EGS-z11-R0: a red, dust-rich galaxy at Cosmic Dawn

Abstract

Context. Galaxies discovered by JWST at z > 10 are predominantly characterized by extremely blue rest-frame UV slopes. Conversely, the existence of dust-reddened systems at such early epochs has remained largely unconfirmed spectroscopically. Aims. We present the spectroscopic confirmation of EGS-z11-R0 at z = 11.45, the most distant red galaxy identified to date, discovered serendipitously through inspection of publicly available JWST/NIRSpec data. Methods. We analyze JWST/NIRSpec PRISM and G395M spectroscopy together with multiwavelength HST, NIRCam, and MIRI photometry. We identify significant detections of the C IV 1548,1551 and C III] 1908 transitions, yielding a redshift of zspec = 11.452 +/- 0.021. We measure rest-frame UV emission-line fluxes and equivalent widths and use these diagnostics to constrain the nature of the ionizing radiation field. Finally, we perform spectral energy distribution modeling with CIGALE, combining spectroscopy with photometry and including stellar, nebular, dust, and active galactic nuclei (AGN) components. Results. EGS-z11-R0 exhibits a red UV continuum slope (betaUV approximately -1.0), placing it well above the canonical MUV-betaUV relation at z~10-12 and making it the highest-z spectroscopically confirmed member of the emerging "red monster" population. Emission-line diagnostics reveal a hard ionizing spectrum consistent with extreme star formation and compatible with a composite stellar plus AGN scenario. The best-fit SED solution favors a stellar mass of log(M*/Msun) ~ 9.2-9.6, a SFR of 10-40 Msun yr^-1, and substantial dust attenuation (AV~1.2 mag). Conclusions. The confirmation of EGS-z11-R0 establishes that chemically evolved, dust-enriched galaxies were already in place at z approximately 11.5. [abridged]

Figures (21)

• Figure 1: Observed 2D spectrum (top panel) and extracted 1D spectrum (bottom panel) of EGS-z11-R0. The pipeline-provided error spectrum is shown in light gray. Emission lines with integrated S/N $> 3$ are highlighted in blue, while the locations of lines for which only a $3\sigma$ upper limit is obtained are marked in red.
• Figure 2: Top and bottom panels: Rest-frame UV spectral regions of EGS-z11-R0 extracted from the NIRSpec prism data. Zoom-in around the Civ emission feature and the Ciii] doublet, respectively. The gray curves show the observed spectrum, while the shaded regions represent the 1$\sigma$ uncertainties.
• Figure 3: Multiwavelength photometric cutouts of EGS-z11-R0: NIRCam bands (upper row, from DJA mosaics) and MIRI (bottom right row). For HST, we report the stacked frame of all the available bands (bottom left), confirming the dropout nature of the source. Each image has a size of 1"$\times$1".
• Figure 4: RGB NIRCam image of EGS-z11-R0. The gray boxes show the MSA shutters of the NIRSpec/PRISM observation. The stamp has a size of 2.5"$\times$2.5" and it is oriented as indicated by the arrows. The image was created using Trilogy and GIMP v 3.0.8 by assigning the F090W, F115W filters to the B channel, F150W, F200W, and F277W to the G channel and F356W, F410M, and F444W to the R channel.
• Figure 5: UV continuum slope $\beta_{\rm UV}$ as a function of absolute UV magnitude $M_{\mathrm{UV}}$ at $z \sim 10$-12. Gray circles show literature measurements from Cullen23Cullen2024, while colored symbols denote individual sources from Casey2024Austin2025Asada2026, and Mitsuhashi2025. Star symbols highlight the subset of Mitsuhashi2025 sources with $\beta_{\rm UV} > -1.5$ and EGS-z11-R0(this work; filled red star for the $\beta_{\rm UV}$ from spectroscopy and empty red star for the $\beta_{\rm UV}$ from the photometry alone). The dashed blue and red lines represent the best-fit $\beta_{\rm UV}$-$M_{\mathrm{UV}}$ relations from Cullen2024 for $10 < z < 11$ and $11 < z < 12$, respectively, with shaded regions indicating their associated uncertainties.