Summary: OTELO, acronym for "OSIRIS Tunable Emission Line Object Survey", is a proposed GTC Key Project that will allow an optimal exploitation of the Spanish Day One GTC instrument OSIRIS, with a high scientific output and international impact. The survey will be made by using the Tunable filters, a unique feature of OSIRIS in 8-10m telescopes, to do co-mobile tomography at a depth allowing to measure the faintest emission line sources, while scanning a volume of Universe containing a statistical significant sample of such objects. With this technique, and given the high sensitivity gained by the use of the Tunable filters, OTELO will be the deepest and richest survey of emission line objects to date, providing an enormous amount of valuable data to tackle a wide variety of first ranked scientific projects.

OTELO, (OSIRIS Tunable Emission Line Object Survey), will survey emission line objects using OSIRIS tunable filters in selected atmospheric windows relatively free of sky emission lines. Different high latitude and low extinction sky regions with enough angular separations will be observed yielding a total area of 1--2 square degrees. A 5σ depth of 8 x 10¹⁸ erg cm^-2 s^-1 will allow detecting objects of equivalent width up to 6, making OTELO the deepest emission line survey to date. OTELO is a deep space probe that will provide a representative sample of the Universe from z = 0.24 through 6.7. Given the observing procedure, OTELO will allow studying clearly defined volumes of Universe at a known flux limit. In this contribution, a review of the project status is presented, together with some of the most significant evolutionary studies that will be tackled.

OTELO will search for emission line objects using the OSIRIS Tunable Filters in selected atmospheric windows relatively free of sky emission lines. Different high latitude and low extinction sky regions with enough angular separations will be observed yielding a total area of 1-2 square degrees. Some of the potential results of the study will be highly exciting. Going 1.5 magnitudes above the deepest survey, it will allow to measure the faint end of the star-forming galaxies function. The possibility of measurement of [NII]lambda658.4 allows a pure Hα flux determination, and as consequence reliable metallicity studies. The large number of expected targets detected will allow studies of statistically significant samples of faint galaxies, and the area covered makes the survey suitable for cosmological studies by avoiding the effect of clustering.

Given the observing procedure, OTELO will allow studying a clearly defined volume of the universe at a known flux limit. Co-ordination with surveys at other wavelengths is expected and already established in some cases, as for examples synergies with the PACS instrument of the Herschel observatory. Although the final aim is producing a catalogue that will be available to the Spanish astronomical community, and whose final version is scheduled for 2010, the first scientific results are expected to be published soon after the start of the OTELO observations.

• The flux limit of the survey, 1.5 magnitudes above the deepest survey, will allow measuring the faint end of the star-forming galaxies luminosity function. This flux limit, on the contrary to other surveys, corresponds to the completeness limit.
• The number of targets, that will allow studies of statistically significant samples of faint galaxies.
• The area covered, that makes it suitable for cosmological studies by avoiding clustering.
• The possibility of measuring [NII], that allows pure Hα flux determinations and metallicity studies.
• The ability of discriminating AGN.
• Also, the ratio [NII]/Hα provided by OTELO will serve:
  • as metallicity indicator to study the chemical evolution of galaxies,
  • to reliably derive SFR from Halpha,
  • to estimate H_2/CO conversion factor for GTM (Gran Telescopio Milimetrico, build in Mexico) and ALMA observations.
  • to search for primordial Helium in low metallicity systems and (v) as an indicator of LINER activity when observed in the nucleus of a spatially resolved galaxy