The Surface Brightness Test

The Surface Brightness Test for the expansion of the Universe

Generally accepted cosmological models are based on the interpretation of the redshift in terms of expansion of the space-time metric. The arguments to support that basic hypothesis are well known: the Hubble law, the abundance of light elements and the background radiation. However, these arguments are all indirect. The classical global tests, aimed to directly probe the nature of the metric, are still awaiting convincing or clear-cut results, or give contradictory answers (Sandage 1988; Moles 1991).

The importance of a direct testing of the metric was already stated by Tolman and Hubble, and recently emphasized by Sandage (Sandage and Perelmuter 1990ab, 1991; SP) among others. Recent developments in the understanding of elliptical galaxies now favors the Surface Brightness Test (SBT) (Kjaergaard, Joergensen and Moles, 1993, KJM). Also this test only depend very weakly on the other cosmological parameters. The test is very sensitive to the redshift, going as (1+z)(-4) in any expanding case, and as (1+z)(-1) in a non-expanding space-time. The difference in SB between the two cases amounts to 0.7 mag for the difference in redshift between z=0.03 and z=0.30.

SP proposed bright cluster galaxies to perform the SBT. We have shown that the use of Fundamental Plane would be more accurate and less sensitive to peculiarities. The higher accuracy is important since the redshift range needed will be much smaller. As a consequence problems with evolution, and technical problems related to small size and low surface brightness will be much smaller.

In order to have proper statistics, to control differences among clusters, and especially to control the universality of the FP we have defined an adequate observing strategy (see KJM). We have selected 36 clusters in 6 redshift bins from z=0.05 to 0.30. The clusters within one redshift bin serve to control the possible cluster-to-cluster differences, in principle due to evolutionary and merging processes (Bender et al 1992, Cappacioli et al. 1992). The comparison of the results at different z, will enable us to control the possible evolutionary effects and, eventually, the universality of the FP relation. Then the Tolman effect could be identified as the diffe- rences in the independent term of the (now universal) FP relation (see KJM).

We have to obtain, for each cluster, deep surface photometry for about the 12-15 brightest early type galaxies in order to determine the surface brightness and the metric size. The photometry will be done mainly in Gunn r, with some exposures in Johnson B to control the color terms in the reduction and calibration. For the nearby clusters the brightest E-type galaxies can be identified from existing work. For the more distant clusters, it will be necessary to make a survey to 1 Abell radius, to identify the target galaxies. Also low resolution spectra from 4000 to 8000AA will be obtained in order to check the galaxies for peculiarities and for membership. These data will be used also to form synthetic color indices and to evaluate the K-correction. Finally, higher resolution spectra will be obtained to determine the velocity dispersions.

The complete program comprise observations for 36 clusters, with detailed photometry, low dispersion spectroscopy and velocity dispersion determination for over 500 galaxies. We plan to observe half of the sample with telescopes in the Northern hemisphere, and half from the South. About 4-6 clusters will be observed from both hemispheres to assess the homogeneity of the data. Indeed, the main goal is the SB test for the space-time metric. However, it is also clear that the program is going to produce important side results on the evolution of galaxies in clusters, cosmical evolutionary effects, and on the spatial distribution and luminosity functions of galaxies in clusters.

We have selected clusters to fulfill the following criteria: Bautz-Morgan class II or later, Rood-Sastry types I and L are excluded. Too poor clusters are also excluded. Finally only clusters at galactic latitude |b| > 40 are considered. These criteria are adopted to avoid 1) cD dominated clusters, 2) non-virialized systems, 3) too poor clusters, 4) problems with interstellar extinction.

References: Bender, R. et al. 1992, Ap.J. 399, 462 Cappacioli et al. 1992, M.N.R.A.S. 259, 323 Kjaergaard, P., Joergensen, Moles, M. 1993, Ap.J. 418, 617 Moles,M. 1991, in The Physical Universe: The Interface between Cosmology, Astrophysics and Particle Physics, eds. J.D. Barrow, XII Autumn School of Physics, Lisboa, Springer Verlag, p. 197 Sandage, A. 1988, Ann. Rev. Astr. Astroph. 26, 561 Sandage, A., Perelmuter,J.-M. 1990a, ApJ, 350, 481 Sandage, A., Perelmuter,J.-M. 1990b, ApJ, 361, 1 Sandage, A., Perelmuter,J.-M. 1991, ApJ, 370, 455

Here you can get some nice images A1643 and A1878


Last updated 04.03.95 by per