  Star-forming galaxies at high redshift show anomalous values of infrared
excess, which can be described only by extremizing the existing relations
between the shape of their ultraviolet continuum emission and their
infrared-to-ultraviolet luminosity ratio, or by constructing \textit{ad-hoc}
models of star formation and dust distribution. We present an alternative
explanation, based on unveiled AGN activity, to the existence of such galaxies.
In fact, the presence of a weak AGN configures as a natural scenario in order
to explain the observed spectral properties of such high-$z$ objects in terms
of a continuum slope distribution rather than altered infrared excesses, due to
the different shape of the AGN continuum emission with respect to quiescent
galaxies. To this aim, we directly compare the infrared-to-ultraviolet
properties of high-redshift galaxies to those of known categories of AGN
(quasars and Seyferts). We also infer the characteristics of their possible
X-ray emission. We find a strong similarity between the spectral shapes and
luminosity ratios of AGN with the corresponding properties of such galaxies. In
addition, we derive expected X-ray fluxes that are compatible with energetics
from AGN activity. We conclude that a moderate AGN contribution to the UV
emission of such high-$z$ objects is a valid alternative to explain their
spectral properties. Even the presence of an active nucleus in each source
would not violate the expected quasar statistics. Furthermore, we suggest that
the observed similarities between anomalous star-forming galaxies and quasars
may provide a benchmark for future theoretical and observational studies on the
galaxy population in the early Universe.
