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Abstract Dual quasars—two active supermassive black holes at galactic scales—represent crucial objects for studying the impact of galaxy mergers and quasar activity on the star formation rate (SFR) within their host galaxies, particularly at cosmic noon when SFR peaks. We present JWST/MIRI mid-infrared integral field spectroscopy of J074922.96+225511.7, a dual quasar with a projected separation of 3.8 kpc at a redshiftz= 2.17. We detect spatially extended [Feii] 5.34μm and polycyclic aromatic hydrocarbon (PAH) 3.3μm emissions from the star formation activity in its host galaxy. We derive the SFR of 10^3.0±0.2M_⊙yr⁻¹using PAH 3.3μm, which is 5 times higher than that derived from the knee of the infrared luminosity function for galaxies atz∼ 2. While the SFR of J0749+2255 agrees with that of star-forming galaxies of comparable stellar mass at the same redshifts, its molecular gas content falls short of expectations based on the molecular Kennicutt–Schmidt law. This discrepancy may result from molecular gas depletion due to the longer elevated stage of star formation, even after the molecular gas reservoir is depleted. We do not observe any quasar-driven outflow that impacts PAH and [Feii] in the host galaxy based on the spatially resolved maps. From the expected flux in PAH-based star formation, the [Feii] line likely originates from the star-forming regions in the host galaxy. Our study highlights the extreme stardust nature of J0749+2255, indicating a potential connection between the dual quasar phase and intense star formation activities. 

ABSTRACT The extreme ultraviolet region (EUV) provides most of the ionization that creates the high equivalent width (EW) broad and narrow emission lines (BELs and NELs) of quasars. Spectra of hypermassive Schwarzschild black holes (HMBHs; MBH ≥ 1010 M⊙) with α-discs, decline rapidly in the EUV suggesting much lower EWs. Model spectra for BHs of mass 106–1012 M⊙ and accretion rates 0.03 ≤ Lbol/LEdd ≤ 1.0 were input to the cloudy photoionization code. BELs become ∼100 times weaker in EW from MBH ∼ 108 M⊙ to MBH ∼ 1010 M⊙. The high-ionization BELs (O vi 1034 Å, C iv 1549 Å, and He ii 1640 Å) decline in EW from MBH ≥ 106 M⊙, reproducing the Baldwin effect, but regain EW for MBH ≥ 1010 M⊙. The low-ionization lines (Mg ii 2798 Å, H β 4861 Å, and H α 6563 Å) remain weak. Lines for maximally spinning HMBHs behave similarly. Line ratio diagrams for the BELs show that high O vi/H β and low C iv/H α may pick out HMBH, although O vi is often hard to observe. In NEL BPT diagrams, HMBHs lie among star-forming regions, except for highly spinning, high accretion rate HMBHs. In summary, the BELs expected from HMBHs would be hard to detect using the current optical facilities. From 100 to 1012 M⊙, the emission lines used to detect active galactic nuclei (AGNs) only have high EW in the 106–109 M⊙ window, where most AGNs are found. This selection effect may be distorting reported distributions of MBH.