2013 looks set to be an interesting year for those of us residing in the Milky Way (page view stats indicate this applies to the majority of visitors to this blog). In the middle of this year, the supermassive black hole in the centre of our galaxy will be paid a visit by a cloud of gas with a mass three times that of the Earth. This could result in a bright flare of X-rays if some of this gas falls too near the black hole and is consumed, allowing us to probe the environment around it better than ever before. But how do we know there’s a black hole there in the first place, and why won’t this gas just get gobbled up without a trace?
The Centaur’s Secret
Despite being… well, black, black holes can be examined through their interactions with the local environment. They might not shine themselves†, but the gas they swirl and toss around and the stars which orbit them certainly can. It’s through these gravitational interactions that we can see the tell-tale signs of the dark monster lurking at the heart of our own galaxy.
In the constellation of Sagittarius lies a region of X-ray emission, seen in the image on the right taken by Nasa’s Chandra X-ray observatory. The three colours in the image (red, yellow and blue) are separate images of different X-ray wavelengths, each snapped through an individual filter. Blue is the highest energy of the three and red the lowest. The white-ish cloud in the middle of the image appears white because it’s shining across the full range of energies, and within it lies the object known as Sagittarius A* (Sgr A*). Here it is in even higher energy X-rays, as seen by the fantastic new NuSTAR observatory, undergoing a flare in brightness. All this X-ray light is telling us something about the gas in that part of the galaxy; it’s very hot. In fact it can be heated up to around 100 million °C or more, which is about 6 times hotter than the core of the Sun! This gas could be a corona – a diffuse but hot plasma – surrounding the central object.
Colder, more dense gas can be pulled in toward a black hole and, in the very closest region around the black hole itself, this in-falling gas can be stretched out and spun around to form a swirling disc about it, called an accretion disc. This dense, fast moving disc of material can emit over a very wide range of wavelengths, from radio to X-rays. Sagittarius A* is a bright radio source and has been viewed using networks of radio telescopes on Earth. These observations have shown that the emission is coming from within a small volume of space, on the scale of the Solar System (which really is small, at least to astronomers). This might well be the signature of an accretion disc, but it is relatively faint compared to some other supermassive black hole candidates in other galaxies. This could quite simply be because our black hole isn’t consuming very much matter; it’s dieting. There are occasional, modest flares like the one seen by NuSTAR, so it could well be snacking now and then.
Yet another way in which black holes can betray their presence is through jets of charged particles shooting out from the poles at nearly the speed of light. These particles, mostly protons and electrons, emit radio waves as they go – called synchrotron radiation. The exact process that produces these jets isn’t well known, but it may be due to the extreme electric and magnetic fields in the disc and/or the spin of the black hole supplying energy to these particles. These jets can be seen coming from the centres of many galaxies, like M87 pictured on the left, though not from our own Milky Way.
In the case of Sagittarius A*, we can see the gas near the centre glowing in radio and X-rays. But there are also stars there too, and when their positions were tracked for the first time something amazing was seen.
This animation was made using observations made by a group of astronomers, led by Dr. Reinhard Genzel at the Max Planck Institute. The data were taken over many years in the infrared part of the spectrum, which allowed them to peer through the dusty space which blocks the visible light coming from the galactic centre. If you study the centre of that animation carefully, you can see that the stars nearest the middle execute sharp slingshots around a conspicuously invisible object. Here is a more focused animation (and here yet more focused) where you can clearly see this motion. By carefully studying these orbits, and in particular that of the closest star — dubbed ‘S2’ — the mass of the central object was calculated using simple Keplerian mechanics. The answer: 4.31 ± 0.38 million times the mass of the Sun! What’s more, this huge mass has to lie within a volume only about 12 light-hours across; roughly the volume enclosed by the orbit of Pluto around the Sun. The presence of such a large mass in such a small volume, at the very region of space where we see the glowing Sagittarius A*, suggests that we do indeed have a supermassive black hole lurking at the centre of our Milky Way.
So what about this gas cloud then?
Over the last few years another object has been observed moving through this region of the galaxy, almost directly towards our slumbering behemoth, at around 1,700 km/s (3.8 million mph)… and it’s speeding up! A paper by Gillessen et al. outlines their observations of this cloud and they predict it will be stretched and ripped apart by the tidal forces near the black hole, allowing some of it to be devoured while the rest is thrown back out into orbit. If this happens, the gas should cause a bright flare as it adds to the accretion disc and ultimately falls inside.
With telescopes all over the world (and above it) we will watch and wait to see whether this indeed happens. Our understanding of the enigmatic centre of our galaxy could be transformed by this rare event. Let’s hope so!