A Nitrogen-rich AGN Powering a Large Ionizing Bubble at z=8.63

Takahiro Morishita, Massimo Stiavelli, Charlotte A. Mason, Roberta Tripodi, Marco Chiaberge, Stefan Schuldt, Chris J. Willott, Yechi Zhang

First listed 2025-08-02 | Last updated 2026-01-15

Abstract

We report the detection of Ly$α$ in CANUCS-LRD-z8.6, a recently discovered AGN at z = 8.63 by Tripodi et al. (2024), in new NIRSpec/MSA G140H/F070LP observations. We detect broad Ly$α$ emission (FWHM $= 1540 \pm 260$ km/s) near the systemic velocity, which suggests a large ionizing bubble considering that the universe is almost fully neutral at the redshift. Through Ly$α$ line-shape modeling assuming a Stromgren sphere, we find a large bubble radius, $R_b = 1.5^{+0.3}_{-0.2}$ pMpc, and a moderately high Ly$α$ escape fraction, $f_{esc} = 11 \pm 3$ %. The intrinsic line width is inferred to be broad ($2200 \pm 280$ km/s), likely originating in the broad-line region. Existing data indicate that CANUCS-LRD-z8.6 is within a mild overdensity, $δ= 1.8^{+3.0}_{-0.6}$, suggesting that other galaxies in its proximity might have contributed to the formation of the bubble. The high N IV]$λ$1488 / C IV$λ$1548 and N IV]$λ$1488 / O III]$λ$1661 line ratios measured in existing NIRSpec/PRISM data indicate nitrogen enrichment in this metal-poor, low-luminosity AGN. The spectroscopic features are overall similar to other nitrogen-rich galaxies discovered in the literature, such as GN-z11 and GHZ2/GLASSz12. This suggests that CANUCS-LRD-z8.6 may represent one of the evolutionary phases of those nitrogen-rich galaxies.

Short digest

New NIRSpec/MSA G140H/F070LP data reveal broad Lyα in CANUCS-LRD-z8.6 (z=8.63) centered near systemic (FWHM 1540±260 km/s), implying transmission through a large reionized region. Simple Strömgren modeling of the line shape yields an ionized-bubble radius Rb=1.5^{+0.3}_{-0.2} pMpc and a Lyα escape fraction fesc=11±3%. The intrinsic width is even broader (≈2200±280 km/s), consistent with a BLR in a low-luminosity AGN, while high N IV]/C IV and N IV]/O III] ratios from PRISM indicate nitrogen enrichment. A mild local overdensity (δ≈1.8) suggests neighboring galaxies may help sustain the bubble, linking this LRD to the emerging class of nitrogen-rich systems like GN-z11 and GHZ2/GLASSz12.

Key figures to inspect

• Figure 1 (NIRCam stamps): Check the blue-UV/red-optical LRD colors and the lensing environment near MACS J1149; verify that the western extended source is a lower-z interloper and does not contaminate the target.
• Figure 2 (G140H Lyα): Inspect the 2D/1D Lyα profile peaking near systemic; compare the fitted model to the dashed intrinsic line to see how the inferred Rb≈1.5 pMpc and modest damping produce the observed shape and the measured FWHM≈1540 km/s.
• Figure 3 (PRISM spectrum): Use the continuum-subtracted spectrum and line labels to verify strong N IV] λ1488 and measure N IV]/C IV and N IV]/O III] ratios that signal nitrogen enrichment in a metal-poor AGN.
• Figure 4 (line widths): Confirm that all narrow UV lines are unresolved at PRISM resolution while the broad component stands out, supporting the BLR-derived intrinsic width ≈2200 km/s.