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Astron. Astrophys. 358, L1-L4 (2000)

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5. Discussion

Although we have detected a number of Ly[FORMULA] emitters in the field of 1138-262, from the present data it is impossible to unambiguously determine whether a significant fraction of these objects form part of the presumed cluster around 1138-262. Note that the observations described here can only detect galaxies with bright Ly[FORMULA] emission lines not attenuated by dust and these might comprise merely a small fraction of the galaxy content of the presumed cluster. First, of the whole population of LBGs as studied by S99 only 20% have a Lya rest EW 20 Å. About 50% of the LBGs have the Lya region even in absorption. Second, only a fraction of the cluster galaxies are likely to be actively forming stars. We assume that the ratio of early to late type galaxies in the cluster around 1138-262 is the same as the 1.1:1 ratio in intermediate redshift ([FORMULA]) clusters as found by Andreon (1998). Taking these two factors into account, the detected Ly[FORMULA] galaxies might comprise only 10% of the cluster galaxies present around 1138-262. In addition to this, we miss galaxies emitting Ly[FORMULA] outside the limited range of wavelength covered by our narrow band filter. Assuming that the radio galaxy is at rest with respect to the cluster, the wavelength of Ly[FORMULA] from galaxies with a positive velocity greater than 860 km s-1 falls outside the FWHM of the filter. Since present day clusters have velocity dispersions as large as [FORMULA] km s-1 (Mazure et al. 1996), we miss 15% of the velocity range of the presumed cluster around 1138-262 if we assume the same velocity dispersion for clusters at high redshift.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000