Abstract

Galaxy morphology has many structures that are suggestive of various processes or stages of secular evolution. Internal perturbations such as bars can drive secular evolution through gravity torques that move gas into the central regions and build up a flattened, disk-like central bulge, or which may convert an open spiral pseudoring into a more closed ring. Interaction between individual components of a galaxy, such as between a bar and a dark halo, a bar and a central mass concentration, or between a perturbation and the basic state of a stellar disk, can also drive secular transformations. In this series of lectures, I review many aspects of galaxy morphology with a view to delineating some of the possible evolutionary pathways between different galaxy types.

Introductory remarks

A principal goal of extragalactic studies has been to understand what drives the morphology of galaxies. It is important to determine the dynamical and evolutionary mechanisms that underlie the bewildering array of forms that define the various galaxy classification schemes used today (e.g., Sandage & Bedke 1994; Buta et al. 2007), because this will allow us to establish the relationships, if any, between different galaxy types. Physical interpretations of galaxy morphology have revolved around two different domains: (1) formative evolution, where rapid, violent processes, such as hierarchical clustering and merging, led to formation of major galactic components, such as bulges, disks, haloes, and presumably, the Hubble sequence (e.g., White & Rees 1978; Firmani & Avila-Reese 2003); and (2) secular evolution, where disk material is slowly rearranged through the collective interaction of instabilities, such as bars, ovals, spirals, and triaxial dark matter haloes (Kormendy & Kennicutt 2004, hereafter KK04).

Abstract

Interstellar medium-related secular evolution of galaxies occurs through continuing infall of gas and removal of gas. Infall can happen through accretion of gas from the intergalactic medium, through interactions and mergers.

Removal of gas can happen through galactic winds, interactions and ram-pressure stripping. In these lectures I will review observational evidence that galaxies accrete and lose gas, quantify the effects and discuss some of the physics involved.

Introductory remarks

When preparing these lectures for a school on secular galaxy evolution I wondered what role the interstellar medium (ISM) plays in secular evolution. On the website of John Kormendy I found a slide which nicely summarises what secular evolution is. The slide, which is shown in John Kormendy's chapter (this volume) is a slightly updated version from the one that appeared in the Annual Review article on secular evolution by Kormendy & Kennicutt (2004). It shows the different processes that drive galaxy evolution. On top are the fast processes, protogalactic collapse and mergers, at the bottom are the slow processes. The slow processes can be subdivided in internal processes, evolution driven by bars, oval distortions and spiral structure and external processes, where evolution is driven by prolonged gas infall, minor mergers, ram-pressure stripping and galaxy harassment. Any slow process, including environmental effects, can be considered part of secular evolution. Internal secular evolution affects the ISM through rearrangement of the gas, most importantly bar-induced inflow.

Abstract

Bars play a major role in driving the evolution of disk galaxies and in shaping their present properties. They cause angular momentum to be redistributed within the galaxy, emitted mainly from (near-)resonant material at the inner Lindblad resonance of the bar, and absorbed mainly by (near-)resonant material in the spheroid (i.e., the halo and, whenever relevant, the bulge) and in the outer disk. Spheroids delay and slow down the initial growth of the bar they host, but, at the later stages of the evolution, they strengthen the bar by absorbing angular momentum. Increased velocity dispersion in the (near-)resonant regions delays bar formation and leads to less strong bars.

When bars form they are vertically thin, but soon their inner parts puff up and form what is commonly known as the boxy/peanut bulge. This gives a complex and interesting shape to the bar which explains a number of observations and also argues that the COBE/DIRBE bar and the Long bar in our Galaxy are, respectively, the thin and the thick part of a single bar.

The value of the bar pattern speed may be set by optimising the balance between emitters and absorbers, so that a maximum amount of angular momentum is redistributed. As they evolve, bars grow stronger and rotate slower. Bars also redistribute matter within the galaxy, create a disky bulge (pseudo-bulge), increase the disk scale-length and extent and drive substructures such as spirals and rings.

Abstract

Self-gravitating systems evolve toward the most tightly bound configuration that is reachable via the evolution processes that are available to them. They do this by spreading – the inner parts shrink while the outer parts expand – provided that some physical process efficiently transports energy or angular momentum outward. The reason is that self-gravitating systems have negative specific heats. As a result, the evolution of stars, star clusters, protostellar and protoplanetary disks, black hole accretion disks and galaxy disks are fundamentally similar. How evolution proceeds then depends on the evolution processes that are available to each kind of self-gravitating system. These processes and their consequences for galaxy disks are the subjects of my lectures and of this Canary Islands Winter School.

I begin with a review of the formation, growth and death of bars. Then I review the slow (‘secular’) rearrangement of energy, angular momentum, and mass that results from interactions between stars or gas clouds and collective phenomena such as bars, oval disks, spiral structure and triaxial dark haloes. The ‘existence-proof’ phase of this work is largely over: we have a good heuristic understanding of how nonaxisymmetric structures rearrange disk gas into outer rings, inner rings and stuff dumped onto the centre. The results of simulations correspond closely to the morphology of barred and oval galaxies. Gas that is transported to small radii reaches high densities. Observations confirm that many barred and oval galaxies have dense central concentrations of gas and star formation.

Abstract

The material in this article was presented in five hours of lectures to the 2011 Canary Islands Winter School. The School's theme was ‘Secular Evolution of Galaxies’ and my task was to present the underlying stellar-dynamical theory. Other lecturers were speaking on the role of bars and chemical evolution, so these topics are avoided here. The material starts with an account of the connections between isolating integrals, quasiperiodicity and angle-action variables – these variables played a prominent and unifying role throughout the lectures. This leads on to the phenomenon of resonant trapping and how this can lead to chaos in cuspy potentials and phase-space mixing in slowly evolving potentials. Surfaces of section and frequency analysis are introduced as diagnostics of phase-space structure. Real galactic potentials include a fluctuating part that drives the system towards unattainable thermal equilibrium. Two-body encounters are only one source of fluctuations, and all fluctuations will drive similar evolution. The orbit-averaged Fokker-Planck equation is derived, as are relations that hold between the second-order diffusion coefficients and both the power spectrum of the fluctuations and the first-order diffusion coefficients. From the observed heating of the solar neighbourhood we show that the second-order diffusion coefficients must scale as ˜ J1/2. We show that periodic spiral structure shifts angular momentum outwards, heating at the Lindblad resonances and mixing at corotation. The equation that would yield the normal modes of a stellar disk is first derived and then used to discuss the propagation of tightly wound spiral waves.

Abstract

What else can be said about star formation rate indicators that has not been said already many times over? The ‘coming of age” of large ground-based surveys and the unprecedented sensitivity, angular resolution and/or field-of-view of infrared and ultraviolet space missions have provided extensive, homogeneous data on both nearby and distant galaxies, which have been used to further our understanding of the strengths and pitfalls of many common star formation rate indicators. The synergy between these surveys has also enabled the calibration of indicators for use on scales that are comparable to those of star-forming regions, thus much smaller than an entire galaxy. These are being used to investigate star formation processes at the sub-galactic scale. I review progress in the field over the past decade or so.

Introductory remarks

My goal for this chapter, based on a series of lectures at the XXIII Canary Islands Winter School of Astrophysics, is to present current understanding and calibrations of star formation rate (SFR) indicators, both on global, galaxy-wide scales, and on local, sub-galactic scales. SFRs are, together with masses, the most important parameters that define galaxies and their evolution across cosmic times. Although SFR calibrations have existed, with various levels of accuracy, for many years and sometimes decades, the past eight to ten years have brought forth major progress, through cohesive, multi-wavelength surveys of nearby and distant galaxies.

Abstract

This is a summary of my lectures during the 2011 Canary Islands Winter School in Puerto de la Cruz. I give an introduction to the field of stellar populations in galaxies, and highlight some new results. Since the title of the Winter School is Secular Evolution in Galaxies I mostly concentrate on nearby galaxies, which are best suited to study this theme. Of course, the understanding of stellar populations is intimately connected to understanding the formation and evolution of galaxies, one of the great outstanding problems of astronomy. We are currently in a situation where very large observational advances have been made in recent years. Galaxies have been detected up to a redshift of ten. A huge effort has to be made so that stellar population theory can catch up with observations. Since most galaxies are far away, information about them has to come from stellar population synthesis of integrated light. Here I will discuss how stellar evolution theory, together with observations in our Milky Way and Local Group, are used as building blocks to analyse these integrated stellar populations.

Introduction

We are living in an era in which we are able to see the light of galaxies at or close to redshift ten. Many galaxies above a redshift of three are already known, and we are starting to discover their clustering properties. These galaxies must undergo an evolution which leads to the galaxies we see around us, of which we have catalogues containing millions of individuals. Understanding how this evolution has taken place is a major task that the current generation of astronomers will have to address. A major tool needed to study galaxy evolution is stellar population synthesis.