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

Sarah E. I. Bosman, Javier Álvarez-Márquez+30 more|arXiv (Cornell University)|Jul 26, 2023

Astrophysical Phenomena and Observations4 citations

TL;DR

This study presents the first rest-frame infrared spectrum of a z > 7 quasar, J1120+0641, using JWST/MRS, revealing hot dust emission at 1.3 µm with a black-body temperature of 1413.5 K, and detecting Hα and Paschen-series broad emission lines. The quasar’s hot dust and broad-line region properties are consistent with lower-redshift quasars, indicating rapid assembly of mature accretion structures less than 760 million years after the Big Bang.

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 ($0.6<λ<3.4μ$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$α$ 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 $λ_{ ext{rest}}\simeq1.3μ$m, leading to a black-body temperature of $T=1413.5^{+5.7}_{-7.4}$K. Compared to similarly-luminous quasars at 06 ($2σ$ significance). We measure the black hole mass of J1120+0641 based on the H$α$ Balmer line, $M_{ ext{BH}}=1.52\pm0.17\cdot 10^9 M_\odot$, 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$α$, Pa-$α$ and Pa-$β$ emission lines are consistent with an origin in a common broad-line region (BLR) with density log$N_H/ ext{cm}^{-3}\geq 12$, 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

Motivation & Objective

• To obtain the first rest-frame infrared spectrum of a quasar at z > 7 using JWST/MRS.
• To investigate the physical conditions of the hot dust torus and broad-line region in a high-redshift quasar.
• To assess whether the properties of early quasars differ significantly from those at lower redshifts, particularly in dust temperature and emission-line characteristics.
• To determine the black hole mass using rest-frame optical emission lines and compare with UV-based measurements.
• To evaluate the presence of extinction and ionisation conditions in the broad-line region via line ratio analysis.

Proposed method

• Acquired high-resolution spectroscopy of J1120+0641 using the James Webb Space Telescope's Mid-Infrared Instrument (MIRI) Medium-Resolution Spectrometer (MRS).
• Fitted the observed continuum with a modified black-body model to derive the hot dust temperature, assuming optically-thin emission with β = 2.
• Measured black hole mass using the Hα Balmer line and Paschen-series lines, applying standard virial mass estimators.
• Compared observed emission line ratios (Hα, Pa-α, Pa-β) to predictions from a one-phase Cloudy photoionisation model to infer BLR density, ionisation parameter, and extinction.
• Conducted statistical comparisons of dust temperature and BLR properties with samples at z < 6 and 6 < z < 6.5 to assess redshift evolution.
• Used archival data from the Mikulski Archive for Space Telescopes and the European JWST Archive for calibration and context.

Experimental results

Research questions

• RQ1 What is the rest-frame infrared spectral energy distribution of the z = 7.0848 quasar J1120+0641, and what does it reveal about its hot dust emission?
• RQ2 How does the temperature of the hot dust torus in J1120+0641 compare to that of similarly luminous quasars at z < 6 and 6 < z < 6.5?
• RQ3 Are the Hα and Paschen-series broad emission lines in J1120+0641 consistent with a common broad-line region, and what are the inferred BLR conditions?
• RQ4 Is there evidence of significant extinction or ionisation anomalies in the broad-line region of this high-redshift quasar?
• RQ5 To what extent do the accretion structures of this z > 7 quasar resemble those of lower-redshift quasars, indicating early maturity?

Key findings

• The hot dust torus in J1120+0641 is detected as a continuum upturn at rest-frame λ ≈ 1.3 µm, with a black-body temperature of 1413.5⁺⁵.⁷₋₇.₄ K.
• The dust temperature is in the top 1% of similarly luminous quasars at z < 6 and in the top 25% among 6 < z < 6.5 quasars, suggesting weak evolution in hot dust temperature at z > 6 at 2σ significance.
• The black hole mass derived from the Hα line is 1.52 ± 0.17 × 10⁹ M☉, in excellent agreement with the previous UV-based Mg II measurement.
• The Paschen-series line-based black hole mass estimate is consistent within uncertainties, indicating no significant extinction biasing the UV-based mass measurement.
• The observed Hα, Pa-α, and Pa-β line ratios are consistent with a single broad-line region with log N_H / cm⁻³ ≥ 12, –7 < log U < –4, and E(B–V) ≤ 0.1 mag.
• The quasar’s hot dust and broad-line region properties are fully consistent with those of luminous quasars at z < 4, indicating that accretion structures were already mature less than 760 million years after the Big Bang.