Method for Determining AGN Accretion Phase in Field Galaxies
  Recent observations of AGN activity in massive galaxies (log Mstar / Msun >
10.4) show that: 1) at z < 1, AGN-hosting galaxies do not show enhanced merger
signatures compared to normal galaxies, 2) also at z < 1, most AGNs are hosted
by quiescent galaxies; and 3) at z > 1, percentage of AGNs in star forming
galaxies increases and becomes comparable to AGN percentage in quiescent
galaxies at z ~ 2. How can major mergers explain AGN activity in massive
quiescent galaxies which have no merger features and no star formation to
indicate recent galaxy merger? By matching merger events in a cosmological
N-body simulation to the observed AGN incidence probability in the COSMOS
survey, we show that major merger triggered AGN activity is consistent with the
observations. By distinguishing between "peak" AGNs (recently merger triggered
and hosted by star forming galaxies) and "faded" AGNs (merger triggered a long
time ago and now residing in quiescent galaxies), we show that the AGN
occupation fraction in star forming and quiescent galaxies simply follows the
evolution of the galaxy merger rate. Since the galaxy merger rate drops
dramatically at z < 1, the only AGNs left to be observed are the ones triggered
by old mergers and are now in the declining phase of their nuclear activity,
hosted by quiescent galaxies. As we go toward higher redshifts the galaxy
merger rate increases and the percentages of "peak" AGNs and "faded" AGNs
become comparable.
