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arXiv · 1703.02044

Magnifying the early episodes of star formation: super star clusters at cosmological distances

E. Vanzella·M. Castellano·M. Meneghetti·A. Mercurio·G. B. Caminha·G. Cupani·F. Calura·L. Christensen·E. Merlin·P. Rosati·M. Gronke·M. Dijkstra·M. Mignoli·R. Gilli·S. De Barros·K. Caputi·C. Grillo·I. Balestra·S. Cristiani·M. Nonino·E. Giallongo·A. Grazian·L. Pentericci·A. Fontana·A. Comastri·C. Vignali·G. Zamorani·M. Brusa·P. Bergamini·P. Tozzi
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Abstract

We study the spectrophotometric properties of a highly magnified (\mu~40-70) pair of stellar systems identified at z=3.2222 behind the Hubble Frontier Field galaxy cluster MACS~J0416. Five multiple images (out of six) have been spectroscopically confirmed by means of VLT/MUSE and VLT/X-Shooter observations. Each image includes two faint (m_uv~30.6), young (<100 Myr), low-mass (<10^7 Msun), low-metallicity (12+Log(O/H)~7.7, or 1/10 solar) and compact (30 pc effective radius) stellar systems separated by ~300pc, after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow (\sigma_v <~ 25 km/s) high-ionization lines. These features may be the signature of very hot (T>50000 K) stars within dense stellar clusters, whose dynamical mass is likely dominated by the stellar component. Remarkably, the ultraviolet metal lines are not accompanied by Lya emission (e.g., CIV / Lya > 15), despite the fact that the Lya line flux is expected to be 150 times brighter (inferred from the Hbeta flux). A spatially-offset, strongly-magnified (\mu>50) Lya emission with a spatial extent <~7.6 kpc^2 is instead identified 2 kpc away from the system. The origin of such a faint emission can be the result of fluorescent Lya induced by a transverse leakage of ionizing radiation emerging from the stellar systems and/or can be associated to an underlying and barely detected object (with m_uv > 34 de-lensed). This is the first confirmed metal-line emitter at such low-luminosity and redshift without Lya emission, suggesting that, at least in some cases, a non-uniform covering factor of the neutral gas might hamper the Lya detection.

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E. Vanzella, M. Castellano, M. Meneghetti, A. Mercurio, G. B. Caminha, G. Cupani, F. Calura, L. Christensen, E. Merlin, P. Rosati, M. Gronke, M. Dijkstra, M. Mignoli, R. Gilli, S. De Barros, K. Caputi, C. Grillo, I. Balestra, S. Cristiani, M. Nonino, E. Giallongo, A. Grazian, L. Pentericci, A. Fontana, A. Comastri, C. Vignali, G. Zamorani, M. Brusa, P. Bergamini, P. Tozzi. 2017-03-06. Magnifying the early episodes of star formation: super star clusters at cosmological distances. https://doi.org/10.3847/1538-4357%2Faa74ae

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