Abstract
A technique perfectly suited to the age of reionization is intensity mapping(IM ). When surveying a large part of the sky, if the main interest is to detect large structures, and their evolution as a function of redshift, it is not necessary to detect all galaxies, but the survey will be carried out much more quickly by making a coarse mapping, with large pixels, and detecting the intensity of each pixel, which may contain the emission of several diluted galaxies. Although this technique has already been applied continuously, gathering all radio sources, the method is being revived for HI lines, and also those of CO (the main tracer of molecular hydrogen), or CII. For the HI line, there is also a crucial challenge : to be able to detect baryonic acoustic oscillations (BAOs), which are the reflection of photon acoustic waves detected in the cosmic background by millimetric satellites (WMAP, PLANCK). The characteristic size of BAOs varies with expansion, and can provide additional information on dark matter and dark energy. Current instruments can only detect an HI galaxy at z < 0.2. Beyond that, it is necessary to use intensity maps. A first detection at z = 0.8 has shown that this is feasible. Similarly, by combining all absorbers in front of quasars at z = 2.3, it was possible to detect BAOs very clearly. Attempts are being made with the H-alpha line, a tracer of star formation. The aim is to detect the entire signal, even from objects too faint to be detected on their own. The survey then provides statistical information on the energy leaving the line. But the big problem is always possible confusion with other lines.
A promising technique is filtering, by correlating two lines, such as HI with optics (H-alpha, etc.), or CO with CII. Correlation makes it easier to get rid of foregrounds, which have no reason to be correlated. Guilaine Lagache describes an intensity map experiment currently being set up in Chile, on the APEX (Atacama Pathfinder Experiment) millimeter telescope. The aim is to observethe CII line of ionized carbon, a tracer of star formation, at 1 mm wavelength. At rest, the line is at 0.158 mm, so the map will target redshifts 5 to 6.
