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arXiv · 1603.01616

High-resolution spectroscopy of a young, low-metallicity optically-thin L=0.02L* star-forming galaxy at z=3.12

E. Vanzella·S. De Barros·G. Cupani·W. Karman·M. Gronke·I. Balestra·D. Coe·M. Mignoli·M. Brusa·F. Calura·G. -B. Caminha·K. Caputi·M. Castellano·L. Christensen·A. Comastri·S. Cristiani·M. Dijkstra·A. Fontana·E. Giallongo·M. Giavalisco·R. Gilli·A. Grazian·C. Grillo·A. Koekemoer·M. Meneghetti·M. Nonino·L. Pentericci·P. Rosati·D. Schaerer·A. Verhamme·C. Vignali·G. Zamorani
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Abstract

We present VLT/X-Shooter and MUSE spectroscopy of an faint F814W=28.60+/-0.33 (Muv=-17.0), low mass (~<10^7 Msun) and compact (Reff=62pc) freshly star-forming galaxy at z=3.1169 magnified (16x) by the Hubble Frontier Fields galaxy cluster Abell S1063. Gravitational lensing allows for a significant jump toward low-luminosity regimes, in moderately high resolution spectroscopy (R=lambda/dlambda ~ 3000-7400). We measured CIV1548,1550, HeII1640, OIII]1661,1666, CIII]1907,1909, Hbeta, [OIII]4959,5007, emission lines with FWHM< 50 km/s and (de-lensed) fluxes spanning the interval 1.0x10^-19 - 2.0x10^-18 erg/s/cm2 at S/N=4-30. The double peaked Lya emission with Delta_v(red-blue) = 280(+/-7)km/s and de-lensed fluxes 2.4_(blue)|8.5_(red)x10^-18 erg/s/cm2 (S/N=38_(blue)|110_(red)) indicate a low column density of neutral hydrogen gas consistent with a highly ionized interstellar medium as also inferred from the large [OIII]5007/[OII]3727>10 ratio. We detect CIV1548,1550 resonant doublet in emission, each component with FWHM ~< 45 km/s, and redshifted by +51(+/-10)km/s relative to the systemic redshift. We interpret this as nebular emission tracing an expanding optically-thin interstellar medium. Both CIV1548,1550 and HeII1640 suggest the presence of hot and massive stars (with a possible faint AGN). The ultraviolet slope is remarkably blue, beta =-2.95 +/- 0.20 (F_lambda=lambda^beta), consistent with a dust-free and young ~<20 Myr galaxy. Line ratios suggest an oxygen abundance 12+log(O/H)<7.8. We are witnessing an early episode of star-formation in which a relatively low NHI and negligible dust attenuation might favor a leakage of ionizing radiation. This galaxy currently represents a unique low-luminosity reference object for future studies of the reionization epoch with JWST.

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E. Vanzella, S. De Barros, G. Cupani, W. Karman, M. Gronke, I. Balestra, D. Coe, M. Mignoli, M. Brusa, F. Calura, G. -B. Caminha, K. Caputi, M. Castellano, L. Christensen, A. Comastri, S. Cristiani, M. Dijkstra, A. Fontana, E. Giallongo, M. Giavalisco, R. Gilli, A. Grazian, C. Grillo, A. Koekemoer, M. Meneghetti, M. Nonino, L. Pentericci, P. Rosati, D. Schaerer, A. Verhamme, C. Vignali, G. Zamorani. 2016-03-04. High-resolution spectroscopy of a young, low-metallicity optically-thin L=0.02L* star-forming galaxy at z=3.12. https://doi.org/10.3847/2041-8205/821/2/l27

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