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[abridged] From a sample of Ly-alpha emitters in the GOODS-S field withuncontaminated photometry and optical (red) spectroscopy, we select a spatiallycompact object at a redshift of 5.563 (Ly-alpha) that shows a second emissionline, identified as N IV] 1486 A. The SED is modelled in a way that accountsfor both the N IV] line emission and the photometry in a self-consistent way.The photoionization code CLOUDY is used to calculate a range of nebular modelsas a function of stellar ionizing source temperature, ionization parameter,density and nebular metallicity. We compare the theoretical and observedmagnitudes and search for the model parameters that also reproduce the observedN IV] luminosity and equivalent width. A nebular model with a hot blackbodyionizing source of around 100 kK and a nebular metallicity of ~5% of solar isable to fit the observed SED and, in particular, explain the large apparentBalmer break which is inferred from the pure stellar population model fittingconventionally applied to multi-band photometric observations. In our model, anapparent spectral break is produced by strong [O III] 4959, 5007 A emissionfalling in one of the IR bands (IRAC1 in this case). A lower limit on the totalbaryonic mass of a model of this type is 3.2 x 10^8 Msun . It is argued thatobjects with Ly-alpha emission at high redshift that show an apparent Balmerbreak may have their SED dominated by nebular emission and so could possibly beidentified with very young starbursting galaxies rather than massive evolvedstellar populations. Detailed studies of these emission nebulae with largetelescopes will provide a unique insight into very early chemical evolution.

Ly-alpha emitters in the GOODS-S field: a powerful pure nebular SED with N IV] emission at z=5.563