Introduction

Some galaxy evolutionary models postulate that powerful starburst galaxies at high-z yield local massive galaxies following the effects induced by an accreting supermassive black hole (SMBH) at their centre (e.g. Di Matteo et al. 2005). However, it is not clear on which spatial and temporal scales and through which physical processes this transition takes place (see Coppin et al. 2008). Here, we investigate this evolutionary scenario by comparing star formation rates (SFRs), AGN activity and stellar masses in high-z (z ∼ 2) active systems.

Spitzer selection of high-z luminous infrared galaxies

For this work, we selected a sample of IR luminous source candidates in a ∼s20 deg2 area obtained by combining the Lockman Hole field (LH, ∼11 deg2, α = 10h 45m, δ = + 58°), and the XMM-LSS field (XMM, ∼9 deg2, α = 02h 21m, δ = −04° 30′) of the Spitzer Wide Area Infrared Extragalactic Survey (SWIRE; Lonsdale et al. 2003). Both fields benefit from multi-band ground-based optical (Ugriz) and Spitzer IR bands (seven bands from 3.6 to 160 μm). IR luminous sources, powered by star formation or AGN activity, are expected to be bright mid-infrared (MIR) sources. Powerful starburst galaxies are characterised by spectral energy distributions (SEDs) that are bright throughout the MIR to millimetre range. Luminous AGNs are bright MIR sources because their emission from AGN-heated dust peaks in the MIR. We thus selected all sources with a 24 μm flux > 400 μJy (corresponding to ≳ 5σ).

Introduction

At redshifts above z ≳ 0.5 extragalactic jet sources are commonly associated with extended emission line regions (for a review see McCarthy 1993; Miley and De Breuck 2008). The most prominent emission line is the hydrogen Lyman α line, but other typical nebular emission lines have also been found. These regions are up to 100 kpc in extent, anisotropic and preferentially aligned with the radio jets (alignment effect). Their properties correlate with those of the radio jets: smaller radio jets (< 100 kpc) have more extended emission line regions with larger velocity widths (1000 km s−1) that are predominantly shock ionised, as diagnosed from their emission line ratios. Larger radio jets (> 100 kpc) have emission line regions even smaller than 100 kpc. Their turbulent velocities are typically about 500 km s−1 and the dominant excitation mechanism is photoionisation. The physical function of these emission line regions can be compared to a detector in a particle physics experiment: in both cases a beam of high-energy particles hits a target. Analysis of the interactions in the surrounding detector, or in astrophysics the emission line gas, provides information about the physical processes of interest. For the astrophysical jets, the information one would like to obtain from such analysis concerns two traditionally separate branches of astrophysics.

The considerable energy release that may be associated with the jet phenomenon is received by a large reservoir of gas surrounding the host galaxy.

Introduction

Only by incorporating various forms of feedback can theories of galaxy formation reproduce the present-day luminosity function of galaxies. It has also been argued that such feedback processes might explain the counterintuitive behaviour of ‘downsizing’ witnessed since redshifts z ≈ 1 − 2. To examine this question, observations spanning 0.4 < z < 1.4 from the DEEP2/Palomar survey (Bundy et al. 2006) are compared with a suite of equivalent mock observations derived from the Millennium Simulation, populated with galaxies using the Galform code (Bower et al. 2006).

Hierarchical assembly

The mock galaxy samples are generated from the population of dark matter halos in the Millennium Simulation (Springel et al. 2005). This simulation consists of approximately 10 billion dark matter particles each of mass 8.6 × 108h−1M⊙ evolving in a cubic volume of side 500h−1 Mpc, assuming a ∧CDM cosmology.

Dark matter halo merger trees are found from this 4-volume using the methods described by Harker et al. (2006). The lowest mass halos contained in these trees, of which there are about 20 million, consist of 20 particles corresponding to a total mass of 5 × 109h−1M⊙. Such halos could contain at most 9 × 108h−1M⊙ of baryonic material, which is well below the lower limit of the stellar mass functions to be considered in this work. Therefore we do not expect the resolution of the Millennium Simulation to affect our results.

Introduction

The idea that AGN activity can detectably influence the evolution of stellar populations in galaxies was advanced about 10 years ago (Silk and Rees 1998). This feedback can either manifest itself in the form of episodes of induced star formation, as originally suggested by Silk and Rees, or one could also imagine that the impressive release of energy from the AGN's jet to the interstellar medium (ISM) of its host galaxy could inhibit stellar formation. The first form is called positive feedback, while the second is often termed negative feedback. Both forms of feedback have durations of the order of a few times 107 years, i.e. the timescale of the AGN's duty cycle. This is a very short timescale in terms of galaxy evolution: thus the detection of negative feedback becomes possible by inspecting the statistical properties of colour–colour and colour–magnitude diagrams in some bands that are sensitive to recent star formation episodes. Only recently, with the massive exploitation of data from large-scale surveys such as the Sloan Digital Sky Survey, has it become possible to obtain galaxy samples large enough to check these effects (see for instance the contribution by Silverman et al. in this volume, Chapter 4).

Only more recently, however, have simulations of the jet–ISM interaction been attempted (see for instance the contributions by Bicknell et al. and Krause and Gaibler in this volume, Chapters 14 and 16).

During the past decade, convincing evidence has been accumulated concerning the effect that AGN activity has on the internal and external environment of host galaxies. At intermediate and relatively high redshifts (z-0.2–1.5) evidence for this interaction comes, for example, from the optical–radio alignment and from the observation of jet-induced star formation. In the nearby universe there is also a series of significant indications: the observation of recent episodes of star formation in otherwise old or early types of ellipticals has emerged from analyses of the SDSS. There is also more direct and circumstantial evidence from the analysis of regions such as the Minkowski object, or the distribution of star-forming regions around the nearby radio envelope of Cen A, and from the enhanced star formation seen in some satellite galaxies of active galaxies at relatively high redshift.

Parallel and somewhat independently from this more direct evidence, the study of galaxy evolution has provided the astrophysical community with challenging new questions. The availability of large-scale photometric and spectral surveys such as the 2dF and the Sloan Digital Sky Survey has made it possible to discover evidence for evolution of the stellar formation features on timescales that are very short, in cosmological terms. The paradigm thus emerging in the astrophysical community is that AGN activity could be tightly connected to these phenomena, and could be capable of affecting the evolution of stellar populations within galaxies.

Introduction

Models invoking only the central AGN to resolve the cooling flow conundrum in galaxy clusters require fine-tuning of highly uncertain microscopic transport properties to distribute the thermal energy over the entire cluster cooling core. A model in which the ICM is heated instead by multiple, spatially distributed AGNs bypasses most of these difficulties (Nusser et al. 2006). The central regions of galaxy clusters are rich in spheroidal systems, all of which are thought to host black holes and could participate in the heating of the ICM via AGN activity of varying strengths. And they do. There is mounting observational evidence for multiple AGNs in cluster environments. Active AGNs drive bubbles into the ICM. We identify three distinct interactions between the bubble and the ICM: (1) Upon injection, the bubbles expand rapidly in situ to reach pressure equilibrium with their surroundings, generating shocks and waves whose dissipation is the principal source of ICM heating. (2) Once inflated, the bubbles rise buoyantly at a rate determined by balance with the viscous drag force, which itself results in some additional heating. (3) Rising bubbles expand and compress their surroundings. This process is adiabatic and does not contribute to any additional heating; rather, the increased ICM density due to compression enhances cooling. Our model sidesteps the “transport” issue by relying on the spatially distributed galaxies to heat the cluster core. We include self-regulation in our model by linking AGN activity in a galaxy to cooling characteristics of the surrounding ICM.

Introduction

A fundamental issue when modeling the evolution of galaxies in a cosmological context is that the majority of the processes driving baryonic evolution (such as star formation, various feedback mechanisms, accretion onto supermassive black holes (SMBHs)) operate or originate on scales well below the resolution of any feasible simulation in a cosmic box. Moreover, these processes are highly nonlinear, poorly understood from a physical point of view, and approximated by means of simplified, often phenomenological, and thus uncertain subgrid prescriptions. Unfortunately, yet unsurprisingly, a number of studies have clearly demonstrated that the results of these models are heavily affected by different choices for such prescriptions (e.g. Benson et al. 2003; Di Matteo et al. 2005), or for parameter values (e.g. Zavala et al. 2008). It is fair to say that first principles or ab-initio models do not exist.

Standard SAMs, their successes and their failures

Extensive comparisons between different scenarios and data are generally conducted by means of semi-analytic modeling (SAMs) for baryons, often grafted onto gravity-only simulations for the dark matter (DM) evolution. By the definition of SAMs, the general behavior of the system is outlined a priori, and then translated into a set of (somewhat) physically grounded analytical recipes – suitable for numerical computation over cosmological timescales – for the processes that are thought to be more relevant to galaxy formation and evolution.

Introduction

The planets and the Sun together form a coupled system, the so-called solar system, which is located in a s piral arm of the Milky Way galaxy. The solar system has existed for 4.6 billion years. Its formation took only between 50 and 100 million years (Chapter 3). According to the nebular hypothesis, a large cloud of gas started to contract under self-gravity. Conservation of angular momentum led to a rotating disk. In the center of this disk mass concentrated into a so-called proto-Sun which grew larger and larger. After reaching a temperature of about 15 million K in the core, nuclear fusion processes started turning hydrogen into helium.

In the inner part of the disk, small planetesimals were formed, which by aggregating more mass became the terrestrial planets (Mercury, Venus, Earth, and Mars). The release of potential energy and the impact of particles produced molten spheres causing a chemical differentiation with denser material sinking to the center and with a loss of volatile components. In the outer disk, lower temperatures prevailed allowing the aggregation of volatile matter such as ices and gases. The result was several larger planets with lower densities (Jupiter, Saturn, Uranus, and Neptune). For a more detailed discussion of the formation and evolution of stars and their planets we refer to Chapter 3.