CFHT Information Bulletin Number 37, Semester 97II

Ultra-deep multispectral surveys for evolution of galaxies with MEGACAM

Brigitte Rocca-Volmerange, Institut d'Astrophysique de Paris, (rocca@iap.fr)

A brief status on galaxy evolution scenarios

Previous analyses of galaxy counts in the visible were firstly based on scenarios of galaxy evolution in which star formation begins early in building blocks while other processes as radio-jets of AGN are responsible for current star formation (see Rocca-Volmerange and Guiderdoni, 1990). These scenarios would imply the evolution of the comoving number of blocks as (1 + z)n with n 1.5. Such a merging factor was not confirmed by recent observations with the HST ( Griffith et al, 1996). Moreover the resulting past star formation history is in excess compared to the classical diagram derived from chemical evolution (Madau, 1990). Our new evolution model PEGASE (Fioc and Rocca-Volmerange, 1997a) continuously fits the galaxy energy distribution of 8 spectral types from the far-UV to the near-IR. Applied to modeling faint counts (Rocca-Volmerange and Fioc, 1996), these main conclusions are confirmed from the visible and the near-infrared. However a new population of faint starbursting galaxies is discovered by Fioc and Rocca-Volmerange, 1997b (FRV2) in the far-UV counts of the FOCA2000 balloon experiment (Armand and Milliard, 1994). As a direct consequence, the detailed star formation scenarios of galaxies have to be revised in detail and need statistical samples between 0 £ z £ 2 before any conclusive results on star formation rates can be obtained. Moreover the detailed history of star formation by type is an essential condition to approach the cosmological parameters.



Perspectives with MEGACAM

Ultra-deep multispectral (UV to visible) surveys, down to V = 29 of about 5 square degrees with MEGACAM will be unique to follow the main signatures of star forming galaxy evolution by spectral type on a long look-back time. Essentially in a pointed mode, these surveys are significantly deeper than most redshift surveys (about V = 26 - 27). As shown in our last analyses, faint counts N(m), color distributions N(C) and the color dependence of the amplitude of the correlation fonction A(w) are the main output data (Figures 11 and 12), able to solve the puzzling question of evolution scenarios. The main advantages of MEGACAM for that basic programme are:

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Fioc M., Rocca-Volmerange B. 1997b, (FRV2), submitted to M.N.R.A.S.

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CFHT Information Bulletin Number 37, Semester 97II

tmca@cfht.hawaii.edu
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