First rest-frame infrared spectrum of a z>7 quasar: JWST/MRS observations of J1120+0641

Abstract

We present a JWST/MRS spectrum of the quasar J1120+0641 at z=7.0848, the first spectroscopic observation of a reionisation-era quasar in the rest-frame infrared ($0.6<\hat{I}»<3.4\hat{I}¼$m). In the context of the mysterious fast assembly of the first supermassive black holes at z>7, our observations enable for the first time the detection of hot torus dust, the H$\hat{I}\pm$ emission line, and the Paschen-series broad emission lines in a quasar at z>7. Hot torus dust is clearly detected as an upturn in the continuum emission at $\hat{I}{»}_{\text{rest}}\simeq 1.3\hat{I}¼$m, leading to a black-body temperature of $T=1413.5_{-7.4}^{+5.7}$K. Compared to similarly-luminous quasars at 0<z<6, the hot dust in J1120+0641 is somewhat elevated in temperature (top 1%). The temperature is more typical among 6<z<6.5 quasars (top 25%), leading us to postulate a weak evolution in the hot dust temperature at z>6 ($2\ddot{I}ƒ$ significance). We measure the black hole mass of J1120+0641 based on the H$\hat{I}\pm$ Balmer line, $M_{\text{BH}}=1.52\pm 0.17\cdot 10^9{M}_{\odot }$, which is in good agreement with the previous rest-UV MgII black hole mass measurement. The black hole mass based on the Paschen-series lines is also consistent, indicating no significant extinction in the rest-frame UV measurement. The broad H$\hat{I}\pm$, Pa-$\hat{I}\pm$ and Pa-${\hat{I}}^2$ emission lines are consistent with an origin in a common broad-line region (BLR) with density log$N_H/{\text{cm}}^{-3}\ge 12$, ionisation parameter $-7<$log$U<-4$, and extinction E(B-V)$\lesssim 0.1$mag. These BLR parameters are consistent with similarly-bright quasars at 0<z<4. Overall, we find that both J1120+0641's hot dust torus and hydrogen BLR properties show no significant peculiarity when compared to luminous quasars down to z=0. The quasar accretion structures must have therefore assembled very quickly, as they appear fully "mature" less than 760 million years after the Big Bang.

19 pages, 10 figures; submitted