Infrared Emission of Normal Galaxies from 2.5 to 12 Micron: Infrared Space Observatory Spectra, Near‐Infrared Continuum, and Mid‐Infrared Emission Features

Abstract

We present ISOPHOT spectra of the regions 2.5-4.9 {μ}m and 5.8-11.6{μ}m for a sample of 45 disk galaxies from the US Infrared SpaceObservatory Key Project on Normal Galaxies. The galaxies were selectedto span the range in global properties of normal, star-forming diskgalaxies in the local universe. The spectra can be decomposed into threespectral components: (1) continuum emission from stellar photospheres,which dominates the near-infrared (NIR; 2.5-4.9 {μ}m) spectral region;(2) a weak NIR excess continuum, which has a color temperature of~10^{3} K, carries a luminosity of a few percent of the totalfar-infrared (FIR) dust luminosity L_{FIR} and most likelyarises from the interstellar medium (ISM); and (3) the well-known broademission features at 6.2, 7.7, 8.6, and 11.3 {μ}m, which are generallyattributed to aromatic carbon particles. These aromatic features inemission (AFEs) dominate the mid-infrared (MIR; 5.8-11.6 {μ}m) part ofthe spectrum and resemble the so-called type A spectra observed in manynonstellar sources and the diffuse ISM in our own Galaxy. The fewnotable exceptions include NGC 4418, where a dust continuum replaces theAFEs in MIR, and NGC 1569, where the AFEs are weak and the strongestemission feature is [S IV] 10.51 {μ}m. The relative strengths of theAFEs vary by 15%-25% among the galaxies. However, little correlation isseen between these variations and either IRAS 60 {μ}m/100 {μ}m fluxdensity ratio R(60/100) or the FIR/blue luminosity ratioL_{FIR}/L_{B}, two widely used indicators of the currentstar formation activity, suggesting that the observed variations are nota consequence of the radiation field differences among the galaxies. Wedemonstrate that the NIR excess continuum and AFE emission arecorrelated, suggesting that they are produced by similar mechanisms andsimilar (or the same) material. On the other hand, as the current starformation activity increases, the overall strengths of the AFEs and theNIR excess continuum drop significantly with respect to that of the FIRemission from large dust grains. In particular, the summed luminosity ofthe AFEs falls from ~0.2 L_{FIR} for the most ``IR-quiescent''galaxies to ~0.1 L_{FIR} for the most ``IR-active'' galaxies.This is likely a consequence of the preferential destruction in intenseradiation fields of the small carriers responsible for the NIR/AFEemission.Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries, France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.