1. INTRODUCTION

Galactic bulges have been generally assumed to be simple components that, morphologically, closely resemble elliptical galaxies. First photometric decompositions of lenticular and spiral galaxies (e.g. Caon et al., 1993) established that the radial behaviour of their surface brightness followed a de Vacouleours (de Vaucouleurs, 1948) or a Sérsic profile (Sersic, 1968) with typically high n values. In the mid 90s, we discovered that bulges in late-type, spiral galaxies were smaller and displayed exponential profiles (Andredakis et al., 1995, Courteau et al., 1996, Carollo, 1999). This difference observed in the light profiles was also present in their colours, with exponential bulges displaying bluer colours than those with larger Sérsic n (e.g. MacArthur et al., 2004, Ganda et al., 2009). Despite the marked distinction in their light profiles, the variation of colour between bulges and their surrounding disks is rather smooth (e.g. Balcells & Peletier, 1994).

Our view of the location of bulges in the major scaling relations (e.g. Faber-Jackson [Faber & Jackson (1976)], Kormendy relation [Kormendy (1977)], or Fundamental Plane [Dressler et al. (1987), Djorgovski & Davis (1987)]) has also evolved over time. The sample selection biases introduced in the first studies (e.g. predominantly early-type galaxies) showed no significant differences between bulges and elliptical galaxies (e.g. Kormendy & Djorgovski, 1989, Jorgensen et al., 1996, Balcells et al., 2007). With samples nowadays including large numbers of spiral galaxies, our understanding of the situation of bulges in those relations has now drastically changed (e.g. Gadotti, 2009, Laurikainen et al., 2010, Erwin et al., 2015).

One aspect in the study of galactic bulges that has radically changed our understanding of their nature (i.e. merger-driven structures around which disks are formed) is their kinematics. While the photometric properties of some bulges already pointed to a high degree of structural similarity with disks (e.g. exponential profiles), this can only be confirmed if their kinematics also follows that displayed by disks (e.g. significant rotation and low velocity dispersions). In a pioneering study Kormendy & Illingworth (1982) investigated the degree of rotational support of a small sample of bulges compared to elliptical galaxies. Figure 1 presents an updated version, from Kormendy & Fisher (2008), of the original figure published in 1982. The figure shows that bulges display a much larger degree of rotation than the elliptical galaxies at a given apparent ellipticity. This was the first piece of evidence in the literature indicating that bulges differed dynamically from their otherwise similarly looking, slow rotating, massive early-type counterparts. While we know now that this picture is not accurate, at the time it led to the realisation that some bulges are actually disks and therefore may not have formed in merger episodes, as most scenarios would assume, but rather formed from internal material through secular processes (Kormendy, 1993). These ideas evolved over time and gave rise to the definition of pseudobulges. We refer the reader to Falcón-Barroso & Knapen (2013) for an extensive review, produced by the lecturers of the XXIII Canary Islands Winter School of Astrophysics, of bulge formation and evolution in the context of secular evolutionary processes.

Figure 1. Historical view of the level of rotational support and anisotropy of a sample of elliptical galaxies (crosses) and bulges (remaining symbols) from Kormendy & Fisher (2008). This is an updated version of the original figure presented in Kormendy & Illingworth (1982). While the physical interpretation of this figure has evolved over time, it was the first piece of evidence suggesting that bulges and massive early-type galaxies were intrinsically different.