Mar Mezcua

PhD Project Supermassive binary black hole systems in active galaxies

According to the hierarchical galaxy formation model, galaxies grow through mergers that boost star formation and activate supermassive black holes (SMBHs) at their centers. As most galaxies harbor an SMBH, mergers likely result in the formation of binary black hole (BBH) systems. Understanding these systems can provide insights into galaxy evolution.

This thesis explores three lines of evidence for BBH systems: X-shaped radio galaxies, ultraluminous X-ray sources (ULXs), and double nucleus post-merger galaxies.

Date: October 18, 2011

Scientific Advisor: Dr. Andrei Lobanov

Collaborators: E. Mediavilla, M. Karouzos, V. H. Chavushyan, J. León-Tavares, M. Kim, L. C. Ho, C. J. Lonsdale, T. P. Roberts, A. D. Sutton

X-shaped Radio Galaxies: These peculiar galaxies feature two pairs of radio lobes that create an X-like shape on radio maps. This morphology may indicate a recent SMBH merger or the presence of a second active black hole. Our study of 38 X-shaped radio galaxies and a control group reveals that these galaxies tend to have higher black hole masses and enhanced star formation, supporting the merger scenario.

TOP: HST image of the extreme ULX NGC 2276-3c. The VLA 5 GHz radio contours are plotted as (5, 6, 7, 8, 9) at 10 times the off-source rms noise of 0.04 mJy/beam. The green circle denotes the Chandra position of the ULX with a diameter of 0.6 arcsec (Mezcua et al. 2013). BOTTOM: EVN 1.6 GHz image of NGC 2276-3c. The synthesized beam size is 16.4 mas × 13.1 mas. The off-source rms noise is 8 microJy/beam. Credit: Mezcua et al. (2015). — **TOP:** HST image of the extreme ULX NGC 2276-3c. The VLA 5 GHz radio contours are plotted as (5, 6, 7, 8, 9) at 10 times the off-source rms noise of 0.04 mJy/beam. The green circle denotes the Chandra position of the ULX with a diameter of 0.6 arcsec (Mezcua et al. 2013). **BOTTOM:** EVN 1.6 GHz image of NGC 2276-3c. The synthesized beam size is 16.4 mas × 13.1 mas. The off-source rms noise is 8 microJy/beam. Credit: Mezcua et al. (2015).

**TOP:** HST image of the extreme ULX NGC 2276-3c. The VLA 5 GHz radio contours are plotted as (5, 6, 7, 8, 9) at 10 times the off-source rms noise of 0.04 mJy/beam. The green circle denotes the Chandra position of the ULX with a diameter of 0.6 arcsec (Mezcua et al. 2013). **BOTTOM:** EVN 1.6 GHz image of NGC 2276-3c. The synthesized beam size is 16.4 mas × 13.1 mas. The off-source rms noise is 8 microJy/beam. Credit: Mezcua et al. (2015).

Ultraluminous X-ray Sources (ULXs): ULXs are bright X-ray sources whose luminosity challenges standard stellar evolution models. They may represent intermediate-mass black holes or secondary black holes from merger events. We estimate black hole masses for 19 ULXs with radio counterparts, finding that many fall within the range expected for intermediate-mass black holes. High-resolution radio observations help refine these estimates.

Double Nucleus Galaxies: In post-merger galaxies with double nuclei, the brightness of each nucleus is influenced by tidal effects and accretion disk disruptions. Our analysis of 54 such galaxies suggests that many have nuclei separated by less than 1 kpc, indicating potential pairs of active galactic nuclei. We find that the nuclear luminosity correlates with the host galaxy's brightness, likely due to tidal enhancements and disk stripping.

Overall, our findings suggest that secondary black holes in post-merger galaxies are quickly quenched, explaining their low detection rates. We outline strategies for more effectively identifying these secondaries in future observations using facilities like the VLT, EVLA, and VLBA.