High-resolution Spectroscopy of a Young, Low-metallicity Optically Thin L = 0.02L* Star-forming Galaxy at z = 3.12
Schaerer, D.; Giavalisco, M.; Castellano, M.; Pentericci, L.; Vignali, C.; Gilli, R.; Grillo, C.; Comastri, A.; Brusa, M.; Balestra, I.; Rosati, P.; Fontana, A.; Vanzella, E.; Grazian, A.; Cristiani, S.; Giallongo, E.; Caputi, K.; Meneghetti, M.; Nonino, M.; Mignoli, M.; De Barros, S.; Cupani, G.; Karman, W.; Gronke, M.; Coe, D.; Calura, F.; Caminha, G. -B.; Christensen, L.; Dijkstra, M.; Koekemoer, A.; Verhamme, A.; Zamorani, G.
Italy, Netherlands, Norway, Germany, United States, Denmark, Switzerland
Abstract
We present VLT/X-Shooter and MUSE spectroscopy of a faint F814W = 28.60 ± 0.33 ({M}{UV}=-17.0), low-mass (≲{10}7{M}⊙ ), and compact (R eff = 62 pc) freshly star-forming galaxy at z = 3.1169 magnified (16×) 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=λ /dλ ∼ 3000{--}7400). We measured C IV λ 1548,1550, He II λ 1640, O III]λ 1661,1666, C III]λ λ 1907,1909, Hβ, [O III]λ λ 4959,5007 emission lines with {FWHM}≲ 50 km s-1 and (de-lensed) fluxes spanning the interval 1.0× {10}-19{--}2× {10}-18 erg s-1 cm-2 at signal-to-noise ratio (S/N) = 4-30. The double-peaked Lyα emission with {{Δ }}v({red}-{blue})=280(±7) km s-1 and de-lensed fluxes {2.4}({blue)}| {8.5}({red)}× {10}-18 erg s-1 cm-2 (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 [O III]λ 5007/ [O II]λ 3727 \gt \quad 10 ratio. We detect C IV λ 1548,1550 resonant doublet in emission, each component with {FWHM}≲ 45 km s-1 and redshifted by +51(±10) km s-1 relative to the systemic redshift. We interpret this as nebular emission tracing an expanding optically thin interstellar medium. Both C IV λ 1548,1550 and He II λ 1640 suggest the presence of hot and massive stars (with a possible faint active galactic nucleus). The ultraviolet slope is remarkably blue, β =-2.95+/- 0.20 ({F}λ ={λ }β ), consistent with a dust-free and young ≲20 Myr galaxy. Line ratios suggest an oxygen abundance 12 + log(O/H)\quad \lt \quad 7.8. We are witnessing an early episode of star formation in which a relatively low N H I 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 the James Webb Space Telescope.
Based on observations collected at the European Southern Observatory for Astronomical research in the Southern Hemisphere under ESO programs P095.A-0840, P095.A-0653, P186.A-0798.