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

Aims. We investigate the nature of the source GDS J033218.92-275302.7 at redshift similar to 5.56. Methods. The spectral energy distribution of the source is well-sampled by 16 bands photometry from UV-optical (HST and VLT), near infrared, near infrared (VLT) to mid-infrared (Spitzer). The detection of a signal in the mid-infrared Spitzer/IRAC bands 5.8, 8.0 mu m - where the nebular emission contribution is less effective - suggests that there is a Balmer break, the signature of an underlying stellar population formed at earlier epochs. The high-quality VLT/FORS2 spectrum shows a clear Ly alpha emission line, together with semi-forbidden N IV] 1483.3-1486.5 also in emission. These lines imply a young stellar population. In particular, the N IV] 1483.3-1486.5 feature (if the source is not hosting an AGN) is a signature of massive and hot stars with an associated nebular emission. Conversely, it may be a signature of an AGN. The observed SED and the Ly alpha emission line profile were modeled carefully to investigate the internal properties of the source. Results. From the SED-fitting with a single and a double stellar population and from the Ly alpha modeling, it turns out that the source seems to have an evolved component with a stellar mass of similar to 5 x 10(10) M-circle dot and age similar to 0.4 Gyr, and a young component with an age of similar to 0.01 Gyr and star formation rate in the range of 30-200 M-circle dot yr(-1). The limits on the effective radius derived from the ACS/z850 and VLT/Ks bands indicate that this galaxy is denser than the local ones with similar mass. A relatively high nebular gas column density is favored from the Ly alpha line modeling (N-HI greater than or similar to 10(21) cm(-2)). A vigorous outflow (similar to 450 km s(-1)) has been measured from the optical spectrum, consistent with the Ly alpha modeling. From ACS observations it turns out that the region emitting Ly alpha photons is spatially compact and has a similar effective radius (similar to 0.1 kpc physical) estimated at the similar to 1400 angstrom rest-frame wavelength, whose emission is dominated by the stellar continuum and/or AGN. The gas is blown out from the central region, but, given the mass of the 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 on an already formed fraction of stellar mass.