The secret of galactic birth control revealed

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Astronomers may have finally discovered the secret of starbirth in large galaxies. 

Young galaxies blaze with bright new stars forming at a rapid rate, but star formation eventually shuts down as a galaxy evolves – but nobody has known why.

A new study shows that the mass of the black hole in the center of the galaxy determines how soon the switch happens. 

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Billions of old stars cause the diffuse glow of the extended central bulge in the Sombrero Galaxy. The very center of the Sombrero glows across the electromagnetic spectrum, and is thought to house a large black hole. A new study shows that the mass of the black hole in the center of the galaxy determines how soon the galaxy stops making new stars

Billions of old stars cause the diffuse glow of the extended central bulge in the Sombrero Galaxy. The very center of the Sombrero glows across the electromagnetic spectrum, and is thought to house a large black hole. A new study shows that the mass of the black hole in the center of the galaxy determines how soon the galaxy stops making new stars

HOW IT WORKS 

The energy pouring into a galaxy from an active galactic nucleus is thought to turn off star formation by heating and dispelling the gas that would otherwise condense into stars as it cooled.  

But observational evidence of a connection between supermassive black holes and star formation has been lacking, until now.

Every massive galaxy has a central supermassive black hole, more than a million times more massive than the sun, revealing its presence through its gravitational effects on the galaxy’s stars and sometimes powering the energetic radiation from an active galactic nucleus (AGN). 

 

Every massive galaxy has a central supermassive black hole, more than a million times more massive than the sun, revealing its presence through its gravitational effects on the galaxy’s stars and sometimes powering the energetic radiation from an active galactic nucleus (AGN). 

The energy pouring into a galaxy from an active galactic nucleus is thought to turn off star formation by heating and dispelling the gas that would otherwise condense into stars as it cooled. 

But observational evidence of a connection between supermassive black holes and star formation has been lacking, until now.

‘We’ve been dialing in the feedback to make the simulations work out, without really knowing how it happens,’ said Jean Brodie, professor of astronomy and astrophysics at UC Santa Cruz and a coauthor of the paper, published in Nature. 

‘This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy.’

The new results show black hole activity and star formation throughout a galaxy’s life is linked.  

The new study focused on massive galaxies for which the mass of the central black hole had been measured in previous studies by analyzing the motions of stars near the center of the galaxy.  

SUPERMASSIVE BLACK HOLES 

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.

Their intense gravitational pull is thought to be what stars in galaxies orbit around.

The researchers are investigating three explanations in hopes to solve the mystery of the snake-like filament near the supermassive black hole, Sagittarius A. Artist's impression pictured 

The researchers are investigating three explanations in hopes to solve the mystery of the snake-like filament near the supermassive black hole, Sagittarius A. Artist's impression pictured 

The researchers are investigating three explanations in hopes to solve the mystery of the snake-like filament near the supermassive black hole, Sagittarius A. Artist’s impression pictured 

How they are formed is still poorly understood.

Astronomers believe they may form when a large cloud of gas up to 100,000 times bigger than the sun, collapses into a black hole.

Many of these black hole seeds then merge to form much larger supermassive black holes.

Alternatively, a supermassive black hole seed could come from a giant star, about 100 times the sun’s mass, that ultimately forms into a black hole after it runs out of fuel and collapses. 

When first author Ignacio Martín-Navarro, a postdoctoral researcher at UC Santa Cruz, compared the star formation histories of galaxies with black holes of different masses, he found striking differences. 

These differences only correlated with black hole mass and not with galactic morphology, size, or other properties.

‘For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. 

‘So star formation lasted longer in those galaxies with smaller central black holes,’ Martín-Navarro said. 

A supermassive black hole is only luminous when it is actively gobbling up matter from its host galaxy’s inner regions. 

Active galactic nuclei are highly variable and their properties depend on the size of the black hole, the rate of accretion of new material falling onto the black hole, and other factors. 

 





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