Mid-Infrared Properties of Nearby Luminous Infrared Galaxies I: Spitzer
  IRS Spectra for the GOALS Sample
  The Great Observatories All-Sky LIRG Survey (GOALS) is a multiwavelength
study of luminous infrared galaxies (LIRGs) in the local universe. Here we
present low resolution Spitzer spectra covering 5-38um and provide a basic
analysis of the mid-IR spectral properties for nearby LIRGs. In a companion
paper, we discuss detailed fits to the spectra. The GOALS sample of 244 nuclei
in 180 luminous and 22 ultraluminous IR galaxies represents a complete subset
of the IRAS RBGS and covers a range of merger stages, morphologies and spectral
types. The majority (>60%) of GOALS LIRGs have high 6.2um PAH equivalent widths
(EQW > 0.4um) and low levels of silicate absorption (s_9.7um >-1.0). There is a
general trend among the U/LIRGs for silicate depth and MIR slope to increase
with LIR. U/LIRGs in the late stages of a merger also have on average steeper
MIR slopes and higher levels of dust obscuration. Together these trends suggest
that as gas & dust is funneled towards the center of a coalescing merger, the
nuclei become more compact and obscured. The sources that depart from these
correlations have very low PAH EQW (EQW < 0.1um) consistent with their MIR
emission being dominated by an AGN. The most heavily dust obscured sources are
the most compact in their MIR emission, suggesting that the obscuring (cool)
dust is associated with the outer regions of the starburst. As the merger
progresses a marked decline is seen for the fraction of high EQW (star
formation dominated) sources while the fraction of composite sources increases
but the fraction of AGN-dominated sources remains low. When compared to the MIR
spectra of submillimeter galaxies (SMGs) at z~2, the average GOALS LIRG is more
absorbed at 9.7um and has more PAH emission. However, when the AGN
contributions to both the local LIRGs and the high-z SMGs are removed, the
average local starbursting LIRG closely resembles the starbursting SMGs.
