A Post-Starburst Pathway for the Formation of Massive Galaxies and Black Holes at z>6

TL;DR

This study uses JWST observations to identify and analyze massive post-starburst galaxies at z>6 hosting quasars, revealing rapid stellar and black hole growth in the early universe.

Contribution

It presents the first detailed spectral and imaging analysis of high-redshift post-starburst galaxies hosting quasars, linking galaxy quenching and black hole growth at z>6.

Findings

Massive post-starburst galaxies at z>6 identified with stellar masses >10^10.6 M_sun.

Evidence of quasar-driven outflows influencing star formation.

One galaxy follows local black hole mass-velocity dispersion relation.

Abstract

Understanding the rapid formation of supermassive black holes (SMBHs) in the early universe requires insight into stellar mass growth in host galaxies. Here, we present NIRSpec rest-frame optical spectra and NIRCam imaging from JWST of two galaxies at z>6, both hosting moderate-luminosity quasars. These galaxies exhibit Balmer absorption lines, similar to low-redshift post-starburst galaxies. Our analyses of the medium-resolution spectra and multiband photometry show bulk of the stellar mass (log (M_* / M_sun) > 10.6) formed in starburst episodes at redshift 9 and 7. One of the galaxies shows a clear Balmer break and lacks spatially resolved H alpha emission. It falls well below the star formation main sequence at z = 6, indicating quiescence. The other is transitioning to quiescence; together, these massive galaxies are among the most distant post-starburst systems known. The blueshifted wings of the quasar [O III] emission lines suggest quasar-driven outflow possibly influencing star formation. Direct stellar velocity dispersion measurements reveal one galaxy follows the local black hole mass-sigma_* relation while the other is overmassive. The existence of massive post-starburst galaxies hosting billion-solar-mass BHs in short-lived quasar phases suggests SMBHs and host galaxies played a major role in each other's rapid early formation.