How HST∕WFC3 and JWST Can Measure Galaxy Assembly and AGN Growth

Abstract

Accretion disks around supermassive black holes (SMBH) in the centers of galaxies are the central engines of active galactic nuclei (AGN), which are observable over the entire electromagnetic spectrum and out to the beginning of galaxy formation. The gradual assembly of galaxies is believed to have resulted in SMBH today. The growth of SMBH is largely hidden by dust, and possibly by large time delays between galaxy mergers and the feeding of the central monster, so the connection between galaxy assembly and SMBH growth is currently at best circumstantial. Facilities like HST WFC3 and the James Webb Space Telescope (JWST) are needed to trace this process from the epoch of reionization to the present.Using panchromatic deep HST WFC3+ACS imaging data, grism spectra, and ground‐based spectroscopy in GOODS and the Hubble Ultra Deep Field (HUDF), we address this issue through the epoch‐dependent rate of major mergers in massive galaxies in the HUDF, and through SED‐fitting of objects with and without (known) AGN in GOODS. On average, the field galaxy population at z = 1–6 has an underlying star‐forming SED with typical ages of 0.1–0.2 Gyr. However, most AGN‐dominated objects at z = 0.5–1.5 have an underlying stellar SED age of ∼1 Gyr on average. This suggests that AGN growth and SMBH feeding may become visible 0.5–1 Gyr after the dynamical event which triggers the dominant starburst at these redshifts. This may also be reflected in the peak in the massive galaxy major merger rate compared to the peak in the redshift distribution of weak AGN. Finally, we discuss how JWST will expand on this topic in the next decade from the epoch of first light to the present.