Gas cloud timelapse: watch the cloud (yellow arrow) move towards the black hole (white +) (Gillessen et al, 2011)
Ed Yong recently started a fun new Tumblr blog called Nature Wants To Eat You, showing pictures of scary-looking animal mouths that may or may not be out to gobble us up. But the scariest and most inescapable example of Nature Wanting To Eat Us is the stuff of astrophysics – in the way that astrophysics tends to kill all the sciences, really: black holes. This week, a team of scientists led by the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching posted their Nature paper to astro-ph, describing their observations of a cloud of gas speeding towards the black hole at the centre of the Milky Way galaxy.
Like with many of these results, the coolest thing isn’t that this is happening. Imagine the size of the Universe: everything you can possibly think of is probably happening right now, somewhere. The mindblowing thing is that we can see it, 27,000 lightyears away, just like you’re probably now watching Strictly Come Dancing or Match of the Day – and believe me, this gas blob is far more exciting.
The central region of our galaxy, which (much evidence suggests) hosts our closest known supermassive black hole, is totally fascinating. It contains some of the most massive stellar clusters known, and star formation is ongoing in the dense and turbulent clouds of molecular gas. In the immediate vicinity of the black hole, Sagittarius A*, there’s a dense cluster of massive stars that the MPE group and others have been observing in detail for over a decade, tracking the stars’ orbits, mapping its structure, figuring out its origins. The gas cloud was found lurking in this dataset.
The Galactic Centre is heavily obscured by dust along the line of sight, so observations at optical wavelengths are useless for this region. All the observations presented in the paper are done in the infrared, in the range of 2 to 5 µm, with a combination of imaging (using NACO on VLT) and spectroscopy (with SINFONI on VLT). Both these instruments use adaptive optics to correct for atmospheric turbulence blurring, which is absolutely crucial for being able to resolve the individual stars in this crowded region. To get an idea of this crowding, each stamp in the above image covers just one arcsecond on the sky – that’s about how wide a human hair would be at a distance of 10 meters. It’s a lot of stuff in a tight space.
So what did Gillessen and colleagues actually see? Comparing the images taken over a number of years, shown in the image above, the arrow points towards the object that appears to be moving across the field in the direction of the black hole (marked by a white +). By comparing images of the blob in several infrared filters they deduced that it’s most likely a cloud of cool ionised gas around 550 K – around 270 degrees C. And it’s moving, fast.
The imaging data allowed them to calculate the apparent movement on the sky over time, and spectral lines in the SINFONI data gave them the object’s radial velocity (along the line of sight) – putting those two together, they were able to constrain the object’s orbit in 3D space. The data suggest that the blob is hurtling along at more than 1000 km/s, its speed doubling to over 2000 km/s over the last 7 years. Excitingly, the orbit they calculated suggests the blob will reach its point of closest approach to the black hole some time in mid-2013. The spectral data suggest that it’s already breaking up under the combination of high velocities, tidal forces and ram pressure.
As it approaches its pericentre, the cloud will become very hot, producing X-ray emission. As it’s likely to have fragmented further by that time, that emission could continue for some time, flaring and dimming as different parts of the cloud pass the pericentre. It’s hard to predict exactly what will happen, as this depends heavily on the conditions inside the cloud – its density and its state of disruption, which are hard to determine with confidence. But watching what happens will give us an amazing “live action” view on how black holes interact with the material around them and how they actually accrete material and grow.
As the Galactic Centre will be nice and high in the Paranal skies in the summer of 2013, I imagine VLT will be pointing its infrared eyes at the region too. Very exciting.
Incidentally, the lead author of this paper, Stefan Gillessen, is Instrument Scientist of GRAVITY, the infrared interferometric instrument I work on at MPIA; co-author Frank Eisenhauer is the project’s PI. The design of GRAVITY, which combines the beams from 4 different telescopes at VLT (combinations of the big Unit Telescopes and the smaller Auxiliary Telescopes), is optimised to carry out exactly this kind of observation: ultra-precise tracking of objects around Sgr A*. As part of the project, MPIA is leading the implementation of infrared wavefront sensing on the adaptive optics systems of each of the VLT 8-m telescopes. I wrote a post some time ago about science with GRAVITY, here.
References
S. Gillessen, R. Genzel, T. K. Fritz, E. Quataert, C. Alig, A. Burkert, J. Cuadra, F. Eisenhauer, O. Pfuhl, K. Dodds-Eden, C. F. Gammie, & T. Ott (2011). A gas cloud on its way towards the super-massive black hole in the Galactic Centre Nature arXiv: 1112.3264v1
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