[Ne v] emission from a faint epoch of reionization-era galaxy: evidence for a narrow-line intermediate mass black hole

TL;DR

GN 42437 at $z=5.59$ hosts an intermediate-mass black hole (IMBH) revealed by a strong [Ne V] emission that cannot be produced by stars, shocks, or X-ray binaries alone. The authors use JWST/NIRSpec high-resolution spectra and photoionization models including IMBHs to reproduce both very-high- and low-ionization lines, finding a best-fit IMBH fraction of ~30% of hydrogen-ionizing photons in a low-metallicity environment ($Z\sim0.1Z_\odot$, $\log U\approx-2$). The black hole mass is constrained to $\log(M_{ m BH}/M_\odot)\sim5{-}7$ through multiple independent methods, and GN 42437 appears offset from the local $M_{ m BH}-M_*$ relation, implying early IMBH growth prior to substantial stellar mass assembly. This work demonstrates that very-high-ionization lines provide a powerful narrow-line AGN diagnostic at high redshift and suggests a potentially substantial population of undetected IMBHs in the early universe.

Abstract

Here we present high spectral resolution $\textit{JWST}$ NIRSpec observations of GN42437, a low-mass (log(M$_\ast/M_\odot)=7.9$), compact ($r_e < 500$pc), extreme starburst galaxy at $z=5.59$ with 13 emission line detections. GN42437 has a low-metallicity (5-10% Z$_\odot$) and its rest-frame H$α$ equivalent width suggests nearly all of the observed stellar mass formed within the last 3 Myr. GN42437 has an extraordinary 7$σ$ significant [Ne V] 3427 $\mathring{\rm A}$ detection. The [Ne V] line has a rest-frame equivalent width of $11\pm2\mathring{\rm A}$, [Ne V]/H$α=0.04\pm0.007$, [Ne V]/[Ne III] 3870$\mathring{\rm A} = 0.26\pm0.04$, and [Ne V]/He II 4687 $\mathring{\rm A} = 1.2\pm0.5$. Ionization from massive stars, shocks, or high-mass X-ray binaries cannot simultaneously produce these [Ne V] and low-ionization line ratios. Reproducing the complete nebular structure requires both massive stars and accretion onto a black hole. We do not detect broad lines nor do the traditional diagnostics indicate that GN42437 has an accreting black hole. Thus, the very-high-ionization emission lines powerfully diagnose faint narrow-line black holes at high-redshift. We approximate the black hole mass in a variety of ways as log(M$_{\rm BH}/M_\odot) \sim 5-7$. This black hole mass is consistent with local relations between the black hole mass and the observed velocity dispersion, but significantly more massive than the stellar mass would predict. Very-high-ionization emission lines may reveal samples to probe the formation and growth of the first black holes in the universe.

Figures (10)

• Figure 1: Three color NIRCam image (blue is F182M, green is F210M, and red is F444W) of GN 42437 with north up and east to the left. GN 42437 has a distinct red color in these filters due to strong H$\alpha$ emission. The location of the three NIRSpec shutters are overlaid in orange. GN 42437 is fully within the slit and shows a compact morphology. There exists a secondary component to the North East of GN 42437 that is not within the slit. Photometric fitting does not definitively distinguish whether the nearby galaxy is a true companion of GN 42437 or a foreground interloper.
• Figure 2: The observed spectral energy distribution (SED) of GN 42437. The NIRCam and HST imaging are shown as dark-blue squares and the gold circles are the NIRSpec synthetic photometry. All data are given in \ref{['tab:lines']}. The bagpipes fit to the photometry and the spectra is given as the gray model. Using both the photometry and the spectra, we more finely sample the SED and probe emission-line-free regions that estimate the star formation history and stellar mass of GN 42437.
• Figure 3: The extracted spectrum of GN 42437. Upper Row: The full G235H (light blue) and G395H (dark blue) observed flux density. The lower x-axis gives the observed wavelength, while the upper x-axis gives the rest-frame wavelength using the redshift derived from the strong [Oiii]4960+5008 Å, H$\alpha$, and [Neiii] 3870 Å emission lines. Lower 2 Rows: Zoom-ins on individual emission lines from both gratings in ascending wavelength order. The rest-frame wavelengths of each feature is marked by a gray vertical line and the fitted Gaussian emission line is overplotted in gold. We detect 13 nebular emission lines at the $>3\sigma$ significance from the spectrum of GN 42437. The [Sii] 6718,6733 Å doublet is not detected at $>3\sigma$, but shown to emphasize the upper-limits on the lines. H$\alpha$ and [Oiii] 5008 have large rest-frame equivalent widths of 901 and 1644 Å, respectively.
• Figure 4: Upper Panel: The two-dimensional spectrum near the [Nev] 3427 Å emission line. Lower Panel: Collapsed 1-dimensional spectrum of the same spectral region. The rest wavelength, defined by the other strong optical emission lines, is indicated by the cyan line in the upper panel. The [Nev] 3427 Å emission is spatially distributed and centered in the middle of the extracted trace. The [Nev] 3427 Å line is spectroscopically resolved and has an integrated $7\sigma$ significance (the error on the flux density is given as the gray band in the lower panel).
• Figure 5: The very-high-ionization emission lines (ionization potentials greater than $54$ eV) in GN 42437. The left panel shows [NeV] 3427 Å (ionization potential greater than 97 eV) and the right panel shows the Heii 4687 Å (ionization potential greater than $54$ eV) features. The profiles are placed into velocity space using the redshift of the strong optical emission lines. Zero-velocity is marked by a gray dashed line. The gray ribbons show the 1$\sigma$ error on the flux density. The [Nev] and Heii lines are detected at the 6.8$\sigma$ and 3.0$\sigma$ significance, respectively. The gold lines show a fit using a velocity width ($\sigma$) that is fixed to the H$\alpha$ velocity width. The light-blue line in the [Nev] 3427 Å panel shows a fit where the velocity width is free to vary. At the depth of the observations, the observed [Nev] profile is 2.5$\sigma$ broader than the H$\alpha$ profile.