Tracing the energetics and evolution of dust with Spitzer : a chapter in the history of the Eagle Nebula

Context. The Spitzer GLIMPSE and MIPSGAL surveys have revealed a wealth of details about the Galactic plane in the infrared (IR) with orders of magnitude higher sensitivity, higher resolution, and wider coverage than previous IR observations. The structure of the interstellar medium (ISM) is tightly connected to the countless star-forming regions. We use these surveys to study the energetics and dust properties of the Eagle Nebula (M16), one of the best known star-forming regions. Aims. We present MIPSGAL observations of M16 at 24 and 70 mu m and combine them with previous IR data. The mid-IR image shows a shell inside the well-known molecular borders of the nebula, as in the ISO and MSX observations from 15 to 21 mu m. The morphologies at 24 and 70 mu m are quite different, and its color ratio is unusually warm. The far-IR image resembles the one at 8 mu m that enhances the structure of the molecular cloud and the pillars of creation. We use this set of IR data to analyze the dust energetics and properties within this template for Galactic star-forming regions. Methods. We measure IR spectral energy distributions (SEDs) across the entire nebula, both within the inner shell and the photodissociation regions (PDRs). We use the DUSTEM model to fit these SEDs and constrain the dust temperature, the dust-size distribution, and the radiation field intensity relative to that provided by the star cluster NGC 6611 (chi/chi(0)). Results. Within the PDRs, the inferred dust temperature (similar to 35 K), the dust-size distribution, and the radiation field intensity (chi/chi(0) < 1) are consistent with expectations. Within the inner shell, the dust is hotter (similar to 70 K). Moreover, the radiation field required to fit the SED is larger than that provided by NGC 6611 (chi/chi(0) > 1). We quantify two solutions to this problem: (1) The size distribution of the dust in the shell is not that of interstellar dust. There is a significant enhancement of the carbon dust-mass in stochastically heated very small grains. (2) The dust emission arises from a hot (similar to 10(6) K) plasma where both UV and collisions with electrons contribute to the heating. Within this hypothesis, the shell SED may be fit for a plasma pressure p/k similar to 5 x 10(7) Kcm(-3). Conclusions. We suggest two interpretations for the M16 inner shell: (1) The shell matter is supplied by photo-evaporative flows arising from dense gas exposed to ionized radiation. The flows renew the shell matter as it is pushed out by the pressure from stellar winds. Within this scenario, we conclude that massive-star forming regions such as M16 have a major impact on the carbon dust-size distribution. The grinding of the carbon dust could result from shattering in grain-grain collisions within shocks driven by the dynamical interaction between the stellar winds and the shell. (2) We also consider a more speculative scenario where the shell is a supernova remnant. In this case, we would be witnessing a specific time in the evolution of the remnant where the plasma pressure and temperature would enable the remnant to cool through dust emission.