Evidence for very massive stars in extremely UV-bright star-forming galaxies at z ∼ 2.2-3.6.

Our recently published work finds evidence for very massive stars (VMS, M > 100M⊙) in extremely UV-bright compact galaxies. Our study reveals a different initial mass function (IMF) with a higher upper mass limit than the normally accepted 100M⊙ value in these sources. These galaxies have very high UV absolute magnitudes (MUV ≃ −24), intense star formation (SFR ≃ 100 − 1000 M⊙/yr), and metallicities in the range of 12+log(O/H) ≃ 8.10 − 8.50. The UV spectral features show strong P-Cygni line profiles of N V λ1240 and C IV λ1550 along with intense and broad He II λ1640 emission with equivalent width (EW) ≃ 1.40 − 4.60 Å, all indicative of massive stellar population. A comparison with known VMS-dominated sources reveals a close spectral match for some UV-bright galaxies. A quantitative comparison of the He II emission strength with new population synthesis models suggests VMS are needed to produce the measured He II strength. Employing an empirical threshold for EW (He II) ≥ 3.0 Å, along with the detection of other VMS-related spectral profiles (N IV λ1486,1719), 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 IMF with upper mass limits between 175M⊙ and 475M⊙.