arXiv subjects

Kastytis Zubovas

Publications and source records attributed to Kastytis Zubovas.

At least 19 recordsLinked to original sources

arXiv2026

Chaotic accretion can (still) explain early supermassive black hole growth

The origin of supermassive black holes (SMBHs) is still debated, with hypotheses ranging from massive seeds to super-Eddington growth to chaotic accretion. Recent discoveries of very high redshift SMBHs constrain their possible origin and growth scenarios. We investigate whether the chaotic accretion scenario can explain the most extreme high-redshift SMBH masses. We predicted the exponential growth timescale of black holes accreting chaotically in a slowly-spinning dark matter halo, in which uncorrelated gas inflows maintain a low spin and hence a low radiative efficiency. We compared this prediction against the growth timescales and seed masses implied by a compilation of all known SMBH mass estimates at $z>7$ and against the feeding and feedback conditions expected in such haloes. We found that in the period $7.6 < z < 10.6$, the most massive SMBHs likely grew very efficiently, with an exponential growth timescale $τ\sim 32$~Myr, which is consistent with chaotic accretion predictions provided that growth was almost continuous. All high-redshift SMBHs with virial mass estimates could have grown from seeds with $M_{\rm seed} \lesssim 100 \, \msun$. We describe the evolution of the mass reservoir feeding such a black hole, suggesting that wind-driven feedback eventually removes most of the gas and changes the growth mode of black holes at $z \sim 7.5$. We make several predictions regarding the spins, host-galaxy correlations, and observable counterparts of high-redshift SMBHs that distinguish our model from its alternatives. These predictions will be testable in the near future. Chaotic accretion from stellar-mass seeds remains a viable origin for the earliest known SMBHs, without invoking massive seeds or super-Eddington growth.

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arXiv2025

Simple relations from complex outflows: How the $M-σ$ relation emerges in a multi-phase environment

The tight empirical $M-σ$ relation between the mass of a SMBH) and the velocity dispersion of the host galaxy bulge is often interpreted as the result of self-regulation by AGN feedback. This picture is motivated by analytical and semi-analytical models in which momentum-driven AGN winds can expel the gas once the SMBH reaches a critical mass. However, these models typically assume idealised conditions: smooth gas distributions, spherical symmetry, and very efficient cooling of the shocked AGN wind. We checked whether AGN outflows can establish the $M-σ$ relation in a multi-phase and turbulent galactic bulge subject to realistic radiative cooling while conserving the shocked AGN wind energy. We calculated a suite of purpose-built hydrodynamical simulations of AGN outflows in turbulent gas shells, covering a wide range of constant AGN luminosities. We tracked the outflow evolution over the course of $\geq1$~Myr. We analysed the effect of AGN outflow on the cold dense gas and SMBH feeding, estimating the luminosity threshold for removing most of the cold gas from the central regions. We find that AGNs with significantly sub-Eddington luminosities cannot suppress SMBH feeding, while luminosities exceeding $\sim 0.7$ times Eddington clear out both the diffuse hot gas and the cold clumps, consistent with the momentum-driven outflow formalism. We also show that dense gas clusters are affected almost exclusively by the AGN wind momentum, while the shocked wind energy escapes through low-density channels and inflates large bubbles of diffuse gas. Therefore, AGN wind-driven energy-conserving feedback in a turbulent multi-phase medium affects the dense gas only via the wind momentum. Thus, the momentum-driven outflow paradigm is applicable for explaining the $M-σ$ relation even in realistic systems.

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arXiv2025

Active galactic nucleus outflows accelerate when they escape the bulge

Large-scale outflows driven by AGNs are an important element of galaxy evolution. Detailed analysis of their properties allows us to probe the activity history of the galactic nucleus and, potentially, other properties of the host galaxy. A recent paper presents detailed radial velocity profiles of outflows in ten AGN host galaxies and shows a common trend of approximately constant velocity in the centre followed by rapid acceleration outside $R_{\rm tr} \sim 1 - 3$ kpc. We show that this is a consequence of the AGN-driven outflows clearing the gaseous bulges of the host galaxies and beginning to expand into a region of negligible gas density. We used a 1D semi-analytical code to calculate outflow propagation in each of the ten galaxies, assuming a constant AGN luminosity and an isothermal bulge density profile, with a finite bulge radius, and leaving the gas fraction and total mass of the bulge as free parameters. We also considered the effect of different gas density profiles, variations in bulge velocity dispersion, AGN luminosity, and the effect of outflow fragmentation. Our simplest model can fit six outflow profiles essentially perfectly, while another can be fit if the bulge gas density profile is shallower than isothermal. A shallower density profile also improves the fit in the central regions of the remaining three outflows, but they accelerate faster than our models predict; this could be evidence of significant gas cooling and star formation that reduce the total mass of outflowing gas. We conclude that a simple AGN-driven wind feedback model can explain the detailed velocity profiles of real outflows in local AGN hosts. The free parameters of our model have values that fall well within reasonable ranges. This suggests that the simple scenario we envisioned is close to the true conditions governing the general trends of large-scale outflow expansion.

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arXiv2024

Slow and steady does the trick: Slow outflows enhance the fragmentation of molecular clouds

Most massive galaxies host a supermassive black hole at their centre. Matter accretion creates an active galactic nucleus (AGN), forming a relativistic particle wind. The wind heats and pushes the interstellar medium, producing galactic-wide outflows. Fast outflows remove the gas from galaxies and quench star formation, and while slower ($v<500$ km s$^{-1}$) outflows are ubiquitous, their effect is less clear but can be both positive and negative. We wish to understand the conditions required for positive feedback. We investigated the effect that slow and warm-hot outflows have on the dense gas clouds in the host galaxy. We aim to constrain the region of outflow and cloud parameter space, if any, where the passage of the outflow enhances star formation. We used numerical simulations of virtual `wind tunnels' to investigate the interaction of isolated turbulent spherical clouds ($10^{3;4;5}$ M$_{\odot}$) with slow outflows ($10$ km s$^{-1} - 400$ km s$^{-1}$) spanning a wide range of temperatures ($10^{4;5;6}$ K). We find that warm outflows compress the clouds and enhance gas fragmentation at velocities ${\leq}200$ km s$^{-1}$, while hot ($T_{\rm out} = 10^6$ K) outflows increase fragmentation rates even at moderate velocities of $400$ km s$^{-1}$. Cloud acceleration, on the other hand, is typically inefficient, with dense gas only attaining velocities of ${<}0.1 v_{\rm out}$. We suggest three primary scenarios where positive feedback on star formation is viable: stationary cloud compression by slow outflows in low-powered AGN, sporadic enhancement in shear flow layers formed by luminous AGN, and self-compression in fragmenting AGN-driven outflows. Our results are consistent with current observational constraints and with previous works investigating triggered star formation in these disparate domains.

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arXiv2023

Life after AGN switchoff: evolution and properties of fossil galactic outflows

Galaxy-wide outflows driven by active galactic nuclei (AGN) are an important ingredient in galaxy evolution. Analytical calculations suggest that such outflows have significant inertia and can persist long after the AGN itself fades away. We use hydrodynamical simulations of outflows in idealised galaxy bulges to investigate the propagation of these `fossil' AGN outflows. We find that fossil outflows should be common in gas-poor galaxies but form only rarely in gas-rich ones; in general, fossil outflows should outnumber driven ones by a factor of a few in the local Universe, and possibly more at high redshift. When they do form, fossil outflows tend to be lopsided and detached from the nucleus, and colder than their driven counterparts, with a more prominent molecular phase. Spatially resolved and/or multiphase observations can help distinguish fossil AGN outflows from star formation-driven ones, which have similar integrated properties. We discuss a number of spatially-resolved observations of outflows, suggesting that most show evidence of fossil outflow existence, sometimes together with driven outflows on smaller scales.

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arXiv2023

The fraction and kinematics of broad absorption line quasars across cosmic time

Luminous quasars are powerful targets to investigate the role of feedback from supermassive black-holes (BHs) in regulating the growth phases of BHs themselves and of their host galaxies, up to the highest redshifts. Here we investigate the cosmic evolution of the occurrence and kinematics of BH-driven outflows, as traced by broad absorption line (BAL) features, due to the C IV ionic transition. We exploit a sample of 1935 quasars quasars at $z=2.1-6.6$ with bolometric luminosity log($L_{\rm bol}/$erg s$^{-1})\gtrsim46.5$, drawn from the Sloan Digital Sky Survey and from the X-shooter legacy survey of Quasars at Reionisation (XQR-30). We consider rest-frame optical bright quasars to minimise observational biases due to quasar selection criteria. We apply a homogeneous BAL identification analysis, based on employing composite template spectra to estimate the quasar intrinsic emission. We find a BAL quasar fraction close to 20\% at $z\sim2-4$, while it increases to almost 50\% at $z\sim6$. The velocity and width of the BAL features also increase at $z\gtrsim4.5$. We exclude that the redshift evolution of the BAL properties is due to differences in terms of quasar luminosity and accretion rate. These results suggest significant BH feedback occurring in the 1 Gyr old Universe, likely affecting the growth of BHs and, possibly, of their host galaxies, as supported by models of early BH and galaxy evolution.

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arXiv2022

Improving Black Hole Accretion Treatment in Hydrodynamical Simulations

The large galactic scales are connected to the many orders of magnitude smaller supermassive black hole (SMBH) scales by an episodic cycle of feeding and feedback. Active galactic nuclei (AGN) are powered by accretion onto SMBH and the majority of AGN energy, in near-Eddington regime, is produced in thin sub-pc accretion discs. Currently, it is very difficult to model processes that occur on vastly different scales, ranging from the circumnuclear gas reservoirs at tens to hundreds of parsecs, down to the accretion disc scales at <0.01 pc. While sub-grid prescriptions used in large-scale or cosmological simulations are able to reproduce large-scale feedback, we propose using a more realistic model in parsec-scale simulations, where it is important to get accurate timescales to understand how feedback affects gas dynamics and star formation in the vicinity of the AGN. To test our approach we use a sub-resolution thin accretion disc model, coupled to the SMBH, in a set of hydrodynamical simulations of a retrograde collision between a gas ring and a molecular cloud in an environment similar to the Galactic centre using the SPH code Gadget-3. The disc-mediated feeding of the SMBH is relatively smooth and delayed compared to an instantaneous feeding prescription. While the reduction of accretion due to feedback is present in both accretion disc and instantaneous feeding simulations, a clear central cavity appears only in accretion disc runs - hinting that a less volatile accretion phase could have a greater impact on the surrounding gas.

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arXiv2022

Determining AGN luminosity histories using present-day outflow properties: a neural-network based approach

Large-scale outflows driven by active galactic nuclei (AGN) can have a profound influence on their host galaxies. The outflow properties themselves depend sensitively on the history of AGN energy injection during the lifetime of the outflow. Most observed outflows have dynamical timescales longer than the typical AGN episode duration, i.e. they have been inflated by multiple AGN episodes. Here, we present a neural-network based approach to inferring the most likely duty cycle and other properties of AGN based on the observable properties of their massive outflows. Our model recovers the AGN parameters of simulated outflows with typical errors $< 25\%$. We apply the method to a sample of 59 real molecular outflows and show that a large fraction of them have been inflated by AGN shining with a rather high duty cycle $δ_{\rm AGN} > 0.2$. This result suggests that nuclear activity in galaxies is clustered hierarchically in time, with long phases of more frequent activity composed of many short activity episodes. We predict that $\sim \! 19\%$ of galaxies should have AGN-driven outflows, but half of them are fossils - this is consistent with currently available data. We discuss the possibilities to investigate AGN luminosity histories during outflow lifetimes and suggest ways to use our software to test other physical models of AGN outflows. The source code of all of the software used here is made public.

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arXiv2021

High-redshift SMBHs can grow from stellar-mass seeds via chaotic accretion

Extremely massive black holes, with masses $M_{\rm BH} > 10^9 M_\odot$, have been observed at ever higher redshifts. These results create ever tighter constraints on the formation and growth mechanisms of early black holes. Here we show that even the most extreme black hole known, Pōniuā'ena, can grow from a $10 M_\odot$ seed black hole via Eddington-limited luminous accretion, provided that accretion proceeds almost continuously, but is composed of a large number of episodes with individually-uncorrelated initial directions. This chaotic accretion scenario ensures that the growing black hole spins slowly, with the dimensionless spin parameter $a \lesssim 0.2$, so its radiative efficiency is also low, $ε\simeq 0.06$. If accretion is even partially aligned, with $20-40\%$ of accretion events happening in the same direction, the black hole spin and radiative efficiency are much higher, leading to significantly slower growth. We suggest that the chaotic accretion scenario can be completely falsified only if a $10^9 M_\odot$ black hole is discovered at $z \geq 9.1$, approximately $150$~Myr before Pōniuā'ena. The space density of extreme quasars suggests that only a very small fraction, roughly one in $4 \times 10^7$, of seed black holes need to encounter favourable growth conditions to produce the observed extreme quasars. Other seed black holes grow much less efficiently, mainly due to lower duty cycles, so are much more difficult to detect.

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arXiv2020

Intermittent AGN episodes drive outflows with a large spread of observable loading factors

The properties of large-scale galactic outflows, such as their kinetic energy and momentum rates, correlate with the luminosity of the active galactic nucleus (AGN). This is well explained by the wind-driven outflow model, where a fraction of the AGN luminosity drives the outflow. However, significant departures from these correlations have been observed in a number of galaxies. This may happen because AGN luminosity varies on a much shorter timescale ($\sim 10^4-10^5$~yr) than outflow properties do ($\sim 10^6$~yr). We investigate the effect of AGN luminosity variations on outflow properties using 1D numerical simulations. This effect can explain the very weak outflow in PDS 456: if its nucleus is currently much brighter than the long-term average luminosity, the outflow has not had time to react to this luminosity change. Conversely, the outflow in Mrk 231 is consistent with being driven by an almost continuous AGN, while IRAS F11119+3257 represents an intermediate case between the two. Considering a population of AGN, we find that very low momentum loading factors $\dot{p}_{\rm out} < L_{\rm AGN}/c$ should be seen in a significant fraction of objects - up to $15\%$ depending on the properties of AGN variability and galaxy gas fraction. The predicted distribution of loading factors is consistent with the available observational data. We discuss how this model might help constrain the duty cycles of AGN during the period of outflow inflation, implications for multiphase and spatially distinct outflows, and suggest ways of improving AGN prescriptions in numerical simulations.

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arXiv2020

The case for the fundamental $M_{\rm BH}$-$σ$ relation

Strong scaling relations between host galaxy properties (such as stellar mass, bulge mass, luminosity, effective radius etc) and their nuclear supermassive black hole's mass point towards a close co-evolution. In this work, we first review previous efforts supporting the fundamental importance of the relation between supermassive black hole mass and stellar velocity dispersion ($M_{\rm BH}$-$σ_{\rm e}$). We then present further original work supporting this claim via analysis of residuals and principal component analysis applied to some among the latest compilations of local galaxy samples with dynamically measured supermassive black hole masses. We conclude with a review of the main physical scenarios in favour of the existence of a $M_{\rm BH}$-$σ_{\rm e}$ relation, with a focus on momentum-driven outflows.

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arXiv2020

Evolution of dwarf galaxy observable parameters

We present a semi-analytic model of isolated dwarf galaxy evolution and use it to study the build-up of observed correlations between dwarf galaxy properties. We analyse the evolution using models with averaged and individual halo mass assembly histories in order to determine the importance of stochasticity on the present-day properties of dwarf galaxies. The model has a few free parameters, but when these are calibrated using the halo mass - stellar mass and stellar mass-metallicity relations, the results agree with other observed dwarf galaxy properties remarkably well. Redshift evolution shows that even isolated galaxies change significantly over the Hubble time and that 'fossil dwarf galaxies' with properties equivalent to those of high-redshift analogues should be extremely rare, or non-existent, in the Local Universe. A break in most galaxy property correlations develops over time, at a stellar mass $M_* \simeq 10^7 M_\odot$. It is caused predominantly by the ionizing background radiation and can therefore in principle be used to constrain the properties of reionization.

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arXiv2019

Slow and fat: low-spin SMBHs are more massive

Active galactic nuclei (AGN) probably control the growth of their host galaxies via feedback in the form of wide-angle wind-driven outflows. These establish the observed correlations between supermassive black hole (SMBH) masses and host galaxy properties, e.g. the spheroid velocity dispersion $σ$. In this paper we consider the growth of the SMBH once it starts driving a large-scale outflow through the galaxy. To clear the gas and ultimately terminate further growth of both the SMBH and the host galaxy, the black hole must continue to grow its mass significantly, by up to a factor of a few, after reaching this point. The mass increment $ΔM_{\rm BH}$ depends sensitively on both galaxy size and SMBH spin. The galaxy size dependence leads to $ΔM_{\rm BH} \propto σ^5$ and a steepening of the $M-σ$ relation beyond the analytically calculated $M \propto σ^4$, in agreement with observation. Slowly--spinning black holes are much less efficient in producing feedback, so at any given $σ$ the slowest--spinning black holes should be the most massive. Current observational constraints are consistent with this picture, but insufficient to test it properly; however, this should change with upcoming surveys.

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arXiv2019

Warm absorbers: supermassive black hole feeding, and Compton-thick AGN

Warm absorbers are found in many AGN and consist of clouds moving at moderate radial velocities, showing complex ionization structures and having moderate to large column densities. Using 1D numerical calculations, we confirm earlier suggestions that the energy released by an AGN pushes the surrounding gas outward in a bubble until this reaches transparency. Typical AGN episode durations of $5\times 10^4$ yr supply enough energy for this, except in very gas-rich and/or very compact galaxies, such as those in the early Universe. In those galaxies, the AGN might remain hidden for many periods of activity, hiding the black hole growth. The typical radii of $0.1-1$ kpc, velocities of $100-1000$ km s$^{-1}$ and resulting optical depths are consistent with observations of warm absorbers. The resulting structure is a natural outcome of outflows driven by AGN buried in an optically thick gas envelope, and has a total mass comparable to the final $M -σ$ mass the central supermassive black hole will eventually reach.These results suggest that AGN can feed very efficiently by agitating this surrounding dense material. This may not be easy to observe, as this gas is Compton thick along many sightlines. The infall may produce episodic star formation in the centre, building up nuclear star clusters simultaneously with the growth of the central black hole.

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arXiv2018

Tidal disruption events can power the observed AGN in dwarf galaxies

In recent years, numerous active galactic nuclei have been discovered in ever smaller galaxies, questioning the paradigm that dwarf galaxies do not harbour central massive black holes. Even if such black holes exist, feeding them by gas streams is difficult, since star formation should be more efficient than AGN feeding in dwarf galaxies. In this paper, I investigate the possibility that tidal disruptions of stars are responsible for the observed AGN in dwarf galaxies. I show that the expected duty cycles of TDE-powered AGN, $f_{\rm AGN} \geq 0.5\%$, are consistent with observed AGN fractions assuming that the occupation fraction in dwarf galaxies is close to unity. Furthermore, I calculate the properties of outflows driven by TDE-powered AGN under idealised conditions and find that they might have noticeable effects on the host galaxies. Outflows themselves might not be detectable, except in gas-poor galaxies, where they can accelerate to $v_{\rm out} > 100$~km/s, but increased gas turbulence, more diffuse density profile and lower star formation efficiency can be discovered and used to constrain the black hole occupancy fraction and more nuanced effects on dwarf galaxy evolution. If massive black holes form from seeds that are much more massive than stellar black holes, then their outflows should be easily detectable; this result, aided by observations of high-redshift dwarf galaxies, provides a potential way of determining seed masses of black holes.

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arXiv2018

AGN must be very efficient at powering outflows

Galaxy evolution is affected by competing feedback processes. Stellar feedback dominates in low-mass galaxies, while AGN feedback predominantly affects massive ones. Recent observational results reveal the dependence of black hole accretion rate (BHAR) and star formation rate (SFR) on galaxy stellar mass, and give information on the galaxy mass at which the changeover between dominant feedback mechanisms occurs. I use this information to derive an empirical estimate of the coupling efficiency, $f_{\rm AGN}$, between AGN luminous energy output and AGN-driven galactic outflows, and the momentum loading factor $f_{\rm p,AGN}$ between the momentum of AGN radiation field and the outflow. The results are independent of any particular model of AGN feedback and show that AGN feedback must be very efficient and/or have very large momentum loading in order to explain current observations. I discuss possible ways of reaching the required efficiency and loading factor, and the selection effects that might result in only weak outflows being observed, while the most powerful ones may be generally obscured. There are significant uncertainties involved in the derivation of the result; I suggest ways of reducing them. In the near future, better estimates of coupling efficiency can help distinguish among AGN feedback models, investigate the redshift evolution and mass dependence of feedback efficiency.

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arXiv2018

Multi-phase outflows as probes of AGN accretion history

Powerful outflows with a broad range of properties (such as velocity, ionization, radial scale and mass loss rate) represent a key feature of active galactic nuclei (AGN), even more so since they have been simultaneously revealed also in individual objects. Here we revisit in a simple analytical framework the recent remarkable cases of two ultraluminous infrared quasars, IRAS F11119+3257 and Mrk 231, which allow us to investigate the physical connection between multi-phase AGN outflows across the ladder of distance from the central supermassive black hole (SMBH). We argue that any major deviations from the standard outflow propagation models might encode unique information on the past SMBH accretion history, and briefly discuss how this could help address some controversial aspects of the current picture of AGN feedback.

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arXiv2018

Sgr A$^*$ envelope explosion and the young stars in the centre of the Milky Way

Sgr A$^*$ is the super massive black hole residing in the centre of the Milky Way. There is plenty of observational evidence that a massive gas cloud fell into the central parsec of the Milky Way $\sim 6$ million years ago, triggering formation of a disc of young stars and activating Sgr A$^*$. In addition to the disc, there is an unexplained population of young stars on randomly oriented orbits. Here we hypothesize that these young stars were formed by fragmentation of a massive quasi-spherical gas shell driven out from Sgr A$^*$ potential well by an energetic outflow. To account for the properties of the observed stars, the shell must be more massive than $10^5$ Solar masses, be launched from inside $\sim 0.01$~pc, and the feedback outflow has to be highly super-Eddington albeit for a brief period of time, producing kinetic energy of at least $10^{55}$~erg. The young stars in the central parsec of the Galaxy may be a unique example of stars formed from atomic rather than molecular hydrogen, and forged by extreme pressure of black hole outflows.

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