Studying bulges of spirals is not restricted to a particular class of galaxies, or even to their central regions. There is a growing body of evidence that knowledge of these bulges is crucial to understanding galaxy formation in general. Indeed, the light distribution of most large galaxies is dominated by two components, a bulge and a disk. Even very-early-type galaxies harbor a variety of luminosity profiles, which are interpreted as due to a varying contribution of a disk component (Saglia et al. 1997; de Jong et al. 2004). Along the Hubble sequence, the variation in bulge magnitude is twice that of the disk (Simien & de Vaucouleurs 1986; de Jong 1996), i.e. the properties of bulges are keys to inferring the nature of the Hubble sequence.
Despite the prospect of their yielding crucial information on galaxy formation and galactic assembly history, bulges have received significantly less attention than elliptical galaxies. This is a direct consequence of the considerable challenge of avoiding contamination from disk light. Figure 27.1 illustrates this point. The data for galaxies, nearby and face-on, are from Jablonka et al. (1996). It appears clearly that, within a fixed aperture, classically of the order R ~ 1-2 arcsec for integrated spectroscopy, it is nearly impossible to get rid of all of the disk light. Even more importantly, one can get very different bulge-to-disk light ratios from one galaxy to another, prejudicing our understanding of trends with physical quantities.
The Metal-rich Universe, eds. G. Israelian and G. Meynet. Published by Cambridge University Press. © Cambridge University Press 2008.
Nevertheless, observational efforts are intensifying, improving our vision of bulges' properties. Although no definitive certainties have emerged yet, new lines of research are now under way.
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