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

Understand

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).