The Role of AGN and Bulges in Quenching the Galaxy Main Sequence

Abstract

Galaxies from the Sloan Digital Sky Survey (SDSS) and the Cosmic Evolution Survey (COSMOS) Deep Field are used to investigate the role of AGN and bulges in quenching star-formation in ‘main sequence’ (MS), star-forming galaxies over the last ~8 Gyr of cosmic time (z < 1.3). Analysis of ~200,000 SDSS galaxies (z = 0.04–0.1) shows that ~60% of all star-formation in massive galaxies (Mstar > 10^10.5 Msun) is associated with galaxies whose optical diagnostic emission lines classify them as AGN - Composites (C), Seyferts (S), or LINERs (L). These AGN form a ‘quenching sequence’ of decreasing mean star-formation (C → S →L), which is simultaneously accompanied by a systematic increase of the galaxy bulge-to-total fraction, B/T , from a median B/T = 0.15 on the MS to B/T = 0.75 in LINERS. Analysis of additional spectral features (Dn4000 and Hδ EW) confirms that the AGN quenching sequence is also an aging sequence, and suggests that there are two quenching pathways, one with and one without passing through the Seyfert phase. A similar analysis of ~100,000 COSMOS galaxies at z = 0.25–1.3 was carried out using AGN identified by mid-infrared colors (MIR), excess X-ray emission (XGs), and excess radio emission (XRGs), and using ultra-deep images obtained with Hyper Suprime-Cam on the Subaru 8m Telescope. The COSMOS analysis confirms the role of AGN quenching (MIR → XG → XRG) as well as the systematic increase in B/T (0.15 → 0.75) as galaxies quench their star-formation by ~2 dex below the MS. Comparison of the observed number density of AGN vs. redshift with the predicted rate of major/minor galaxy mergers from state-of-the-art cosmological simulations shows reasonable agreement, suggesting that galaxy mergers provide a viable hypothesis for triggering quenching in massive galaxies. A comparison of SDSS galaxies with recent surveys of atomic (HI) and star-forming molecular (H2) gas shows sufficient reservoirs of HI gas (~10^10 Msun) in massive MS galaxies as well as an increasing H2/HI gas fraction vs. stellar mass. The total gas supply appears to decrease by ~1–2 dex as galaxies quench into the LINER phase. These results strengthen the merger paradigm in which bulges, AGN activity and gas depletion are directly associated with quenching star-formation in massive gas-rich disk galaxies on the MS.