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Astron. Astrophys. 353, 498-506 (2000)

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5. Discussion

In a recent work, Wu et al. (1999) have used the largest sample of clusters of galaxies with good broad band X-ray spectroscopy from the literature to discuss the [FORMULA] and the [FORMULA] relationships 3. Their sample is mainly composed by clusters at [FORMULA] (only 5 clusters out of 142 used are at higher redshift). By comparing the clusters at [FORMULA] with those at [FORMULA] they do not find convincing evidence for a significant evolution in the [FORMULA] and the [FORMULA] relationship out to [FORMULA], a result which was first pointed out by Mushotzky & Scharf (1997).

As summarized in Table 4, distant cluster temperatures which have been measured to date are limited to a few systems at [FORMULA]. At these redshifts the lookback time approaches half the age of the Universe and, therefore, the time leverage to measure evolution in cluster properties is large.


[TABLE]

Table 4. Clusters at [FORMULA] with a temperature measurement.
Notes:
a) For consistency with the other measurements we have reported in this column the 90% confidence range.
Ref :
WXF: Wu et al. (1999); M+S: Mushotzky & Scharf (1997); D96: Donahue (1996); D98: Donahue et al. (1998); G99: Gioia et al. (1999); D99: Donahue et al. (1999); HB98: Hughes & Birkinshaw (1998); H97: Hattori et al. (1997).


In Fig. 7, we plot the high redshift cluster temperatures known to date in the [FORMULA] plane. The error on the temperature represent the 90% confidence range, while we have used a realistic 15% absolute error for the bolometric luminosity. Since MS 2053.7-0449 and AXJ 2019-1127 have very large errors on the measured temperature, we have not reported these objects in Fig. 7.

[FIGURE] Fig. 7. Clusters at z [FORMULA] 0.5 with a temperature measurement (see Table 4). The filled square represents the Beppo SAX measured temperature of RXJ 0152.7-1357 reported in this paper. The objects have been labeled in order of increasing redshift. Error bars on the temperature represent the 90% confidence range, while error bars on the bolometric luminosity represent a realistic 15% absolute error. The solid line represents the derived [FORMULA] relationship obtained by Wu et al. (1999) using a sample of 142 clusters from the literature; the dotted lines represent the [FORMULA] scatter on the slope. The short dashed line represents the evolving [FORMULA] relationship with [FORMULA] at [FORMULA], while the long dashed line represents the evolving [FORMULA] relationship with [FORMULA] at [FORMULA]. See Sect. 5 for details.

The [FORMULA] relationship for clusters at [FORMULA] obtained by Wu et al. (1999) ([FORMULA]) is also shown (solid line), together with the [FORMULA] on the slope (dotted lines). The short-dashed line represents the evolving [FORMULA] relationship with [FORMULA] [FORMULA] at [FORMULA], while the long dashed line represents the evolving [FORMULA] relationship with [FORMULA] [FORMULA] at [FORMULA]. The two redshifts enclose all the objects shown in Fig. 7. For A1 and A2 we have assumed the values 1 and 3, respectively; these two values have been determined by Borgani et al. (1999), and represent the 90% confidence level required to fit the lack of observed evolution of the XLF in the RDCS cluster sample in a [FORMULA] = 1 universe. Low density models, instead, can easily be accommodated with a non-evolving [FORMULA] relation, or mild ([FORMULA]) evolution. The new cluster temperature we have determined for RXJ0152.7-1357 is not consistent with a strong evolution of the [FORMULA] relation out to [FORMULA]. Since all the data points in Fig. 7 lie to the right of the [FORMULA] line, according to the parameterization of Borgani et al. (1999), the cluster temperatures measured so far at [FORMULA] lend considerable support to cosmological models with a low density parameter. Similar results have been recently obtained by Donahue et al. (1999) and by Donahue & Voit (1999). Using a complete sample of high redshift EMSS clusters, Donahue et al. (1999), have shown that the cluster temperature function reveals modest evolution, a result which implies a low [FORMULA] value (Donahue & Voit 1999).

The metal abundance of the ICM in rich clusters of galaxies has been recently investigated by Mushotzky & Loewenstein (1997). They found that the Fe abundance shows little or no evolution out to [FORMULA] ([FORMULA]), suggesting that most of the enrichment of the ICM occurred at [FORMULA]. Given the present uncertainty on the ICM abundance in RXJ 0152.7-1357 we cannot set strong constraints on the cosmological evolution of the Fe abundance. However, within the large uncertainty ([FORMULA]; 68% confidence interval), these data suggest that the bulk of the Fe enrichment was completed by [FORMULA].

Finally it is worth comparing RXJ 0152.7-1357 with MS1054.4-0321. Provided that temperature measurements are not biased by cooling flows, strong deviations from isothermality or by the presence of contaminating AGN, it is interesting to note that the temperature of MS1054.4-0321 is significantly higher than that of RXJ 0152.7-1357, although the two clusters have very similar luminosities. With the present X-ray data we are unable to discuss any further the difference in temperature between these two distant clusters which have nonetheless many similarities. The highly spatially resolved X-ray spectroscopy and high throughput that Chandra and XMM will provide are needed to clarify this problem as well as to study in detail distant clusters of galaxies.

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Online publication: December 17, 1999
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