The Quest for Active Galactic Nuclei Feedback in Local and Distant Galaxies
The mechanisms for quenching star formation in galaxies are not yet well understood. Identifying these mechanisms is one of the paramount endeavors in the current state of Astrophysics. The fundamental requirement for quenching is that the cold gas that fuels star formation must be depleted or removed, or heated. AGN feedback is one of the hypothesized quenching agents. In this dissertation, we present our observational studies on AGN feedback.
In chapter 2, we will identify very rare galaxy candidates going through a rapid merger evolutionary sequence from disturbed starbursts, followed by fading and relaxed AGN, and to eventually young and quiescent post-starburst galaxies. Most nearby galaxies today are evolving slowly. The era of major galaxy mergers and rapid black hole growth is almost over. However, post-starbursts (PSBs) are rapidly evolving from the blue cloud to the red sequence today. Although they are rare today, integrated over time they may be an important pathway to the red sequence. The transition PSBs have stellar properties that are predicted for fast-quenching starbursts and morphological characteristics that are already typical of early-type galaxies. The active galactic nucleus (AGN) fraction, as estimated from optical line ratios, of these post-starbursts is about three times higher (> 36 +- 8) than that of normal star forming galaxies of the same mass, but there is a significant delay between the starburst phase and the peak of nuclear optical AGN activity (median age difference of 200 +- 100 Myr). We also find that starbursts and post-starbursts are significantly more dust obscured than normal star forming galaxies in the same mass range. The time delay between the starburst phase and AGN activity suggests that AGNs do not play a primary role in the original quenching of starbursts but may be responsible for quenching later low-level star formation by removing gas and dust during the post-starburst phase.
In chapter 3, we study the cold gas contents of PSBs. We undertook new CO(2-1) observations of 24 Seyfert post-starburst galaxies and together with our data analyzed about 100 previously studied PSBs. When combined with the other samples, our sample is indispensable in sampling the entire starburst-AGN-quenched post-starburst evolutionary sequence. Unlike the previous studies, we find that both star-formation and molecular gas evolutions in PSBs are rapid. These galaxies do not need to linger in the green valley for a longer period of time as previous studies suggested. We find a significantly lower molecular gas detection rate (25%) in our sample than do previous PSBs studies (50-90%). The distribution of gas fraction in Seyfert PSBs is significantly different from young star-forming galaxies. We observe a rapid decline in gas fraction around 0.7Gyr after the starburst. We interpret this far removed event from the peak of the starburst as evidence for a delayed AGN feedback.
A key physical manifestation of active galactic nuclei (AGN) feedback is predicted to be powerful galactic winds. However, the relative roles between AGN activity and star formation in driving such winds remain largely unexplored at redshifts z ~ 1, near the peak of cosmic activity for both. In chapter 4, we study winds in 12 X-ray AGN host galaxies at z ~ 1 in the CANDELS fields using deep Keck rest-frame UV spectroscopy. We use the low-ionization Fe II 2586 absorption profile in their stacked spectrum to trace the cool gas kinematics. Our wind model accounts for both the galactic self-absorption and wind absorption imprinted on the absorption profile. We find that the AGN show a median centroid velocity shift of -124 km/s and a median velocity dispersion of 106 km/s. For comparison, a star-forming, non-AGN sample at a similar redshift, matched roughly in stellar mass and galaxy inclination, show the outflows to have a median centroid velocity of -148 km/s and a median velocity dispersion of 168 km/s. Thus, winds in the AGN are similar in velocities to those found in star-formation-driven winds, and are too weak to escape and expel substantial cool gas from galaxies. Thus, we do not find evidence to support bulk velocities having greater than 500 km/s predicted by some AGN feedback models. Future studies of winds in older and less star-forming AGN than the current sample will be useful to discriminate between delayed AGN wind gas removal or gas heating by AGN and will tell us how the AGN feedback operates.