PRISMS. UNCOVER-26185, a metal-poor SFG at z=10.05 with no evidence for a X-ray-luminous AGN

TL;DR

PRISMS uses JWST/MIRI LRS to obtain rest-frame optical spectroscopy of a sample of galaxies around $z\\sim 10$, focusing on intermediate UV luminosities to reveal ISM conditions and chemical evolution. For UNCOVER-26185, deep spectroscopy combined with NIRSpec, NIRCam, and ALMA data yields a two-component SFH, extremely low metallicity ($12+\\log(O/H) \\approx 7.3$–$7.5$) and a low ionization parameter ($\\log U \\approx -2.5$), with a high ionizing photon production efficiency ($\\log \xi_{ion} \\approx 25.50$). The emission-line diagnostics show a metal-poor, star-forming ISM with negligible dust and no evidence for a luminous AGN, challenging the prior interpretation of UNCOVER-26185 as hosting a Compton-thick AGN. The galaxy, with $M_∗ \\approx 1.7\\times 10^{8} M_⊙$ and SFR \\approx 1.3 M_⊙$ yr$^{-1}$, lies on the high-$z$ star-forming main sequence and represents a typical, faint population at cosmic dawn, underscoring PRISMS’ goal to map metallicity and ISM properties across a broader UV-luminosity range at $z\\sim 10$.

Abstract

This work presents the first results of the PRImordial galaxy Survey with MIRI Spectroscopy (PRISMS), a JWST cycle 4 program (PID 8051) aimed at the characterization of a relatively large sample of ten galaxies about 500 Myr after the Big Bang. Here, we present deep (13.9 hours) spectroscopy with the MIRI LRS of the lensed galaxy UNCOVER-26185 at a redshift of z=10.054. It is a faint UV galaxy (UV absolut magnitude of -18.83 mag) previously identified as a X-ray luminous AGN. MIRI LRS detects the H$β$+[OIII]4960,5008 complex and H$α$ emission line with a significance of 10$σ$ and 8$σ$, respectively, as well as the optical continuum emission at rest-frame 0.45 $μ$m and 0.57 $μ$m with a signal-to-noise ratio of 6-7. The UV-to-optical spectral energy distribution, combining continuum and emission lines, is compatible with: (i) a low stellar (A$_V$= 0.2) and nebular (A$_V$=0.0) extinction, (ii) a SFH composed by a young (7 Myr) starburst and an intermediate-age (65 Myr) stellar population, and (iii) a total stellar mass of 1.7$\times$10$^{8}$ M$_{\odot}$. The H$α$-derived star-formation rate is 1.3 M$_{\odot}$ yr$^{-1}$. The low optical emission line ratios locate UNCOVER-26185 as the most metal-poor (Z = 0.04 Z$_{\odot}$), and as outlier with the lowest ionization (logU=-2.5) galaxy identified so far at redshifts above 9. With no evidence of an active galactic nuclei in the rest-frame UV-to-optical spectrum, UNCOVER-26185 has the properties of a metal-poor, main-sequence star-forming galaxy at redshift 10, with ISM and ionization properties very different than those of the already studied UV-bright galaxies at redshifts beyond 10. PRISMS is starting to explore the population of intermediate-UV luminosity galaxies at z=10, covering UV absolute magnitudes in the range of -17.9 to -20.5, fainter than those of UV-bright galaxies studied so far.

Figures (5)

• Figure 1: MIRI LRS spectrum and emission line fitting of U28165 galaxy at a redshift of 10.054 $\pm$ 0.011. Upper left panel: area of the MIRI LRS 2D detector combined image with a total exposure time of 13.9 hours. The dashed black lines define the aperture of 0.44 arcsec used to extract the 1D spectrum. Bottom left panel: MIRI LRS 1D extracted spectrum and continuum fluxes. Black and gray area: 1D extracted spectrum showing the detection of the H$\beta$+[OIII] complex and the H$\alpha$ emission line. Red dots and triangle: stacked MIRI LRS continuum fluxes and 3$\sigma$ upper-limit at 5.0, 6.3, and 7.8 $\mu$m. Orange dashed line: 1$\sigma$ error of the MIRI LRS spectrum. Upper right panel: three-component Gaussian fit of the H$\beta$+[OIII] complex (purple line and area). Green, cyan, and blue lines represent the H$\beta$, [OIII]4960$\AA$, and [OIII]5008$\AA$, respectively. Bottom right panel: one-component Gaussian fit of H$\alpha$ (purple line and area). Bottom panels of right panels are the residual of the line fitting.
• Figure 2: SED-fitting analysis using the magnified, scaled fluxes of all detected lines and continuum emission of U26185 at redshift of 10.05. The analysis combine the NIRCam photometry (Blue dots), NIRSpec R100 spectrum (gray line), MIRI LRS derived continuum photometry and [OIII]$\lambda$5008$\AA$ and H$\alpha$ fluxes (Red dots for detections and red triangles for 3$\sigma$ upper-limits), and the ALMA Band 7 continuum and [OIII]$\lambda$88$\mu$m 3$\sigma$ upper-limits (Purple triangles). The best-fit CIGALE model is represented with dashed back line, and its synthetic NIRCam photometry, and MIRI LRS and ALMA continuum fluxes and emission lines fluxes are represented by yellow crosses.
• Figure 3: Line ratio diagnostic diagrams. Red star and colored squares and circles represent U26185 and the high-z and N-rich samples, as presented in Sect. \ref{['Sect:lineratios']}. Gray, blue and red points display a sample of local metal-poor galaxies Izotov+06 and high-z SF and AGN galaxies from the main JWST surveys (Sect. \ref{['Sect:lineratios']} for further details), respectively. Colored squares and circles represent recent JWST-observed $z>9$ galaxies and $z>5$ N-rich galaxies, respectively. Blue, orange, red, green and purple diamonds display the stacked values for the $z>$ [5, 6, 7, 8, 9] galaxy subsamples from Roberts-Borsani+24, respectively. Solid and dashed lines represent the CLOUDY photo-ionization models presented in Calabro+23 for the AGN and SF scenarios, respectively. Increasingly darker blue lines represent an increasing log($U$)=[-3, -2.5, -2] values while the metallicities are displayed as colored markers.
• Figure 4: Metallicity relations based on the different line ratios. Grey points and colored squares and circles display the samples of local metal-poor and high-$z$ galaxies, as in Fig. \ref{['fig:line_ratios']}. Solid, dashed and dotted line represent the best-fit expressions derived for each line ratio by Sanders+25, Chakraborty+25 and Nakajima+22, respectively. Red horizontal line and shade mark the U26185 value and its uncertainty, respectively.
• Figure 5: The stellar mass-metallicity plane. The dotted black lines represent the observationally inferred relationships at $z$$\approx$ 0.08 curti+20, $z$$\approx$ 3 Li2023_MZR, and $z\approx4-9$Nakajima+23Curti+24Sarkar+25. Squares represent $z$$>$ 9 galaxies with NIRSpec and/or MIRI detections (color coded following Figure \ref{['fig:line_ratios']}). The red star is, U 26185, as constrained in this work. For comparison, we also include $z\sim9-10$ predictions from simulations as follows: FIRE Ma2016, ILLUSTRIS-TNG Torrey2019, ASTRAEUS Ucci2023, FIRE-2 Marszewski2024.