Abstract
Abstract For star-forming galaxies, we investigated a global relation between the polycyclic aromatic hydrocarbon (PAH) emission luminosity at 3.3μm, LPAH3.3, and the infrared (8–1000μm) luminosity, LIR, to understand how the PAH3.3μm feature relates to star-formation activity. With AKARI, we performed near-infrared (2.5–5μm) spectroscopy of 184 galaxies having LIR ∼ 108–1013L⊙. We classified the samples into infrared galaxies (IRGs: LIR < 1011L⊙), luminous infrared galaxies (LIRGs: LIR ∼ 1011–1012L⊙) and ultra-luminous infrared galaxies (ULIRGs: LIR > 1012L⊙). We excluded sources likely to be contaminated by AGN activity, based on the restframe equivalent width of the PAH emission feature (<40 nm) and the power-law index, representing the slope of continuum emission (Γ > 1; Fv ∝ λΓ). Of these samples, 13 IRGs, 67 LIRGs, and 20 ULIRGs show the PAH emission feature at λrest = 3.3μm in their spectra. We find that the LPAH3.3/LIR ratio considerably decreases toward the luminous end. Utilizing the mass and temperature of dust grains as well as the Brα emission for the galaxies, we discuss the cause of the relative decrease in the PAH emission with LIR.
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