From dual to binary supermassive black holes
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The study of dual active galactic nuclei (AGN) and supermassive black hole binaries (SMBHB), at parsec-to-kpc and sub-pc scale separation respectively, is among the current observational and theoretical astrophysics hottest topics. It provides unique insights into the dynamics of galactic mergers, the evolution of galaxies, the growth of SMBH and the (low frequency) gravitational wave signatures.
Dual AGN (pc to kpc separation) can be spatially resolved and several systems have been observed. On the contrary, SMBHB (at milli-parsec) are very difficult to resolve and search for spectroscopically (using the Doppler shift in emission lines, or looking for periodicities in the light curves), with only a handful of candidates currently identified. So far definitive smoking-gun signatures of SMBHB have not been yet established when compared to single AGN.
Binary SMBHs separated at milli-parsec are gravitationally bound and can be loud emitters of gravitational waves in the low-frequency ranges (PTA and LISA). Recently, three different PTAs, EPTA, PPTA and NANOGrav, have independently reported on evidence for the presence of a background of low-frequency gravitational waves in their pulsar timing data. This background could either result from the incoherent superposition of gravitational waves from a large number of sources at cosmological distances, from one or a few supermassive black hole binaries in the local Universe, or from the addition of all these contributions. In the future, PTAs might be able to detect individual sources of gravitational wave emission. The mergers of SMBHBs are expected to be the loudest sources for the gravitational wave observatory LISA.
Novel techniques, such as probing the broad line regions with IR interferometry (e.g., GRAVITY, GRAVITY+) or the wealth of time-domain data that LSST with the Rubin Telescope promise important progress for binaries, while JWST is discovering many candidate dual AGN even at high redshift. The VLTI, in its current status, can directly detect chromatic shifts in emission regions down to microarcsecond scales. This is actively being used to directly observe the broad line regions (BLR) of AGN for the first time. This presents an opportunity for SMBHB detection when the binary is on sub-parsec scales and the detection signatures with SMBHB separation is an active and young field spurred by the upcoming GW facilities and recent ongoing upgrades to the VLTI.
At the same time simulations, from circumbinary disc scale to cosmological scales are including the modeling of binary and dual AGN. Understanding the dynamics of massive black hole pairs and binaries is a challenging feat, but recent progress has nevertheless predicted substantial populations of dual AGN, while the population of observable binaries is expected to be smaller. There are now several predictions for the populations of binary and dual AGN within reach of telescopes and surveys, but it’s still unclear which signatures are actually observable. For instance, periodicities are predicted, but observationally the intrinsic variability of AGN may mimic or hide such periodic signatures. Probing the SMBH merging process from dual system to milli-parsec system is key to our understanding of the last stage of galaxy mergers and for the expected LISA detection rate. Simulations also suggest that many SMBH binaries are surrounded by matter and therefore should provide for electromagnetic signatures from radio to X-rays/gamma-rays.
This workshop aims at bringing together the French community working on dual AGN and binary SMBHs, gathering PNCG, PNHE and GDR “Ondes gravitationnelles”.
SOC: Lucas Guillemot, Astrid Lamberts, James Leftley, Anne-Laure Melchior, Delphine Porquet, Marta Volonteri