Ph.D. Research
When galaxies merge, gravitational forces can disturb the gas within them, funneling it to their centers. This causes bursts of star formation (SF) and increased fueling of the SMBHs, and their possible activation as AGNs. This increased activity in galaxies’ centers can cause outflows of gas from galaxies’ centers. Outflows can eventually remove gas from the entire galaxy, stopping both SF and AGN activity. Geometrically, outflows can take the form of biconical jets (Rosario et al. 2010) or of a one-sided conical outflow located near a normal emission line region (Rupke et al. 2013); regardless, the two emitting regions are moving at different velocities, causing emission lines to Doppler shift to different wavelengths. When spatially unresolved, both of these outflow scenarios can appear as double-peaked emission lines, such those as shown in the left panel of the Figure.
On the other hand, if galaxy mergers trigger AGN activity, the SMBHs of both galaxies should be bright in some galaxy mergers. Hundreds of AGN pairs are known at wide separations (>49,000 lightyears; ly) (e.g. Myers et al. 2007), while there are currently only ~20 AGN pairs known with close separations <49,000 ly (commonly known as dual AGNs). When both AGNs, moving at different velocities, emit an emission line, we would also observe double-peaked emission lines. If we cannot spatially resolve the two galaxies, we cannot distinguish whether the double-peaked emission line is due to an outflow or a dual AGN (McGurk et al. 2011; Fu et al. 2011, 2012).
As both of the cases outlined above can produce double-peaked emission lines, my research addresses the question, how often do these double-peaked emission lines originate in outflows and how often in dual AGNs? If the double-peaked emission lines are caused by outflows, how does the relative masses and separation of the two merging galaxies affect the outflows’ strength, velocity, separation from the black hole, and origin (SF or AGN driven)? If each emission line comes from one of the merging galaxies, does each galaxy host an AGN? I seek to answer these questions using various visible, near-infrared (NIR), and X-ray techniques.
We studied a sample of double-peaked [O III] emitting SDSS AGNs. By obtaining new and archival high spatial resolution images taken with the Keck Laser Guide Star Adaptive Optics (AO) system and the Near-InfraRed (IR) Camera2, we showed that 30% of double- peaked [O III] SDSS AGNs have two spatial components within a 3" radius (McGurk et al. 2015). However, spatially resolved spectroscopy or X-ray observations are needed to confirm these galaxy pairs as systems containing two AGNs. We followed up these spatially- double candidate dual AGNs in the near-IR and the optical, respectively, with integral field spectroscopy from Keck OSIRIS and Gemini GMOS and with long-slit spectroscopy from Keck NIRSPEC and Lick’s Shane Kast Double Spectrograph. We find double-peaked emitters are caused sometimes by dual AGN (25%) and sometimes by outflows (42%), narrow line kinematics (25%), or AGN/starbursting galaxy pairs (8%).
X-ray observations are the golden standard of AGN confirmation; if each spatial component matches a luminous X-ray nucleus, the object is a dual AGN. We performed Chandra ACIS-S observations on 12 candidate dual AGNs. Using our observations and 8 archival observations, we compared the distribution of X-ray photons to our spatially double near-IR images and measured X-ray luminosities and hardness ratios. We detect 5 companions out of the 20 candidates (25%), which matches the 25% rate of dual AGN that we find from our spectroscopic confirmation.