Lyα emitters in the GOODS-S field A powerful pure nebular SED with NIV] emission at z=5.563

Context. The Great Observatories Origins Deep Survey (GOODS) has provided us with one of the deepest multi-wavelength views of the distant universe. The combination of multi-band photometry and optical spectroscopy has resulted in the identification of sources whose redshifts extend to values in excess of six. Amongst these distant sources are Ly alpha emitters whose nature must be deduced by clearly identifying the different components that contribute to the measured SED. Aims. From a sample of Ly alpha emitters in the GOODS-S field with uncontaminated photometry and optical (red) spectroscopy, we select a spatially compact object at a redshift of 5.563 (Ly alpha) that shows a second emission line, identified as Niv] 1486 angstrom. The SED is modelled in a way that accounts for both the Niv] line emission and the photometry in a self-consistent way. Methods. The photoionization code CLOUDY is used to calculate a range of nebular models as a function of stellar ionizing source temperature, ionization parameter, density and nebular metallicity. We compare the theoretical and observed magnitudes and search for the model parameters that also reproduce the observed Niv] luminosity and equivalent width. Results. A nebular model with a hot blackbody ionizing source of around 100 kK and a nebular metallicity of similar to 5% of solar is able to fit the observed SED and, in particular, explain the large apparent Balmer break which is inferred from the pure stellar population model fitting conventionally applied to multi-band photometric observations. In our model, an apparent spectral break is produced by strong [OIII] 4959, 5007 angstrom emission falling in one of the IR bands (IRAC1 in this case). A lower limit on the total baryonic mass of a model of this type is 3.2 x 10(8) M-circle dot. Conclusions. It is argued that objects with Ly alpha emission at high redshift that show an apparent Balmer break may have their SED dominated by nebular emission and so could possibly be identified with very young starbursting galaxies rather than massive evolved stellar populations. Detailed studies of these emission nebul with large telescopes will provide a unique insight into very early chemical evolution.