High-resolution VLBI imaging of nearby supermassive black holes and jets
Supermassive black holes are believed to exist at the center of most galaxies (including the one at the center of our own Galaxy, Sgr A*). Some active galaxies launch powerful jets of plasma traveling close to the speed of light. These jets are active radio (synchrotron) emitters and are thought to be powered by the accretion of matter onto the central supermassive black hole (“the central engine”).
Very Long Baseline Interferometry (VLBI) provides the highest angular resolution in astronomy today. At (sub) millimeter wavelengths, global VLBI imaging can penetrate the opacity barrier of synchrotron self-absorption in the nuclear regions and thus offers a unique opportunity to directly probe the immediate vicinity of supermassive black holes. Recent developments in (sub)mm-VLBI have led to the successful detection of event-horizon-scale structures in Sgr A* and the first-ever image of a black hole in the nearby radio galaxy M87 with the Event Horizon Telescope (EHT), providing a key confirmation of Einstein’s General relativity in a strong gravitational field.
Left: the first image of a black hole — at the centre of the galaxy M87 (credit: EHT Collaboration). Right: schematic diagram of the 1.3 mm VLBI observations of Sgr A* in the Galactic centre reported in Lu et al. 2018, where the insets show possible shapes of the event-horizon-scale structure that are consistent with the measurements (credit: Eduardo Ros/Thomas Krichbaum (MPIfR)
Our research focuses on detailed VLBI imaging of nearby Active Galactic Nuclei (Quasars,BL Lacs, Radio Galaxies, etc.) with the highest possible spatial resolution with VLBI arrays around the world (e.g., the EHT, the Global Millimeter VLBI Array, the Very Long Baseline Array, the East Asian VLBI Array). Together with our collaborators, we are not only interested in black hole imaging for the Galactic center and the radio galaxy M87, but also in studying how the relativistic jets are launched, collimated and accelerated in more distant sources.
Ongoing Collaborations
Feng Yuan and Luis Ho collaborate on a wide range of observational and theoretical studies on the physics of black hole accretion when their accretion rates are highly sub-Eddington. Their work concerns the nature of radiatively inefficient accretion flows, their predicted radiation signatures across the electromagnetic spectrum, the interpretation of observational data based on the theoretical model, and the generation of jets from accretion flows by magnetic field.
Examples of their joint research include:
Younes, G., Ptak, A., Ho, L. C., Xie, F.-G., Terashima, Y., Yuan, F., et al. 2019, ApJ, 870, 73: NuSTAR Hard X-ray View of Low-luminosity Active Galactic Nuclei: High-energy Cutoff and Truncated Thin Disk
Yu, Z., Yuan, F., & Ho, L. C. 2011, ApJ, 726, 87: On the Origin of Ultraviolet Emission and the Accretion Model of Low-luminosity AGNs
Yuan, F., Yu, Z., & Ho, L. C. 2009, ApJ, 703, 1034: Revisiting the "Fundamental Plane" of Black Hole Activity at Extremely Low Luminosities
Yuan, F., Lin, J., Wu, K., & Ho, L. C. 2009, MNRAS, 395, 2183: A Magnetohydrodynamic Model for the Formation of Episodic Jets