|Annu. Rev. Astron. Astrophys. 2005. 43:
Copyright © 2005 by Annual Reviews. All rights reserved
A number of conclusions are now clear from the analysis of the identified sources in the CIB:
Although these findings are answering the basic questions about the sources that make up the CIB, there are still observational difficulties to be overcome to complete these answers. The SEDs of LIRGs and ULIRGs are quite variable and often not very well constrained in their ratio of far-infrared to mid-infrared or to submillimeter wavelengths. The far-infrared, where most of the energy is radiated, requires cryogenically cooled telescopes. These have small diameters and, hence, poor angular resolution and severe confusion limits for blind surveys. Establishing proper SEDs for the different classes of infrared galaxies detected either in mid-infrared (with ISOCAM at 15 µm or MIPS at 24 µm) or in millimeter-submillimeter surveys is one of the challenges of the coming decade. Making sure that no class of sources that contribute significantly to the CIB at any wavelength has been missed is an other observational challenge. The submillimeter galaxies not found through the radio-selected sources and the question of the warm submillimeter galaxies are also two of those challenges.
Multiwavelength observations of high-z infrared galaxies give a number of new insights on the galaxy formation and evolution problem. As an example, the gas masses and total masses of SMGs are found to be very high. There is a first indication that the number of such high-mass object at redshifts between 2 and 3 is uncomfortably large compared to semianalytical models of galaxy formation based on the standard hierarchical structure-formation frame. The evolution of the luminosity function is dominated by more luminous sources as redshift increases. This is surprising because the mass function of the collapsed structure is expected to be dominated by smaller and smaller objects as redshift increases.
The populations of infrared galaxies concentrated at z 1 and at z 2.5 studied so far reveal rather different type of sources. The lower redshift ones seem to be starburst phases of already-built massive, late-type field galaxies accreting gas or gas-rich companions forming the disks. We see today a rapid decrease of this activity probably associated with a dry out of the gas reservoir in their vicinity. The larger redshift ones, which are also more luminous, seem to belong to more massive complex systems involving major merging. These systems could be located in the rare larger amplitude peaks of the large-scale structures leading to massive elliptical galaxies at the center of rich clusters. The redshift distribution of these seems quite similar to the redshift distribution of quasars.
Interesting problems that are central to the understanding of galaxy formation and evolution have to be solved in the next decade:
Current observations all point in the direction of a possible strong effect of the large-scale environment and the need for models of hierarchical formation and evolution that include properly star versus gas ratio in the accreted material.
Finally the connection between the starburst phenomenon and the AGN activity is an old question still largely unresolved. Recent observations of infrared/submillimeter galaxies have reinforced the link but have not much improved our understanding of the physical link.
We are very grateful to Alexandre Beelen, Karina Caputi, David Elbaz, François Hammer, and George Helou for very useful discussions during the writing of this manuscript. We also thank Pierre Chanial for providing us the M82 SED. Finally, we warmly thank the scientific editor, who found a large number of typos and mistakes in our use of English. The reading of the paper has been significantly improved by his detailed corrections.