Models with mergers

In order to check the effect of possible gaseous mergers triggering star formation during the lifetime of elliptical galaxies on their chemical and photometric properties, Pipino & Matteucci (2006) computed some cases with various merging epochs and amounts of accreted matter. In Figure 44.5 we show the predicted and observed ([Mg/Fe])V versus MV (visual magnitude) relation (the equivalent of the ([Mg/Fe])* versus mass relation) for ellipticals. The quantity ([Mg/Fe])V represents the stellar [Mg/Fe] ratio averaged over MV instead of over the mass.

Figure 44.5 contains the predictions of the best model of PM04 for galaxies of various masses, which lie well inside the area of existence of the observations, whereas the predictions of models with mergers tend to fall well outside the observed region unless the merger is unimportant. In particular, the agreement with observations worsens with increasing merged mass and consequent star formation.

Thomas et al. (1999) studied a scenario in which the formation of ellipticals occurs by merging of two spirals like the Milky Way. They concluded that this scenario fails to reproduce the a-element-enhanced abundance ratios in the metal-rich stars of ellipticals, unless the IMF is flattened during the burst ignited by the merger.

Figure 44.5. The observed ([Mg/Fe]}v versus MV for ellipticals (shaded area) compared with models without and with mergers. The big empty stars represent the best model of PM04 whereas the filled squares represent models with mergers. The squares lying further from the shaded area represent models in which mergers with large amounts of gas and consequent star formation are allowed.

Figure 44.5. The observed ([Mg/Fe]}v versus MV for ellipticals (shaded area) compared with models without and with mergers. The big empty stars represent the best model of PM04 whereas the filled squares represent models with mergers. The squares lying further from the shaded area represent models in which mergers with large amounts of gas and consequent star formation are allowed.

It is worth mentioning that recently De Lucia et al. (2006) studied the starformation histories, ages, and metallicities of ellipticals by means of the Millennium Simulation of the concordance ACDM cosmology. They also suggested that more-massive ellipticals should have shorter star-formation timescales, but lower assembly (by dry mergers) redshift than less-luminous systems. This is the first hierarchical-paper admitting "downsizing" in the star-formation process in ellipticals. However, the lower assembly redshift for the most-massive system is still in contrast to what is concluded by Cimatti et al. (2006), who show that the downsizing trend should be extended also to the mass assembly, in the sense that the most-massive ellipticals should have assembled before the less-massive ones. This is in agreement with the model of PM04, which assumes an increasing timescale for the assembly of less-massive ellipticals.

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