GALAXY MERGERS AND DUAL AGN
It is widely accepted that galaxies are not isolated objects in the Universe, but are rather observed to undergo galaxy merging. This can have a significant impact on the morphology and kinematics of the galaxies, e.g. in the form of gas inflows, winds that can clear the galaxies of their gas, a significant increase of the star formation, and reshaping the stellar disks into a bulge, as predicted by several numerical simulations. If all massive galaxies host a supermassive black hole (SMBH) at their center, a pair of bound SMBHs is expected to form in the course of a galaxy merger. Binary SMBHs play thus an important role in the final SMBH and bulge formation process, and their demographics are important to constrain frequency of major mergers and growth of SMBHs through BH-BH mergers.
If enough gas is available for accretion onto the progenitor SMBHs, the gas funneled towards the galactic centers during the merger may initiate and fuel nuclear activity. It is therefore expected that at some point the two SMBHs may be simultaneously actively accreting, creating a binary active galactic nucleus (AGN).
With the aim of finding observational evidence of binary AGN, I investigated X-shaped radio galaxies and double nucleus disk galaxies using optical photometry and spectroscopy.
From top to bottom, left to right: color gri SDSS DR8 images of the sources NGC 5058, NGC 3773, Mrk 1114, Mrk 712, Mrk 721, Mrk 116, Mrk 104, NGC 3758, Mrk 1263, NGC 7468, NGC 5860, Mrk 423, NGC 5256, Mrk 212, and MCG +00-12-073. Sources are ordered in ascending nuclear separation. The field of view is different for each object (i.e., 25 arcsec × 25 arcsec, 51 arcsec × 51 arcsec, and 100 arcsec × 100 arcsec) so that the morphological type of the host galaxy can be appreciated (Mezcua et al. 2014a).
X-SHAPED RADIO GALAXIES
X-shaped radio galaxies exhibit a peculiar radio morphology produced by two pairs of misaligned radio lobes associated to the same galaxy. This morphology was suggested to reflect, among other scenarios, a recent merger of two SMBHs or the presence of a second AGN. This was tested in my thesis by a multifaceted study that combined estimates of the radio lobes age with the black hole mass and starburst history estimated from stellar population synthesis modeling. A statistical study of all these properties revealed that the X-shaped sources analyzed are found in elliptical galaxies, their black hole mass is on average higher than that of a control sample of classical radio-loud AGN, and they exhibit enhanced star-formation activity on timescales expected for galactic mergers. These results support the merger scenario as the origin of the X-shaped radio morphology. See:
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We also initiated a radio observational campaign with the European VLBI Network aimed at detecting two compact core components and tracing weak, existing or relic jets in a pilot sample of X-shaped radio galaxies. Stay tuned to the results!
DOUBLE-NUCLEUS DISK GALAXIES
In Mezcua et al. (2014) we studied a sample of double-nucleus disk galaxies considered as candidates for minor mergers. The luminosity of each of the nuclei and their relative separation was derived from a multicomponent photometric fit to the galaxies in the SDSS optical images. We found that the nuclei in most sources have projected separations ≤ 4 kpc. The ratio of nuclear luminosities indicates that most systems are in the coalescence stage of a major merger. This is supported by the existence of a single galaxy disk in 65% of the systems studied and the finding of a merger-triggered enhancement of nuclear luminosity as expected from simulations. We also identified 19 double-nucleus galaxies in which the two nuclei are physically separated by ≤ 1.1 kpc, constituting thus a potential sample of sub-kpc binary AGN. See:
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One-dimensional vector (solid black line) extracted along the position of the nuclei of Mrk 38 (top) and NGC 5256 (bottom). The two-dimensional photometric fit of a disk (for Mrk 38) and 2 disks (for NGC 5256) is shown (red dashed line) decomposed into the host fit (green dotted line) and the fit to the two nuclei (brown dotted-dashed line). The fractional residuals are shown at the bottom of each plot (Mezcua et al. 2014a).
Radio contours at 1.4 GHz of the X-shaped radio galaxy NGC 326, showing the low-surface-brightness wings oriented almost perpendicular to the active lobes (Murgia et al. 2001).