The Nature of Star Formation in Distant Ultraluminous Infrared Galaxies Selected in a Remarkably Narrow Redshift Range

We present mid-infrared spectra of 32 high-redshift ultraluminous infrared galaxies, selected via the stellar photospheric feature at rest-frame 1.6 μm, and an observed-frame 24 μm flux of >500 μJy. Nearly all the sample reside in a redshift range of ⟨ z⟩ = 1.71 ± 0.15 and have rest-frame 1-1000 μm luminosities of 10 12.9 -10 13.8 L ☉ . Most of the spectra exhibit prominent polycyclic aromatic hydrocarbon emission features and weak silicate absorption, consistent with a starburst origin for the IR emission. Our selection method appears to be a straightforward and efficient way of finding distant, IR-luminous, star-forming galaxies in narrow redshift ranges. There is, however, evidence that the mid-IR spectra of our sample differ systematically from those of local ULIRGs; our sample have comparable PAH equivalent widths but weaker apparent silicate absorption, and (possibly) enhanced PAH 6.2 μm/7.7 μm and 6.2 μm/11.2 μm flux ratios. Furthermore, the composite mid-IR spectrum of our sample is almost identical to that of local starbursts with IR luminosities of 10 10 -10 11 L ☉ , rather than that of local ULIRGs. These differences are consistent with a reduced dust column, which can plausibly be obtained via some combination of (1) star formation that is extended over spatial scales of 1-4 kpc and (2) star formation in unusually gas-rich regions.