Positive and negative feedback by AGN jets in high-redshift galaxies

Simulations of feedback by jets from active galactic nuclei (AGN) in the past mostly focused on the interaction at large scales as the circumgalactic medium or intra-cluster medium for clusters of galaxies. Only in recent years, simulations have included the interaction of jets with a highly inhomogeneous medium as required by a multi-phase interstellar medium (ISM). At the same time, feedback by AGN has become a common component for cosmological simulations of galaxy evolution to form massive galaxies compatible with observations. I will present some of our recent results and will put them into further context of other feedback simulations and how the opposing effects of positive and negative feedback by jets might be understood in terms of different properties of the ISM.Comment: 6 pages, 1 figure, published in final form in Reviews in Modern Astronomy, Vol. 26, AN 335, 531. Based on invited talk at annual meeting 2013 of the Astronomische Gesellschaf

Stellar Signatures of AGN Jet Triggered Star Formation

To investigate feedback between relativistic jets emanating from Active Galactic Nuclei (AGN) and the stellar population of the host galaxy, we analyze the long-term evolution of the galaxy-scale simulations by Gaibler et al. (2012) of jets in massive, gas-rich galaxies at z ~ 2 - 3 and of stars formed in the host galaxies. We find strong, jet-induced differences in the resulting stellar populations of galaxies that host relativistic jets and galaxies that do not, including correlations in stellar locations, velocities, and ages. Jets are found to generate distributions of increased radial and vertical velocities that persist long enough to effectively extend the stellar structure of the host. The jets cause the formation of bow shocks that move out through the disk, generating rings of star formation within the disk. The bow shock often accelerates pockets of gas in which stars form, yielding populations of stars with significant radial and vertical velocities, some of which have large enough velocities to escape the galaxy. These stellar population signatures can serve to identify past jet activity as well as jet-induced star formation

External pressure-triggering of star formation in a disc galaxy: a template for positive feedback

Feedback from active galactic nuclei (AGN) has often been invoked both in simulations and in interpreting observations for regulating star formation and quenching cooling flows in massive galaxies. AGN activity can, however, also over-pressurise the dense star-forming regions of galaxies and thus enhance star formation, leading to a positive feedback effect. To understand this pressurisation better, we investigate the effect of an ambient external pressure on gas fragmentation and triggering of starburst activity by means of hydrodynamical simulations. We find that moderate levels of over-pressurisation of the galaxy boost the global star formation rate of the galaxy by an order of magnitude, turn stable discs unstable, and lead to significant fragmentation of the gas content of the galaxy, similar to what is observed in high redshift galaxies.Comment: submitted to MNRA

Numerical modelling of the lobes of radio galaxies in cluster environments II : Magnetic field configuration and observability

Copyright The Authors 2014. Published by Oxford University Press on behalf of the Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly citedWe describe three-dimensional magnetohydrodynamical modelling of powerful radio galaxies in realistic poor cluster environments. This modelling extends our earlier work on the hydrodynamics of radio galaxies as a function of their cluster environment to consider the magnetic field configuration in the lobes and its observational consequences, using a realistic model for the magnetic field in the intracluster medium, very high density contrast in the lobes and high numerical resolution. We confirm, now with a realistic magnetic field model, that lobes have characteristic trajectories in the radio power/linear size diagram which depend strongly on their environment. We investigate the detailed evolution of polarized emission, showing that the lobes evolve from the initially ordered field configuration imposed by our boundary conditions to one in which the longitudinal field comes to dominate. We obtain simulated observations of polarization whose properties are quantitatively consistent with observations. The highly spatially intermittent magnetic field also reproduces the observation that inverse-Compton emission from lobes is much smoother than synchrotron. Our simulations allow us to study the depolarizing effect of the external medium on the lobes, and so to demonstrate that Faraday depolarization from environments of the type we consider can reproduce the integrated fractional polarization properties of large samples and the observed preferential depolarization of the receding lobe.Peer reviewe

Physics and fate of jet related emission line regions

At high redshift, extragalactic jets are associated with extended emission line regions. We present global and local hydrodynamic simulations of the interaction of jets with their environment relevant to this phenomenon. We determine the fraction of kinetic energy and momentum of the jet that should appear in the emission line gas. From momentum considerations, we argue that the gas seen in outward motion in many examples must to a large part originate from within the galaxy. The host galaxies should have been massive, gas rich and heavily star forming galaxies. The expelled gas is not expected to return to the galaxy quickly. The observed kinetic energy in the emission line gas constrains the jet powers to be greater than 10^47 erg/s.Comment: 12 pages, 5 figure

Evolution of the ISM and Galactic Activity

We study the effects of time-dependent mass injection and heating on the evolution of the interstellar medium (ISM) in elliptical galaxies. As the large and luminous ellipticals have supermassive black holes at their cores, which were probably much less massive in the young universe, feeding these black holes is essential. We examine steady state solutions and describe the impact of the initial starburst on the evolution of the ISM and consequences for galactic activity, based on results from Starburst99.Comment: 6 pages, 5 figures, poster contribution for the 1st Arizona/Heidelberg Symposium "The High Redshift Frontier", November 30 - December 3, 2004, Tucson, AZ, USA. Proceedings available at http://highz.ita.uni-heidelberg.de/Proceedings.pd

Asymmetries in extragalactic double radio sources: clues from 3D simulations of jet-disc interaction

V. Gaibler, S. Khochfar, and M. Krause, 'Asymmetries in extragalactic double radio sources: clues from 3D simulations of jet–disc interaction', Monthly Notices of the Royal Astronomical Society, Vol. 411, pp. 155-161, first published online 21 January 2011. The version of record is available online at doi::10.1111/j.1365-2966.2010.17674.x. Published by Oxford University Press on behalf of the Royal Astronomical Society. © 2010 The Author(s). Journal compilation © 2010 RAS.Observational and theoretical studies of extragalactic radio sources have suggested that an inhomogeneous environment may be responsible for observed arm-length asymmetries of jets and the properties of extended emission-line regions in high-redshift radio galaxies. We perform 3D hydrodynamic simulations of the interaction of a powerful extragalactic bipolar jet with a disc-shaped clumpy interstellar medium (ISM) of lognormal density distribution and analyse the asymmetry. Furthermore, we compute the relation between jet asymmetry and the ISM properties by means of Monte Carlo simulations based on a 1D propagation model for the jet through the dense medium. We find that the properties of the ISM can be related to a probability distribution of jet arm-length asymmetries: disc density and height are found to have the largest effect on the asymmetry for realistic parameter ranges, while the Fourier energy spectrum of the ISM and turbulent Mach number only have a smaller effect. The hydrodynamic simulations show that asymmetries generally may be even larger than expected from the 1D model due to the complex interaction of the jet and its bow shock with gaseous clumps, which goes much beyond simple energy disposal. From our results, observed asymmetries of medium-sized local radio galaxies may be explained by gas masses of 109– 1010 M in massive elliptical galaxies. Furthermore, the simulations provide a theoretical basis for the observed correlation that emission-line nebulae are generally found to be brighter on the side of the shorter lobe in high-redshift radio galaxies. This interaction of jets with the cold gas phase suggests that star formation in evolving high-redshift galaxies may be affected considerably by jet activity.Peer reviewe