Dust and Molecules at High Redshift

Abstract

In the last years, progress has been very rapid in the domain of molecules at high redshift, and we know in better detail now the molecular content in several systems beyond z = 1 and up to z~ 5, either through millimeter and sub-millimeter emission lines, continuum dust emission, or millimeter absorption lines in front of quasars. The first discovery in 1992 by Brown and van den Bout of CO lines at z = 2.28 in a gravitationally lensed starburst galaxy, strongly stimulated searches of other systems, but these were harder than foreseen, and less than 10 other systems have been discovered in CO emission. Redshifts range between 2 and 5, the largest being BR1202-0725 at z = 4.69. Most of these systems, if not all, are gravitationally amplified objects. Some have been discovered first through their dust emission, relatively easy to detect because of the negative K-correction effect. The detection of all these systems could give an answer about the debated question of the star-formation rate as a function of redshift. The maximum of star-formation rate, found around z = 2 from optical studies, could shift to higher z if the most remote objects are hidden by dust.

Absorption in front of quasars can also probe cold gas at high redshift, taking advantage of very high spatial (≤ 10^-3 arcsec) and spectral (30m/s) resolutions, and sampling column-densities between N(H₂)= 10²⁰ and 10²⁴ cm^-2. Up to now four absorption systems have been discovered in the millimeter range, and up to 20 molecular lines have been detected in a single object (HCO⁺, HNC, HCN, N₂H⁺, C¹⁸O, CS, H₂ CO, CN, etc....). From the diffuse components, one can measure the cosmic black body temperature as a function of redshift. The high column density component allows observation of important molecules not detectable from the ground, like O₂, H₂O and LiH for example.

All these preliminary studies will be carried out at large scales with future millimeter instruments, and some perspectives are given.