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Collisions between galaxies can leave the black holes at their centres with nothing to feast on

A cataclysmic collision between two galaxies could leave the supermassive black hole at the centre of the larger one ‘starved of material’, astronomers claim.

Experts from the University of Tokyo created the ‘most accurate simulation’ of a range of scenarios to understand what happens when galaxies merge. 

It was previously thought that collisions between galaxies would necessarily add to the activity of the massive black holes (MBH) at their centres, by providing more matter to them feast on.

However, the Japanese team say under certain ‘head on collision’ scenarios the galactic nuclei could be cleared of matter that would otherwise fuel the MBH. 

Artist’s impression of gas being pulled away from a galactic nucleus after a collision with another galaxy – resulting in the black hole being starved of matter to feast on

Galactic archaeologists, looking at the make-up of stars in the Milky Way, say there is evidence of a large scale head on collision some 11 billion years ago. 

While the researchers didn’t specifically modelled the Milky Way in this study, senior author Yohei Miki told MailOnline the activity level of the massive black hole at the centre of the Milky Way is relatively low compared with black holes at the core of other galaxies.

This suggests that our own black hole, named Sagittarius A*, may have at some point been starved of material after a head-on collision with another galaxy. 

‘When you think about gargantuan phenomena such as the collision of galaxies, it might be tempting to imagine it as some sort of cosmic cataclysm, with stars crashing and exploding, and destruction on an epic scale,’ said author Yohei Miki.

‘The reality is that it is actually closer to a pair of clouds combining, usually a larger one absorbing a smaller one,’ the research associate added. 

Under that scenario, it is unlikely any stars within the galaxies would collide themselves; however the wider consequences of the ‘merging clouds’ can be enormous.  

Galaxies collide in different ways – sometimes a small galaxy will graze the outer part of a larger one and either pass through it completely or the two will merge. In either case, the two galaxies will exchange a lot of stars in the process.

Galaxies can also collide head-on, where the smaller of the two will be torn apart by overpowering tidal forces of the larger one – this is where the MBH could be starved. 

‘For as long as astronomers have explored galactic collisions, it has been assumed that a collision would always provide fuel for an MBH in the form of matter within the nucleus,’ said Miki.

Visualizations of the dynamic model simulating two different scenarios. The top row shows a collision reducing core activity, the bottom row shows a collision increasing core activity

Visualizations of the dynamic model simulating two different scenarios. The top row shows a collision reducing core activity, the bottom row shows a collision increasing core activity

Adding that it was also assumed this fuel would feed the MBH and ‘significantly increase its activity’ – visible through UV and X-ray light.

‘However, we now have good reason to believe that this sequence of events is not inevitable and that in fact the exact opposite might sometimes be true,’ Miki wrote.


One solar mass is equivalent to  2 times 10 to the 30th of a single kilogram. 

The entire galaxy is 1.5 trillion times greater (1.5 multiplied by ten to the power of 12) than this. 

That means the the sun weighs 3 x 10^42 kg.  

This equates to 3 x 10^39 tonnes. 

In non-mathematics, this means the Milky Way’s weight is therefore equal to 3,000 trillion trillion trillion tonnes. 

It seems logical that a galactic collision would only increase the activity of an MBH, but Miki and his team were curious to test this notion. 

They constructed highly detailed models of galactic collision scenarios and ran them on supercomputers. 

The team was pleased to see that in some circumstances, an incoming small galaxy might actually strip away the matter surrounding the MBH of the larger one. 

This would reduce instead of increase its activity.

‘We computed the dynamic evolution of the gaseous matter which surrounds the MBH in a torus, or donut, shape,’ said Miki. 

‘If the incoming galaxy accelerated this torus above a certain threshold determined by properties of the MBH, then the matter would be ejected and the MBH would be starved. 

‘These events can last in the region of a million years, though we are still unsure about how long the suppression of MBH activity may last.’

This could help astronomers understand the evolution of our own Milky Way as they are confident the Milky Way has collided with many smaller ones in the past. 

It could also help to understand what might happen in 4.5 billion years when the Milky Way is due to collide with the larger spiral galaxy – Andromeda.

Future research will look to discover how long any suppression of an MBH might last, Miki told MailOnline. 

The findings have been published in the journal Nature Astronomy. 


The Galactic centre of the Milky Way is dominated by one resident, the supermassive black hole known as Sagittarius A* (Sgr A*).

Supermassive black holes are incredibly dense areas in the centre of galaxies with masses that can be billions of times that of the sun.

They act as intense sources of gravity which hoover up dust and gas around them. 

Evidence of a black hole at the centre of our galaxy was first presented by physicist Karl Jansky in 1931, when he discovered radio waves coming from the region. 

Pre-eminent yet invisible, Sgr A* has the mass equivalent to some four million suns.  

At just 26,000 light years from Earth, Sgr A* is one of very few black holes in the universe where we can actually witness the flow of matter nearby.

Less than one per cent of the material initially within the black hole’s gravitational influence reaches the event horizon, or point of no return, because much of it is ejected. 

Consequently, the X-ray emission from material near Sgr A* is remarkably faint, like that of most of the giant black holes in galaxies in the nearby universe.

The captured material needs to lose heat and angular momentum before being able to plunge into the black hole. The ejection of matter allows this loss to occur.