Evidence for Very Massive Stars in extremely UV-bright star-forming galaxies at z ~ 2.2-3.6

We present a comprehensive analysis of the presence of very massive stars (VMS > 100 M_) in the integrated spectra of 13 UV-bright star-forming galaxies at 2.2 z 3.6 taken with the Gran Telescopio Canarias (GTC). These galaxies have very high UV absolute magnitudes (MUV -24), intense star formation (SFR 100-1000 M_ yr⁻¹), and metallicities in the range of 12+log(O/H) 8.10-8.50 inferred from strong rest-optical lines. The GTC rest-UV spectra reveal spectral features indicative of very young stellar populations with VMS, such as strong P-Cygni line profiles in the wind lines N~{ v} λ 1240 and C~{ iv} λ 1550 along with intense and broad He~{ ii} λ 1640 emission with EW₀ 1.40-4.60 , and FWHM 1150-3170 km \ s⁻¹. A Comparison with known VMS-dominated sources and typical galaxies without VMS reveals that some UV-bright galaxies closely resemble VMS-dominated clusters (e.g., R136 cluster). 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 EW₀ () ≥ 3.0 , along with the detection of other VMS-related spectral profiles (N~{ iv} λ 1486, 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 LUV* 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 175 M_ and 475 M_.