Supermassive binary black hole systems in active galaxies

According to the hierarchical galaxy formation model, galaxies grow through mergers that enhance star formation and trigger active galactic nucleus (AGN) activity. If nearly all galaxies host a supermassive black hole (SMBH) in their center, it is expected that pairs of SMBHs will be formed in the course of a merger event. The detection and number estimates of binary black hole (BBH) systems can, thus, help us to understand how galaxies form and grow, and shed light on the evolutionary models that rule the Universe. With this purpose, we pursue, in this thesis, three different lines of observational evidence for BBH systems: in X-shaped radio galaxies, in ultraluminous X- ray sources, and in double nucleus post-merger galaxies. X-shaped radio galaxies are a class of peculiar radio sources which exhibit two pairs of radio lobes that pass symmetrically through the center of the host galaxy, giving the galaxy the X-shaped morphology seen on radio maps. This morphology has been suggested to reflect either a recent merger of two SMBHs or the presence of a second active BH in the galactic nucleus. This scenario is tested in this thesis by a multifaceted study that combines measurements of radio and optical luminosity, estimates of the black hole masses and ages of the radio lobes, and assessments of the starburst histories in the X-shaped radio sources and their host galaxies. The study covers a sample of 38 X-shaped radio galaxies and a control sample of 36 radio-loud active nuclei with similar redshift and optical and radio luminosities. The optical spectra of the host galaxies are modeled using a linear combination of synthetic stellar populations that provides the stellar velocity dispersion (from which the BH mass can be derived) and the light fraction, mass fraction, and metallicity of the stellar populations. From these, the starburst history of the galaxies is derived. The dynamic age of the active lobes is also estimated, from their angular size on the radio maps. A statistical study of all these properties reveals that all the X-shaped sources in the sample are found in elliptical galaxies, their BH masses are on average higher than those in a control sample of radio-loud AGN, and they exhibit enhanced star formation activity on timescales expected in galactic mergers. These results yield further support to the merger scenario as the origin of the peculiar X-shaped radio morphology. Ultraluminous X-ray sources (ULX) have also been suggested to be related to BBH systems in post-merger galaxies. The high X-ray luminosity of ULXs cannot be explained by stellar evolution models, unless they were X-ray binaries with super-Eddington accretion, strong beaming effects, or in an ultraluminous state. Alternatively, ULXs have been suggested to be intermediate-mass black holes (IMBH) accreting at sub-Eddington rates, young supernovae, or secondary BHs in the course a merger event with the ULX host galaxy. In order to clarify which is the most likely scenario, the BH mass of those ULXs with radio counterparts detected has been estimated from the location of the ULXs in the black hole fundamental plane. We find that the 19 ULXs studied have estimated BH masses in the range 10^3–10^8 M⊙, and that 58% of them have estimated BH masses in the range 10^2–10^5 M⊙ expected for IMBHs. We also perform high-resolution VLBI radio observations of three ULXs with radio counterparts. This yields a better constraint of the BH mass of two of them and the resolved structure of one of the youngest known supernova remnants. The third class of objects likely to host BBH systems are the galaxies with double nucleus (double nucleus galaxies). According to the models that describe the evolution of BBH systems, the luminosity of the two nuclei is expected to be affected by an enhancement of the accretion rate due to tidal effects and by accretion disk disruption. This is tested, in this thesis, with the study of the optical luminosities of a sample of 54 post-merger galaxies with double nucleus. The luminosity of each of the nuclei and their relative separation is derived from the fit of two gaussian components (PSF fitting) to the g, r, and u-band images of the center of these post-merger galaxies, while the luminosity of the host galaxy is obtained from SDSS optical photometry. As a result of the PSF fitting, we find that in 19 post-merger galaxies the two nuclei are physically separated by ≤ 1 kpc. These sources are, thus, potential sub-kpc scale AGN pairs. Secondly, we find that the nuclear luminosity is correlated with the luminosity of the host galaxy due to a tidal enhancement of the accretion rate, but that another mechanism is also affecting the luminosity trends observed. We suggest that accretion disk stripping is also at play, and estimate that it should take place at a binding radius between 1 and 3 kpc. A fit by a BBH model, which takes into account tidal enhancement as well as accretion disk disruption, supports these results, obtaining that the accretion disk of both the primary and the secondary nucleus are being disrupted in all the sources. The model also finds that the two nuclei become gravitationally bound when they are at a projected separation of 2.3 kpc, which is the case for 60% of the galaxies studied. We also find that the value of the power-law spectral index of the accretion disk derived from the fit is in agreement with the ones of AGN with a peak in the optical-UV band (big blue bump) of their spectral energy distribution. In order to test the BBH scenario in the ULXs with radio counterparts detected, the results of the BBH fit obtained for double nucleus post-merger galaxies are also applied to the X-ray luminosity dependence of the ULXs on the nuclear separation to their host galaxy. The observed trend between ULX luminosity and nuclear distance is in agreement with the BBH model, supporting the BBH scenario for the ULX sources studied. Overall, the results of our study indicate that the activity of the secondary black holes is rapidly quenched in post-merger galaxies, which explains the very low detection rate for the secondaries. Based on the results of our study, more effective approaches for detecting the secondaries have been derived, and these approaches will be now pursued at a number of optical and radio astronomical facilities, including the VLT, the EVLA, and the VLBA