GOODS-Herschel & CANDELS: The Morphologies of Ultraluminous Infrared Galaxies at z~ 2 (original) (raw)
2011, Arxiv preprint arXiv: …
Using deep 100 and 160 µm observations in GOODS-South from GOODS-Herschel, combined with high resolution HST/WFC3 near-infrared imaging from CANDELS, we present the first detailed morphological analysis of a complete, far-infrared (FIR) selected sample of 52 Ultraluminous Infrared Galaxies (ULIRGs: L IR > 10 12 L ⊙ ) at z ∼ 2. By using the Herschel observations in the FIR we are able to identify true ULIRGs, unaffected by the biases of color selections or 24 µm selections that have been problematic for previous studies. We also make use of a comparison sample of galaxies with lower IR luminosities (undetected by Herschel) but with the same redshift and H-band magnitude distribution. Our visual classifications of these two samples indicate that the fractions of objects with disk and spheroid morphologies is roughly the same but that there are significantly more mergers, interactions, and irregular galaxies among the ULIRGs (72 +5 −7 % versus 32 ± 3%). The combination of disk and irregular/interacting morphologies suggests that early stage interactions and minor mergers could play an important role in ULIRGs at z ∼ 2. We compare these fractions with those of a z ∼ 1 sample selected from GOODS-H and COSMOS across a wide luminosity range and find that the fraction of disks decreases systematically with L IR while the fraction of mergers and interactions increases, as has been observed locally. At comparable luminosities, the fraction of ULIRGs with various morphological classifications is similar at z ∼ 2 and z ∼ 1. We investigate the position of the z ∼ 2 ULIRGs, along with 70 z ∼ 2 LIRGs, on the specific star formation rate versus redshift plane, and find 52 systems to be starbursts (i.e., they lie more than a factor of 3 above the main sequence relation). The morphologies of these systems are dominated by interacting and merging systems (∼ 50%). If irregular disks are included as potential minor mergers, then we find that up to ∼ 73% of starbursts are involved in a merger or interaction at some level. Although the final coalescence of a major merger may not be required for the high luminosities of ULIRGs at z ∼ 2 as is the case locally, the large fraction of interactions at all stages and potential minor mergers suggest that the high star formation rates of ULIRGs are still largely externally triggered at z ∼ 2.
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