SHELLQs-JWST: Revealing the Spectra of Extended Emission in 12 z > 6 Quasar Host Galaxies using the JWST NIRSpec Fixed Slit

Camryn L. Phillips, Michael A. Strauss, Masafusa Onoue, Xuheng Ding, John D. Silverman, Yoshiki Matsuoka, Takuma Izumi, Junya Arita, Kentaro Aoki, Shunsuke Baba, Masatoshi Imanishi, Nobunari Kashikawa, Toshihiro Kawaguchi, Chien-Hsiu Lee, Mahoshi Sawamura, Yoshiki Toba, Feige Wang, Jinyi Yang

First listed 2025-10-25 | Last updated 2025-10-25

Abstract

We present an analysis of the rest frame optical JWST NIRSpec Fixed Slit spectra of extended host galaxy emission in 12 quasars from the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) sample at redshifts 6.0 < z < 6.4. The spatial point spread function is modeled primarily by a sum of two Gaussians as a function of wavelength and is used to fit and subtract the quasar from the 2D spectra, leaving only extended galaxy emission which we analyze. Ten of 12 systems show spatially extended line emission and five of 12 systems show an extended stellar continuum. From the extended [OIII]5008 emission line, we measure a 132 ${\pm}$ 19 km/s ionized outflow in one system and 52 ${\pm}$ 12 km/s rotation, suggesting a coherent disk, in another. From the extended narrow H$α$ emission, which we hypothesize is ionized by star-forming regions rather than the quasar, we measure star formation rates ranging from ${\sim}$ 7 to 111 M${_\odot}$/yr, the majority of which are consistent with the star-forming main sequence at z ${\approx}$ 6. The positions of our host galaxies on the log10[OIII]5008/H$β$ vs. log10[NII]6584/H$α$ (R3N2) Baldwin-Phillips-Terlevich (BPT) diagram indicate ionization rates typical of AGN activity in the low-redshift universe, but are consistent with the placement of similar z ${\approx}$ 6 quasar host galaxies, suggesting that the R3N2 line ratios cannot distinguish AGN and star-formation powered line emission at high redshifts. We conclude from the consistency between our quasar host sample with z ${\sim}$ 6 galaxies that the presence of a low-luminosity AGN causes little significant change in the properties of galaxies at z ${\approx}$ 6 on 10 Myr timescales.

Short digest

JWST/NIRSpec Fixed Slit (F290LP/G395M) spectra of 12 SHELLQs quasars at 6.0 < z < 6.4 are PSF-modeled with a wavelength-dependent double-Gaussian and subtracted to isolate host emission, revealing extended lines in 10/12 and extended stellar continuum in 5/12. The hosts show [OIII]5008 kinematics including a 132 ± 19 km/s ionized outflow in one system and 52 ± 12 km/s rotation in another, while narrow Hα implies SFRs ≈7–111 M⊙/yr, largely consistent with the z ≈ 6 main sequence. On the R3N2 BPT plane, line ratios fall in AGN-like regions by low-z standards but overlap with other z ≈ 6 quasar hosts, indicating these ratios cannot cleanly separate AGN from star-formation ionization at high z. Together, the results argue that low-luminosity AGN produce little short-timescale (~10 Myr) impact on typical galaxy properties at z ≈ 6.

Key figures to inspect

• Figure 2 (PSF residuals comparison): Verify that the double-Gaussian plus smoothed residuals model removes the nuclear light across wavelength, minimizing wing residuals so that extended [OIII]/Hα features are not PSF leakage.
• Figure 4 (PSF width vs. wavelength): Inspect the chromatic ‘wiggles’ and Level-3 broadening; this justifies modeling/subtracting the PSF in individual Level-2 dithers before coadding to preserve faint, spatially extended host emission.
• Figure 3 (single-wavelength PSF fit): Check that the summed double-Gaussian closely matches the spatial profile with near-zero residuals—key for trustworthy kinematic measurements (e.g., the 132 km/s outflow, 52 km/s rotation).
• Figure 1 (centroid/amplitude/width with λ): See the chromatic centroid shifts and width trends in the calibration star that motivate a wavelength-dependent PSF and explain systematic patterns in the rectified data.
• BPT (R3N2) placement figure: Identify where hosts lie on log10([OIII]5008/Hβ) vs. log10([NII]6584/Hα); note their apparent AGN-like locus at low z yet overlap with z ≈ 6 hosts, underscoring the diagnostic’s ambiguity at early times.