The evolution of bulges

Regarding the bulges of spiral galaxies, one generally distinguishes between true bulges, hosted by S0-Sb galaxies, and the "pseudobulges" hosted in later-type galaxies (Renzini 2006). Generally, the properties (luminosity, colors, line strengths) of true bulges are very similar to those of elliptical galaxies. In the following, we will refer only to true bulges and in particular to the bulge of the Milky

Figure 44.6. The predicted [a/Fe] versus [Fe/H] relations for the bulge (upper curve), the Solar vicinity (middle curve) and irregular galaxies (lower curve). Data for the bulge are shown for comparison. Data for the LMC and damped Lyman-a (DLA) systems are also shown for comparison, indicating that DLA systems are probably irregular galaxies. Figure from Matteucci (2001).

Figure 44.6. The predicted [a/Fe] versus [Fe/H] relations for the bulge (upper curve), the Solar vicinity (middle curve) and irregular galaxies (lower curve). Data for the bulge are shown for comparison. Data for the LMC and damped Lyman-a (DLA) systems are also shown for comparison, indicating that DLA systems are probably irregular galaxies. Figure from Matteucci (2001).

Way. The bulge of the Milky Way is, in fact, the best-studied bulge and several scenarios for its formation have been put forward in past years. As summarized by Wyse & Gilmore (1992), the proposed scenarios are (1) the bulge formed by accretion of extant stellar systems, which eventually settled in the center of the Galaxy; (2) The bulge was formed by accumulation of gas at the center of the Galaxy and subsequent evolution with either fast or slow star formation; and (3) The bulge was formed by accumulation of metal-enriched gas from the halo, thick disk, or thin disk in the center of the Galaxy.

The metallicity distribution of stars in the bulge and the [a /Fe] ratios greatly help in selecting the most probable scenario for formation of the bulge. In Figure 44.6

we present the predictions by Matteucci (2001) of the [a/Fe] ratios as functions of [Fe/H] for galaxies of various morphological types. In particular, for the bulge or an elliptical galaxy of the same mass, for the region of the Solar vicinity, and for an irregular Magellanic galaxy (the LMC and SMC). The underlying assumption is that different objects undergo different histories of star formation, this being very fast in the spheroids (bulges and ellipticals), moderate in spiral disks, and slow and perhaps gasping in irregular gas-rich galaxies. The effect of different star-formation histories is evident in Figure 44.6, where the predicted [a/Fe] ratios in the bulge and ellipticals stay high and almost constant over a large interval of [Fe/H]. This is due to the fact that, since star formation is very intense, the bulge very soon reaches Solar metallicity thanks only to the SNe II; then, when SNe Ia start exploding and restituting Fe into the ISM, the change in the slope occurs at larger [Fe/H] than that in the Solar vicinity. In the extreme case of irregular galaxies the situation is the opposite: here the star formation is slow and when the SNe Ia start exploding the gas is still very metal-poor. This scheme is quite useful since it can be used to identify galaxies merely by looking at their abundance ratios. A model for the bulge behaving as shown in Figure 44.6 is able to reproduce also the observed metallicity distribution of bulge stars (see Matteucci & Brocato 1990; Matteucci et al. 1999). The scenario suggested in these papers favors the formation of the bulge by means of a short and strong starburst, in agreement with Elmegreen (1999) and Ferreras et al. (2003). A similar model, although updated with the inclusion of the development of a galactic wind and more recent stellar yields, is presented in Chapter 48 by Ballero et al., who show how a bulge model with intense starformation (star-formation efficiency ~20 Gyr-1) and rapid assembly of gas (within 0.1 Gyr) can best reproduce the most recent accurate data on abundance ratios and the metallicity distribution.

Previous attempts to model the galactic bulge were presented by Molla et al. (2000) and by Samland et al. (1997). Both these models, although they differ in other respects, assumed a more prolonged period of star formation than in the models discussed above, which produces [a/Fe] ratios behaving more akin to those in the Solar vicinity. Very recently, Zoccali et al. (2006) derived oxygen and iron abundances for 50 K giants in the Galactic bulge. The spectra were taken with the UVES at the VLT and have quite a high resolution (R = 45,000). These data show a longer plateau for [O/Fe] than in the Solar vicinity, with a change in slope in the [O/Fe] versus [Fe/H] relation occurring at [Fe/H] ---0.2 dex, in very good agreement with the predictions of Ballero et al. in Chapter 48. Also dynamical models by Immeli et al. (2004), who simulated the formation of galaxies from clouds with various dissipation efficiencies, can explain the bulge abundance ratios by means of a short starburst occurring when the dissipation efficiency was quite high.

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