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F. Combes (Observatoire de Paris)
The observations of high-z galaxies, to probe their formation and evolution, will make a giant step with ALMA. It is now believed that the star formation rate has increased by at least an order of magnitude between z=0 and 1, and most galaxies experienced a starburst beyond z=2. These objects will be quite easy to detect in submm continuum; their identification, plus the full study of their physics, dynamical equilibrium, mass, excitation, will be conducted with the CO lines.
With bolometers such as SCUBA or MAMBO, it is possible at present to detect about 1-2 sources per arcmin2 at high redshift (with limitations due to confusion, sensitivity, identification problems..). ALMA will not suffer confusion (with spatial res. 0.1"), and will be of the order of two orders of magnitude more sensitive than present instruments. Much more moderate starbursts, such as Lyman-Break Galaxies (LBG) which are observed today in the optical with a density of about 150 per arcmin2 at z=2.5-3.5, will then be easily detectable.
Hierarchical formation scenarios predict a larger comoving density of galaxies in the past. Until which redshift (up to 30?) galaxies can be observed? When ALMA is operational, there will exist a great number of deep fields, with known spectroscopic redshifts. Objects detected only in the submm will be identified with mm CO lines, with wide-bande receivers. CO maps will be crucial to determine the kinematics and mass of early star-forming galaxies, since their optical lines are rarely a virialised tracer.
The author(s) of this abstract have provided an email address for comments about the abstract: francoise.combes@obspm.fr