Annu. Rev. Astron. Astrophys. 2005. 43: xxx-xxx Copyright © 2005 by Annual Reviews. All rights reserved

5. GALAXIES AT REDSHIFTS z 1.5

Analysis of the CIB in the light of the ISO observations shows that, as we go to wavelengths much longer than the emission peak, the CIB should be dominated by galaxies at higher redshifts as illustrated in Figure 4. The comoving infrared production rate needed to fill the CIB around 1mm at a redshift centered around 2.5 to 3 remains comparable to the one from galaxies detected in the ISOCAM surveys and filling 65% of the peak of the CIB. In this section we discuss the rapidly growing observational evidence that this picture is basically correct. The main source of these observations has been the SCUBA submillimeter observations at 850 µm and 450 µm (see Blain et al 2002 for a review) and observations from the MAMBO instrument on the IRAM 30-m telescope at 1.2 mm (Greve et al. 2004). The negative K-correction becomes very effective at these wavelengths, leading to an almost constant observed flux for galaxies of the same total infrared luminosity between redshifts 1 and 5. More recently, the Spitzer observatory has produced a wealth of early observations showing that this observatory will contribute much to our understanding of infrared galaxies at z 1.5.

5.1. Number Counts, Contribution to the CIB

Blank-field deep surveys combined with mapping of areas lensed by clusters lead to number counts at 850 µm down to 0.5 mJy (e.g., Smail et al. 2002; Wang et al. 2004). At 1.2 mm counts have been obtained down to 2.5 mJy (e.g., Greve et al. 2004). The number counts shapes at 850 µm and 1.2 mm are compatible with the assumption that they are made of the same population with a flux ratio F₈₅₀ / F₁₂₀₀ = 2.5. For a typical ULIRG SED, a 5mJy source at 850 µm has a luminosity of 10¹² L at a redshift of about 2.5. The large fraction of the background resolved at 850 µm (see Section 3.2) has interesting consequences. It shows very directly that if the sources are at redshift larger than 1 (as confirmed by the redshift surveys discussed below), the infrared luminosity of the sources that dominate the background is larger than 10¹² L. This is a population with a very different infrared luminosity function than the local or even the z = 1 luminosity function. The link between this population at high z, and what has been seen at z ~ 1 (as discussed in Section 4) will be done by Spitzer/MIPS observations at 24 µm. Figure 4 shows that the building on the bulk of the CIB near its peak (at 150 µm) with redshift is expected to be similar to the building of the 24 µm background when the history of the 15 and 70 µm CIB have larger contributions from redshift-1 sources. The K-correction plots (Figure 6) show for 15 µm a hump at z = 1 associated with the coincidence of the 6-9 µm aromatic features in the ISOCAM filter and a hump at the same redshift for the 24 µm MIPS filter associated with the 11-14 µm set of aromatic features in the MIPS filter. For the MIPS filter a second hump is visible at z ~ 2 that corresponds to 6-9 µm features centered on the 24 µm MIPS filter. ISOCAM galaxies contribute to about 2/3 of the energy peak of the CIB. Following the previous considerations, it is easy to understand why the remaining fraction is likely to be made of sources in the redshift range 1.5-2.5. The presently detected SMGs with luminosity 10¹² L have an almost constant flux between redshift 1.7 and redshift 2.5 at 24 µm (similar to the constant flux at 850 µm between redshift 1 and 5). The MIPS 24 µm deep surveys (e.g., Papovich et al. 2004) reach a sensitivity of 50 µJy and thus can detect all these galaxies when they are starburst-dominated. Considering the speed of the MIPS it is likely that 24 µm surveys will become the most efficient way to search for luminous starburst galaxies up to z = 2.5 and up to 3 for the most luminous ones.