Kastytis Zubovas

AGN outflows trigger starbursts in gas-rich galaxies

Theoretical astrophysics research in Leicester is supported by an STFC Rolling Grant. KZ thanks the UK STFC for support successively in the form of a studentship and a postdoctoral research associate position. MIW acknowledges the Royal Society for financial support.

Galaxy-wide outflows: cold gas and star formation at high speeds

Several active galaxies show strong evidence for fast (vout ∼ 1000 km s −1 ) massive ( u M = several × 1000 Myr −1 ) gas outflows. Such outflows are expected on theoretical grounds once the central supermassive black hole reaches the mass set by the M−σ relation, and may be what makes galaxies become red and dead. Despite their high velocities, which imply temperatures far above those necessary for molecule dissociation, the outflows contain large amounts of molecular gas. To understand this surprising result, we investigate the gas cooling and show that the outflows cannot stably persist in high-temperature states. Instead, the outflowing gas forms a two-phase medium, with cold dense molecular clumps mixed with hot tenuous gas, as observed. We also show that efficient cooling leads to star formation, providing an observable outflow signature. The central parts of the outflows can be intrinsically luminous gamma-ray sources, provided that the central black hole is still strongly accreting. We note also that these outflows can persist for ∼10 8 yr after the central AGN has turned off, so that many observed outflows (particularly with high speeds) otherwise assumed to be driven by starbursts might also be of this type.

Sgr A* flares: tidal disruption of asteroids and planets?

It is theoretically expected that a supermassive black hole (SMBH) in the centre of a typical nearby galaxy disrupts a solar-type star every ∼10 5 yr, resulting in a bright flare lasting for months. Sgr A ∗ , the resident SMBH of the Milky Way, produces (by comparison) tiny flares that last only hours but occur daily. Here we explore the possibility that these flares could be produced by disruption of smaller bodies – asteroids. We show that asteroids passing within an au of Sgr A ∗ could be split into smaller fragments which then vaporize by bodily friction with the tenuous quiescent gas accretion flow on to Sgr A ∗ . The ensuing shocks and plasma instabilities may create a transient population of very hot electrons invoked in several currently popular models for Sgr A ∗ flares, thus producing the required spectra. We estimate that asteroids larger than ∼10 km in size are needed to power the observed flares, with the maximum possible luminosity of the order of 10 39 erg s −1 . Assuming that the asteroid population per parent star in the central parsec of the Milky Way is not too dissimilar from that around stars in the solar neighbourhood, we estimate the asteroid disruption rates, and the distribution of the expected luminosities, finding a reasonable agreement with the observations. We also note that planets may be tidally disrupted by Sgr A ∗ as well, also very infrequently. We speculate that one such disruption may explain the putative increase in Sgr A ∗ luminosity

Quasar feedback: accelerated star formation and chaotic accretion

Growing supermassive black holes (SMBHs) are believed to influence their parent galaxies in a negative way, terminating their growth by ejecting gas before it can turn into stars. Here we present some of the most sophisticated SMBH feedback simulations to date, showing that the effects of quasars on galaxies are not always negative. We find that when the ambient shocked gas cools rapidly, the shocked gas is compressed into thin cold dense shells, filaments and clumps. Driving these high-density features out is much more difficult than analytical models predict. However, in this regime quasars have another way of affecting the host – by triggering a massive star-formation burst in the cold gas by overpressurizing it. Under these conditions SMBHs actually accelerate star formation in the host, having a positive rather than negative effect on their host galaxies. The relationship between SMBHs and galaxies is thus even more complex and symbiotic than currently believed. We also suggest that the instabilities found here may encourage a chaotic active galactic nucleus feeding mode.

Energy- and momentum-conserving AGN feedback outflows

KZ acknowledges the UK STFC for support successively in the form of a PhD studentship and a postdoctoral research position, both at the University of Leicester. This research is partially supported by the Research Council Lithuania grant no. MIP-062/2013. Numerical simulations presented in this work were carried out on two computing clusters. Some computations were performed on resources at the High Performance Computing Center HPC Sauletekis in Vilnius University Faculty of Physics. This work also used the DiRAC Complexity system, operated by the University of Leicester, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by a BIS National E-Infrastructure capital grant ST/K000373/1 and DiRAC Operations grant ST/K0003259/1. DiRAC is part of the UK National E-Infrastructure.

Outflows of stars due to quasar feedback

Quasar feedback outflows are commonly invoked to drive gas out of galaxies in the early gas-rich epoch to terminate growth of galaxies. Here we present simulations that show that AGN feedback may drive not only gas but also stars out of their host galaxies under certain conditions. The mechanics of this process is as following: (1) AGN-driven outflows accelerate and compress gas filling the host galaxy; (2) the accelerated dense shells become gravitationally unstable and form stars on radial trajectories. For the spherically symmetric intial conditions explored here, the black hole needs to exceed ]

The M-sigma relation in different environments

Galaxies become red and dead when the central supermassive black hole (SMBH) becomes massive enough to drive an outflow beyond the virial radius of the halo. We show that this final SMBH mass is larger than the final SMBH mass in the bulge of a spiral galaxy by up to an order of magnitude. The M - \sigma relations in the two galaxy types are almost parallel (M \propto \sigma^{4+\beta}, with \beta < 1) but offset in normalization, with the extra SMBH mass supplied by the major merger transforming the galaxy into an elliptical, or by mass gain in a galaxy cluster. This agrees with recent findings that SMBH in two Brightest Cluster Galaxies are \sim 10\times the expected M-\sigma mass. We show that these results do not strongly depend on the assumed profile of the dark matter halo, so analytic estimates found for an isothermal potential are approximately valid in all realistic cases. Our results imply that there are in practice actually {\it three} M - \sigma relations, corresponding to spiral galaxies with evolved bulges, field elliptical galaxies and cluster centre elliptical galaxies. A fourth relation, corresponding to cluster spiral galaxies, is also possible, but such galaxies are expected to be rare. All these relations have the form M_{\rm BH} = C_n\sigma^4, with only slight difference in slope between field and cluster galaxies, but with slightly different coefficients C_n. Conflating data from galaxies of different types and fitting a single relation to them tends to produce a higher power of \sigma.

SUPERNOVAE IN THE CENTRAL PARSEC: A MECHANISM FOR PRODUCING SPATIALLY ANISOTROPIC HYPERVELOCITY STARS

Several tens of hypervelocity stars (HVSs) have been discovered escaping our Galaxy. These stars share a common origin in the Galactic center and are distributed anisotropically in Galactic longitude and latitude. We examine the possibility that HVSs may be created as the result of supernovae (SNe) occurring within binary systems in a disk of stars around Sgr A* over the last 100 Myr. Monte Carlo simulations show that the rate of binary disruption is {approx}10{sup -4} yr{sup -1}, comparable to that of tidal disruption models. The SN-induced HVS production rate ({Gamma}{sub HVS}) is significantly increased if the binaries are hardened via migration through a gaseous disk. Moderate hardening gives {Gamma}{sub HVS} {approx_equal} 2 Multiplication-Sign 10{sup -7} yr{sup -1} and an estimated population of {approx}20 HVSs in the last 100 Myr. SN-induced HVS production requires the internal and external orbital velocity vectors of the secondary binary component to be aligned when the binary is disrupted. This leaves an imprint of the disk geometry on the spatial distribution of the HVSs, producing a distinct anisotropy.

BAL QSOs AND EXTREME UFOs: THE EDDINGTON CONNECTION

We suggest a common physical origin connecting the fast, highly ionized winds (UFOs) seen in nearby active galactic nuclei (AGNs), and the slower and less ionized winds of broad absorption line (BAL) QSOs. The primary difference is the mass-loss rate in the wind, which is ultimately determined by the rate at which mass is fed toward the central supermassive black hole (SMBH) on large scales. This is below the Eddington accretion rate in most UFOs, and slightly super-Eddington in extreme UFOs such as PG1211+143, but ranges up to ~10-50 times this in BAL QSOs. For UFOs this implies black hole accretion rates and wind mass-loss rates which are at most comparable to Eddington, giving fast, highly ionized winds. In contrast, BAL QSO black holes have mildly super-Eddington accretion rates, and drive winds whose mass-loss rates are significantly super-Eddington, and so are slower and less ionized. This picture correctly predicts the velocities and ionization states of the observed winds, including the recently discovered one in SDSS J1106+1939. We suggest that luminous AGNs may evolve through a sequence from BAL QSO through LoBAL to UFO-producing Seyfert or quasar as their Eddington factors drop during the decay of a bright accretion event. LoBALs correspond to a short-lived stage in which the AGN radiation pressure largely evacuates the ionization cone, but before the large-scale accretion rate has dropped to the Eddington value. We show that sub-Eddington wind rates would produce an M-σ relation lying above that observed. We conclude that significant SMBH mass growth must occur in super-Eddington phases, either as BAL QSOs, extreme UFOs, or obscured from direct observation.

Collapse and fragmentation of molecular clouds under pressure

Recent analytical and numerical models show that AGN outflows and jets create ISM pressure in the host galaxy that is several orders of magnitude larger than in quiescent systems. This pressure increase can confine and compress molecular gas, thus accelerating star formation. In this paper, we model the effects of increased ambient ISM pressure on spherically symmetric turbulent molecular clouds. We find that large external pressure confines the cloud and drives a shockwave into it, which, together with instabilities behind the shock front, significantly accelerates the fragmentation rate. The compressed clouds therefore convert a larger fraction of their mass into stars over the cloud lifetime, and produce clusters that are initially more compact. Neither cloud rotation nor shear against the ISM affect this result significantly, unless the shear velocity is higher than the sound speed in the confining ISM. We conclude that external pressure is an important element in the star formation process, provided that it dominates over the internal pressure of the cloud.