2011A&A...534A..15M

Using extremely deep PACS 100- and 160µm Herschel data from the GOODS-Herschel program, we identify 21 infrared bright galaxies previously missed in the deepest 24µm surveys performed by Spitzer/MIPS. These MIPS dropouts are predominantly found in two redshift bins, centred at z∼0.4 and ∼1.3. Their S₁₀₀/S₂₄ flux density ratios are similar to those of local (ultra-) luminous infrared galaxies (LIRGs and ULIRGs), whose silicate absorption features at 18µm (at z∼0.4) and 9.7µm (at z∼1.3) are shifted into the 24µm MIPS band at these redshifts. The high-z sub-sample consists of 11 infrared luminous sources, accounting for ∼2% of the whole GOODS-Herschel sample and putting strong upper limits on the fraction of LIRGs/ULIRGs at 1.0<z<1.7 that are missed by the 24µm surveys. We find that a S₁₀₀/S₂₄>43 colour cut selects galaxies with a redshift distribution similar to that of the MIPS dropouts and when combined with a second colour cut, S₁₆/S₈ > 4, isolates sources at 1.0<z<1.7. We show that these sources have elevated specific star formation rates (sSFR) compared to main sequence galaxies at these redshifts and are likely to be compact starbursts with moderate/strong 9.7µm silicate absorption features in their mid-IR spectra. Herschel data reveal that their infrared luminosities extrapolated from the 24µm flux density are underestimated, on average, by a factor of ∼3. These silicate break galaxies account for 16% (8%) of the ULIRG (LIRG) population in the GOODS fields, indicating a lower limit in their space density of 2.0x10^–5/Mpc³. Finally, we provide estimates of the fraction of z<2 MIPS dropout sources as a function of the 24-, 100-, 160-, 250- and 350µm sensitivity limits, and conclude that previous predictions of a population of silicate break galaxies missed by the major 24µm extragalactic surveys have been overestimated.