In this thesis we explore the OTELO (OSIRIS Tunable Filter Emission Line Object) survey. OTELO is a very deep, pencil beam survey for the search of emission line sources by means of the tunable filters of the OSIRIS instrument, installed at the Gran Telescopio Canarias. A tomography scan method results in 2D, low-resolution pseudo-spectra for all the sources in the observed field. The reached line flux of 4:6 10^19 erg s^-1 cm^2 makes it the deepest survey in terms of the line flux. In particular, we present the morphological catalogue of the OTELO sources detected in high-resolution images from the ACS instrument of the Hubble Space Telescope. We have employed a parametric method to fit 2D model composed of single-Sérsic profile using a multi-wavelength version of GALFIT software. This software fits simultaneously selected model to all given images using Chebyshev polynomial functions. We show that this multi-wavelength method provides better results in the low signal-to-noise, recovering a substantial fraction of sources in this regime, as compared to a standard version of the same software. We also provide a preliminary visual morphology of the selected sample (up to magnitude I=24.5 mag). The results of this analysis are consistent with the obtained from the parametric method. We have tested various methods for separation of early-type galaxies from late-type galaxies, as well as the mass-size relation. We also found a median stellar mass evolution up to z = 2 finding good agreement with previous works. From the morphological catalogue we selected a population of early-type emission line galaxies (ET-ELG). Even if under investigation for many decades, there is lack of the clear consensus on the nature and origin of the emission lines found in the ET-ELG. We study a sample of 14 such objects at intermediate redshift. Due to the limited spectral information available, we compared the number densities of our sample with a carefully selected sample of local counterparts from SDSS. We found that OTELO detects a considerably higher ET-ELG density than it would be expected from the local SDSS sample. This result holds even considering a cosmic variance – the most important source of uncertainties for a pencil beam survey ( 56 arcmin^2) as OTELO. Finally, we studied a stellar-mass–metallicity relation (MZR) of selected Ha emission line sources at z~0.4. Particularly, and taking advantage of the depth reached by the OTELO survey, we focus on the very low-mass end of this relation, with the low-mass limit (10^7Msun ). Most of the galaxies in the Ha sample are morphologically classified as disk-like sources with low Sérsic index, as expected for the star-forming galaxies. We have found no evidence for the evolution of the MZR since z = 0.4.
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