Alma and Cosmology

ALMA is a privileged instrument to tackle high redshift galaxies , due to the negative K-correction in the millimeter domain. Many dusty star-forming galaxies, invisible in the optical or NIR, will be detected easily through the peak of their emission in the FIR redshifted in the submm between z=10 and z=5. Their mass and dynamics will be determined through the CO lines, together with the efficiency of star formation. Normal intervening galaxies at all z will be studied through absorption lines in front of quasars, exploring the dense tail of the column density spectrum. CMB anisotropies could be detected at the arcsecond scale, the secondary effects (SZ, Vishniak-Ostriker) could test the re-ionization and the nature of dark energy. The detection of the SZ effect on a few arcsec scales will allow to map in detail clusters and proto-clusters. Star forming galaxies emit most of their light in the far infrared around 100 mi-crons, due to emission from dust heated by the recent stars formed (e.g. Sanders & Mirabel 1996). At high redshift, the peak of the SED (spectral energy distribution) of galaxies is moving to the submillimeter range, which is at the origin of the negative K-correction effect: it is as easy to detect the continuum emission from z = 1 and z = 10 objects. Already at the present sensitivity of submm instruments (SCUBA on JCMT, MAMBO on IRAM-30m), a few hundreds of galaxies have been detected at high redshift, and many of them are not identified in the visible domain. Above a flux of 1mJy, deep surveys detect about 1 source per arcminute (e.g. Greve et al 2004). These SMG (submillimeter galaxies) are very actively forming stars, as luminous as ULIRGs. At the moment, it is not possible to detect more modest starbursts, and in particular the typical Lyman break galaxies at redshift around 3 are not detected. With the 100 times better sensitivity of ALMA, they will all be detected, and the number of sources in deep surveys is expected to be multiplied by 2 orders of magnitude (100 objects per arcminute). For very high redshifts objects (z > 7), the sensitivity of ALMA will be larger than in the visible/NIR, larger than for the JWST.