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Abstract

The quasar lifetime t _Q is one of the most fundamental quantities for understanding quasar evolution and the growth of supermassive black holes (SMBHs), but remains uncertain by several orders of magnitude. In a recent study we uncovered a population of very young quasars (t _Q ≲ 10⁴-10⁵ yr), based on the sizes of their proximity zones, which are regions of enhanced Lyα forest transmission near the quasar resulting from its own ionizing radiation. The presence of such young objects poses significant challenges to models of SMBH formation, which already struggle to explain the existence of SMBHs (̃10⁹ M _☉) at such early cosmic epochs. We conduct the first comprehensive spectroscopic study of the youngest quasar known, SDSS J1335+3533 at z = 5.9012, whose lifetime is t _Q = 10^3.0±0.8 yr (95% confidence). A careful search of our deep optical and near-infrared spectra for H I and metal absorption lines allows us to convincingly exclude the possibility that its small proximity zone results from an associated absorption system rather than a short lifetime. From the Mg II emission line we measure a black hole mass {M}_BH}=({4.13}_-3.02^+10.54)× {10}⁹ {M}_☉ , implying an Eddington ratio of {0.30}_-0.22^+0.77—comparable to other co-eval quasars of similar luminosity. The only possible anomaly associated with SDSS J1335+3533's youth are its weak emission lines, but larger samples are needed to shed light on the potential causality. We discuss the implications of short lifetimes for various SMBH growth and formation scenarios, and argue that future observations of young quasars with the James Webb Space Telescope could distinguish between them.