Retrospective: IAU Symposium 322 - The Multi-messenger Astrophysics of the Galactic Centre

Sun, sand and serious science: the prescription for a week in the tropical paradise of Far North Queensland as, for the first time ever, the IAU symposium on the Galactic Center visits Australian shores.

The weather even cooperated with the organisers, with an entire week of almost wall-to-wall rain keeping the 150 or so astronomers from all over the world focussed on a wide range of interesting talks as opposed to long walks on the beach.  

To pick some of my favorite talks to highlight is somewhat self-indulgent and selfish, given my relatively narrow field of interest. I’ll try to be as unbiased as possible (so, still quite biased!).

To kick off, Wednesday’s second session was home of an outstandingly good natured set of talks on positron astrophysics. Thomas Siegert, who has been responsible together with the SPI/INTEGRAL team at MPA Garching for most of the recent analysis of the 11 years of data on positron annihilation in the Galaxy, gave an excellent overview of the topic. The new analysis of the SPI data has revealed some surprising truths about the location of most annihilating antimatter in the Galaxy, in particular the fact that as much antimatter is being annihilated in the Galactic disk as the Galactic bulge. This is contrary to the results from the first 5-8 years of data, which showed a very faint disk component to the emission, half of which could be explained by the annihilation of positrons being produced from the long-lived radioisotope Aluminum-26. Now as it stands, barely 10% of Galactic positrons can originate from this isotope, synthesised by massive stars, and the origin of most bulge positrons remains unexplained. Siegert gives a convincing argument for multiple origins for Galactic positrons, an argument I’ve certainly disagreed with in the past. Also just in are his excellent results on detecting 511keV emission in dwarf satellite galaxies of the Milky Way, which almost certainly deserves a blog post of it’s own. Read the just-published A&A paper here.

I was perhaps a little star-struck by meeting Nidhal Guessoum, whose work on positron microphysics and transport I’ve been using to inform my own work on simulating positron transport, energy losses and annihilation. Guessoum’s talk focussed on the plausibility of a source close to Sgr A* being the source of Galactic bulge positrons: positrons produced by Sgr A* itself, or a source nearby, can be diffusively transported out of the Galactic nucleus by MHD turbulence. The preliminary results presented look exciting, and I’m looking forward to seeing the final results.

Moving away from positrons and back to my old stomping ground of the Nuclear Star Cluster, I feel the need to highlight two excellent talks. First by Marion Grould, who discussed how observations using the GRAVITY instrument on the ESO’s VLT of the star S2, which occupies an elliptical orbit with a period of ~15 years about the central supermassive black hole in our Galaxy, can be used as a test of General Relativity. The simulations seem to say yes, so it will be exciting if the precession of the star due to the warped spacetime around Sgr A* can indeed be detected with this instrument despite the Newtonian perturbations on the star due to unresolved members of the NSC.

The origin of the NSC was also up for debate in two talks that occupied the Friday morning session. Oleg Gnedin’s talk on the formation of the NSC had me almost wishing I had taken the offer of a PhD place at University of Queensland investigating just that. The theory talk by Gnedin was beautifully backed up by a talk by Tuan Do on the observational constraints on the NSC’s formation. Do described the incredibly detailed data now available on the massive stars that orbit the monster black hole in the Galactic Center, and the take home message seems to be “watch this space” as these data are added to and utilized to investigate whether the NSC formed in-situ, or from the disruption of a Galactic bulge globular cluster.

David Nataf took an informative step back from 5 days of focus on, largely, the central 200pc of the Galaxy with his comparative review of the Galactic Bulge stellar population. Highlighted was this lovely work by Melissa Ness and Dustin Lang on the X-shaped feature in the Galactic bulge revealed in beautiful detail in WISE data.

Another particular highlight was the afternoon dedicated to a detailed discussion of the Galactic GeV excess. An observed excess of gamma rays at ~2GeV, a signal observed by the Fermi telescope that is sometimes referred to as the “Hooper Bump” has had two posited explanations: annihilation of a ~10GeV dark matter particle, or an unresolved population of millisecond pulsars. Convincing arguments were presented from both sides, from Richard Bartels, Dan Hooper and Chris Gordon. Francesca Calore presented an updated analysis of the gamma ray data while Doug Finkbeiner presented a thorough Bayesian look at the data.

I’ve somewhat skipped over some of the other topics covered. Remarkable things are being done in radio observations, probing accretion onto Sgr A*, the structure of the SMBH itself and the radio-emitting filaments of gas in the central regions of the Galaxy using incredibly high resolution studies. X-ray data from the likes of NuSTAR is also shedding new light on the Galactic Center region, although the untimely demise of ASTRO-H has disappointed many who were hoping that the new instrument would obtain more detailed observations of the 3.5keV line near the Galactic Center. The Event Horizon Telescope, which sees many of the world’s radio telescopes joined together as a colossal interferometer to probe in fine detail the structure of Sgr A*, was also discussed, along with how the new Cherenkov Telescope Array will provide an even sharper look at VHE cosmic rays being emitted from the Galactic Center region and perhaps shed light on the “Pevatron”.

The great strength of the conference, which brings together scientists with an interest in anything related to the central regions of our Galaxy, is that it opens up the opportunity for interdisciplinary collaboration. Theorists present their work alongside observers, and as the name of the conference suggests, every observation technique from radio through to gamma rays is well represented. The outcome of most of the well-attended panel discussions clear: We don’t really understand the central regions of our Galaxy (or most of our Galaxy, really) and in order to do so, we need to build a unified, self-consistent picture of how it works.