Insight into the starburst nature of Galaxy GN-z11 with JWST MIRI spectroscopy

¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, E-28850 Madrid, Spain
² Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA
³ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155A, 2200 Copenhagen, Denmark
⁴ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁵ Departamento de Física de la Tierra y Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain
⁶ Department of Astronomy, Stockholm University, Oscar Klein Centre, AlbaNova University Centre, 106 91 Stockholm, Sweden
⁷ Centro de Astrobiología (CAB), CSIC-INTA, Camino Viejo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁸ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁹ Cosmic Dawn Centre (DAWN), Copenhagen, Denmark
¹⁰ European Space Agency, Space Telescope Science Institute, Baltimore, Maryland, USA
¹¹ DTU Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs., Lyngby, Denmark
¹² Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800 9700 AV Groningen, The Netherlands
¹³ Telespazio UK for the European Space Agency, ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
¹⁴ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁶ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

^⋆ Corresponding author; jalvarez@cab.inta-csic.es

Received: 31 July 2024
Accepted: 10 December 2024

Abstract

This paper presents a deep MIRI/JWST medium-resolution spectroscopy (MRS) covering the rest-frame optical spectrum of the GN-z11 galaxy. The [O III] 5008 Å and Hα emission lines are detected and spectroscopically resolved. The line profiles are well modeled by a narrow Gaussian component with intrinsic full widths at half maximum of 189 ± 25 and 231 ± 52 km s⁻¹, respectively. We do not find any evidence of a dominant broad Hα emission line component tracing a broad-line region in a type 1 active galactic nucleus (AGN). The existence of an accreting black hole dominating the optical continuum and emission lines of GN-z11 is not compatible with the measured Hα and [O III] 5008 Å luminosities. If the well-established relations for low-z AGNs apply in GN-z11, the [O III] 5008 Å and Hα luminosities would imply extremely high super-Eddington ratios (λ_E > 290), and bolometric luminosities ∼20 times those derived from the UV/optical continuum. However, a broad (∼430–470 km s⁻¹) and weak (< 20–30%) Hα line component, tracing a minor AGN contribution in the optical, cannot be completely ruled out with the sensitivity of the current data. The physical and excitation properties of the ionized gas are consistent with a low-metallicity starburst with a star formation rate of 24 ± 3 M_⊙ yr⁻¹. The electron temperature of the ionized gas is T_e (O⁺⁺) = 14 000 ± 2100 K, while the direct-T_e gas-phase metallicity is 12 + log(O/H) = 7.91 ± 0.07 (Z = 0.17 ± 0.03 Z_⊙). The optical line ratios locate GN-z11 in the starburst or AGN region, but they are more consistent with those of local low-metallicity starbursts and high-z luminous galaxies detected at redshifts similar to GN-z11. We conclude that the MRS optical spectrum of GN-z11 is consistent with that of a massive, compact, and low-metallicity starburst galaxy. Its high star formation and stellar mass surface densities are close to those of the densest stellar clusters, and we therefore speculate that GN-z11 might undergo a feedback-free, highly efficient starburst phase. Additional JWST data are needed to validate this scenario and other recently proposed alternatives to explain the existence of bright compact galaxies in the early Universe.