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

Abstract

We present a comprehensive analysis of the presence of very massive stars (VMS > $100 M_ odot $) 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 UV -24$), intense star formation (star formation rate $ odot $ yr$^ $), and metallicities in the range of 12+log(O/H) $ 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 1240$ and C iv 1550$ along with intense and broad He ii 1640$ emission with equivalent width and full width half maximum (FWHM) $ 1150-3170$ $km \ $. 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 $EW_ along with the detection of other VMS-related spectral profiles (N iv 1486, 1719$), 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<!PCT!>$) contrasts significantly with the negligible presence of VMS in typical UV $ LBGs at similar redshifts ($<1<!PCT!>$). 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_ odot $ and $475 M_ odot