VIRMOS: Deep Redshift Surveys with the VLT

We present the status of the feasibility study of the Visible InfraRed Multi Object Spectrograph, conducted under contract with ESO for the VLT. The aim of this study is to establish the scientific objectives, identify the opto-mechanical concepts, and predict the performances of a wide-field multi-object spectrograph dedicated to the deepest galaxy and AGN redshift surveys that will be possible with the VLT. We have identified that a concept based on a four quadrant imaging-spectrograph, using multi-slit aperture plates, is the most efficient concept in this context. The maximum number of slits 10 arcseconds in length, that can be observed simultaneously, is in excess of 800, offering a spectacular multiplexing gain in a field of 4 × 7 × 8 arcmin2, for a wavelength coverage from 0.37 to 1.8μm. We are proposing a dedicated wide-field deep redshift survey, to collect ~ 105 galaxies brighter than I = 24, in the redshift range 0.3 4 or more, in less than 200 nights at one Nasmyth focus of the VLT. The timescale of the instrument development is expected to be 4 years, with first light in early 2001, provided that the VIRMOS concept is selected by ESO early in 1997.