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

GRIPS, Gamma-Ray Burst Investigation via Polarimetry and Spectroscopy, had been proposed in 2007 in response to the ESA Cosmic Vision call as a new-generation Compton-and-pair telescope. Though it was not selected for further study, a variety of investigations are being performed to improve the concept and to verify the performance.

With the Compton scattering being dependent on the polarization of the incoming photons, any Compton telescope is, per se, a decent polarimeter. Beyond this, such detectors can be tailored to have a particularly high polarization sensitivity by obeying some simple design principles.

Polarization is the last property of high-energy electromagnetic radiation which has not been utilized to its full extent, and promises to uniquely determine the emission processes of a variety of astrophysical sources, among them pulsars, anomalous X-ray pulsars (AXP) and soft-gamma repeaters (SGR), or gamma-ray bursts (GRB).

GRIPS would carry two major telescopes: the gamma-ray monitor (GRM) and the X-ray monitor. The GRM is a combined Compton-scattering and pair-creation telescope for the energy range 0.2–50 MeV. It will thus follow the successful concepts of imaging high-energy photons used in COMPTEL (0.7–30 MeV) as well as EGRET (>30 MeV) and Fermi (>100 MeV) but combines them into one instrument. The following deals exclusively with the GRM concept, and its capability to measure polarization at unprecedented sensitivity.

Introduction

The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne, Compton-based polarimeter, with an energy range of 25–80 keV. In the pathfinder instrument to be flown in August 2010, the detector system will employ 61 phoswich detector cells (PDC) and 30 side anticoincidence shield (SAS) detectors situated in an unbroken ring around the PDCs. The full size PoGOLite instrument will contain 217 PDCs. The previous tests and simulations of the detector system are explored in more detail in.

The 61 PDC and 30 SAS detectors must be arranged in a way which optimizes the detection efficiency of valid events, while also allowing for the virtually complete rejection of background. For this purpose, the light yield of each component of each PDC and SAS unit was measured, using radioactive sources with particles and energies to which the detector materials are most sensitive. The light yield of a detector indicates its efficiency and is given by the peak channel number in the spectrum.

Introduction

Polarization of light originating from different regions of a black hole accretion disc and detected by a distant observer is influenced by strong gravitational field near a central black hole.

Introduction

The presence of a ‘soft excess’, i.e. soft X-ray emission above the extrapolation of the absorbed nuclear emission, is very common in low-resolution spectra of nearby X-ray obscured active galactic nuclei (AGN). It is generally very difficult to discriminate between thermal emission, as expected by gas heated by shocks induced by AGN outflows or episodes of intense star formation, and emission from a gas photoionized by the AGN primary emission. However, the high energy and spatial resolution of XMM-Newton and Chandra have allowed us to make important progress in the last few years.

The photoionization signatures

The high-resolution spectra of the brightest obscured AGN, made available by the gratings aboard Chandra and XMM-Newton, revealed that the ‘soft excess’ observed in CCD spectra was due to the blending of strong emission lines, mainly from He- and H-like transitions of light metals and L transitions of Fe (see Figure 19.1, e.g).

Introduction

Despite the wealth of sources accessible to polarization measurements and the importance of these measurements, there has been a paucity of missions with dedicated instrumentation. The most recent was a measurement of the Crab at 2.6 keV and 5.2 keV by an experiment on the OSO-8 satellite in 1976. Measurements using instruments on-board the INTEGRAL satellite have reinvigorated the field of late. At soft gamma-ray energies, nonthermal processes are likely to produce high degrees of polarization.

Introduction

A recent flurry of new mission proposals has renewed interest in X-ray polarization from a variety of astrophysical sources, hopefully marking the “coming of age of X-ray polarimetry” in the very near future. The Gravity and Extreme Magnetism SMEX (GEMS) mission, for example, should be able to detect a degree of polarization δ < 1% for a flux of a few mCrab (e.g. and these proceedings). A similar detector for the International X-ray Observatory (IXO) could achieve sensitivity roughly 10× greater (δ <0.1%, Alessandro Brez in these proceedings). In this talk, based on our recent paper, we focus on the polarization signal from accreting stellar-mass black holes (BHs) in the thermal state, which are characterized by a broad-band spectrum peaking around 1 keV.