Infrared luminosities and aromatic features in the 24μm flux-limited sample of 5muses

Abstract

We study a 24 μm 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 newempirical spectral templates from Spitzer data. The infrared luminosities of this sample range mostly from 109 L⊙ to 1013.5 L⊙,with 83% in the range 1010 L⊙ < LIR < 1012 L⊙. The redshifts range from 0.008 to 4.27, with a median of 0.144. The equivalentwidths of the 6.2μm aromatic feature have a bimodal distribution,probably related to selection effects. We use the 6.2μm 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 EWobjects (SB-dominated) tend to have steeper mid-IR to far-IR spectral slopes and lower LIR and redshifts. The low EW objects (AGN-dominated) tend to have less steep spectral slopes and higher LIR 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(LPAH7.7 μm/LPAH11.3 μm) ratios than star-forming galaxies, possibly due to preferential destruction of the smaller aromatics by the AGN. The log(LPAH7.7 μm/L PAH11.3 μm) 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 LIR for different types of objects. © 2010. The American Astronomical Society.