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MIRI spectrophotometry of GN-z11: Detection and nature of an optical red continuum component

A. Crespo Gómez, L. Colina, P. G. Pérez-González, J. Álvarez-Márquez, M. García-Marín, A. Alonso-Herrero, M. Annunziatella, A. Bik, S. Bosman, A. J. Bunker, A. Labiano, D. Langeroodi, P. Rinaldi, G. Östlin, L. Boogaard, S. Gillman, G. Barro, S. L. Finkelstein, G. C. K. Leung

TL;DR

GN-z11, a galaxy at $z \approx 10.6$, is studied with JWST to extend rest-frame optical coverage up to $0.86\,\mu$m using new MIRI imaging and archival spectroscopy. The continuum shows a rise beyond rest-frame $0.66\,\mu$m that cannot be explained by a single stellar population; SED fitting with CIGALE and SYNTHESIZER-AGN favors a mixed stellar population plus a red component consistent with either a dusty torus around a type 2 AGN or hot dust associated with a compact starburst. Type 1 AGN scenarios are disfavoured unless an ad-hoc red component is invoked; the data imply a possible torus with $R_T \sim 3$–$9$ pc and $M_{\mathrm{dust}} \sim 0.5$–$1.5 \times 10^5\,M_\odot$, while X-ray non-detections are compatible with this picture. The authors emphasize the need for deeper MIRI imaging (e.g., F1000W, F1280W) and spectroscopy to confirm the 0.86 μm flux and to discriminate between AGN and extreme starburst interpretations, with important implications for the growth of black holes and dusty environments in the early universe.

Abstract

We present new MIRI F560W, F770W and F1000W imaging of GN-z11, extending the previous rest-frame coverage from 0.38 to 0.86$μ$m. We report significant detections (14$σ$) in the F560W and F770W images, and a marginal detection (3.2$σ$) in F1000W. Here, we analyse its SED combining new MIRI imaging data with archival NIRSpec/Prism and MRS spectroscopy, and NIRCam imaging. The continuum emission shows a flat energy distribution, in f$_ν$, up to 0.5$μ$m, compatible with the presence of a mixed stellar population of young (4$\pm$1 Myr) and mature (63$\pm$23 Myr) stars that also account for the [O III], H$β$ and H$α$ emission lines. The continuum at rest-frame 0.66$μ$m shows a 36$\pm$3% flux excess above the predicted flux for a mixed stellar population, pointing to the presence of an additional source contributing at these wavelengths. This excess increases to 91$\pm$28% at rest-frame 0.86$μ$m, although with a large uncertainty due to the marginal detection in the F1000W filter. We consider that hot dust emission in the dusty torus around a type 2 AGN could be responsible for the observed excess. Alternatively, this excess could be due to hot dust emission or to a photoluminiscence dust process (Extended Red Emission, ERE) under the extreme UV radiation field, as observed in some local metal-poor galaxies and in young compact starbursts. The presence of a type 1 AGN is not supported by the observed SED since high-z QSOs contribute at wavelengths above rest-frame 1$μ$m, and an additional ad-hoc red source would be required to explain the observed flux excess at 0.66 and 0.86$μ$m. Additional deep MIRI imaging covering the rest-frame near-IR are needed to confirm the flux detection at 10$μ$m with higher significance, and to discriminate between the different hot dust emission in the extreme starburst and AGN scenarios with MIRI imaging at longer wavelengths.

MIRI spectrophotometry of GN-z11: Detection and nature of an optical red continuum component

TL;DR

GN-z11, a galaxy at , is studied with JWST to extend rest-frame optical coverage up to m using new MIRI imaging and archival spectroscopy. The continuum shows a rise beyond rest-frame m that cannot be explained by a single stellar population; SED fitting with CIGALE and SYNTHESIZER-AGN favors a mixed stellar population plus a red component consistent with either a dusty torus around a type 2 AGN or hot dust associated with a compact starburst. Type 1 AGN scenarios are disfavoured unless an ad-hoc red component is invoked; the data imply a possible torus with – pc and –, while X-ray non-detections are compatible with this picture. The authors emphasize the need for deeper MIRI imaging (e.g., F1000W, F1280W) and spectroscopy to confirm the 0.86 μm flux and to discriminate between AGN and extreme starburst interpretations, with important implications for the growth of black holes and dusty environments in the early universe.

Abstract

We present new MIRI F560W, F770W and F1000W imaging of GN-z11, extending the previous rest-frame coverage from 0.38 to 0.86m. We report significant detections (14) in the F560W and F770W images, and a marginal detection (3.2) in F1000W. Here, we analyse its SED combining new MIRI imaging data with archival NIRSpec/Prism and MRS spectroscopy, and NIRCam imaging. The continuum emission shows a flat energy distribution, in f, up to 0.5m, compatible with the presence of a mixed stellar population of young (41 Myr) and mature (6323 Myr) stars that also account for the [O III], H and H emission lines. The continuum at rest-frame 0.66m shows a 363% flux excess above the predicted flux for a mixed stellar population, pointing to the presence of an additional source contributing at these wavelengths. This excess increases to 9128% at rest-frame 0.86m, although with a large uncertainty due to the marginal detection in the F1000W filter. We consider that hot dust emission in the dusty torus around a type 2 AGN could be responsible for the observed excess. Alternatively, this excess could be due to hot dust emission or to a photoluminiscence dust process (Extended Red Emission, ERE) under the extreme UV radiation field, as observed in some local metal-poor galaxies and in young compact starbursts. The presence of a type 1 AGN is not supported by the observed SED since high-z QSOs contribute at wavelengths above rest-frame 1m, and an additional ad-hoc red source would be required to explain the observed flux excess at 0.66 and 0.86m. Additional deep MIRI imaging covering the rest-frame near-IR are needed to confirm the flux detection at 10m with higher significance, and to discriminate between the different hot dust emission in the extreme starburst and AGN scenarios with MIRI imaging at longer wavelengths.

Paper Structure

This paper contains 17 sections, 1 equation, 5 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Observation and data processing
  3. JWST/MIRI data and calibration
  4. Ancillary JWST data: photometry and spectroscopy
  5. Analysis
  6. Aperture photometry and NIRSpec renormalization
  7. Continuum-only fluxes and EWs
  8. SED fitting
  9. Results and discussion
  10. Continuum-only fluxes and colour-colour diagrams
  11. SF-only SED modelling
  12. SF + AGN SED modelling
  13. Origin of the red excess emission
  14. Dust in AGN: Dusty torus emission
  15. Dust emission in compact starbursts
  16. ...and 2 more sections

Figures (5)

  • Figure 1: GN-z11 images in the NIRCam/F200W, MIRI/F560W, MIRI/F770W and MIRI/F1000W filters. The NIRCam/F200W image was convolved to match the MIRI/F560W resolution (see Sect. \ref{['subsec:phot']}). Dashed circles represent the 0.5$\arcsec$ aperture used for the photometry extraction. White circles on the lower right corners of each panel represent the FWHM for each filter.
  • Figure 2: Continuum-only colour-colour diagrams. Black squares represent the GN-z11 colours based on the NIRCam and MIRI photometry after the subtraction of the emission lines contribution measured with NIRSpec and MRS (i.e., continuum-only colours) presented in Table \ref{['tab:fluxes']}. Continuum (dashed) lines display the continuum-only colours derived considering a stellar population colour-coded by their age assuming an instantaneous (continuum) SFH. These colours include the contribution from the stellar continuum calculated using the stellar models from BPASSv2.2 and its associated nebular continuum derived with Cloudy (see Sect. \ref{['subsec:Cont-only']}). Thin and thick lines represent the 0.05 and 0.2 $Z_\mathrm{\odot}$ tracks, respectively. Orange, green and purple diamonds show the derived continuum-only colours from the composed $z$$\sim$ 1 QSO (Glikman+06Selsing+16, see Sect. \ref{['subsec:SED_fitting']}), the Seyfert 2 Mrk3 Spinelli+06 and a $z$ = 7.1 QSO Bosman2024Bosman+25. Red, blue and brown diamonds represent the values derived from a stacked spectra of $\sim$ 150 LRDs (PerezGonzalez-LRD2024, Pérez-González et al. in prep.), the COSMOS-Webb median-stacked LRD Akins+25 and the unique LRD known as 'The Cliff' deGraaff+25, respectively. In the left and middle panels the brown diamond lie outside the colour range displayed. Black arrows in the lower right corners represent the vector magnitude for $A_\mathrm{V}$ = 0.3 mag.
  • Figure 3: NIRCam and MIRI photometry compared with different red SED objects. Grey squares and black circles represent the NIRCam+MIRI observed and continuum-only (see Sect. \ref{['subsec:Cont.-results']}) fluxes, respectively. Coloured lines represent the SEDs for different AGNs types and high-$z$ objects with optical-red colours presented in Sect. \ref{['subsec:Cont.-results']}. Red line represents the stacked spectra for $\sim$ 150 local and high-$z$ LRDs (PerezGonzalez-LRD2024, Pérez-González et al. in prep.). Brown line displays the spectra from the exotic $z$$\sim$ 3.5 LRD 'The Cliff' while brown diamonds represent its re-normalised fluxes from deGraaff+25 at rest- 0.9 and 1.9 $\mu$m. Blue line shows the COSMOS-Webb median-stacked LRD SED model presented in Akins+25. Orange and purple lines show the $z$$\sim$ 1 QSO composite spectrum, created by combining the UV+optical and near-IR spectral templates from Selsing+16 and Glikman+06, and the NIRSpec+MRS spectrum for the $z$ = 7.1 QSO J1120+0641 (Bosman2024Bosman+25), respectively. The HST/STIS spectrum for the Sy2 Mrk3 presented by Spinelli+06 is displayed as a green line. All observed spectra have been redshifted to $z$ = 10.6 and normalised at 4800Å.
  • Figure 4: CIGALE SED models derived from the single and mixed stellar population scenarios described in Sect. \ref{['subsec:Onlystellar']}. Blue (orange) line and squares represent the SED models and the expected fluxes for NIRCam/MIRI filters derived using a single $\tau$-decaying (mixed) stellar population. Black line and triangles represent the NIRSpec/Prism spectra and the NIRCam+MIRI photometry, respectively. The blue and orange lines have been convolved to match de NIRSpec/Prism resolution (R=100).
  • Figure 5: SED best-fit models considering the star formation and the different AGN scenarios. Top panel show the best-fit SEDs obtained with CIGALE assuming a type 1 and type 2 AGNs as blue and green lines, respectively. Red line displays the best-fit derived with SYNTHESIZER-AGN using a QSO template and an additional dusty torus. Black line and black circles represent the NIRSpec/Prism data presented in Bunker+23 and the NIRCam and MIRI photometry as derived in Sect. \ref{['subsec:phot']}, respectively. coloured squares show the modelled NIRCam and MIRI fluxes from the different best-fit models, while their residuals with the measured photometry are displayed below. Green, blue and red lines have been convolved to match the spectral resolution from NIRSpec/Prism (i.e., R=100). Middle panel shows the CIGALE best-fit models (in black) and the contributions of the nebular and stellar components (in blue) and the Fritz+06 torus emission (in brown). Dashed and continuos lines differentiate the results drawn from the SED fitting assuming a type 1 and type 2 AGN, respectively. Bottom panel displays the SYNTHESIZER-AGN best-fit (in black) along with the contributions of the nebular and stellar components (in blue), the QSO template (in magenta) and the dusty torus (in brown).