Advanced Lectures on the Starburst-AGN Connection

The following sections are included:

• Contents

• Preface

• List of Participants

From the political posters (see Fig 1) in the streets of Tonantzintla, no attendee at the summer school can have failed to notice that there is an election taking place. The opening of this advanced school seems a good time to provide the background to this election…

Continuum and emission-line variability of active galactic nuclei provides a powerful probe of microarcsecond scale structures in the central regions of these sources. In this contribution, we review basic concepts and methodologies used in analyzing AGN variability. We develop from first principles the basics of reverberation mapping, and pay special attention to emission-line transfer functions. We discuss application of cross-correlation analysis to AGN light curves. Finally, we provide a short review of recent important results in the field.

In recent years researchers have begun to realize that the classification system used for active galaxies requires some changes. “Unification” refers to the merging of two or more AGN subclasses into a single subclass, usually by understanding that inclination effects or some other relatively uninteresting parameter controls how the object is classified. Once these superficial effects are understood we can get past them to the real physics of the AGN. This chapter describes the current state of unification of several common subclasses of AGN, with a view to showing what researchers can look for and avoid in classifying and understanding these objects.

The physics of the ionized gas in active galactic nuclei (AGN) is governed by the source luminosity, the spectral energy distribution (SED) and the gas density and distribution. Photoionization is the dominant heating and excitation mechanism throughout the central ~ 1 kpc except for nuclear star-forming regions that are dominated by local heating processes. The physics of “pure” star-forming regions is very different. The energy input is mostly mechanical and the micro-physics of the ionized gas is governed by collisional processes. This review gives a short account of the steps required to construct photoionization and collisional models and apply them to three observed “regions” in AGN: the broad line region (BLR), the narrow line region (NLR) and the highly ionized gas (HIG). It also discusses the physical conditions in a typical starburst region. Specifying the gas distribution and properties in AGN reproduces the observed properties of all three regions, although extra ingredients, such as dust, are sometimes required. The emergent model is that of a “messy” AGN, with gas of different properties at almost all regions. The last part of this review deals with the AGN-starburst connection. It shows that both are intimately related in luminous AGN as well as in several ultra luminous IR galaxies (ULIRGs). New spectral tools are required to understand the physics of such objects. The most useful diagnostics for distinguishing accretion dominated from starburst dominated sources are likely to be found in the X-ray part of the spectrum.

The UV-X continuum, the X-ray spectral features, and the variability in these bands provide powerful tools for studying the innermost regions of AGNs from which we gain an insight into the accretion process. In this chapter the discussion focusses on luminous AGN, i.e. Seyfert galaxies and quasars. The standard accretion disk model (a stationary geometrically thin disk) is described, and vertically averaged solutions for the radial structure are given. The emission of the standard disk is discussed using different approximations, and it is compared to the observations. This leads to the conclusion that more complex models are required, such as the irradiated disk and the disk-corona models. The advantage of this last model is that it explains the overall UV-X spectral distribution. In the framework of these disk models, the profile, intensity, and variability properties of the X-ray iron line can be explained by reprocessing at the surface of the cold disk very close to the black hole (the “relativistic disk model”). An alternative possibility is discussed, where the UV-X continuum is produced by a quasi-spherical distribution of dense clouds surrounded by (or embedded in) a hot medium. In such a model the iron line profile could be due to Comptonization instead of relativistic effects.

Active Galactic Nuclei are fueled from material (gas or stars) that are in general far away from the gravitational influence of the central black hole, the engine thought to be responsible for their activity. The required material has a lot of angular momentum that, a priori, is quite difficult to evacuate. The various dynamical mechanisms that may play a role in this game are reviewed, including m = 2 perturbations (bars and spirals), m = 1 perturbations (spirals, warps, lopsidedness), and tidal interactions between galaxies and mergers. In the latest stages of the merger, a binary black hole could be formed, and its influence on the dynamics and fueling is discussed. Starbursts are often associated with AGN, and the nature of their particular connection, and their role in the nuclear fueling is described. Evolution of the fueling efficiency with redshift is addressed.

I will discuss the evidence for the presence of young massive stellar clusters in the central regions of AGN. Some of the possible consequences of massive nuclear Starbursts in AGN will also be discussed

I review the most recent results on the cosmological evolution of the QSO population. I focus on the ensemble average properties of the QSO population including their space density/luminosity, clustering, galaxy environment and host galaxies. I show that the cosmological evolution of QSOs closely parallels that of galaxies and that many of the properties of QSOs are consistent with a picture in which QSO activity may be linked to the formation of galaxy spheroidal components in the early Universe.