Deep images of blank fields at 6.75 and 15 µm done with ISOCAM provide a different view on the distant universe. While the ISOCAM-LW3 filter, centered at 15 µm , can probe dust emission up to z ~ 2 for luminous objects, the redshift range to probe star formation from dust emission with the ISOCAM-LW2 band, centered at 6.75 µm , is limited to the relatively nearby universe due to k-correction. However the emission of the old stellar component, which peaks in the near IR, being brought to this wavelength range for high-z galaxies, their stellar masses are better constrained from this flux density.
Sato et al. (2004) derived stellar masses around M* ~ 1011 M for galaxies with z ~ 0.2-3 from the correlation between rest-frame near IR luminosity and stellar mass. The stellar mass-to-light ratios were derived from model fit of the set of observed magnitudes depending on the galaxies star formation histories.
The contribution of the 6.75 µm selected galaxies to the cosmic stellar mass density of galaxies per unit comoving volume as a function of redshift were estimated to be comparable to those inferred from observations of UV bright galaxies (see Fig. 3). Given the narrow mass ranges, these estimates were obtained as simple summations of the detected sources and should therefore be considered as lower limits only. The faint 6.7 µm galaxies generally had red colors. A comparison with a particular population synthesis model suggests that they have experienced vigorous star formation at high redshifts. The derived large stellar masses for the faint 6.7 µm galaxies also support such star forming events at the past. However these events can only be detected at longer wavelengths using the 15 µm ISOCAM or 90 and 170 µm ISOPHOT bands with ISO (or SCUBA at 850 µm and now the Spitzer Space Telescope). The next sections are devoted to these surveys and their consequences on our understanding of the history of star formation in the universe.
Figure 3. (left) ISOCAM 6.75 µm image of the SSA13 field (Fig. 2 from Sato et al. 2003, 23 hour observation). This image reaches an 80% completeness limit of 16µJy in the central 7 arcmin2 region. North is top and east to the left in J2000. The center is (RA, Dec) = (13h 12m 26s, 42° 44' 24".8). The map shows signal-to-noise ratio (S/N) per 0.6 arcsec sub-pixel. The signal is an average, weighted by the inverse of the assigned variance, and the noise is a normalized standard deviation. (right) Stellar mass density in the universe as a function of redshift (Fig. 13 from Sato et al. 2004). The right axis shows densities normalized to the critical density of the universe. The contributions of the 6.75 µm galaxies are shown with solid and double circles for the "combined" (including photometric redshifts) and purely spectroscopic samples, respectively. The double circles are plotted at slightly lower redshifts. The horizontal bars represent the redshift ranges of the bins and the vertical bars show one sigma errors, taking account of Poisson noise and uncertainties in stellar mass and Vmax. Several other estimates are overlaid (see Sato et al. 2004 for references). The empty/solid squares and the diamond are obtained from full integration of a Schechter fit to their respective luminosity or stellar mass function at each redshift bin. The X marks and the empty circles are quasi-fully integrated values with a finite integration range from 10L* to 1/20 L*, and 10.5 < log(Mstar[h65-2 M]) < 11.6, respectively. The triangles are simply summed values of the detected sources. The two dashed curves are deduced by integrating the star formation rate density in the universe, which is derived from the UV luminosity density as a function of redshift (Cole et al. 2001). The upper curve is an extinction corrected case for E(B - V) = 0.15, and the lower one has no dust correction.