The debate of whether or not a supermassive black hole (SMBH) was kicked out of the centre of a galaxy continues in the Black Holes I session at the AAS. According to Stefanie Komossa and her team at the Max Plank Institute for extraterrestrial Physics (MPE) back in May 2008, spectroscopic data of a galactic core appeared to show a collision event between two SMBHs. In this case, the smaller SMBH was propelled out of its host galaxy by an intense and focused "superkick" by gravitational waves.
However, the delegates attending Session 328 have other ideas…
Tamara Bogdanovic, University of Maryland, kicked off the Black Hole I Session with an investigation into the spectroscopic data derived by Komossa et al. Bogdanovic presented her research on the possibility that rather than showing a superkick, the data could be showing the motion of binary SMBHs around the galactic core after a galactic merger. She made the rather sobering statement that there were, "more publications than data," highlighting the fact that far from being conclusive evidence of a superkick, that more subtle mechanisms may be at work. Model data of orbiting binaries appear to fit the same spectroscopic analysis just as well as the superkick situation. As binary SMBHs would be long-lived objects, there's a good (statistical) chance of observing them. Further work is required, however, possibly using the Very Long Baseline Array (VLBA).
Dipanker Maitra, of the University of Amsterdam, then presented his results of time-dependent modelling of Sagittarius A* (the SBH at the centre of our galaxy). It turns out that there are more high energy flare events detected from Sag A* than expected from the predicted accretion rate. Maitra models the time lag observed in radio data between the first high-energy flares and the following low energy flares.
Jen Blum, from the University of Maryland, then took on the emissions from a stellar black hole in the X-ray binary GRS 1915+105. Key to Blum's research is to investigate the strange asymmetric iron emission line. It looks like this asymmetry can be explained by a combination of special relativity and general relativity effects near the space-time warping black hole.
David Garofalo, who works at JPL/Caltech, then followed quickly with his research of the "central engine" inside galactic nuclei, investigating how strong a SMBH's magnetic field can be. In his models, he finds the spin of the black hole is key to magnetic field strength. Counter-intuitively, Garofalo's work suggests that the fastest spinning black holes may have the weakest magnetic field. Also, slowly spinning SMBHs appear to have a larger gap region. He is quick to point out that his model only shows us what configurations are possible, but concludes with the suggestion that you don't need a fast-spinning SMBH for powerful jets to be generated. "[It's a] tug-o-war between gravity and the Lorentz forces," he said when referring to his model, "but other [unaccounted for] physics may significantly modify the model."
Avery Broderick, from the Canadian Institute for Theoretical Astrophysics, examines jets produced by the Milky Way's SMBH and M87. Both are fantastic objects to study as they are relatively close. However, the angular resolution of instrumentation needs to be boosted, or new techniques are needed to understand jet mechanisms.
Massimo Dotti, from the University of Michigan, re-explored Komossa's research, also supporting Tamara Bogdanovic's work that a superkick may not have caused the emissions studied by Komossa. He also shows that a galactic merger and then SMBH binary can generate similar red-shifted and blue-shifted components of emission profiles. Dotti then showed details of his model and proposed some observational constrains.
Bonus speaker and NASA scientist Teddy Cheung then discussed his search for "offset galactic nuclei" that may be evidence for SMBH collisions in the centre of galaxies. According to Cheung, the calculations to find the black hole masses can be "done on the back of an envelope… the flap of the envelope!" He then showed some results of the observation campaign, pointing to a few candidates that might reveal a SMBH binary partner may have achieved escape velocity (i.e. been kicked out of the galaxy), but he emphasised that this number was small. Radio data of pre-merger and post-merger lobes were also presented, helping future studies characterize collision and merger events.
All in all, Session 328 was a superb start to the conference for me, really opening my eyes to the cutting edge supermassive black hole research going on around the world. There's a lot more where that came from…
Written by Ian O'Neill
