Spitzer Spectroscopy of Infrared-Luminous Galaxies: Diagnostics of AGN
  and Star Formation and Contribution to Total Infrared Luminosity
  We use Spitzer IRS spectroscopy to study the nature 65 IR-luminous galaxies
at 0.02 < z < 0.6 with F(24micron) > 1.2mJy. The IRS spectra cover wavelengths
spanning the PAH features and important atomic diagnostic lines. Our sample
corresponds to L(IR) = L(8-1000micron) = 10^10-10^12 Lsun. We divide our
galaxies into those with Spitzer IRAC colors indicative of warm dust heated by
an AGN (IRAGN) and those whose colors indicate star-formation processes
(non-IRAGN). Compared to the non-IRAGN, the IRAGN show smaller PAH emission
EWs, which we attribute to an increase in mid-IR continuum from the AGN. We
find that in both the IRAGN and non-IRAGN samples, the PAH luminosities
correlate strongly with the [Ne II] emission, from which we conclude that the
PAH luminosity directly traces the instantaneous SFR in both the IRAGN and
non-IRAGN galaxies. We compare the ratio of PAH luminosity to the total IR
luminosity and show that for most IRAGN star-formation accounts for 10-50% of
the L(IR). We also find no measurable difference between the PAH luminosity
ratios of L(11.3)/L(7.7) and L(6.2)/L(7.7) for the IRAGN and non-IRAGN,
suggesting that AGN do not significantly excite or destroy PAH molecules on
galaxy-wide scales. A small subset of galaxies show excess of [O IV] emission
compared to their PAH emission, which indicates the presence of
heavily-obscured AGN, including 3 galaxies that are not otherwise selected as
IRAGN. The low PAH emission and low [Ne II] emission of the IRAGN and [O
IV]-excess objects imply they have low SFRs and their IR luminosity is
dominated by processes associated with the AGN.
