1. Introduction

2. Observations
2.1. Spectroscopy
2.2. Photometry and SED fitting

3. The He II emitters

4. Analysis
4.1. AGN
4.2. Wolf-Rayet stars
4.3. Gravitational cooling radiation from gas accretion
4.4. Pop III star formation
4.5. Peculiar stellar populations
4.6. Other mechanisms

5. Discussion

6. Conclusions