Astron. Astrophys. 353, 124-128 (2000)

4. Discussion and conclusion

Comparing the gas mass fraction for Cl0024+17 with the typical values of 20-30% at larger radii for nearby clusters (e.g. Böhringer 1994, David et al. 1995b, White & Fabian, 1995) we find that the 6-8 keV models give quite consistent results. At temperatures lower than about 5 keV the gas mass fractions are becoming too high, larger than 35% (In the sample of White & Fabian of 19 well studied nearby clusters for example non of the clusters has an observed gas mass fraction larger than 26% and even the values extrapolated to large radii never exceed 35%). We should note, however, that the gas masses at the outer radii are obtained from largely extrapolated X-ray surface brightness profiles.

Compared to the weak lensing mass of within Mpc (Bonnet et al. 1994) and the virial mass (Schneider et al. 1986) of within Mpc the mass deduced here from the X-ray observations is lower by a factor of 4. This discrepancy cannot be reconciled by a moderate increase of the gas temperature, we would rather have to make this the hottest cluster ever observed to obtain consistency. The X-ray gas temperature has actually been determined from ASCA observations by Soucail et al. (1999) with some uncertainty due to contaminating sources yielding keV. Note, that in comparing with the result on large scales by Bonnet et al., we have extrapolated the gas density profile from the observed outer radius of Mpc to Mpc. Since a temperature increase at large radii is phyically unlikely (see e.g. Markevitch et al. 1998) a larger mass could be obtained if the slope of the gas density profile steepens significantly. A steepening of the value by about a factor of 1.5 from the small value observed at small radii is not impossible. On the other hand the result by Bonnet et al. (1994) is derived on the assumption of spherical symmetry up to large radii. Clumping in the mass distribution can help to reduce the mass required to reproduce the observations. Another source of uncertainty is the assumed redshift of the lensed objects. All these effects could reduce but not remove the discrepancy between the two results.

A comparison with the central lensing masses is more encouraging. The lensing mass from the strong lensing model of Kassiola et al. (1992) and Smail et al. (1997) with kpc and the weak shear estimate by Smail et al. (1996) with kpc are roughly consistent with the upper limit of the X-ray results, while the result of Tyson et al. (1998) is a bit higher with kpc . Since in these models the most probable distance to the source was generally assumed to be slightly lower than now measured the masses reduce insignificantly for our discussion by of the order of 10%. Broadhurst et al. (1999) who also applied a lens model for a mass estimate with the newly measured arc redshift find kpc very similar to the earlier results by Kassiola et al. (1992) and Smail et al. (1997).

While the X-ray mass may be consistent with the mass of the cluster core, there could be much more mass in an unrelaxed state surrounding the cluster. Thus the cluster could well be a somewhat scaled-up version of the Virgo cluster for which a core mass of has been deduced from X-ray observations but a much larger mass is indicated by the large diffuse and irregular X-ray halo (Böhringer et al. 1994) and a mass of is deduced from the Virgo infall velocity.

The most interesting morphological result is the small core radius of the cluster. In some cases equally small core radii in the X-ray surface brightness have been measured for other massive clusters with cooling flows (e.g. Perseus (Schwarz et al. 1992) or some of the the clusters analyzed by Durret et al. 1994 and Mohr et al. 1999). In this case the central surface brightness peak is related to the mean temperature drop of the gas in the cooling flow region and does not necessarily reflect a small core radius of the cluster potential. This can for example be compared to a large sample of mostly nearby clusters analysed by White et al. (1997) in which the core radius of the gravitational potential of the clusters was estimated such that consistent image deprojection and hydrostatic solutions were obtained. For none of the clusters a core radius smaller than 100 kpc was implied (the only exception in the sample is the radio galaxy Fornax A which is not a proper cluster). Smaller core radii for the distant lensing clusters have on the contrary often been implied by lensing studies (e.g. Miralda-Escude 1991, Mellier et al. 1993). In the present case we do not expect a significant influence of central cooling. The small core radius is therefore most certainly reflecting the shape of the gravitational potential. The clusters with small core radii and cooling flows usually have dominant, central cD galaxies, which is also not found for Cl0024+17. Therefore it is very assuring that we recover a very similar core radius as the lensing models of kpc, while Tyson et al. (1998) find kpc and Smail et al. (1996) find kpc. It is probably this small core radius of the gravitational potential rather than the overall mass which makes Cl0024+17 such a spectacular gravitational lens.

© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999
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