GOODS-Herschel: a population of 24   μm dropout sources atz <  2

Abstract

Using extremely deep PACS 100-and 160 μm Herschel data from the GOODS-Herschel program, we identify 21 infrared bright galaxies previously missed in the deepest 24 μm surveys performed by Spitzer/MIPS. These MIPS dropouts are predominantly found in two redshift bins, centred at z ∼ 0.4 and ∼1.3. Their S 100 /S 24 flux density ratios are similar to those of local (ultra-) luminous infrared galaxies (LIRGs and ULIRGs), whose silicate absorption features at 18 μm (at z ∼ 0.4) and 9.7 μm (at z ∼ 1.3) are shifted into the 24 μm 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 24 μm surveys. We find that a S 100 /S 24 > 43 colour cut selects galaxies with a redshift distribution similar to that of the MIPS dropouts and when combined with a second colour cut, S 16 /S 8 > 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 9.7 μm silicate absorption features in their mid-IR spectra. Herschel data reveal that their infrared luminosities extrapolated from the 24 μm 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 2.0 × 10 −5 Mpc −3 . Finally, we provide estimates of the fraction of z < 2 MIPS dropout sources as a function of the 24-, 100-, 160-, 250-and 350 μm sensitivity limits, and conclude that previous predictions of a population of silicate break galaxies missed by the major 24 μm extragalactic surveys have been overestimated.

Hidden starbursts and active galactic nuclei at 0 < z < 4 from theHerschel-VVDS-CFHTLS-D1 field: Inferences on coevolution and feedback

“…Due to the shallowness of the SWIRE imaging in 24 μm relative to other similar fields (e.g., COSMOS), this 24 μm-"orphaned" Herschel population comprised a large fraction of the SPIRE sample over the full SPIRE mosaic (∼40% of significantly detected objects), though some are likely galaxies which have low f 24 μm / f SPIRE ratios (Magdis et al 2011). In order to prevent severely limiting our sample size, a second round of matching was performed on the orphaned SPIRE sample.…”

The far-infrared/radio correlation and radio spectral index of galaxies in the SFR–M∗plane up toz~2

“…This method provides reliable and highly complete PACS/SPIRE source catalogues (Lutz et al 2011;Magnelli et al 2013). We note that the small fraction of Herschel sources without a MIPS counterpart and thus missing in our source catalogues (Magdis et al 2011) will be considered via our stacking analysis which is based on the positions of complete stellar-mass-selected samples (Sect. 3.1).…”

STAR FORMATION AND AGN ACTIVITY IN GALAXY CLUSTERS FROM z = 1–2: A MULTI-WAVELENGTH ANALYSIS FEATURING HERSCHEL/PACS

“…redshift range relevant to this work, these MIPS dropouts will occur preferentially at z ∼ 1.3 due to silicate absorption (e.g., Magdis et al 2011), making it more challenging to interpret changes in cluster populations around this epoch. Given the depth of the SDWFS catalog, there is typically one IRAC source per PACS 100 μm beam and we use visual inspection to identify the rare cases of blending.…”