Hidden activity in high-redshift spheroidal galaxies from mid-infrared and X-ray observations in the GOODS-North field

We exploit very deep mid-infrared (MIR) and X-ray observations by Spitzer and Chandra in the GOODS-North field to identify signs of hidden [either starburst or active galactic nucleus (AGN)] activity in spheroidal galaxies between z similar or equal to 0.3 and 1. Our reference is a complete sample of 168 morphologically classified spheroidal (elliptical/lenticular) galaxies with z(AB) < 22.5 selected from GOODS Advanced Camera for Surveys imaging. Of these 19 have 24-mu m detections in the GOODS catalogue, half of which have an X-ray counterpart in the 2-Ms Chandra catalogue (six detected in the 2-10 keV X-rays hard band), while about 25 per cent have 1.4-GHz fluxes larger than 40 mu Jy. Traces of hidden activity in the spheroidal population are also searched for in the deep X-ray images and 14 additional galaxies are detected in X-rays only. The nature of the observed MIR emissions is investigated by modelling their spectral energy distributions based on the available multiwavelength photometry, including X-ray, ultraviolet, optical, near-infrared, MIR and radio fluxes, and optical spectroscopy. The amount of dust derived from the infrared emission observed by Spitzer appears in excess of that expected by mass loss from evolved stars. When the available independent diagnostics are compared, in general, they provide consistent classifications about the nature of the activity in our spheroidal population. Given that, in principle, none of these diagnostics alone can be considered as conclusive, only trough a panchromatic comparison of them, we can reach an accurate comprehension of the underlying physical processes. In particular, our multiwavelength analysis of the X-ray and MIR properties leads us to conclude that at least eight of the 19 24-mu m bright sources should hide an obscured AGN, while the X-ray undetected sources are more likely dominated by star formation. We conclude that similar to 30 objects (similar to 20 per cent) of the original flux-limited sample of 168 spheroidal galaxies in the GOODS-North are detected during phases of prominent activity, of both stellar and quasar origin. Due to the short expected lifetimes of the infrared and X-ray emissions, this fraction might imply a significant level of activity in this class of galaxies during the relatively recent cosmic epochs - z similar to 0.3-1 - under investigation.