The unusual NIV]-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 J033218.92-275302.7at
redshift ~ 5.56. Methods: The SED of the source is well sampled by 16 bands
photometry, from UV-optical, near infrared and mid-infrared (MID-IR).The
detection of signal in the MID-IR Spitzer/IRAC bands 5.8, 8.0 um -- where the
nebular emission contribution is less effective -- suggests the presence of a
Balmer break, signature of an underlying stellar population formed at earlier
epochs. The optical spectrum shows a clear Lya emission line together with
semi-forbidden NIV] 1483.3-1486.5 also in emission. Results: From the SED
fitting and the Lya modelling it turns out that the source seems to have an
evolved 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 30-200
Msolar 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
Lya line modelling (NHI>=10^(21) cm^(-2)). A vigorous outflow (~ 450 km/s) has
been measured from the optical spectrum,consistent with the Lya modelling. From
ACS observations it turns out that the region emitting Lya photons is spatially
compact and of the same order of the effective radius estimated at the ~1400A
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 to an already formed fraction of stellar mass (abridged).
