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`Pseudobulges' are supposed to rotate and have an exponential light profile, akin to the disk material from which they formed (Bardeen 1975; Hohl 1975; Hohl & Zhang 1979; Combes & Sanders 1981). The topic of their light profiles was already addressed in Section 2.2 with a large galaxy sample, where it was both explained why and shown that there is no physical divide at n = 2 in the structural diagrams, although see Fisher & Drory (2008) for an alternative view. It was also pointed out that the scaling relations involving the `effective' structural parameters are curved and therefore cannot on their own be used to identify different bulge (formation) type.

5.1. Rotation

Bulges have of course been known to rotate for many years (e.g. Pease 1918; Babcock 1938; Rubin, Ford & Kumar 1973).

Merger events can create rotating elliptical galaxies (e.g. Naab, Khochfar & Burkert 2006; González-García et al. 2009; Hoffman et al. 2009), and merger simulations can also create bulges which rotate (Bekki 2010; Keselman & Nusser 2012). Classical bulges can be spun up by a bar (Saha et al. 2012). Bar dynamics may give the illusion of rotation in classical bulges (Babusiaux et al. 2010). Williams et al. (2010) report that some boxy bulges, (previously) thought to be bars seen in projection (Combes & Sanders 1981), do not display cylindrical rotation as expected and can have stellar populations different to their disk. Qu et al. (2011) report on how the rotational delay between old and young stars in the disk of our Galaxy may be a signature of a minor merger event.

For the above reasons, rotation is not a definitive sign of bulges built via secular disk processes, and as such it can not be used to definitively identify bulge type.

5.2. Ages

From optical and near-IR colours, Peletier et al. (1999) concluded (after avoiding dusty regions) that the bulges of S0-Sb galaxies are old and cannot have formed from secular evolution more recently than z = 3. Bothun & Gregg (1990) had previously argued that bulges in S0 galaxies are typically 5 Gyr older than their disks. Bell & de Jong (2000) reported that bulges tend to be older and more metal rich than disks in all galaxy types, and Carollo et al. (2007) found that roughly half of their late-type spirals had old bulges. Gadotti & dos Anjos (2001) found that ≈ 60% of Sbc galaxies have bulge colours which are redder than their disks. For reference, it is noted that the average Sbc spiral has a Sérsic index n < 2 (Graham & Worley 2008).

From spectra, Goudfrooij, Gorgas & Jablonka (1999) reported that bulges in their sample of edge-on spiral galaxies are old (like in elliptical galaxies), and have super-solar α/Fe ratios similar to those of giant elliptical galaxies. They concluded that their observations favor the `dissipative collapse' model rather than the `secular evolution' model. Thomas & Davies (2006) concluded, from their line strength analysis, that secular evolution is not a dominant mechanism for Sbc and earlier type spirals (see also González-Delgado et al. 2004). MacArthur, González & Courteau (2009) revealed that most bulges in all spiral galaxies have old mass-weighted ages, with < 25% by mass of the stars being young.

Given that most bulges have old mass-weighted ages, it favours a prevalence of classical bulges, many of which may co-exist with a secular-driven pseudobulge, as advocated in Erwin et al. (2003) for the S0 population, and appears to also be the case for the Milky Way (Dékány et al. 2013).

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