due to current instrumental limitations, it becomes progressively easier to spectroscopically confirm only the emitters (e.g. Dow-Hygelund et al. 2007).

On one hand the  bright phase could represent a stochastic event in the life of any galaxy (as in the duty cycle model e.g. Nagamine et al. 2008); alternatively it could be related to some specific physical property of the galaxies, such as the (young) age, the dust or gas content.\
In principle, the Ly$\alpha$ line can be used to probe star-formation rates, clustering properties (Kovac et al. 2007), and even for cosmological applications such as re-ionization studies (e.g. Dijkstra et al. 2007, Dayal et al. 2008). In practice, this effort is far from trivial because of the above stated complications. Understanding what determines the presence of the emission line is therefore fundamental to interpret the bias introduced by the observational selection techniques.

A comparison between the physical and morphological properties of LBGs and LAEs is necessary but not straightforward. Because of the different selection techniques, LAEs tend to be much fainter than LBGs and therefore modelling their spectral energy distributions (SEDs) to constrain the relevant physical properties has always been a difficult task (see Gawiser 2009 for a review). 