  We report the initial results from a Spitzer GO-1 program to obtain low
resolution, mid-infrared spectra of infrared luminous galaxies at z~1-2. This
paper presents the spectra of eight sources observed with the Spitzer InfraRed
Spectrograph (IRS). Of the eight spectra, six have mid-IR spectral features,
either emission from Polycyclic Aromatic Hydrocarbon (PAH) or silicate
absorption. Based on these mid-IR features, the inferred six redshifts are in
the range of 1.8-2.6. The remaining two spectra detect only strong continua,
thus do not yield redshift information. Strong, multiple PAH emission features
are detected in two sources, and weak PAH emission in another two. These data
provide direct evidence that PAH molecules are present and directly observable
in ULIRGs at z~2.
  The six sources with measured redshifts are dusty, infrared luminous galaxies
at z~2 with estimated $L_{bol} \sim 10^{13}L_\odot$. Of the eight sources, two
appear starburst dominated; two with only power law continua are probably type
I QSOs; and the remaining four are likely composite systems containing a buried
AGN and a starburst component. Since half of our sample are optically faint
sources with R>25.5mag (Vega), our results demonstrate the potential of using
mid-infrared spectroscopy, especially the Aromatic and silicate features
produced by dust grains to directly probe optically faint and infrared luminous
populations at high redshift.

Astrophysics