Probing the Interstellar Medium of z~1 Ultra-luminous Infrared Galaxies
  through Interferometric Observations of CO and Spitzer Mid-infrared
  Spectroscopy
  We explore the relationship between gas, dust and star formation in a sample
of 12 ultra-luminous infrared galaxies (ULIRGs) at high redshift compared to a
similar sample of local galaxies. We present new CO observations and/or Spitzer
mid-IR spectroscopy for 6 70 micron selected galaxies at z~1 in order to
quantify the properties of the molecular gas reservoir, the contribution of an
active galactic nuclei (AGN) to the mid-IR luminosity and the star formation
efficiency (SFE=LIR/L'CO). The mid-IR spectra show strong polycyclic aromatic
hydrocarbon (PAH) emission and our spectral decomposition suggests that the AGN
makes a minimal contribution (<25%) to the mid-IR luminosity. The 70 micron
selected ULIRGs which we find to be spectroscopic close pairs, are observed to
have high SFE, similar to local ULIRGs and high redshift submillimeter
galaxies, consistent with enhanced IR luminosity due to an ongoing major
merger. Combined with existing observations of local and high redshift ULIRGs,
we further compare the PAH, IR and CO luminosities. We show that the ratio
LPAH6.2/LIR decreases with increasing IR luminosity for both local and high
redshift galaxies but the trend for high redshift galaxies is shifted to higher
IR luminosities; the average LPAH6.2/LIR ratio at a given LIR is ~3 times
higher at high redshift. When we normalize by the molecular gas, we find this
trend to be uniform for galaxies at all redshifts and that the molecular gas is
correlated with the PAH dust emission.The similar trends seen in the [CII] to
molecular gas ratios in other studies suggests that PAH emission, like [CII],
continues to be a good tracer of photodissociation regions even at high
redshift. Together the CO, PAH and far-IR fine structure lines should be useful
for constraining the interstellar medium conditions in high redshift galaxies.
