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:
- The high angular resolution which allows identification of dwarf galaxies and to better determine the faint end of the luminosity functions.
- A few 5 square degree fields, very large when compared to the Hubble Deep Field or to the half square degree field of the largest redshift surveys (ESO/Sculptor, Hawaii surveys). This field is needed to establish statistical samples by type and to confirm the relation of the correlation function amplitude with colors (Landy et al, 1996, FRV2)
- The necessity of the U band is clearly demonstrated as a unique possibility to access the starbursting population recently revealed in the ultra-violet counts (FRV2).
- The image quality in the U band with an ADC is required because it will allow to identify starbursting galaxy populations, not detectable at other wavelengths.
- Another reason in favor of an ADC with the U band is to accurately count Ly
galaxies at z 2 on a large field size. According to Shaver et al, 1996, the AGN distribution peaks at z = 2 while from Madau, 1990, the galaxy distribution peaks at z = 1; it is essential to understand the meaning of the redshift differences between the two peaks.
As a conclusion, our understanding of galaxy evolution is depends strongly on the depth of selected fields; sufficient depth is only possible in pointed mode of MEGACAM which will then bring definite conclusions in this domain.
Armand C., Milliard B. 1994, A&A 282, 1.
Cowie L.L., Songaila A., Hu E.M., Cohen J.G. 1996, AJ 112, 839.
Fioc M., Rocca-Volmerange B. 1997a, in press in Astronomy & Astrophysics (SISSA).
Fioc M., Rocca-Volmerange B. 1997b, (FRV2), submitted to M.N.R.A.S.
Landy S.D., Szalay A.S., Koo D.C. 1996, ApJ 460, 94.
Rocca-Volmerange B., Fioc M., 1996, in ``Critical dialogues in Cosmology'', Princeton, in press.
Rocca-Volmerange B., Guiderdoni B. 1990, MNRAS 247, 166
Shaver P., Wall J.V., Kellerman K.J., Jackson C.A., Hawkins, M R S, 1996 Letter to Nature, 384, 439.
Williams R.E., Blacker B., Dickinson M., Van Dyke Dixon W., Ferguson H.C., Fruchter A.S., Giavalisco M., Gilliland R.L., Heyer I., Katsanis R., Levay Z., Lucas R.A., McElroy D.B., Petro L., Postman M., Adorf H.-M., Hook R.N. 1996, AJ 112, 1335.
CFHT Information Bulletin Number 37, Semester 97II
tmca@cfht.hawaii.edu
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