SPITZER SPECTROSCOPY OF INFRARED-LUMINOUS GALAXIES: DIAGNOSTICS OF ACTIVE GALACTIC NUCLEI AND STAR FORMATION AND CONTRIBUTION TO TOTAL INFRARED LUMINOSITY.

We use mid-infrared (MIR) spectroscopy from the Spitzer Infrared Spectrograph to study the nature of starformation and supermassive black hole accretion for a sample of 65 IR-luminous galaxies at 0.02 < z < 0.6 with F(24μm) > 1.2mJy. The MIR spectra cover wavelengths 5–38 μm, spanning the polycyclic aromatic hydrocarbon (PAH) features and important atomic diagnostic lines. Our sample of galaxies corresponds to a range of total IR luminosity, L_IR = L(8–1000 μm) = 10¹⁰–10¹² L⊙ (median LIR of 3.0 × 1011 L⊙). We divide our sample into a subsample of galaxies with Spitzer Infrared Array Camera 3.6–8.0 μm colors indicative of warm dust heated by an active galactic nucleus (AGN; IRAGN) and those galaxies whose colors indicate star-formation processes (non-IRAGN). Compared to the non-IRAGN, the IRAGN show smaller PAH emission equivalent widths, which we attribute to an increase in mid-IR continuum from the AGN. We find that in both the IRAGN and star-forming samples, the luminosity in the PAH features correlates strongly with [Ne II] λ12.8 μm emission line, from which we conclude that the PAH luminosity directly traces the instantaneous star-formation rate (SFR) in both the IRAGN and star-forming galaxies.We compare the ratio of PAH luminosity to the total IR luminosity, and we show that for most IRAGN star-formation accounts for 10%–50% of the total IR luminosity. 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. Interestingly, a small subset of galaxies (8 of 65 galaxies) show a strong excess of [Oiv] λ25.9 μm 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 the IR luminosity of these objects is dominated by processes associated with the AGN. Because these galaxies lie in the "green valley" of the optical color–magnitude relation and have low implied SFRs, we argue their hosts have declining SFRs and these objectswill transition to the red sequence unless some process restarts their star-formation. [ABSTRACT FROM AUTHOR]