The general topic of galaxy evolution is enjoying widespread attention in the literature. The aim is to answer, through observations and numerical modelling, how galaxies have evolved from the earliest stages of the Universe, or from a subsequent epoch of formation, to the shape in which we observe them in the local Universe. The two main strands in this wide topic are cosmological evolution of galaxies, which deals with their formation and early evolution, and secular evolution, with which we mean the internal evolution of galaxies, under the influence of the dynamical actions of, e.g., bars or spiral arms. Secular evolution, as comprehensively reviewed by Kormendy & Kennicutt (2004), is a relatively slow process compared to the more rapid evolution undergone by galaxies in the early Universe, the latter often due to mergers and galaxy-galaxy interactions.
Because the early stages of galaxy formation and evolution are hard or sometimes impossible to observe due to the combined effects of distance, redshift, and dust extinction, the detailed study of nearby galaxies is one of the very few ways to confirm the detailed predictions of models of large-scale galaxy formation and evolution. This kind of study can be nicknamed `galactic palaeontology', because in local galaxies we study the `fossil record' of billions of years of galaxy evolution - both cosmological and secular. To read and interpret this fossil record, a combination of many different observational and interpretational techniques must be used, from observations of individual stars in our own Milky Way and the most nearby galaxies to photometric and kinematic observations across many different wavelengths in external galaxies, all combined with a wide range of interpretational, analytical and numerical tools.
One of the major drivers of the internal evolution is the flow of gaseous material, from the disk to the central regions of the galaxy. To move inwards, rotating gas must lose angular momentum, and it can do so by shocking and under the influence of a non-axisymmetric potential. That in turn gets set up by a bar, by interactions or minor mergers, or even by less obvious deviations from axisymmetry like spiral arms, ovals, or lenses (e.g., Schwartz 1984; Shlosman et al. 1989, 1990; Knapen et al. 1995; Kormendy & Kennicutt 2004; Comerón et al. 2010). Bars are very common in galaxies, about two thirds of local galaxies have a bar (de Vaucouleurs et al. 1991; Sellwood & Wilkinson 1993; Moles et al. 1995; Ho et al. 1997; Mulchaey & Regan 1997; Hunt & Malkan 1999; Knapen et al. 2000; Eskridge et al. 2000; Laine et al. 2002; Laurikainen et al. 2004a; Menéndez-Delmestre et al. 2007; Marinova & Jogee 2007; Sheth et al. 2008; Laurikainen et al. 2009). As we will see below, in Sect. 3, nuclear rings can occur in unbarred galaxies, and they seem to prove that the non-axisymmetry induced by a spiral or oval may well be enough to induce gas inflow, and thus lead to secular evolution. Although most galaxies are barred, secular evolution is thus not dependent on the presence of a bar.
Galactic interactions are rare, occurring in only about 2% of local
galaxies (up to 4% if merely bright galaxies are considered;
Knapen & James 2009).
Close companions to local galaxies are much more common, and
Knapen & James (2009)
found that some 15% of local
galaxies have a companion not more than 3 mag fainter than itself
within a radius of five times the diameter of the galaxy under
consideration, and within a range of
± 200 km s-1 in systemic velocity. The
effects on the star formation rate in galaxies of the presence of a
close companion, and even of interactions, is, perhaps surprisingly,
limited (observations by
Bushouse 1987;
Smith et al. 2007;
Woods & Geller 2007;
Li et al. 2008;
Knapen & James 2009;
Jogee et al. 2009;
Rogers et al. 2009;
Ellison et al. 2010,
corroborated by numerical simulations by
Mihos & Hernquist
1996;
Kapferer et al. 2005;
Di Matteo et al. 2007,
2008;
Cox et al. 2008).
Statistically, the star formation rate is raised by a factor of just
under two by the presence of a close companion, but the
H
equivalent width is hardly increased at all
(Knapen & James 2009).
This implies that even
though galaxies with close companions tend to form stars at a higher
rate, they do so over extended periods of time, and not as a burst. Even
the majority of the truly interacting galaxies in
Knapen & James's
(2009)
sample have unremarkable star formation properties. The reason
that extreme star-forming galaxies, such as ultra-luminous infrared
galaxies (ULIRGs) are found to be almost ubiquitously interacting must
surely be a selection effect. In general, interactions do not always
cause starbursts, and starbursts do not always occur in interacting
galaxies.
In fact, Knapen & James (2009) used their sample of 327 nearby disk galaxies to explore how one might best define the term `starburst'. They concluded that none of the definitions that are in common use in the literature can be considered to be objective and generally discriminant. For instance, selecting galaxies on the basis of their high star formation rate yields large star-forming disk galaxies. Selecting those with high equivalent widths (apparently a bona fide starburst discriminator as this selects galaxies with a much enhanced current star formation rate as compared to the average rate in the past), yields primarily late-type galaxies of very small mass, whose star formation activity is caused by one or a few H II regions (and which will have very low impact on the intergalactic medium through, e.g., stellar winds). And selecting galaxies with the shortest gas depletion timescales does not only select galaxies with very high current star formation rates, but also gas-poor early-type galaxies with a very small star formation rate. The conclusion of Knapen & James (2009) is that starbursts are very hard to define properly, and the use of the term should be restricted to well-described small numbers of objects.
This review deals with aspects of secular evolution, and how we can trace its actions back through the detailed study of structural components, particularly bars and rings, in nearby galaxies. The interpret the effects of these agents, the tools we will use here are primarily optical and infrared imaging and two-dimensional kinematic mapping. Other authors have presented reviews on galactic evolution, and in particular the paper by Kormendy & Kennicutt (2004) presents an authoritative review of the subject of secular evolution. We will supplement that by presenting selected recent results on bars and rings that highlight the intricate and detailed connections that exist between the different structural components of a galaxy and the overall galactic evolution.
In Section 2 of this paper, we will review how the strength of a bar is connected to many of the basic properties of the bar, such as its length, or the shape of its dust lanes. We will also see that bars are indeed connected to spirals, and how bars in S0 galaxies may be different from those in spirals. Section 3 describes how the basic physical properties of the host galaxy and, where present, its bar condition the location and morphology of a nuclear ring, thus highlighting the close physical connections between these components. Section 4 discusses how the Spitzer Survey of Stellar Structure in Galaxies (S4G; Sheth et al. 2010) will deliver the data which should allow us to make significant further progress in the study of galaxy evolution by means of detailed analyses of the stellar component in a large sample of nearby galaxies. We briefly present our conclusions in Section 5.