First Spectroscopic Study of a Young Quasar
  The quasar lifetime $t_{\rm\,Q}$ 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 ($t_{\rm
Q}\lesssim10^4-10^5$ yr), based on the sizes of their proximity zones, which
are regions of enhanced Ly$\alpha$ 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 at such early cosmic epochs. We
conduct the first comprehensive spectroscopic study of the youngest quasar
known, $\rm SDSS\,J1335+3533$ at $z=5.9012$, whose lifetime is $t_{\rm Q}<10^4$
yr ($95\%$ confidence). A careful search of our deep optical and near-infrared
spectra for HI and metal absorption lines allows us to convincingly exclude
that its small proximity zone results from an associated absorption system
rather than a short lifetime. We use the MgII emission line to measure its
black hole mass $M_{\rm BH}=(4.09\pm0.58)\times 10^9 M_{\odot}$, implying an
Eddington ratio of $0.30\pm0.04$ -- comparable to other co-eval quasars. We
similarly find that the relationship between its black hole mass and dynamical
mass are consistent with other $z\sim6$ quasars. The only possible anomaly
associated with youth are its weak emission lines, but larger samples are
needed to shed light on a potential causal connection. We discuss the
implications of short lifetimes for various SMBH growth scenarios, and argue
that future observations of young quasars with JWST could distinguish between
them.
