A Relation of the PAH 3.3 µm Feature with Star-forming Activity for Galaxies with a Wide Range of Infrared Luminosity

Abstract

For star-forming galaxies, we investigated a global relation between the polycyclic aromatic hydrocarbon (PAH) emission luminosity at 3.3 μm, L_PAH3.3, and the infrared (8-1000 μm) luminosity, L_IR, to understand how the PAH 3.3 μm feature relates to star-formation activity. With AKARI, we performed near-infrared (2.5-5 μm) spectroscopy of 184 galaxies having L_IR ∼ 10⁸-10¹³ L_⊙. We classified the samples into infrared galaxies (IRGs: L_IR < 10¹¹ L_⊙), luminous infrared galaxies (LIRGs: L_IR ∼ 10¹¹ -10¹² L_⊙) and ultra-luminous infrared galaxies (ULIRGs: L_IR > 10¹² L_⊙). We excluded sources likely to be contaminated by AGN activity, based on the rest-frame equivalent width of the PAH emission feature (< 40 nm) and the power-law index, representing the slope of continuum emission (Γ > 1 F_ν ∝ λ^Γ). Of these samples, 13 IRGs, 67 LIRGs, and 20 ULIRGs show the PAH emission feature at λ _rest = 3.3 μm in their spectra. We find that the L_PAH3.3/L_IR 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 L_IR.