Properties of $z\sim3-6$ Lyman Break Galaxies. II. Impact of nebular
  emission at high redshift
  We present a homogeneous, detailed analysis of the spectral energy
distribution (SED) of $\sim$ 1700 LBGs from the GOODS-MUSIC catalogue with deep
multi-wavelength photometry from $U$ band to 8 $\mu$m to determine stellar
mass, age, dust attenuation, and star formation rate. Using our SED fitting
tool, which takes into account nebular emission, we explore a wide parameter
space. We also explore a set of different star formation histories. Nebular
emission is found to significantly affect the determination of the physical
parameters for the majority of $z \sim $ 3--6 LBGs. We identify two populations
of galaxies by determining the importance of the contribution of emission lines
to broadband fluxes. We find that $\sim$ $65\%$ of LBGs show detectable signs
of emission lines, whereas $\sim$ $35\%$ show weak or no emission lines. This
distribution is found over the entire redshift range. We interpret these groups
as actively star forming and more quiescent LBGs, respectively. We find that it
is necessary to considerer SED fits with very young ages ($<50$ Myr) to
reproduce some colours affected by strong emission lines. Other arguments
favouring episodic star formation and relatively short star formation
timescales are also discussed. Considering nebular emission generally leads to
a younger age, lower stellar mass, higher dust attenuation, higher star
formation rate, and a large scatter in the SFR-$M_{\star}$ relation. Our
analysis yields a trend of increasing specific star formation rate with
redshift, as predicted by recent galaxy evolution models. The physical
parameters of approximately two thirds of high redshift galaxies are
significantly modified when we account for nebular emission. The SED models
which include nebular emission shed new light on the properties of LBGs with
numerous important implications.
