A principal process that regulates the stellar content of a galaxy and drives its evolution is energetic feedback from stars, supernovae, and the accreting supermassive black holes at their cores. Via powerful mass outflows, this feedback injects energy and momentum into a galaxy’s interstellar gas and can drive powerful winds on scales ranging from the nucleus to the entire galaxy. These winds play a key role in the evolution of galaxies through the suppression or enhancement of star formation throughout the galaxy.

We studied powerful outflows of gas from the nuclei of nearby gas-rich galaxies, called Ultra-Luminous Infrared Galaxies (ULIRGS), in order to gain insight into outflows and feedback driven by active black holes and by vigorous star formation. In particular we focused on merging or colliding galaxies, since violent merger dynamics can cause gas to flow into the massive black holes that are at the center of most galaxies. For much of this work we used a technology called “Adaptive Optics”, which sharpens the images of ground-based telescopes to remove blurring due to turbulence in the earth’s atmosphere.

In a study of 22 ULIRGS, we showed that merger-induced star formation, which can be a powerful driver of outflows and galactic winds, becomes more vigorous and pervasive the closer the galaxy pair is to its final merger event. For 11 of these galaxies we were able to use the stellar motions in each galaxy nucleus to measure the masses of their central black holes. We found that this sample of black holes are over-massive (100 million to a billion times the mass of the Sun) compared to the masses based on the statistical black hole scaling relations for normal non-merging galaxies, which would suggest that the major epoch of black hole growth occurs in early stages of a merger. However this conclusion would be modified if there is a lot of dense molecular gas in the galaxy nucleus, a possibility which we explored for a galaxy merger called NGC 6240 using the ALMA radio telescope.

We also studied several individual galaxy mergers in more detail. For the galaxy merger called MCG+08-11-002, we used the age of young stellar clusters to show that the most vigorous star formation occurred about 20 million years ago, when it was associated with the final coalescence of the two merging galaxies. For the ULIRG IRAS F17207-0014, we found an inner strong shock to be evidence of a collimated outflow from the nuclear regions, whereas weak shocks throughout the galaxy merger were indicative of the interstellar gas in the two merging galaxies crashing into each other.

The southern nucleus of the late-stage galaxy merger Mrk 273 has been known to contain an accreting black hole, or active galactic nucleus (AGN), in its southern nucleus. Under a previous NSF grant we had found evidence for a second AGN in the northern nucleus, as well as evidence for an outflow from that nucleus which contained highly excited Silicon atoms likely due to irradiation by energetic photons from that AGN. Under the no-cost extension of the current NSF grant, that hypothesis was confirmed (with colleagues from several other institutions) using the Chandra X-Ray observatory to observe the northern AGN in high-energy xrays which can penetrate the high column densities usually found in the central regions of gas-rich galaxy mergers.

We investigated a proposed method for identifying many more double active galactic nuclei (AGNs) through the presence of double-peaked [O III] emission lines in the spectrum of the (apparently single) galactic nucleus. Using high-resolution infrared imaging and spatially resolved spectroscopy in both optical and infrared light, we showed that this was only a partially reliable way to find actual dual AGN. We found that the double-peaked [O III] AGNs with close companions are only 17% dual AGNs; 42% of them are outflows, 25% unresolved substructure, and 17% AGN/galaxy pairs.

Last Modified: 06/29/2020
Modified by: Claire E Max