First detections of stellar light from quasar host galaxies at z > 6

November 26, 2022

 The existence of high-redshift supermassive black holes (SMBHs) -- shining brightly as quasars during the first billion years of our universe -- has presented a conundrum in astrophysics. A broad variety of physical mechanisms have been proposed for the formation and rapid growth of these early SMBHs. Promising diagnostics include the relative properties of the black hole and its host galaxy. However, up to now, the detection of stars in quasar host galaxies has been elusive beyond redshift z>2, even with deep HST observations. The first detections of the stellar component of the host galaxies of two relatively low-luminosity quasars at z>6 have recently been reported, from observations with JWST. After modeling and subtracting the glare from the quasar itself,  the host galaxies are found to be massive (stellar masses of a few times 10 billion M⊙), compact, and disk-like. Unlike most SMBHs in the nearby universe, these quasars are displaced from the centers of their host galaxies in the rest-frame optical, in one case by nearly 1 kpc. These first positive detections of quasar hosts at z>6 are a pivotal milestone; we can now assess the stellar environment along with star formation and black hole accretion to determine the physical conditions that govern the formation and evolution of the first SMBHs.

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