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3.5. Future prospect for space-based Lyalpha surveys

Because of the existence of only two medium-band filters in WFPC2 suitable for finding compact emission-line objects (F410M and F467M), we are limited in the number of redshifts we could search (z appeq 2.4 and z appeq 2.85). The Cycle 5 data offered a unique opportunity to examine the z appeq 2.4 epoch, but similar observations with the F467M filter on HST can be carried out at z appeq 2.85. The scientific return from a modest investment of exposure time in the HDF in the F410M and F467M filters would be considerable, since all the broadband UBVI data is in hand. This would be an excellent direct determination of the space density of z appeq 2.4 and z appeq 2.85 subgalactic clumps. A more extensive medium-band filter set on the Next Generation Space Telescope will be needed to find these objects at z appeq 2-7. Indeed, more random observations with WFPC2 in F410M are necessary to confirm these initial findings at z appeq 2.4, and to better determine the luminosity function of the z appeq 2.4 clumps. We also note that these studies can no longer be done with HST if the WFPC2 gets turned off after installation of the ACS in 1999, since the ACS does not have the required medium-band filter set, or ramp-filters with a large enough FOV to do the exercise.