• active galactic nuclei
• emission lines
• high redshift
• HII regions
• interstellar medium
• massive black holes
• photoionization

The Thesis work concerns the study of possible signatures of the presence of supermassive black holes in the first galaxies formed in the Epoch of Reionization (redshift>6, or 1 Gyr from the Big Bang). Some recent evidences seem to suggest that black holes formed extremely rapidly during cosmic evolution and many of them can be largely obscured or are too faint to be detected by even the deepest X-ray observations. A class of UV emission lines, such as NV 1238.82Å, 1242.80Å, CIV 1548.19Å, 1550.77Å, HeII 1640.42Å, OIII] 1660.81Å, 1666.15Å, [CIII] 1906.68Å and CIII] 1908.73Å might be the smoking gun of the accretion onto a black hole, and therefore they may guide the search via more refined experiments. With this thesis work we want to answer the question: is the stellar radiation sufficient to excite all the emission lines or is extra power from the accreting black holes required? After a first analysis of the necessary conditions to produce the ions in the interstellar medium, we have implemented the relevant physics in photoionization models, using the software CLOUDY, to reconstruct the expected properties of the emission lines produced in HII regions. We find that the stellar radiation produced by a young(age<6.5 Myr) and metal poor (Z <0.4Z_sun) population can excite some of the lines, but this possibility can be excluded for NV and HeII emission lines. We conclude that at least some of the sources in a galaxy sample that we selected from literature should host an active galactic nucleus.