Scaling relations central indices

With the exception of the Milky Way and M31, in which we can resolve individual stars, studies of bulges have to deal with integrated properties.

Spectroscopic studies of the central parts of bulges were pioneered by Bica (1988). He gave the first evidence for a relative independence of the bulges' spectral properties with respect to the morphological type of the parent galaxies. He also showed that changes in age/metallicity were linked to the luminosity of the galaxy. During the following years, a central metallicity-luminosity (Z-L) relation for bulges was more firmly established and authors stressed its similarity to the relation derived for ellipticals (Jablonka et al. 1996; Idiart et al. 1996). This similarity appears both in the slope of the Z-L relation and in similar [a/Fe] ratios. Interestingly, the authors of both of the above studies observed face-on spirals and varied their integration apertures, either by adapting their spectroscopic apertures at the time of the observations, fixing a low and constant bulge-to-disk light ratio for all galaxies, or by inspecting the light profiles along the slit width when extracting the spectra.

Authors of subsequent works sampled inclined galaxies and advocated distinctions between late-type and early-type spiral bulges. Prugniel et al. (2001) found bulges located below the Mg2-a relation obtained for ellipticals. Falcon-Barroso et al. (2002) found a 20% steeper slope than for ellipticals and S0 galaxies. Proctor & Sansom (2002) reported that small bulges (low a) depart from the relation between spectral indices and a obeyed by large bulges: while large bulges populate the same region as elliptical galaxies, the smaller ones have relatively lower spectral indices. However, Thomas & Davies (2006), by re-analyzing Proctor and Sansom's sample, showed that this apparent discrepancy vanishes when the same range of central velocity dispersion is considered for both types of system, i.e. when low-a bulges are compared with low-a ellipticals.

Figure 4 in Falcon-Barroso et al. (2002) could serve as a warning: the dispersion in data from the various studies is rather large, likely due to the various observational strategies. In particular, it is of the order of the difference claimed to exist between different types of bulges and with elliptical galaxies. Nevertheless, there are true points of convergence among the studies quoted here, which can be summarized as follows. There is a range of properties of the bulge stellar populations as sampled by their inner regions. They are related to the bulge mass or maybe even more to the total gravitational potential of the parent galaxy. Indeed, Prugniel et al. (2001) and more recently Moorthy & Holtzman (2006) found a tighter relation between Mg2 and the galaxy-rotation velocity than with the central bulge-velocity dispersion, for example. The bulge central-luminosity-weighted metallicities range from -— 0.5 to ~+0.5 dex and the luminosity-weighted [a/Fe] ranges from the Solar value to ~0.4 dex. Ages are more subject to debate, but a broad consensus would certainly be reached for a range between the age of a very old stellar population and a few gigayears younger.

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