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

TL;DR

CANUCS-LRD-z8.6, a nitrogen-rich AGN at z=8.63, exhibits broad Lyα emission near the systemic velocity, implying a large ionizing bubble in a largely neutral IGM. Through Lyα line-profile modeling within a Strömgren-sphere framework and supporting NIRSpec Prism lines, the study derives a bubble radius of $R_b = 1.5_{-0.2}^{+0.3}$ pMpc and a Lyα escape fraction of $f_{esc} = 11 \\pm 3\%$, with an intrinsic Lyα width of $v_{intr} \,\approx\, 2200 \,\pm\, 280$ km s$^{-1}$, consistent with an AGN broad-line region of $v_{BLR} \sim 3600 \,\pm\, 500$ km s$^{-1}$. The source lies in a mild overdensity with $\delta = 1.8_{-0.6}^{+3.0}$, suggesting nearby galaxies may contribute to the bubble, while strong N IV] 1488 emission indicates nitrogen enrichment in metal-poor gas, aligning CANUCS-LRD-z8.6 with other nitrogen-rich high-z galaxies. Together, these findings illuminate an early phase of nitrogen-enriched AGN activity and demonstrate how AGN and environment jointly shape the ionization state during the epoch of reionization.

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.

Figures (8)

• Figure 1: Postage stamps of CANUCS-LRD-z8.6 in JWST/NIRCam filters in the cutout size of $3.\!"2$. A pseudo color image (F150W/F200W/F480M for blue, green, and red) is also shown. The extended object to the west is a foreground source at $z_{\rm phot}\sim0.4$.
• Figure 2: (Top): Two-dimensional NIRSpec G140H/F070LP spectrum of CANUCS-LRD-z8.6. The wavelength locations of ${\rm Ly\alpha}$ and $\textrm{N}~\textsc{v}_{\lambda1240}$ are indicated (red lines). (Bottom): One-dimensional spectrum, in the original (gray solid lines) and binned (red) pixel scales. The forward-modeled ${\rm Ly\alpha}$ line profile (green solid line), and its intrinsic model (cyan dashed line), inferred from the IGM transmission modeling (Sec. \ref{['sec:emi_g140']}), are shown. An asymmetric Gaussian model (Eq. \ref{['eq:1']}) is also shown (orange solid line).
• Figure 3: Ionized bubble size distributions of CANUCS-LRD-z8.6 and galaxies at $z>6$ in the literature. Sizes for COLA1 matthee18mason20 and JADES-GN-z8-0-LA/JADES-GS-z8-0-LA/JADES-GS-z8-1-LA witstok25 were measured from analysis of the ${\rm Ly\alpha}$ line profile, similar to the one here. The size for EGS_910_44164 was inferred by the fact that it is located at $\approx1$ Mpc away from another ${\rm Ly\alpha}$ emitter larson22. umeda25 obtained the median bubble size of galaxies at each redshift through a ${\rm Ly\alpha}$ transmission and ${\rm Ly\alpha}$ damping wing absorption analysis performed on stacked NIRSpec/PRISM spectra.
• Figure 4: (Top): Two-dimensional PRISM spectrum of CANUCS-LRD-z8.6. (Bottom): One-dimensional PRISM spectrum of CANUCS-LRD-z8.6 (black solid line) along with flux uncertainties (black shaded region). The best-fit SED model (stellar model only; blue line) used for continuum subtraction and NIRCam photometric data points (blue diamonds), to which the PRISM spectrum is normalized, are shown. Detected lines ($S/N>3$) are labeled in red, others in gray. ${\rm Ly\alpha}$ is also shown in Fig. \ref{['fig:grating']}.
• Figure 5: Width measurements of the lines detected in the prism spectrum ($S/N>3$; circles with error bars). The dashed curve compared is a nominal resolution limit for NIRSpec PRISM. All lines, except for the broad ${\rm H\beta}$ component (green), are unresolved.