravitationally unstable for the accretion rates needed to explain super-massive black hole (SMBH) activity. We propose that SMBH are fed by a succession of mass deposition events with randomly directed angular momenta. Because of incomplete angular momentum cancellation a warped accretion disc forms in the inner few parsec. The orientation of the disc performs a random walk. Deposition of new material promotes SMBH accretion at rates much faster than viscous. Observational implications of this picture include: (i) lighter accretion discs that can fuel AGN and quasars and yet avoid star formation at R >> 0.1 pc; (ii) star formation inside the disc is not a function of mass accretion rate only. It can take place at high or low accretion rates, e.g., when too few clouds arrive in the inner region. An example of this might be the central parsec of our Galaxy. (iii) The discs can form Compton-thick obscuring structures of ~ parsec size as required in AGN unification models; (iv) faster black hole growth resulting from misalignment of the disc and the black hole spin in the early Universe; (v) Isotropic deposition of SMBH energy and momentum feedback in the galaxy bulge. This may help explain the high efficiency with which it seems to be operating in the Universe. (vi) No correlation between SMBH activity and the presence of kiloparsec scale bars or gaseous discs in galactic bulges; (vii) Bodily collisions between gaseous components of merging galaxies facilitate production of gas streams feeding the centre of the combined galaxy. Mergers should thus be catalysts of SMBH growth. (viii) Conversely, galaxies experiencing fewer mergers are more likely to form massive nuclear star clusters than feed their SMBHs.

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arXiv2007

Can the soft excess in AGN originate from disc reflection?

The soft excess seen in many AGN is most probably due to partially ionized material moving at relativistic speeds close to the black hole. There are currently two potential geometries for this material, one where it is out of the line of sight, seen via reflection, e.g. the accretion disc, the other where it is in the line of sight, seen in absorption e.g. a wind above the disc. Both models require apparent fine tuning of the ionization parameter of this material in order to produce the large jump in opacity at $\sim 0.7$ keV associated with OVII/VIII, as required to make the soft excess. However, Chevalier et al (2006) show that these states rather naturally dominate the absorption spectrum for soft X-ray spectra if the illuminated material is (at least approximately) in pressure balance. Here we explore whether hydrostatic pressure equilibrium in a disc can likewise naturally select the required ionization states in reflection. We find the opposite. The soft excess X-ray excess is much weaker in the hydrostatic models than it is in the constant density models. Since even the constant density models cannot fit the largest soft excesses seen without the intrinsic continuum being hidden from view, this means that reflection from a hydrostatic disc cannot realistically match the data. Even if the disc structure is instead more like a constant density atmosphere, the required fine--tuning of the ionization parameter still remains a problem for reflection models.

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arXiv2007

Stellar processes near AGN

Precise mechanisms by which Active Galactic Nuclei (AGN) receive their gaseous fuel is still a mystery. Here I draw attention to the extra ordinary star formation event that took place in the central ~ 0.5 parsec of our Galaxy. The most reliable explanation of the event seems to be that two somewhat massive nearly co-eval gaseous disks failed to accrete on Sgr A*, the super-massive black hole (SMBH) in our Galaxy, and instead cooled down and gravitationally collapsed, forming the stars observed now. This emphasises that star formation must be an important part of AGN feeding puzzle. I also discuss a model in which stellar winds create the observed obscuration of AGN. These winds are cold, clumpy and dusty, as required by the observations, but they are Compton-thin unless wind outflow rate is highly super-Eddington. This argument is in fact a general one, independent of the wind driving mechanism. I thus suggest that winds may be important for optically thin absorbers, and that a better model for optically thick AGN obscuration is a warped accretion/star forming disk.

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arXiv2007

Simulations of star formation in a gaseous disc around sgr A* - a failed AGN

We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation of the young massive stars observed in the central parsec of our Galaxy. In this study, we adopt a simple prescription with a locally constant cooling time. We find that, for cooling times just short enough to induce disc fragmentation, stars form with a top-heavy Initial Mass Function (IMF), as observed in the Galactic Centre (GC). For shorter cooling times, the disc fragments much more vigorously, leading to lower average stellar masses. Thermal feedback associated with gas accretion onto protostars slows down disc fragmentation, as predicted by some analytical models. We also simulate the fragmentation of a gas stream on an eccentric orbit in a combined Sgr A* plus stellar cusp gravitational potential. The stream precesses, self-collides and forms stars with a top-heavy IMF. None of our models produces large enough co-moving groups of stars that could account for the observed ``mini star cluster'' IRS13E in the GC. In all of the gravitationally unstable disc models that we explored, star formation takes place too fast to allow any gas accretion onto the central super-massive black hole. While this can help to explain the quiescence of `failed AGN' such as Sgr A*, it poses a challenge for understanding the high gas accretion rates infered for many quasars.

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