MIRI spectrophotometry of GN-z11: Detection and nature of an optical red continuum component

Abstract

We present new MIRI F560W, F770W and F1000W imaging of GN-z11, extending the previous rest-frame coverage from 0.38 to 0.86$\mu$m. We report significant detections (14$\sigma$) in the F560W and F770W images, and a marginal detection (3.2$\sigma$) in F1000W. Here, we analyse its SED combining new MIRI imaging data with archival NIRSpec/Prism and MRS spectroscopy, and NIRCam imaging. The continuum emission shows a flat energy distribution, in f$_\nu$, up to 0.5$\mu$m, compatible with the presence of a mixed stellar population of young (4$\pm$1 Myr) and mature (63$\pm$23 Myr) stars that also account for the [O III], H$\beta$ and H$\alpha$ emission lines. The continuum at rest-frame 0.66$\mu$m shows a 36$\pm$3% flux excess above the predicted flux for a mixed stellar population, pointing to the presence of an additional source contributing at these wavelengths. This excess increases to 91$\pm$28% at rest-frame 0.86$\mu$m, although with a large uncertainty due to the marginal detection in the F1000W filter. We consider that hot dust emission in the dusty torus around a type 2 AGN could be responsible for the observed excess. Alternatively, this excess could be due to hot dust emission or to a photoluminiscence dust process (Extended Red Emission, ERE) under the extreme UV radiation field, as observed in some local metal-poor galaxies and in young compact starbursts. The presence of a type 1 AGN is not supported by the observed SED since high-z QSOs contribute at wavelengths above rest-frame 1$\mu$m, and an additional ad-hoc red source would be required to explain the observed flux excess at 0.66 and 0.86$\mu$m. Additional deep MIRI imaging covering the rest-frame near-IR are needed to confirm the flux detection at 10$\mu$m with higher significance, and to discriminate between the different hot dust emission in the extreme starburst and AGN scenarios with MIRI imaging at longer wavelengths.