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Using the Chandra space telescope, astronomers at the University of Alabama led by Ka-Wah Wong have photographed and analysed, in unprecedented detail in the X-ray domain, the flux of gas falling towards the central super-massive black hole in the centre of the lenticular galaxy NGC 3115, which lies at a distance of about 32 million light years.
The above image shows the X-ray emission (blue) overlayed on an optical image taken by ESO's Very Large Telescope (orange).
At a certain distance from the black hole, known as the "Bondi radius", whose value depends on the mass of the black hole, the motion of the gas starts to be dominated by the gravitational pull of the cosmic monster. At this point the gas is accelerated and compressed, increasing significantly its temperature.
In the case of the super-massive black hole nesting on the centre of NGC 3115 (at the centre of the inset which is 4150 light years across) Wong and colleagues have measured a net increase in the temperature of the infalling gas at a distance of 700 light years, a radius that suggests the mass of the black hole is around 2 billion solar masses. The closest to Earth of that size!
From the data obtained directly as well as theoretical models, Wong's team has calculated the inward rate of passage of material though the Bondi radius that is expected to fall onto the black hole: 2% of a solar mass per year. Inserting this value into models that predict how much energy will be converted into radiation, they find that the source observed by Chandra should actually be a million times more luminous than is observed.
This is not the first time this problem has been encountered: the X-ray luminosity of the regions surrounding super-massive black holes is in fact normally much lower than predicted. There are two possible solutions: the quantity of material that falls onto the black hole could be, for some unknown reason, much less than that which falls within the Bondi radius; or, the energy conversion producing the radiation is much less efficient than thought. In fact, there are also contrasting ideas as to how quickly the gas density increases towards the black hole, so further work is clearly needed before this mystery is solved.
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