Ly-alpha emitters in the GOODS-S field: a powerful pure nebular SED with
  N IV] emission at z=5.563
  [abridged] 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 N IV] 1486 A. The SED is modelled in a way that accounts
for 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 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
N IV] luminosity and equivalent width. A nebular model with a hot blackbody
ionizing source of around 100 kK and a nebular metallicity of ~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 [O III] 4959, 5007 A 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 Msun . 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 nebulae with large
telescopes will provide a unique insight into very early chemical evolution.
