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

Abstract

We present a comprehensive analysis of the presence of very massive stars (VMS > $100M_{\odot }$) in the integrated spectra of 13 UV-bright star-forming galaxies at $2.2\lesssim z\lesssim 3.6$ taken with the Gran Telescopio Canarias (GTC). These galaxies have very high UV absolute magnitudes ($M_{\mathrm{UV}}\simeq -24$), intense star formation (SFR $\simeq 100-1000$ $M_{\odot }$ yr${}^{-1}$), and metallicities in the range of 12+log(O/H) $\simeq 8.10-8.50$ 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~{\sc v} $\lambda 1240$ and C~{\sc iv} $\lambda 1550$ along with intense and broad He~{\sc ii} $\lambda 1640$ emission with $E{W}_0$ $\simeq 1.40-4.60$ Å, and FWHM $\simeq 1150-3170$ $km{s}^{-1}$. 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~{\sc ii} emission with population synthesis models with and without VMS, where models with VMS are clearly preferred. Employing an empirical threshold for $E{W}_0$ (\heii) $\ge 3.0$ Å, along with the detection of other VMS-related spectral profiles (N~{\sc iv} $\lambda 1486,1719$), 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 ($\approx 70\%$) contrasts significantly with the negligible presence of VMS in typical $L_{\mathrm{UV}}^{\ast }$ 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 $175M_{\odot }$ and $475M_{\odot }$.

20 pages, 11 Figures, Accepted for Publication in Astronomy & Astrophysics

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