GMOS SV program 64

We propose to combine the ultra-deep optical, IR and Chandra X-ray imaging in the WHDF with the light-grasp and multiplex advantage of GMOS to attack three front-line research areas simultaneously: the epoch of galaxy formation, the origin of the X-ray background, and the search for early-type galaxies at z~1. We propose to observe up to 10 galaxy candidates with redshifts between 5 and 7. detected by R-dropout to a limit of I = 24 and a further ~20 galaxy candidates with z between 2.5 and 4.5 detected by B- and U-dropout, together with up to ~10 X-ray sources detected in a deep 75ksec Chandra observation of this same field. The magnitude limit is R = 24.5 for the B-, U-dropouts and R = 24 for the Chandra targets. We shall also simultaneously observe as many examples as possible of a brighter sample of ~30 redder early-type galaxies with R < 23.5 (including all candidate ERO's in the WHDF) to detect the `missing population' of early-type galaxies at z~1.

We propose a productive and _practicable_ observation for SV making use of the excellent images already obtained during commissioning and learning from our experience during commissioning. This will provide a critical test of GMOS's ability to detect faint objects whose exact spectral characteristics are not known a priori. The MOS observations of WHDF during commissioning were not ideal because of the use of a grating, R400, with too high a dispersion to obtain the desired sensivity and the use of the i' filter which limited the wavelength range to 700-850nm. With both the enhanced sensitivity of the R150 grating and the maximisation of the wavelength range by removing the filter, the probability of measuring object redshifts will be greatly improved. Chances of success will be further increased by biassing our selection to brighter objects, ~1 mag brighter than those used for commissioning. This will give us a good hit rate compatible with the current capabilities of the instrument and the state of the mask design software which indicate that 20-30 slits will be achievable. We plan to observe ~5 R-dropouts out of ~10 possible plus 10/20 B- and U-dropouts plus 5/10 Chandra sources plus 10/30 z~1 early types, giving a total of of 30 targets. However, we hope to increase this by careful mask design.

The pre-imaging multiband WHDF observations are excellent and a good basis for mask-making. We will use the same target centre and position angle as the commissioning observation so no further pre-imaging is required. The spectroscopy requires a 6x1800s exposure with the R150 grating using a single mask. The GMOS ITC indicates that in a 5A increment, the S/N peaks at 5 for an R=24.5 source, and is larger than 8 for the Chandra targets (R < 24) and larger than 12 for the z~1 early-types (R < 23.5). Thus we can be fairly sure to detect the ~15 targets with R < 24, while the remaining ~15 targets still have R < 24.5 and include candidates which may allow GMOS to claim the first galaxy with z > 6!