Clusters of galaxies, the largest virialized objects in the Universe, are an ideal laboratory for studying the physical processes that shape the cosmic evolution of galaxies and their surrounding dark matter halos. Recent observational progress has revealed a stunning complexity of the gas dynamical processes in the intra-cluster medium (ICM), including the discovery of phenomena which pose significant challenges for theoretical modelling. Perhaps the most puzzling among these observational facts is the so-called "cooling-flow" problem. Hot intra-cluster gas emits diffuse X-ray radiation, making clusters of galaxies extremely bright X-ray sources shining with up to a few times 10⁴⁵ erg/s, an energy loss that should reduce the thermal energy content of the gas. While the estimated cooling time in the bulk of a cluster due to these radiative losses is typically longer than the age of the Universe, this is not the case for the dense gas in the cluster cores, where the cooling times are short due to the sensitive dependence of the cooling rate on density. This implies that the central gas should cool out of the cluster atmosphere, leading to central mass deposition rates as high as 1200 solar masses per year in some cases. However, the new generation of X-ray telescopes, XMM-Newton and Chandra, failed to detect the predicted huge amount of cold central gas, ruling out the simple cooling-flow picture. Thus, a physical explanation for this apparent paradox needs to be found.

Over the last couple of years, it has been recognized that probably all galaxies with a spheroidal stellar component conceal a supermassive black hole (BH) at their centers. This suggests the existence of an important link between the growth of supermassive black holes and the formation of their host galaxies, which presumably originates in the energy released by an accreting BH and the impact this has on the formation of the stellar spheroid. Similarly, when gas is funneled towards the center of a cluster by cooling, a fraction of the gas will be accreted by the BH, resulting in a substantial release of energy. Provided the released energy can heat the central cluster gas efficiently, the cooling losses may be offset. At the same time, the feeding of the BH would be stopped, so we expect that this should give rise to a periodic triggering of the AGN, establishing a self-regulated activity pattern between cooling and AGN activity. Direct support for the existence of energy input by AGN into cluster cores comes from observations which show that AGN can inflate hot, buoyant bubbles in the cluster atmospheres during their active phases. The bubbles are thought to detach and rise from the cluster center, and to interact with the surrounding gas, possibly providing the needed heating mechanism.

In order to test whether this physical scenario of bubble heating actually works, scientists from the Max-Planck-Institute for Astrophysics have performed a series of hydrodynamical simulations in which they incorporated periodic heating by AGN bubbles. For the first time, they followed the formation and assembly of a set of galaxy clusters in fully self-consistent cosmological simulations, analyzing in particular how the properties of the intracluster medium and the cluster galaxies change when AGN heating is included.

The simulations reproduce the characteristic features observed for the bubble morphology, as can been seen in Figure 1, where we show a temperature map of the cluster central region (see also the movie). The bubbles develop a mushroom shaped structure when they rise in the cluster atmosphere, pushing the intra-cluster gas above them and also entraining some of the cooler central gas. As a result, they mix colder and hotter gas components initially present at different radii, heating up the central region of the cluster. However, this AGN heating mechanism is comparatively gentle, generating no significant shock waves, such that the gas on top of the bubbles forms cold rims, as observed in a number of cases.

Moreover, after the thermal energy is released in a bubble, the subsequent expansion generates sound waves that travel into the ICM. These sound waves cause faint ripples in the X-ray emissivity (see Figure 2) and can reach even the cluster outskirts within 10⁹ yrs. Depending on the amount of viscosity of the gas, the bubble-induced sound waves will dissipate their energy on different spatial scales, possibly providing an additional mechanism for a non-local heating of the cluster gas.

The use of fully self-consistent simulations of cluster formation allowed the scientists from the Max-Planck-Institute for Astrophysics to study how the AGN ``bubble-feedback'' influences the gaseous and stellar properties over cosmic time. In particular, they analyzed the cooling time of gas (illustrated in Figure 3), comparing the simulations without (blue line) and with (red line) bubble heating. Clearly, the gas residing in the cluster core is efficiently heated, showing much higher values for the estimated cooling time. Along with the cooling time, other thermodynamic properties of the intracluster gas change as well, and the trends of these changes are all in the direction of improving the match of the simulated systems with observations of real galaxy clusters.

Interestingly, not only the state of the intra-cluster gas changes due to the AGN activity, but also the properties of the central cluster galaxy are altered. Most importantly, the number of stars forming in the massive central cD galaxy is considerably reduced, up to the point of a complete termination of star formation at late times. This is highly relevant for another long standing problem of hydrodynamical simulations of galaxy formation. They typically predicted the existence of too massive and too blue central cluster galaxies compared with observations. Bubble feedback, however, decreases the amount of stars formed, and the heating of the inner regions together with the accompanying reduction of the recent star formation rate makes the central cluster galaxies redder, resulting in much better agreement with observations.

It is a stunning revelation that objects as small as black holes can affect the state of the largest virialized objects known in the Universe, namely rich clusters of galaxies that often harbor more than a thousand systems. Yet the outflows from accreting supermassive black holes are so energetic that this is indeed possible, and as the results in this work confirm, it might provide the right physical solution for the "cooling-flow" paradox. Black holes thus appear as a crucial component for understanding galaxy formation as a whole.

Debora Sijacki and Volker Springel

Original publication:

D. Sijacki and V. Springel: Hydrodynamical simulations of cluster formation with central AGN heating, 2005, MNRAS, accepted
astro-ph/0509506


Further information:

Colliding galaxies light up dormant black holes
Tiziana Di Matteo, Volker Springel and Lars Hernquist

Magnetic Turbulence in the Hearts of Clusters of Galaxies
Torsten Ensslin and Corina Vogt

The parallel lives of super-massive black holes and their host galaxies
Andrea Merloni

The Connection Between Active Galactic Nuclei and Starbursts
Guinevere Kauffmann