A quasar-galaxy merger at $z\sim 6.2$: black hole mass and quasar properties from the NIRSpec spectrum

Abstract

We present JWST/NIRSpec integral field data of the quasar PJ308-21 at $z=6.2342$. As shown by previous ALMA and HST imaging, the quasar has two companion sources, interacting with the quasar host galaxy. The high-resolution G395H/290LP NIRSpec spectrum covers the $2.87-5.27\hat{\mathrm{I}}¼\mathrm{m}$ wavelength range and shows the rest-frame optical emission of the quasar with exquisite quality ($S/N\sim 100-400$ per spectral element). Based on the H${\hat{I}}^2$ line from the broad line region, we obtain an estimate of the black hole mass $M_{\mathrm{BH},\mathrm{H}{\hat{\mathrm{I}}}^2}\sim 2.7\times 10^9{\mathrm{M}}_{\odot }$. This value is within a factor $\lesssim 1.5$ of the H$\hat{I}\pm$-based black hole mass from the same spectrum ($M_{\mathrm{BH},\mathrm{H}\hat{\mathrm{I}}\pm }\sim 1.93\times 10^9{\mathrm{M}}_{\odot }$) and is consistent with a previous estimate relying on the MgII $\hat{I}»2799$ ($M_{\mathrm{BH},\mathrm{MgII}}\sim 2.65\times 10^9{\mathrm{M}}_{\odot }$). All these $M_{\mathrm{BH}}$ are within the $\sim 0.5$ dex intrinsic scatter of the adopted mass calibrations. The high Eddington ratio of PJ308-21 $\hat{I}{»}_{\mathrm{Edd},\mathrm{H}{\hat{\mathrm{I}}}^2}\sim 0.67$ ($\hat{I}{»}_{\mathrm{Edd},\mathrm{H}\hat{\mathrm{I}}\pm }\sim 0.96$) is in line with the overall quasar population at $z\gtrsim 6$. The relative strengths of the [OIII], FeII and H${\hat{I}}^2$ lines are consistent with the empirical "Eigenvector 1" correlations as observed for low redshift quasars. We find evidence for blueshifted [OIII] $\hat{I}»5007$ emission with a velocity offset $\hat{I}”v_{[\mathrm{OIII}]}=-1922\pm 39$ km s${}^{-1}$ from the systemic velocity and a $\mathrm{FWHM}([\mathrm{OIII}])=2776_{-74}^{+75}$ km s${}^{-1}$. This may be the signature of an outflow from the nuclear region, despite the true values of $\hat{I}”v_{[\mathrm{OIII}]}$ and $\mathrm{FWHM}([\mathrm{OIII}])$ are likely more uncertain due to the blending with H${\hat{I}}^2$ and FeII lines. Our study demonstrates the unique capabilities of NIRSpec in capturing quasar spectra at cosmic dawn and studying their properties in unprecedented detail.

17 pages, 9 figures, accepted for publication in A&A