Spectroscopic signatures of sub-parsec massive black hole.

The focus of this thesis is on the electromagnetic signatures associated with massive black hole (MBH) binaries (MBHBs). In current models of structure formation, galaxies assemble in a bottom-up hierarchical fashion. Observations show that MBHs are hosted in the nuclei of almost all nearby massive galaxies. The existence of close MBHBs is therefore expected as the result of galaxy mergers, although clear observational evidences of MBHBs remain elusive. Most of the current sub-pc MBHB candidates have been spectroscopically selected through the Doppler-shift technique. This method consists in the identification of broad emission lines (BELs) shifted with respect to the galaxy reference frame, as traced by the set of narrow lines (NELs). This shift is due to the hypothesized motion of the active MBH orbiting in a close binary. However velocity off-sets between BELs and NELs are not a unique feature of close MBHBs, and cannot be searched for beyond z ∼ 0.8 since most NELs fall out of the wavelength range covered by current optical surveys. This work presents our contribution to (i) the prediction of new spectroscopic features associated with MBHBs at different evolutionary stages in the aftermath of a gas-rich merger, and to (ii) the search for new secure candidates at different redshifts, ruling out other possible interpretations of their physical nature. In the first part of this thesis we discuss a MBHB model envisaging a set of narrow lines characterized by the same Doppler-shift displayed by the BELs. This additional NEL system originates from the lower density gas orbiting in the so called gap region excavated by the gravitational interaction between the sub-pc MBHB and a surrounding thin circum-binary disk. We propose this model to interpret the features observed in the spectrum of the quasar SDSSJ092712.65+294344.0 (SDSSJ0927 hereafter). For the case of this peculiar source we show that the binary hypothesis is favored against the other possible physical scenarios of a recoiling AGN or of a chance superposition of two distinct objects in a galaxy cluster. However further follow up observations are mandatory to monitor on longer timescales (∼ > 10 yr) the periodic drift of the BELs expected over the binary orbital period, and to gather additional information on the quasar host galaxy and its close environment. Our study on SDSSJ0927 contributed to the realization of a systematic search for MBHB candidates at z ∼ < 0.8 through the development of automatic procedures by different groups including ours. The new algorithm implemented by our group will be also adapted to look for new candidates at higher redshifts based on the presence of two displaced peaks in the profile of the broad CIV line interpreted in terms of two active binary MBHs. In the second part of this work, we devise a new spectroscopic approach to select MBHBs based on peculiarly reduced flux ratios between broad lines of lower and higher ionization potentials. In particular, we show that MBHBs can display anomalously small flux ratios between the MgII and CIV BELs, i.e. FMgII/FCIV ∼ < 0.15, in a limited range of sub-pc separations due to the erosion of the BEL emitting material bound to the secondary active MBH. This erosion is due to the tidal field of the more massive binary companion. The advantage of our newly proposed technique is twofold: (i) it can select new candidates at higher redshifts, z ∼ 2, where both the considered BELs are visible in the optical spectrum, and (ii) the detection of a reduced FMgII/FCIV would be a strong indication of the presence of a sub-pc MBHB since it is not expected in the other physical scenarios alternative to the binary hypothesis. We show that it is possible to use our flux ratio criterion to select peculiar AGN spectra at z ∼ 2 from the database of the Sloan Sky Digital Survey. These peculiar sources can be observed again in the IR band to look for a shift between the broad lines of Hβ, MgII and CIV and the narrow [OIII]λ5007. We start this new MBHB search with a small sample of 9 targets observed in the NIR-H band with the Very Large Telescope. Two sources are selected as binary candidates because the lower ionization lines of the MgII and of the Hβ have reduced fluxes relative respectively to the higher ionization line of the CIV and of the [OIII], and the three observed BELs show moderate shifts up to 1000 kms−1 relative to the narrow [OIII]. Hence we plan to perform new optical observations of these two sources to confirm and monitor the measured shifts, and to complete the search for MBHBs at z ∼ 2 to check the reliability of our newly proposed spectroscopic approach.