Origin of the Heavy Elements in the Central Region

With XMM-Newton and Chandra observations the information on the heavy element distribution has increased dramatically (good information on more elements, most importantly on oxygen and well resolved abundance profiles) and we can now investigate the origin of the heavy elements in more detail. The central abundance peak in cooling flow clusters with its high surface brightness and the most detailed spectra provide a special clue for this study.

We know two sources for the enrichment of the ICM with heavy elements: core collapse supernovae, type II, which produce a broad spectrum of element masses with a bias towards the lighter elements like O and Mg, and type Ia supernovae, thermonuclear explosions of white dwarfs, which dominantly yield Fe group elements and lighter elements like Si and S but very little O and Mg. Checking now which of the elements follow the central abundance peak shows that it is traced by the heavier SN Ia products but not by the lighter elements like O and Mg (Fig. 23.20, [95]. This is consistent with the picture that SN II activity happens in the early history of cluster formation when the stellar populations of the cluster galaxies are still young. These elements have time to mix well in the ICM and show a more homogeneous distribution. SN Ia are still occurring and are observed in present day cluster ellipticals and in the central cD galaxies. The more recent yields obviously lead to more local enrichments. Thus the massive stellar population of the cD galaxies dominating the centers of cooling core clusters are responsible for the central enrichment [17,41].

Fig. 23.20 Abundance profiles of O (left) and Si (right) in the M87 X-ray halo studied with XMM-Newton by Matsushita et al. [95]. The thick line symbols show the results of a more realistic two-temperature model fit (compared to a one temperature model, thin symbols) while the crosses and diamonds mark the data from the pn and MOS detectors, respectively. The Si abundance profile is also shown in the left panel as dashed line for comparison

Fig. 23.20 Abundance profiles of O (left) and Si (right) in the M87 X-ray halo studied with XMM-Newton by Matsushita et al. [95]. The thick line symbols show the results of a more realistic two-temperature model fit (compared to a one temperature model, thin symbols) while the crosses and diamonds mark the data from the pn and MOS detectors, respectively. The Si abundance profile is also shown in the left panel as dashed line for comparison

A more careful inspection of the chemical composition of the central ICM reveals also slight increases in the relative abundances of O and Mg as found, for example, for the M87 halo and the Centaurus cluster [95,96]. The ICM abundances of these elements are comparable to the stellar abundances which implies that the ICM in the very central region most probably reflects the stellar mass loss of the central galaxy. Consideration of the budget of the central ICM, for example, for the case of M87 shows that the inner 10 kpc region of the ICM, roughly the region of the scale radius of the cD galaxy (or approximately half light radius), contains a gas mass of about 2 x 109 M0 and an iron mass of about 6 x 106 M0. Both can be replenished by stellar mass loss from M87 if we adopt a stellar mass loss rate of 2.5 x 10-11 x LB M0 yr-1 (Ciotti et al. [36]) and a Fe production with a supernova rate of 0.15 SNU (Cappelaro et al. [31]). It is important, however, for this enrichment to happen that the gas is not cooling and condensing with the classically calculated cooling flow rates, since in this case the central ICM produced by stellar mass loss would disappear faster than it can be replenished. Both, the evidence described in the previous section and the observed abundances of O and Mg support the notion that the central ICM is not condensing at the high rates inferred previously.

The central abundance peak extends much further than the central 10 kpc region inside the central galaxy. Since the central galaxy dominates the stellar population out to radii of the order of 100 kpc. It is thus responsible for most of the secular heavy elements in this zone. This implies some transport of the centrally enriched ICM to larger radii and larger enrichment times than inferred above for the 10 kpc region. Calculations for the observations of four nearby cooling core clusters (M87, Centaurs, Perseus, A1795), with constant SN Ia rate and alternatively with an increasing SN Ia rate in the past, imply very large enrichment times of 5-9 Gyrs for the region inside ^ 50 kpc and 7-12 Gyrs for <100 kpc. If these estimates are correct the results imply that the central regions of the cool core clusters and their ICM can not have suffered major disturbances during the past 10 Gyrs [17]!

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