JWST NIRSpec spectroscopy of the triply-lensed $z=10.17$ galaxy MACS0647$-$JD

Abstract

We present JWST/NIRSpec prism spectroscopy of MACS0647-JD, the triply-lensed $z\sim 11$ candidate discovered in HST imaging and spatially resolved by JWST imaging into two components A and B. Spectroscopy of component A yields a spectroscopic redshift $z=10.17$ based on 7 detected emission lines: CIII] $\lambda \lambda$1907,1909, [OII] $\lambda$3727, [NeIII] $\lambda$3869, [NeIII] $\lambda$3968, H$\delta$ $\lambda$4101, H$\gamma$ $\lambda$4340, and [OIII] $\lambda$4363. These are the second-most distant detections of these emission lines to date, in a galaxy observed just 460 million years after the Big Bang. Based on observed and extrapolated line flux ratios we derive a gas-phase metallicity $Z=$ log(O/H) = $7.5-8.0$, or $(0.06-0.2)$ $Z_{\odot }$, ionization parameter log($U$) $\sim -1.9\pm 0.2$, and an ionizing photon production efficiency $\log ({\xi }_{\mathrm{ion}})=25.2\pm 0.2$erg${}^{-1}$ Hz. The spectrum has a softened Lyman-$\alpha$ break, evidence for a strong Ly$\alpha$ damping wing, suggesting that MACS0647-JD was unable to ionize its surroundings beyond its immediate vicinity ($R_{\text{HII}}\ll 1$ pMpc). The Ly$\alpha$ damping wing also suppresses the F150W photometry, explaining the slightly overestimated photometric redshift $z=10.6\pm 0.3$. MACS0647-JD has a stellar mass log($M/M_{\odot }$) = $8.1\pm 0.3$, including $\sim$ 6$\times 10^7{M}_{\odot }$ in component A, most of which formed recently (within $\sim$ 20 Myr) with a star formation rate $2\pm 1M_{\odot }$ / yr, all within an effective radius $70\pm 24$pc. The smaller component B ($r\sim 20$) pc is likely older ($\sim$100 Myr) with more dust ($A_V\sim 0.1$ mag), as found previously. Spectroscopy of a fainter companion galaxy C separated by a distance of \about\ 3kpc reveals a Lyman break consistent with $z=10.17$. MACS0647-JD is likely the most distant galaxy merger known.

22 pages, 6 figures, accepted for publication in ApJ

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