UV bright star-forming galaxies spectra

The presence of VMS is further supported by a quantitative comparison of the observed strength of the He ii emission with population synthesis models with and without VMS, where models with VMS are clearly preferred. Employing an empirical threshold for EW_0_ (HeII)>=3.0{AA}, along with the detection of other VMS-related spectral profiles (NIV {lambda}1486; {lambda}1719), we classify nine out of 13 UV-bright galaxies as VMS-dominated sources.

This high incidence of VMS-dominated sources in the UV-bright galaxy population (~70%) contrasts significantly with the negligible presence of VMS in typical L*_UV_ LBGs at similar redshifts (<1%). Our results thus indicate that VMS are common in UV-bright galaxies, suggesting a different initial mass function (IMF) with upper mass limits between 175M_{sun}_ and 475M_{sun}_.