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Figure 6.15. Upper panel: [O/Fe] versus [Fe/H] observed in a sample of BCGs by Thuan et al. (1995) (filled circles); open triangles and asterisks are data for disk and halo stars shown for comparison. Adapted from Thuan et al. (1995). Lower panel: new data from Izotov et al. (2006). The large filled circles represent the BCGs whereas the dots are for the SDSS galaxies. Abundances in the left panel are calculated as in Thuan et al. (1995) whereas those in the right panel are calculated as in Izotov et al. (2006) (see the original papers for details). Adapted from Izotov et al. (2006).

Figure 6.16. Predicted abundances for the H II region in IZw18 (dashed lines represent a model adopting the yields of Meynet & Maeder (2002) for Z = 10~B, whereas the continuous line refers to a higher metallicity (Z = 0.004). Observational data are represented by the shaded areas. From Recchi et al. (2004).

Figure 6.16. Predicted abundances for the H II region in IZw18 (dashed lines represent a model adopting the yields of Meynet & Maeder (2002) for Z = 10~B, whereas the continuous line refers to a higher metallicity (Z = 0.004). Observational data are represented by the shaded areas. From Recchi et al. (2004).

et al. 2003). In particular, Aloisi et al. (2003) found the largest difference relative to the H II data.

Recchi et al. (2001), using chemo-dynamical (two-dimensional) models, studied first the case of IZw18 with only one burst at the present time and concluded that the star-burst triggers a galactic outflow. In particular, the metals leave the galaxy more easily than does the unprocessed gas and, among the types of enriched material, the SN Ia ejecta leave the galaxy more easily than do other ejecta. In fact, Recchi et al. (2001) had reasonably assumed that Type Ia SNe can transfer almost all of their energy to the gas, since they explode in an already hot and rarefied medium after the SN II explosions. As a consequence of this, they predicted that the [a/Fe] ratios in the gas inside the galaxy should be larger than the [a/Fe] ratios in the gas outside the galaxy. They found, at variance with results of previous studies, that most of the metals are already in the cold gas phase after 8-10 Myr since the superbubble does not break immediately and thermal conduction can act efficiently. Recchi et al. (2004) extended the model to a two-burst case, still with the aim of reproducing the characteristics of IZw18. The model well reproduces the chemical properties of IZw18 with a relatively long episode of SF lasting 270 Myr plus a recent burst of SF that is still going on. In Figure 6.16 we show the predictions of Recchi et al. (2004) for the abundances in the H II regions of IZW18 and in Figure 6.17 those for the H I region, showing that there is little difference between the H II and H I abundances, which is more in agreement with the data of Lecavelier des Etangs et al. (2004).

Figure 6.17. Predicted abundances for the H I region. The models are the same as in Figure 6.16. Observational data are represented by the shaded areas. The upper shaded area in the panel for oxygen and the lower shaded area in the panel for N/O represent the data of Lecavelier des Etangs et al. (2003). From Recchi et al. (2004).

Figure 6.17. Predicted abundances for the H I region. The models are the same as in Figure 6.16. Observational data are represented by the shaded areas. The upper shaded area in the panel for oxygen and the lower shaded area in the panel for N/O represent the data of Lecavelier des Etangs et al. (2003). From Recchi et al. (2004).

6.4. Elliptical galaxies—quasars—ICM enrichment

6.4.1 Ellipticals

We recall here some of the most important properties of ellipticals or early-type galaxies (ETG), which are systems made of old stars with no gas and no ongoing SF. The metallicity of ellipticals is measured only by means of metallicity indices obtained from their integrated spectra, which are very similar to those of K giants. In order to pass from metallicity indices to [Fe/H] one needs then to adopt a suitable calibration, which is often based on population-synthesis models (Worthey 1994). We also summarize the most common scenarios for the formation of ellipticals.

6.4.2 Chemical properties

The main properties of the stellar populations in ellipticals are as follows.

• There exist the well-known color-magnitude and color-ao (velocity dispersion) relations indicating that the integrated colors become redder with increasing luminosity and mass (Faber 1977; Bower et al. 1992). These relations are interpreted as a metallicity effect, although there exists a well-known degeneracy between metallicity and age of the stellar populations in the integrated colors (Worthey 1994).

• The index Mg2 is normally used as a metallicity indicator since it does not depend much upon the age of stellar populations. There exists for ellipticals a well-defined Mg2-ao relation, equivalent to the already-discussed mass-metallicity relation for star-forming galaxies (Bender et al. 1993; Bernardi et al. 1998; Colless et al. 1999).

• Abundance gradients in the stellar populations inside ellipticals are found (Carollo et al. 1993; Davies et al. 1993). Kobayashi & Arimoto (1999) derived the average gradient for ETGs from a large compilation of data, for which it is A[Fe/H]/Ar > —0.3, with the average metallicity in ETGs of ([Fe/H])* >—0.3 dex (from —0.8 to +0.3 dex).

• A very important characteristic of ellipticals is that their central dominant stellar population (dominant in the visual light) exhibits an overabundance, relative to the Sun, of the Mg/Fe ratio, ([Mg/Fe])* > 0 (from 0.05 to +0.3 dex) (Peletier 1989; Worthey et al. 1992; Weiss et al. 1995; Kuntschner et al. 2001).

• In addition, the overabundance increases with increasing galactic mass and luminosity, ([Mg/Fe])* versus ao, (Worthey et al. 1992; Matteucci 1994; Jorgensen 1999; Kuntschner et al. 2001).

6.4.3 Scenarios for galaxy formation The most common ideas on the formation and evolution of ellipticals can be summarized as follows.

• They formed by an early monolithic collapse of a gas cloud or early merging of lumps of gas where dissipation plays a fundamental role (Larson 1974; Arimoto & Yoshii 1987; Matteucci & Tornambe 1987). In this model SF proceeds very intensively until a galactic wind is developed and SF stops after that. The galactic wind is voiding the galaxy of all its residual gas.

• They formed by means of intense bursts of star formation in merging subsystems made of gas (Tinsley & Larson 1979). In this picture SF stops after the last burst and gas is lost via ram pressure stripping or galactic wind.

• They formed by early merging of lumps containing gas and stars in which some dissipation is present (Bender et al. 1993).

• They formed and continue to form in a wide redshift range and preferentially at late epochs by merging of stellar systems that formed early (e.g. Kauffmann et al. 1993, 1996).

Pipino & Matteucci (2004), by means of recent revised monolithic models taking into account the development of a galactic wind (see Section 6.3), computed the relation [Mg/Fe] versus mass (velocity dispersion) and compared it with the data published by Thomas et al. (2002). Thomas (1990) already showed how hierarchical semi-analytical models cannot reproduce the observed [Mg/Fe] versus mass trend, since in this scenario massive ellipticals have longer periods of star formation than do smaller ones. In Figure 6.18, the original figure from Thomas et al. (2002) is shown, on which we have also plotted our predictions. In the Pipino & Matteucci (2004) model it is assumed that the most-massive galaxies assemble faster and form stars faster than do less-massive ones. The IMF adopted is the Salpeter one. In other words, more-massive ellipticals seem to be older than less-massive ones, in agreement with what has been found for spirals (Boissier et al. 2001). In particular, in order to explain the observed ([Mg/Fe])* > 0 in giant ellipticals, the dominant stellar population should have formed on a timescale no longer than (3-5) x 108 yr (Weiss et al. 1995; Pipino & Matteucci 2004).

6.4.4 The ellipticals-quasars connection We know now that most if not all massive ETGs host an AGN for some time during their life. Therefore, there is a strict link between quasar activity and the evolution of ellipticals.

Figure 6.18. The relation [a/Fe] versus velocity dispersion (mass) for ETGs. The continuous line represents the prediction of the Pipino & Matteucci (2004) model. The shaded area represents the prediction of hierarchical models for the formation of ellipticals. The symbols are the observational data. Adapted from Thomas et al. (2002).

Figure 6.18. The relation [a/Fe] versus velocity dispersion (mass) for ETGs. The continuous line represents the prediction of the Pipino & Matteucci (2004) model. The shaded area represents the prediction of hierarchical models for the formation of ellipticals. The symbols are the observational data. Adapted from Thomas et al. (2002).

6.4.5 Thee chemical evolution of QSOs It is very interesting to study the chemical evolution of QSOs by means of the broad emission lines in the QSO region. In the first studies Wills et al. (1985) and Collin-Souffrin et al. (1986) found that the abundance of Fe in QSOs, as measured from broad emission lines, turned out to be about a factor of ten more than the Solar one, which constituted a challenge for chemical-evolution model makers. Hamman & Ferland (1992) from N v/C IV line ratios for QSOs derived the N/C abundance ratios and inferred the QSO metallicities. They suggested that N is overabundant by factors of 2-9 in the high-redshift sources (z > 2). Metallicities 3-14 times the Solar one were also suggested in order to produce such a high N abundance, under the assumption of a mainly secondary production of N. To interpret their data they built a chemical-evolution model, a Milky Way-like model, and suggested that these high metallicities are reached in only 0.5 Gyr, implying that QSOs are associated with vigorous star formation. At the same time, Padovani & Matteucci (1993) and Matteucci & Padovani (1993) proposed a model for QSOs in which QSOs are hosted by massive ellipticals. They assumed that after the occurrence of a galactic wind the galaxy evolves passively and that for massess > 1011 M& the gas restored by the dying stars is not lost but feeds the central black hole. They showed that in this context the stellar mass-loss rate can explain the observed AGN luminosities. They also found that Solar abundances are reached in the gas in no more

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