Detection of stellar light from quasar host galaxies at redshifts above 6

Abstract

The detection of starlight from the host galaxies of quasars during the reionization epoch ($z>6$) has been elusive, even with deep HST observations. The current highest redshift quasar host detected, at $z=4.5$, required the magnifying effect of a foreground lensing galaxy. Low-luminosity quasars from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) mitigate the challenge of detecting their underlying, previously-undetected host galaxies. Here we report rest-frame optical images and spectroscopy of two HSC-SSP quasars at $z>6$ with JWST. Using NIRCam imaging at 3.6$μ$m and 1.5$μ$m and subtracting the light from the unresolved quasars, we find that the host galaxies are massive (stellar masses of $13\times$ and $3.4\times$ $10^{10}$ M$_{\odot}$, respectively), compact, and disk-like. NIRSpec medium-resolution spectroscopy shows stellar absorption lines in the more massive quasar, confirming the detection of the host. Velocity-broadened gas in the vicinity of these quasars enables measurements of their black hole masses ($1.4\times 10^9$ and $2.0\times$ $10^{8}$ M$_{\odot}$, respectively). Their location in the black hole mass - stellar mass plane is consistent with the distribution at low redshift, suggesting that the relation between black holes and their host galaxies was already in place less than a billion years after the Big Bang.

Figures (10)

• Figure 1: NIRCam images of the field around J2236+0032 and J2255+0251. Both JWST filters F356W ( left panels) and F150W ( right panels) are shown. The middle panels are zoomed-in regions of each image. One of the PSF stars used for 2D image decomposition is indicated in each quasar field.
• Figure 1: Distribution of known quasars at redshift $\mathbf{z>5.8}$. The y-axis indicates the absolute magnitudes at rest-frame 1450 Å. The two target quasars in this work are shown in red (J2236+0032 at $z=6.40$, and J2255+0251 at $z=6.34$), while other low-luminosity quasars from the HSC-SSP are shown in blue. The JWST 12 Cycle 1 targets in GO #1967 are highlighted with open circles. Other known quasars are shown in black.
• Figure 2: 2D decomposition into quasar and host-galaxy emission. Panels are as follows from left to right: quasar image (data), best-fit model (quasar + host galaxy), host galaxy only (data $-$ model quasar point-source component), and normalized residual image, i.e., (data $-$ model) / $\sigma$, where $\sigma$ is the flux uncertainty of each pixel. The target name is above each row of panels and the filter is indicated on the left. The alignment of the NIRSpec slit ($0\hbox{$.\!\!^{\prime\prime}$}{}2\times0\hbox{$.\!\!^{\prime\prime}$}{}6$) is shown in the left panel of the F356W image for each target. Pixel units are Mega-Janskys per steradian. A scale bar of $0\hbox{$.\!\!^{\prime\prime}$}{}5$ corresponds to $2.8$ kpc at $z\sim6.4$.
• Figure 2: Quasar-subtracted images of J2236+0032 based on 2D decomposition by two independent codes. The inferred host magnitude from each code is indicated. For this comparison, the point_source_supersampling_factor is set to 2 for galight. The same PSF is used for this comparison. Coordinates are in pixel units; the pixel scales for F356W and F150W are $0\hbox{$.\!\!^{\prime\prime}$}{}0315$ and $0\hbox{$.\!\!^{\prime\prime}$}{}0153$, respectively.
• Figure 3: NIRSpec rest-frame optical spectra of J2236+0032 and J2255+0251. For each quasar, the left panel shows the full spectrum while the top-right panels zoom in on the H$\beta$+[O iii] $\lambda\lambda\ 4960,5008$ doublet. The flux density $F_\lambda$ is in units of $10^{-20}$ erg cm$^{-2}$ s$^{-1}$ Å$^{-1}$. The data are shown in black with the errors per pixel in grey. The stellar absorption lines in J2236+0032 are identified in H$\gamma$, H$\delta$ and H$\epsilon$, which are also presented in the inset zoom-in panel. Best-fit models of power-law continuum (blue), Fe ii (green), broad H$\beta$ (magenta), narrow H$\beta$ + [O iii] (orange thick line), and broad [O iii] (orange thin line) are overplotted in the top-right panels. The sum of the continuum and emission line models is shown in red. The residuals are displayed in the bottom right panels.