Monserrat Martinez (Swinburne Uni of Technology)

Abstract: An excess of massive quiescent galaxies (MQGs) is found at the early universe (z~3-4). MQGs at this epoch suggest their stellar content was already in place in the first 1.5 Gyr of the Universe. These findings indicate that MQGs must have experienced an event that caused rapid quenching. Although observational evidence is scarce, AGNs are the most suitable candidate for quenching these galaxies. In this talk, we present a sample of massive galaxies (10 < log(𝑀★/𝑀⊙ ) < 11) at 3 < z < 4 from JWST NIRSpec and the ZFOURGE survey with K–band spectra of the Keck/MOSFIRE, VLT/KMOS. We find that a large fraction have strong [OIII]5007 and H𝛽 emission lines with large line widths (𝜎 100 – 450 km/s). These results show a significantly high AGN fraction in massive galaxies, ranging from 60–70%, pointing to the period between 3 < z < 4 being a time of heightened activity for the development of supermassive black holes in the massive end of the galaxy population and provide evidence for their role in the emergence of the first massive quenched galaxies at this epoch.