Observational evidence for the presence of PAHs in distant Luminous Infrared Galaxies using ISO and Spitzer

Abstract

We present ISOCAM 15 μm and MIPS 24 μm photometry of a sample of 16 distant Luminous Infrared Galaxies (LIRGs) characterized by a median luminosity L_IR ∼ 2 × 10¹¹~L_⊙ and redshift z = 0.7 (distributed from z = 0.1 to 1.2). While some sources display 24/15 μm flux ratios also consistent with a featureless continuum dominating their mid-infrared (MIR) spectral energy distributions (SEDs), the presence of prominent emission features such as the Polycyclic Aromatic Hydrocarbons is clearly required to explain the observed colors for more than half of the sample. As a result, a general good agreement is observed between the data and predictions from the local starburst-dominated SEDs that have been used so far to constrain IR galaxy evolution. This is consistent with the star-forming nature of LIRGs derived from previous works, even though our approach cannot rule out the dominance of an AGN in some cases. Our study also supports the possibility of tracing the total IR luminosity of distant galaxies (up to z∼ 1) from their MIR emission.