Christian R. Kaiser

AGN-controlled cooling in elliptical galaxies

A long-standing problem for models of galaxy formation has been the mismatch between the predicted shape of the mass function of dark matter halos and the observed shape of the luminosity function of galaxies. The number of massive halos is predicted to decrease as a power law (N proportional to M^-2) out to very large masses, while the galaxy luminosity function cuts off exponentially at luminosities above L*. This implies that the efficiency with which gas cools onto massive systems is lower than expected. This letter investigates the role of radio-loud active galactic nuclei (AGN) in continually re-heating the cooling gas. By combining two observational results, the time-averaged energy output associated with recurrent radio source activity is determined, as a function of the black hole mass of the host galaxy: H = 10^21.4 (M_BH / M_sun)^1.6 W. It is shown that for massive elliptical galaxies this radio-source heating balances the radiative energy losses from the hot gas surrounding the galaxy. The recurrent radio-loud AGN activity may therefore provide a self-regulating feedback mechanism capable of controlling the rate of growth of galaxies.

On the prevalence of radio-loud active galactic nuclei in brightest cluster galaxies: implications for AGN heating of cooling flows

The prevalence of radio-loud AGN activity in present-day massive halos is determined using a sample of 625 nearby groups and clusters selected from the Sloan Digital Sky Survey. Brightest group and cluster galaxies (BCGs) are more likely to host a radio– loud AGN than other galaxies of the same stellar mass (by below a factor of two at a stellar mass of » 5 × 10 11 M¯, but rising to over an order of magnitude below 10 11 M¯). The distribution of radio luminosities for BCGs does not depend on mass, however, and is similar to that of field galaxies of the same stellar mass. Neither the radio–loud fraction nor the radio luminosity distribution of BCGs depends strongly on the velocity dispersion of the host cluster. The radio-AGN fraction is also studied as a function of distance from the cluster centre. Only within 0.2 r200 do cluster galaxies exhibit an enhanced likelihood of radio–loud AGN activity, which approaches that of the BCGs. In contrast to the radio properties, the fraction of galaxies with optical emission–line AGN activity is suppressed within r200 in groups and clusters, decreasing monotonically towards the cluster centre. It is argued that the radio–loud AGN properties of both BCGs and non-BCGs can naturally be explained if this activity is fuelled by cooling from hot gas surrounding the galaxy. Using observational estimates of the mechanical output of the radio jets, the time–averaged energy output associated with recurrent radio source activity is estimated for all group/cluster galaxies. Within the cooling radius of the cluster, the radio–mode heating associated with the BCG dominates over that of all other galaxies combined. The scaling between total radio–AGN energy output and cluster velocity dispersion is observed to be considerably shallower than the » ¾ 4 v scaling of the radiative cooling rate. Thus, unless either the mechanical–to–radio luminosity ratio or the efficiency of converting AGN mechanical energy into heating increases by 2–3 orders of magnitude between groups and rich clusters, radio–mode heating will not balance radiative cooling in systems of all masses. In groups, radio–AGN heating probably overcompensates the radiative cooling losses, and this may account for the observed entropy floor in these systems. In the most massive clusters, an additional heating process (most likely thermal conduction) may be required to supplement the AGN heating.

A dark jet dominates the power output of the stellar black hole Cygnus X-1

Black holes undergoing accretion are thought to emit the bulk of their power in the X-ray band by releasing the gravitational potential energy of the infalling matter. At the same time, they are capable of producing highly collimated jets of energy and particles flowing out of the system with relativistic velocities. Here we show that the 10-solar-mass (10M[circdot]) black hole in the X-ray binary Cygnus X-1 (refs 3–5) is surrounded by a large-scale (∼5 pc in diameter) ring-like structure that appears to be inflated by the inner radio jet. We estimate that in order to sustain the observed emission of the ring, the jet of Cygnus X-1 has to carry a kinetic power that can be as high as the bolometric X-ray luminosity of the binary system. This result may imply that low-luminosity stellar-mass black holes as a whole dissipate the bulk of the liberated accretion power in the form of ‘dark’, radiatively inefficient relativistic outflows, rather than locally in the X-ray-emitting inflow.

Hot bubbles from active galactic nuclei as a heat source in cooling-flow clusters

Hot, X-ray-emitting plasma permeates clusters of galaxies. The X-ray surface brightness often shows a peak near the centre of the cluster that is coincident with a drop in the entropy of the gas. This has been taken as evidence for a ‘cooling flow’, where the gas cools by radiating away its energy, and then falls to the centre. Searches for this cool gas have revealed significantly less than predicted, indicating that the mass deposition rate is much lower than expected. Most clusters with cooling flows, however, also host an active galactic nucleus at their centres. These active galactic nuclei can inflate large bubbles of hot plasma that subsequently rise through the cluster ‘atmosphere’, thus stirring the cooling gas and adding energy. Here we report highly resolved hydrodynamic simulations which show that buoyant bubbles increase the cooling time in the inner regions of clusters and significantly reduce the deposition of cold gas.

Evolutionary tracks of FRII sources through the P-D diagram

This is the second in a series of papers presenting an analytical model for the evolution of FRII radio sources. In this paper we evaluate the expected radio emission from a radio source incorporating energy loss processes for the relativistic electrons. By combining these results with our earlier dynamical model we calculate evolutionary tracks through the Power–Linear size diagram. These tracks are in good agreement with the observed distribution of sources in this diagram. The effects of different forms for the evolution of the magnetic field in the cocoon, the redshift of the source, the environment of the source and the defining parameters of the jet are investigated. The evolutionary tracks are found to be insensitive to the assumed form of the magnetic field evolution. Some evidence against protons as a major constituent of the jet material is also found.

Buoyant radio plasma in clusters of galaxies

Radio galaxies are known to inflate lobes of hot relativistic plasmas into the intergalactic medium. Here we present hydrodynamical and magnetohydrodynamical simulations of these hot plasma bubbles in FR II objects. We focus on the later stages of their evolution after the jet has died down and after the bow shock that surrounded the lobes at earlier stages has vanished. We investigate the evolution of the plasma bubbles as they become subject to Rayleigh–Taylor instabilities. From our simulations we calculate the radio and X-ray emissivities of the bubbles and discuss their appearance in observations. Finally, we investigate the influence of large-scale magnetic fields on the evolution of the bubbles. The issues of re-acceleration and diffusion of relativistic particles are briefly discussed.

Simulation of radio plasma in clusters of galaxies

We present three-dimensional hydrodynamical simulations of buoyant gas in a typical cluster environment. The hot matter was injected continuously into a small region off-set from the cluster centre. In agreement with previous analytic estimates we found that the bubbles evolve very differently depending on their luminosity. Using tracer particles we computed radio maps of the bubbles based on different assumptions about the magnetic field. In the radio band the bubbles closely resemble FRI sources. For the bubbles to be detectable for long enough to account for FRI sources, we found that reacceleration has to take place. The bubbles are generally difficult to detect, both, in the radio and in the X-ray band. Thus it is possible to hide a significant amount of energy in the form of bubbles in clusters. Finally, we compute the efficiency of the bubbles to stir the ICM and find that recurrent low-power sources may be fairly effective in mixing the inner cluster region.

Simple models of cooling flows

A semi-analytic model of cluster cooling flows is presented. The model assumes that episodic nuclear activity followed by radiative cooling without mass-dropout cycles the cluster gas between a relatively homogeneous, nearly isothermal post-outburst state and a cuspy configuration in which a cooling catastrophe initiates the next nuclear outburst. Fitting the model to Chandra data for the Hydra cluster, a lower limit of 284 Myr until the next outburst of Hydra A is derived. Density, temperature and emission-measure profiles at several times prior to the cooling catastrophe are presented. It proves possible to fit the mass M(?) with entropy index P?-? less than ? to a simple power-law form, which is almost invariant as the cluster cools. We show that radiative cooling automatically establishes this power-law form if the entropy index was constant throughout the cluster gas at some early epoch or after an active galactic nucleus (AGN) activity cycle. To high precision, the central value of ? decreases linearly with time. The fraction of clusters in a magnitude-limited sample that have gas cooler than T is calculated, and is shown to be small for T= 2 keV. Similarly, only 1 per cent of clusters in such a sample contain gas with Pp-? < 2 keV cm2. Entropy production in shocks is shown to be small. The entropy that is radiated from the cluster can be replaced if a few per cent of the cluster gas passes through bubbles heated during an outburst of the AGN.

Luminosity function, sizes and FR dichotomy of radio‐loud AGN

The radio luminosity function (RLF) of radio galaxies and radio-loud quasars is often modelled as a broken power law. The break luminosity is close to the dividing line between the two Fanaroff-Riley (FR) morphological classes for the large-scale radio structure of these objects. We use an analytical model for the luminosity and size evolution of FR type II (FR II) objects together with a simple prescription for FR type I (FR I) sources to construct the RLF. We postulate that all sources start out with a FR II morphology. Weaker jets subsequently disrupt within the quasi-constant density cores of their host galaxies and develop the turbulent lobes of FR I. With this model, we recover the slopes of the power laws and the break luminosity of the RLF determined from observations. The rate at which active galactic nuclei (AGN) with jets of jet power Q appear in the universe is found to be proportional to Q -1.6 . The model also roughly predicts the distribution of the radio lobe sizes for FR II objects, if the radio luminosity of the turbulent jets drops significantly at the point of disruption. We show that our model is consistent with recent ideas of two distinct accretion modes in jet-producing AGN if radiative efficiency of the accretion process is correlated with jet power.

A "combination nova" outburst in Z Andromedae: nuclear shell burning triggered by a disk instability

We describe observational evidence for a new kind of interacting binary star outburst that involves both an accretion instability and an increase in thermonuclear shell burning on the surface of an accreting white dwarf. We refer to this new type of eruption as a combination nova. In late 2000, the prototypical symbiotic star Z Andromedae brightened by roughly 2mag in the optical. We observed the outburst in the radio with the VLA and MERLIN, in the optical both photometrically and spectroscopically, in the far-ultraviolet with FUSE, and in the X-rays with both Chandra and XMM-Newton. The 2 year long event had three distinct stages. During the first stage, the optical rise closely resembled an earlier, small outburst that was caused by an accretion disk instability. In the second stage, the hot component ejected an optically thick shell of material. In the third stage, the shell cleared to reveal a white dwarf whose luminosity remained on the order of 10(4) L-circle dot for approximately 1 yr. The eruption was thus too energetic to have been powered by accretion alone. We propose that the initial burst of accretion was large enough to trigger enhanced nuclear burning on the surface of the white dwarf and the ejection of an optically thick shell of material. This outburst therefore combined elements of both a dwarf nova and a classical nova. Our results have implications for the long-standing problem of producing shell flashes with short recurrence times on low-mass white dwarfs in symbiotic stars.