We present mid-infrared spectroscopy obtained with the Spitzer Space Telescope of a sample of 11 optically faint , infrared luminous galaxies selected from a Spitzer MIPS 70 \mu m imaging survey of the NDWFS Boötes field .
These are the first Spitzer IRS spectra presented of distant 70 \mu m -selected sources .
All the galaxies lie at redshifts 0.3 < z < 1.3 and have very large infrared luminosities of L _ { IR } \sim 0.1 - 17 \times 10 ^ { 12 } ~ { } L _ { \odot } .
Seven of the galaxies exhibit strong emission features attributed to polycyclic aromatic hydrocarbons ( PAHs ) .
The average IRS spectrum of these sources is characteristic of classical starburst galaxies , but with much larger infrared luminosities .
The PAH luminosities of \nu L _ { \nu } ( 7.7 \mu m ) \sim 0.4 - 7 \times 10 ^ { 11 } ~ { } L _ { \odot } imply star formation rates of \sim 40 - 720 M _ { \odot } yr ^ { -1 } .
Four of the galaxies show deep 9.7 \mu m silicate absorption features and no significant PAH emission features ( 6.2 \mu m equivalent widths < 0.03 \mu m ) .
The large infrared luminosities and low \nu f _ { \nu } ( 70 \mu m ) / \nu f _ { \nu } ( 24 \mu m ) flux density ratios suggests that these sources have AGN as the dominant origin of their large mid-infrared luminosities , although deeply embedded but luminous starbursts can not be ruled out .
If the absorbed sources are AGN-dominated , a significant fraction of all far-infrared bright , optically faint sources may be dominated by AGN .