Energy- and momentum-conserving AGN feedback outflows

It is usually assumed that outflows from luminous AGN are either in the energy-conserving (non-radiative) or in the momentum-conserving (radiative) regime. We show that in a non-spherical geometry the effects of both regimes may manifest at the same time, and that it is the momentum of the outflow that sets the $M_{\rm BH}-\sigma$ relation. Considering an initially elliptical distribution of gas in the host galaxy, we show that a non-radiative outflow opens up a wide ``escape route'' over the paths of least resistance. Most of the outflow energy escapes in that direction. At the same time, in the directions of higher resistance, the ambient gas is affected mainly by the incident momentum from the outflow. Quenching SMBH growth requires quenching gas delivery along the paths of highest resistance, and therefore, it is the momentum of the outflow that limits the black hole growth. We present an analytical argument showing that such energy-conserving feedback bubbles driving leaky ambient shells will terminate SMBH growth once its mass reaches roughly the $M_\sigma$ mass derived earlier by King (2003) for momentum-conserving AGN outflows. Our simulations also have potentially important implications for observations of AGN jet feedback and starburst galaxy feedback. The collimation of the wide angle AGN outflow away from the symmetry plane, as found in our simulations, indicates that credit for work done by such outflows may sometimes be mistakenly given to AGN jets or star formation feedback since wide angle $v \sim 0.1 c$ outflows are harder to observe and the phase when they are present may be short.Comment: 13 pages, 7 figures. Accepted for publication in MNRA

Quasar feedback: accelerated star formation and chaotic accretion

Growing Supermassive Black Holes (SMBH) are believed to influence their parent galaxies in a negative way, terminating their growth by ejecting gas out before it could turn into stars. Here we present some of the most sophisticated SMBH feedback simulations to date showing that quasar's effects 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 since dense filaments are resilient to the feedback. However, in this regime quasars have another way of affecting the host -- by triggering a massive star formation burst in the cold gas by over-pressurising 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 SMBH and galaxies is thus even more complex and symbiotic than currently believed. We also suggest that the instabilities found here may encourage the chaotic AGN feeding mode.Comment: 7 pages. Submitted to MNRAS (version addressing referee's comments

Reignited star formation in dwarf galaxies quenched during reionization

Irregular dwarf galaxies of the Local Group have very varied properties and star formation histories. Some of them formed the majority of their stars very late compared to the others. Extreme examples are Leo A and Aquarius which reached the peak of star formation at $z 6 Gyr after BB). This fact seemingly challenges the LCDM cosmology because the dark matter halos of these galaxies on average should assemble the majority of their masses before z~2 (<3 Gyr after BB). In this work we investigate whether the delayed star formation histories of some irregular dwarf galaxies could be explained purely by the stochasticity of their mass assembly histories coupled with the effect of cosmic reionization. We develop a semi-analytic model to follow the accretion of baryonic matter, star formation and stellar feedback in dark matter halos with present day virial masses 10^9 M_Sun < M < 10^11 M_Sun and with different stochastic growth histories obtained using the PINOCCHIO code based on Lagrangian perturbation theory. We obtain the distributions of observable parameters and the evolution histories for these galaxies. Accretion of baryonic matter is strongly suppressed after the epoch of reionization in some models but they continue to accrete dark matter and eventually reach enough mass for accretion of baryonic matter to begin again. These "reborn" model galaxies show very similar delayed star formation histories to those of Leo A and Aquarius. We find that the stochasticity caused by mass assembly histories is enhanced in systems with virial masses ~10^10 M_Sun because of their sensitivity to the photoionizing intergalactic radiation field after the epoch of reionization. This results in qualitatively different star formation histories in late- and early-forming galaxies and it might explain the peculiar star formation histories of irregular dwarf galaxies such as Leo A and Aquarius.Comment: 10 pages, 5 figures, accepted for publication in A&

BAL QSOs and Extreme UFOs: the Eddington connection

We suggest a common physical origin connecting the fast, highly ionized winds (UFOs) seen in nearby AGN, and the slower and less ionized winds of 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 towards 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 $\sim 10-50$ 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 AGN 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 $M - \sigma$ 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.Comment: 8 pages, 5 figures; a higher quality version of Figure 5 available on reques

Supernovae in the Central Parsec: A Mechanism for Producing Spatially Anisotropic Hypervelocity Stars

Several tens of hyper-velocity stars (HVSs) have been discovered escaping our Galaxy. These stars share a common origin in the Galactic centre and are distributed anisotropically in Galactic longitude and latitude. We examine the possibility that HVSs may be created as the result of supernovae occurring within binary systems in a disc of stars around Sgr A* over the last 100 Myr. Monte Carlo simulations show that the rate of binary disruption is ~10^-4 yr^-1, comparable to that of tidal disruption models. The supernova-induced HVS production rate (\Gamma_HVS) is significantly increased if the binaries are hardened via migration through a gaseous disc. Moderate hardening gives \Gamma_HVS ~ 2*10^-7 yr^-1 and an estimated population of ~20 HVSs in the last 100 Myr. Supernova-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 disc geometry on the spatial distribution of the HVSs, producing a distinct anisotropy.Comment: 7 pages, 4 figures. Accepted for publication in the Astrophysical Journa