Dark Energy, Dark Matter, and Cosmology
Measurements of Cluster Abundances
A crucial constraint on the formation of structure is the determination of where and when the metals were created. The latest optical and X-ray data on massive clusters indicate that most of the massive galaxies were in place and already old at z ~1 and that the Fe abundance at z~1 is similar to that at lower redshifts (Tozzi et al. 2003). However, we have no direct knowledge of of how the abundance in groups evolves or of the oxygen abundance, which is crucial to determining the Type II SN contribution to the metallicity. Only Constellation- X can give us this information.
Unlike galaxies, clusters are essentially closed boxes, providing an unbiased record of nucleosynthesis in the Universe. Thus, measurement of the elemental abundances in clusters and their evolution provide fundamental data about the origin of the elements. The distribution of the elements in clusters also reveals how the metals were transferred from stellar systems into the intergalactic medium.
While many of the emission lines from Fe, Mg, and Ne are blended with CCD resolution, they are easily resolved with the 4-eV Constellation-X spectral resolution as are the He-like triplets of all the abundant elements. The combination of high spectral resolution and high collecting area will give definitive measurements out to z~1 for O, Si, and Fe over a wide mass range allowing a true measure of the metal formation in the universe.
Recent XMM, Chandra, and ASCA analysis (Baumgartner et al. 2005, Jones et al 2004, Ettori et al. 2004) have improved our understanding of the elemental composition of the cluster gas and its evolution with redshift. While there appears to be a narrow range of cluster Fe abundances at low redshift, the ratios of Fe/Si/S/Ni are not consistent with simple supernova nucleosynthesis models nor with the abundances seen in the two other samples of old material: the damped Lyman-α galaxies and the metal-poor halo stars in the Milky Way galaxy. Further, there is a variation of Fe/O by a factor of two from cluster to cluster with no simple relationship with cluster mass (e.g., Peterson et al. 2003). These data cannot be explained by a simple model of the superposition of Type I and Type II SN (Finoguenov et al. 2003), severely testing our understanding of metal formation in the universe and the nature of supernova metal enrichment.
XMM and Chandra have measured Fe abundances out to z~1.2 (Hashimoto et al. 2004, Tozzi et al. 2003), showing that little Fe abundance evolution has occurred over the last nine billion years. Combined with the lack of star formation in massive galaxies in clusters at these redshifts (e.g. Rosati et al 2004), this shows that most of the Fe in clusters is produced at z > 2, requiring an enormous star-formation rate at very early times. However, the XMM and Chandra results are limited to the most luminous clusters at z > 0.6, and it is not clear how representative these results are. To more fully comprehend the implication of these new data requires extension to lower-mass systems, a wider redshift range, and the measurement of elements other than Fe. Constellation-X has the collecting area, angular resolution and sensitivity to probe the creation of the elements to early times.
While the recent results on cluster chemical abundances are exciting, they do not represent the bulk of the material in the Universe. Most of the cosmic baryons lie in groups of galaxies, and we have little or no information on their chemical abundance evolution. Also because most of these systems have kT~1 keV, many of the emission lines are not resolved in CCD data, leading to a wide range of published abundances (e.g., Buote et al. 2003). If we are to truly understand the evolution of most of the material in the Universe we must expand high-resolution X-ray spectroscopy to the groups allowing robust chemical abundance determinations out to z > 0.3 for a wide range of elements. Since groups are much less luminous than clusters, it will require the large collecting area of Constellation-X to obtain a large sample at moderate to high redshift.
References
Baumgartner, W., Loewenstein, M., Horner, D. J., and Mushotzky, R. F., 2005, ApJ, 620, 680Buote, D. A., Lewis, A. D., Brighenti, F., and Mathews, W. G., 2003, ApJ, 595, 151
Ettori, S., et al., 2004, MNRAS, 354, 111
Finoguenov, A., Burkert, A., and Böhringer, H, 2003, ApJ, 594,136
Hashimoto, Y, et al., 2004, A&A, 417, 819
Jones, L. R., et al., 2004, in Carnegie Observatories Astrophysics Series, Vol. 3, edited by J. S. Mulchaey, A. Dressler & A. Oemler
Peterson, J. R., et al., 2003, ApJ, 590, 207
Rosati, P. et al., 2004, AJ, 127, 230
Tozzi, P., et al., 2003, ApJ, 593, 705
