INFRARED LUMINOSITIES AND AROMATIC FEATURES IN THE 24μm FLUX-LIMITED SAMPLE OF 5MUSES

We study a 24 mu m selected sample of 330 galaxies observed with the infrared spectrograph for the 5 mJy Unbiased Spitzer Extragalactic Survey. We estimate accurate total infrared luminosities by combining mid-IR spectroscopy and mid-to-far infrared photometry, and by utilizing new empirical spectral templates from Spitzer data. The infrared luminosities of this sample range mostly from 10(9) L-circle dot to 10(13.5) L-circle dot, with 83% in the range 10(10) L-circle dot < L-IR < 10(12) L-circle dot. The redshifts range from 0.008 to 4.27, with a median of 0.144. The equivalent widths of the 6.2 mu m aromatic feature have a bimodal distribution, probably related to selection effects. We use the 6.2 mu m polycyclic aromatic hydrocarbon equivalent width (PAH EW) to classify our objects as starburst (SB)-dominated (44%), SB-AGN composite (22%), and active galactic nucleus (AGN)-dominated (34%). The high EW objects (SB-dominated) tend to have steeper mid-IR to far-IR spectral slopes and lower L-IR and redshifts. The low EW objects (AGN-dominated) tend to have less steep spectral slopes and higher L-IR and redshifts. This dichotomy leads to a gross correlation between EW and slope, which does not hold within either group. AGN-dominated sources tend to have lower log(L-PAH7.7 (mu m)/ L-PAH11.3 (mu m)) ratios than star-forming galaxies, possibly due to preferential destruction of the smaller aromatics by the AGN. The log(L-PAH7.7 (mu m)/ L-PAH11.3 (mu m)) ratios for star-forming galaxies are lower in our sample than the ratios measured from the nuclear spectra of nearby normal galaxies, most probably indicating a difference in the ionization state or grain size distribution between the nuclear regions and the entire galaxy. Finally, we provide a calibration relating the monochromatic continuum or aromatic feature luminosity to L-IR for different types of objects.