Narrow He II emission in star-forming galaxies at low metallicity. Stellar wind emission from a population of Very Massive Stars

Abstract

In a recent study star-forming galaxies with HeII emission between redshifts 2 and 4.6 have been found to occur in two modes, distinguished by the width of their HeII emission lines. Broad HeII emission has been attributed to stellar emission from a population of evolved Wolf-Rayet (WR) stars while narrow HeII emission has been attributed to nebular emission excited by a population of very hot PopIII stars formed in pockets of pristine gas at moderate redshifts. In this work we propose an alternative scenario for the origin of the narrow HeII emission, namely very massive stars (VMS) at low metallicity (Z) which form strong but slow WR-type stellar winds due to their proximity to the Eddington limit. We estimate the expected HeII line fluxes and equivalent widths based on wind models for VMS and population synthesis models, and compare the results with recent observations of star-forming galaxies at moderate redshifts. The observed HeII line strengths and equivalent widths are in line with what is expected for a population of VMS in one or more young super-clusters located within these galaxies. In our scenario the two observed modes of HeII emission originate from massive stellar populations in distinct evolutionary stages at low Z. If this interpretation is correct there is no need to postulate the existence of PopIII stars at moderate redshifts to explain the observed narrow HeII emission. An interesting possibility is the existence of self-enriched VMS with similar WR-type spectra at extremely low Z. Stellar HeII emission from such very early generations of VMS may be detectable in future studies of star-forming galaxies at high redshifts with the James Webb Space Telescope. The fact that the HeII emission of VMS is largely neglected in current population synthesis models will generally affect the interpretation of the integrated spectra of young stellar populations.

4 pages, 1 figure, A&A letters (accepted)

References in corpus (9)

• The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
• The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
• Effects of rotation on the evolution of primordial stars
• Mass loss from late-type WN stars and its Z-dependence: very massive stars approaching the Eddington limit
• On the Star Formation Rate - Brightest Cluster Relation: Estimating the peak SFR in post-merger galaxies
• The Galactic WC stars: Stellar parameters from spectral analyses indicate a new evolutionary sequence
• The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence
• The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136
• Narrow He II emission in star-forming galaxies at low metallicity. Stellar wind emission from a population of Very Massive Stars

Cited by in corpus (23)

• On the nature of massive helium star winds and Wolf-Rayet-type mass loss
• Theory and Diagnostics of Hot Star Mass Loss
• New Insights into the Evolution of Massive Stars and Their Effects on Our Understanding of Early Galaxies
• Winds from stripped low-mass Helium stars and Wolf-Rayet stars
• Grids of stellar models with rotation IV. Models from 1.7 to 120 Msun at a metallicity Z = 0.0004
• The X-SHOOTER Lyman-$\alpha$ survey at z=2 (XLS-z2) I: What makes a galaxy a Lyman-$\alpha$ emitter?
• Exploring HeII$\lambda$1640 emission line properties at $z\sim 2-4$
• Narrow He II emission in star-forming galaxies at low metallicity. Stellar wind emission from a population of Very Massive Stars
• Very massive stars and Nitrogen emitting galaxies
• Spectroscopic evolution of very massive stars at Z = 1/2.5 Zsun
• Clues on the presence and segregation of very massive stars in the Sunburst Lyman-continuum cluster at z=2.37
• Stars and gas in the most metal-poor galaxies I: COS and MUSE observations of SBS 0335-052E
• Physics and evolution of the most massive stars in 30 Dor. Mass loss, envelope inflation, and a variable upper stellar mass limit
• X-ray properties of He II 1640 emitting galaxies in VANDELS
• Inferring the presence of very massive stars in local star-forming regions
• Spectroscopic evolution of massive stars near the main sequence at low metallicity
• An IFU investigation of possible Lyman continuum escape from Mrk 71/NGC 2366
• UNCOVERing the High-redshift AGN Population among Extreme UV Line Emitters
• The hydrogen clock to infer the upper stellar mass
• Deciphering stellar metallicities in the early Universe: Case study of a young galaxy at z = 4.77 in the MUSE eXtremely Deep Field
• Exploring the Correlation between $\mathrm{H}\alpha$-to-UV Ratio and Burstiness for Typical Star-forming Galaxies at $z\sim 2$
• A meeting at z~3: Young massive galaxies and an AGN within 30kpc of the luminous QSO LBQS0302-0019
• The significant contribution of supersoft X-ray Sources to the nebular HeII line emission