Age

Figure 6.19. The temporal evolution of the abundances of several chemical elements in the gas of an elliptical galaxy with luminous mass of 1O11M0. Feedback effects are taken into account in the model (Pipino & Matteucci 2004), as described in Section 6.3. The arrow pointing down indicates the time for the occurrence of the galactic wind. After this time, the SF stops and the elliptical evolves passively. All the abundances after the time for the occurrence of the wind are those that we observe in the broad-emission-line region. The shaded area indicates the abundance sets which best fit the line ratios observed in the QSO spectra. From Maiolino et al. 2006.

than 108 yr, explaining in a natural way the standard emission lines observed in high-z QSOs. The predicted abundances could explain the data available at that time and solve the problem of the quasi-similarity of QSO spectra at different redshifts. Finally, they suggested also a criterion for establishing the ages of QSOs on the basis of the [a/Fe] ratios observed from broad emission lines; see also Hamman & Ferland (1993).

Much more recently, Maiolino et al. (2005, 2006) used more than 5000 QSO spectra from SDSS data to investigate the metallicity of the broad-emission-line region in the redshift range 2 < z < 4.5 and over the luminosity range —24.5 < MB < —29.5. They found substantial chemical enrichment in QSOs already at z = 6. Models for ellipticals by Pipino & Matteucci (2004) were used as a comparison with the data and they well reproduce the data, as one can see in Figure 6.19. In this figure the evolutions of the abundances of several chemical elements in the gas of a typical elliptical are shown. The elliptical suffers a galactic wind at around 0.4 Gyr since the beginning of star formation. This wind voids the galaxy of all the gas present at that time. After this time, the SF stops and the galaxy evolves passively. All the gas restored after the galactic-wind event by dying stars can in principle feed the central black hole, thus the abundances shown in Figure 6.19, after the time of the wind, can be compared with the abundances measured in the broad-emission-line region. As one can see, the predicted Fe abundance after the galactic wind is always higher than that of O, owing to the Type Ia SNe which continue to produce Fe even after SF has stopped. On the other hand, O and a-elements cease to be produced when the SF halts. The predicted abundances and those derived from the QSO spectra are in very good agreement and indicate ages for these objects of between 0.5 and 1 Gyr.

Finally, in the context of the joint formation of QSOs and ellipticals we recall the work of Granato et al. (2001), who include the energy feedback from the central AGN in ellipticals. This feedback produces outflows and stops the SF in a downsizing fashion, in agreement with the chemical properties of ETGs indicating a shorter period of SF for the more-massive objects.

6.4.6 The chemical enrichment of the ICM

The X-ray emission from galaxy clusters is generally interpreted as thermal bremsstrahb-lung in a hot gas (107-108 K). There are several emission lines (O, Mg, Si, S) including the strong Fe K-line at around 7 keV which was discovered by Mitchell et al. (1976). Iron is the best-studied element in clusters. For kT > 3 keV the intracluster medium (ICM) Fe abundance is constant and ~0.3XFe0 in the central cluster regions; the existence of metallicity gradients seems evident only in some clusters (Renzini 2004). At lower temperatures, the situation is not so simple and the Fe abundance seems to increase. The first works on chemical enrichment of the ICM even preceded the discovery of the Fe line (Gunn & Gott 1972; Larson & Dinerstein 1975). In the following years other works appeared, such as those of Vigroux (1977), Himmes & Biermann (1988), and Matteucci & Vettolani (1988). In particular, Matteucci & Vettolani (1988) started a more detailed approach to the problem that was followed by David et al. (1991), Arnaud (1992), Renzini et al. (1993), Elbaz et al. (1995), Matteucci & Gibson (1995), Gibson & Matteucci (1997), Lowenstein & Mushotzky (1996), Martinelli et al. (2000), Chiosi (2000), and Moretti et al. (2003). In the majority of these papers it was assumed that galactic winds (mainly from ellipticals and S0 galaxies) are responsible for the chemical enrichment of the ICM. In fact, ETGs are the dominant type of galaxy in clusters and Arnaud (1992) found a clear correlation between the mass of Fe in clusters and the total luminosity of ellipticals. No such correlation was found for spirals in clusters. Alternatively, the abundances in the ICM are due to ram pressure stripping (Himmes & Biermann 1988) or derive from a chemical enrichment from pre-galactic Population III stars (White & Rees 1978).

In Matteucci & Vettolani (1988) the Fe abundance in the ICM relative to the Sun, XFe/XFe0, was calculated as (MFe)pred/(Mgas)obs to be compared with the observed ratio (XFe/Xpe0)obs = 0.3-0.5 (Rothenflug & Arnaud 1985). They found a good agreement with the observed Fe abundance in clusters if all the Fe produced by ellipticals and S0, after SF has stopped, is eventually restored into the ICM and if the majority of gas in clusters has a primordial origin. Low values for [Mg/Fe] and [Si/Fe] at the present time were predicted, due to the short period of SF in ETGs and to the Fe produced by Type Ia SNe. With the Salpeter IMF they found that the Type Ia SNe contribute >50% of the total Fe in clusters. This leads to a bimodality of the [a/Fe] ratios in the stars and in the gas in the ICM, since the stars have overabundances of [a/Fe] > 0 whereas the ICM should have [a/Fe] < 0. The same conclusion was drawn and given more emphasis later by Renzini et al. (1993). More recently, Pipino et al. (2002) computed the chemical enrichment of the ICM as a function of redshift by considering the evolution of the cluster luminosity function and an updated treatment of the SN feedback. They adopted Woosley & Weaver (1995) yields for Type II SNe and the Nomoto et al. (1997) W7 model for Type Ia SNe and a Salpeter IMF. They also predicted Solar or undersolar

Figure 6.20. Observed Fe abundance and predicted Fe abundance in the ICM as a function of redshift: data from Tozzi et al. (2003)(dark circles), with a model (continuous line) from Pipino et al. (2002), in which the formation of ETGs was assumed to occur at z = 8.

Figure 6.20. Observed Fe abundance and predicted Fe abundance in the ICM as a function of redshift: data from Tozzi et al. (2003)(dark circles), with a model (continuous line) from Pipino et al. (2002), in which the formation of ETGs was assumed to occur at z = 8.

Figure 6.21. New data (always relative to Fe) from Balestra et al. (2006) showing an increase of the Fe abundance in the ICM on going from z = 1 to z = 0. Error bars refer to the 1a confidence level. The big shaded area represents the rms dispersion. From Balestra et al. (2006).

[a/Fe] ratios in the ICM. The observational data on abundance ratios in clusters are still uncertain and vary from cluster centers, where they tend to be Solar or undersolar, to the outer regions, where they tend to be oversolar (e.g. Tamura et al. 2004). So, no firm conclusions can be drawn on this point. Concerning the evolution of the Fe abundance in the ICM as a function of redshift, most of the above-mentioned models predict very little or no evolution of the Fe abundance from z = 1 to z = 0 (Pipino et al. 2002). This prediction seemed to be in good agreement with data from Tozzi et al. (2003) as shown in Figure 6.20. However, more recently, more data on Fe abundance for high-redshift clusters have appeared, showing a different behavior.

In Figure 6.21 we show the data of Balestra et al. (2006), who claim to find an increase, by at least a factor of two, of the Fe abundance in the ICM on going from z = 1 to z = 0. Clearly, if we assume that only ellipticals have contributed to the Fe abundance in the ICM, this effect is difficult to explain unless we assume that recent SF has occurred in ellipticals. Another possible explanation could be that spiral galaxies contribute Fe when they become S0 as a consequence of ram pressure stripping, and this morphological transformation might have started just at z = 1.

6.4.7 Conclusions on the enrichment of the ICM From what has been discussed before, we can draw the following conclusions.

• Elliptical galaxies are the dominant contributors to the abundances and energetic content of the ICM. A constant Fe abundance of ~0.3 ZFe0 is found in the central regions of clusters hotter than 3 keV (Renzini 2004).

• Good models for the chemical enrichment of the ICM should reproduce the iron mass measured in clusters plus the [a/Fe] ratios inside galaxies and in the ICM as well as the Fe mass-to-light ratio (IMLR = MFeICM/LB, with LB being the total blue luminosity of member galaxies, as defined by Renzini et al. (1993). Abundance ratios are very powerful tools for imposing constraints on the evolution of ellipticals and of the ICM.

• Models in which a top-heavy IMF for the galaxies in clusters is not assumed (a Salpeter IMF can reproduce best the properties of local ellipticals) predict [a/Fe] > 0 inside ellipticals and [a/Fe] < 0 in the ICM. Observed values are still too uncertain to allow one to draw firm conclusions on this point.

Acknowledgments

This research has been supported by INAF (the Italian National Institute for Astrophysics), Project PRIN-INAF-2005-1.06.08.16

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