  We present mid-infrared spectra of thirty two high redshift ultraluminous
infrared galaxies, selected via the stellar photospheric feature at rest-frame
1.6um, and an observed-frame 24um flux of >500muJy. Nearly all the sample
reside in a redshift range of <z>=1.71+/-0.15, and have rest-frame 1-1000um
luminosities of 10^12.9 - 10^13.8 Lsun. 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
6.2um/7.7um and 6.2um/11.2um 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 Lsun 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 1-4Kpc, and (2) star formation in unusually gas-rich regions.

Astrophysics