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 mu m, and an observed-frame 24 mu m flux of > 500 mu Jy. Nearly all the sample reside in a redshift range of < z > 1.71 +/-0.15 and have rest-frame 1-1000 mu m luminosities of 10(12.9)-10(13.8) L-circle dot. 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 mu m/7.7 mu m and 6.2 mu m/11.2 mu m 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-circle dot, 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.