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Aims: We investigate the nature of the source GDS J033218.92-275302.7atredshift ~ 5.56. Methods: The SED of the source is well sampled by 16 bandsphotometry, from UV-optical, near infrared and mid-infrared (MID-IR).Thedetection of signal in the MID-IR Spitzer/IRAC bands 5.8, 8.0 um -- where thenebular emission contribution is less effective -- suggests the presence of aBalmer break, signature of an underlying stellar population formed at earlierepochs. The optical spectrum shows a clear Lya emission line together withsemi-forbidden NIV] 1483.3-1486.5 also in emission. Results: From the SEDfitting and the Lya modelling it turns out that the source seems to have anevolved component with stellar mass of ~5 x10^(10) Msolar and age ~ 0.4 Gyrs,and a young component with an age of ~ 0.01 Gyrs and SFR in the range of 30-200Msolar yr^(-1). The limits on the effective radius derived from the ACS/z850and VLT/Ks bands indicate that this galaxy is denser than the local ones withsimilar mass. A relatively high nebular gas column density is favored from theLya line modelling (NHI>=10^(21) cm^(-2)). A vigorous outflow (~ 450 km/s) hasbeen measured from the optical spectrum,consistent with the Lya modelling. FromACS observations it turns out that the region emitting Lya photons is spatiallycompact and of the same order of the effective radius estimated at the ~1400Arest-frame wavelength, whose emission is dominated by the stellar continuumand/or AGN. The gas is blown out from the central region,but given the mass ofthe galaxy it is uncertain whether it will pollute the IGM to large distances.We argue that a burst of star formation in a dense gas environment is active(possibly containing hot and massive stars and/or a low luminosity AGN),superimposed to an already formed fraction of stellar mass (abridged).

The unusual NIV]-emitter galaxy GDS J033218.92-275302.7: star formation or AGN-driven winds from a massive galaxy at z=5.56