DREAMS. JWST Spectroscopy of a $z=8.3$ Galaxy with an ALMA Dust Continuum Detection: Early Dust, Very High $T_{\mathrm{dust}}$, and a Multi-wavelength [OIII] Ratio Discrepancy

Abstract

We present a deep DREAMS JWST/NIRSpec MSA medium-grating spectrum of MACS0416-Y1, a galaxy at $z=8.312$ with the highest-redshift ALMA dust continuum detection to date, in order to characterize its properties together with archival IFU and ALMA data. The deep NIRSpec spectrum reveals a broad H${\hat{I}}^2$ line with a width of $\sim 1100$ km s${}^{-1}$. We interpret it as a broad-line AGN whose line diagnostics are consistent with AGN activity across its clumpy structure, given the absence of little red dot signatures. MACS0416-Y1 clearly shows [OIII]4363 emission, suggesting a moderately low metallicity of $12+\log (O/H)=7.86_{-0.08}^{+0.09}$ ($0.15Z_{\odot }$). The combination of [CII]158$\hat{I}¼$m and dust continuum emission indicates low dust mass ratios of $\log (M_{\mathrm{dust}}/M_{\mathrm{gas}})=-3.60_{-0.22}^{+0.29}$ and $\log (M_{\mathrm{dust}}/M_{\mathrm{metal}})=-0.95_{-0.20}^{+0.29}$. Because the metallicity of MACS0416-Y1 is around the critical metallicity of $0.1\text{-}0.2Z_{\odot }$, the system is expected to undergo dust growth, explaining these low dust mass ratios as well as its small dust mass, $M_{\mathrm{dust}}\sim 10^6{M}_{\odot }$. The intense UV radiation from the AGN may contribute to a high dust temperature of $T_{\mathrm{dust}}\simeq 91_{-35}^{+62}$ K, boosting the dust-continuum emission above the ALMA detection limit despite the small $M_{\mathrm{dust}}$ at $z>8$. We find a very high total flux ratio of [OIII]88$\hat{I}¼$m/[OIII]5007 = $0.26\pm 0.06$ in MACS0416-Y1, above predictions from single ionized nebular models at any electron density. This discrepancy suggests that the [OIII]88$\hat{I}¼$m and [OIII]5007 trace largely distinct regions, with the optical line suppressed in dusty nebulae, and thus requires careful interpretation when combining optical and infrared emission lines in JWST+ALMA studies.

18 pages, 18 figure, Submitted to ApJ