Morphology is the key to understanding secular processes. Galaxies are susceptible to internal instabilities or component interactions that can affect a galaxy's basic structure and slowly change it over time. Even subtle interactions with the environment can produce long-term secular evolution (Kormendy & Bender 2012). Relevant questions are like this: when you see a galaxy of type SA(s)b or SB(r)c, etc., has the galaxy always had this type, or have these types evolved from other types? Is there a specific direction of evolution, e.g., from late to early type, from barred to nonbarred, from pseudoringed to ringed, from spiral to nonspiral? What guidance can we get from theory regarding these questions?
It is not hard to find speculative examples of possible morphological evolution. For example, Fig. 1 shows two morphologically similar galaxies that differ in a few ways: one (NGC 3351) is a clear intermediate-type spiral while the other (NGC 2859) is a `late' S0; the spiral has very little bulge while the S0 has a more prominent bulge; the bar in the S0 looks weaker than that in the spiral, while the rings in the spiral are better described as pseudorings compared to what is seen in the S0. Looking at these two galaxies, one might wonder if a galaxy like NGC 2859 might be a possible end-product of some long-term evolutionary process in a galaxy like NGC 3351. Indeed, it was from examining such possible relationships that Kormendy (1979) first proposed the idea that secular evolution takes place in barred galaxies: he noticed a special relationship between bars and features called `lenses' that suggested to him that bars may dissolve over time into a more axisymmetric state, the engine of dissolution being an interaction between the spheroidal component and the bar. This idea was not far off the mark: Bournaud & Combes (2002) examined bar dissolution and rejuvenation in models with and without external gas accretion. In the models without accretion, the bar evolves to a lens-like structure.
Figure 1. Does the similarity between these two galaxies, one an intermediate-type spiral and the other a late S0, imply an evolutionary connection?
In examining the impact of secular evolution on galaxy morphology, we should be mindful of the intrinsic and extrinsic factors that have an impact on morphology. Here is a brief summary of these factors:
Random orientations of symmetry axes. Inclination of the symmetry plane to the line of sight, and the accompanying projection effects and enhanced influence of dust obscuration, is probably the most important extrinsic factor affecting the morphology of nearby galaxies. As a disk-shaped galaxy is viewed from a face-on orientation to an edge-on orientation, the appearance of familiar morphological features can change. For example, a bar may become so foreshortened that it is not recognisable. If a bar has significant three-dimensional structure, its face-on shape can be lost while its edge-on shape becomes its distinguishing characteristic. Rings and spiral patterns can be lost or less recognisable, although, as shown in B13, these features may still be evident even at inclinations as high as 81°.
Wavelength of observation. The influence of dust and star formation on spiral galaxy morphology has a strong wavelength dependence (Fig. 2). The blue (B) band, the historical waveband of galaxy classification studies, is sensitive to reddening and extinction by dust, and to the hot blue stars associated with star-forming regions. As wavelength increases from B to the near-infrared (IR), the dust becomes more transparent, reddening and extinction are reduced, and the influence of star-forming regions diminishes, giving spiral galaxies a smoother appearance in the red and near-IR. However, a curious thing happens in the mid-IR. In this wavelength domain, the ultraviolet energy absorbed by dust grains in star-forming regions is re-emitted strongly, and is already evident at 3.6μm by the return of the prominence of star-forming regions even as the extinction diminishes to only 5% of that in the V-band. As is shown in Lecture 3 (Section 6), 3.6 μm galaxy morphology is astonishingly similar to B-band morphology, absent the effects of extinction and reddening.
Figure 2. NGC 5364 in two passbands. The B-band emphasises dust and star formation, while the I-band is less sensitive to dust and emphasises an older, more smoothly distributed stellar population.
Total mass and luminosity. Figure 3 shows the strong dependence of galaxy morphology on total mass and luminosity. M 81 is a giant spiral having a B-band absolute magnitude of -21.1 and shows extremely organised and well-developed high-surface brightness structure. DDO 155, a dwarf having MBo = -12.1, is in contrast a very small, low surface brightness, irregular-shaped galaxy. Van den Bergh (1960a, b) and Sandage & Tammann (1981) effectively used such differences to define galactic luminosity classes (van den Bergh 1998; dVA; B13).
Environmental density, interaction, and merger history. The strong correlation between environmental density and galaxy morphology was first described by Hubble & Humason (1935) and studied in greater detail by Dressler (1980). The morphology-density relation, as it is called, is such that denser environments like rich galaxy clusters have a preponderance of early-type (E, S0) galaxies compared to lower-density environments (Fig. 4). Even in lower-density environments where spirals are abundant, such as the Virgo cluster, morphology can show evidence of an interaction with the intra-cluster medium (e.g., Koopmann & Kenney 2004; see Lecture 4, Section 7).
Figure 4. A high-density environment like the central region of the Coma cluster is rich in early-type galaxies, while spirals are more prevalent in less dense environments. The Analysis of the Interstellar Medium in Isolated Galaxies (AMIGA) project is described by Verdes-Montenegro et al. (2005).
Star formation history. Galaxies that formed all of their stars many Gyr ago tend to look very different from those that did not. Galaxies which are not currently forming any stars are redder, smoother, more centrally concentrated, and more symmetric than those which are. The bluest normal galaxies are Magellanic spirals and irregulars.
Lookback time. Morphology can be significantly affected by the lookback time to a galaxy. When the redshift is high, the lookback time can be so great that we see an early phase of morphological evolution. Galaxies tend to have more irregular shapes and relatively small linear sizes for z ≥ 1 (see B13 and Lecture 4, Section 7).