Black holes kill their own galaxies

Galaxy NGC 1275

Galaxy NGC 1275 is a nearby equivalent to many of the distant massive galaxies studied by the Nottingham / University College London team. It shows the phenomenal power of supermassive black holes to disrupt the gas of a galaxy, and represents a window onto a violent past of the lives of galaxies.

Black holes are thought to be at the centre of almost every galaxy, with some growing to more than a billion times the mass of the Sun.

Now a team of UK astronomers believe that these supermassive black holes are commonplace, release more than enough energy to rip their host galaxies apart, and in the process shut down these galaxies’ star formation for good.

Asa Bluck of the University of Nottingham, who led the research, explained the dramatic impact of these monster black holes in a talk at the Royal Astronomical Society’s National Astronomy Meeting in Glasgow.

For many years black holes have fascinated scientists and the public alike, with their peculiar ability to warp space and time and their almost sinister tendency to devour everything they encounter.

Before it falls in, as matter swirls around the black hole it forms a whirlpool called an ‘accretion disc’, where it heats up and radiates energy outwards.

These “supermassive” black holes have such a strong gravitational field that the infalling matter releases a vast amount of energy, making each accretion disc far brighter than the combined output of the hundreds of billions of stars in the galaxy around it.

One of the consequences of this outpouring of energy is that it drives away any cool gas and dust clouds, the raw ingredients of new stars. This permanently shuts down star formation in the surrounding galaxy, dooming it to a slow death, wherein the remaining stars age, grow red, end their lives and are never replaced.

Artist's impression of the "accretion disc" whirlpool surrounding a black hole.

Artist's impression of the "accretion disc" whirlpool surrounding a black hole.

Huge outpouring of energy

The new study considered the role of supermassive black holes in the development of galaxies.

To search for them, the team used the Hubble Space Telescope and the Chandra X-ray Observatory to image the Universe to unprecedented depth and resolution at optical, near infra-red and X-ray wavelengths.

In particular, the astronomers looked for galaxies that have a very high emission of X-rays, a classic signature of black holes devouring gas and dust.

The team found that at least one third of all the massive galaxies in the Universe not only contain supermassive black holes, but that at some point in their histories the outpourings from the holes’ accretion discs far outshines their host galaxies.

The energy output of regions around the black holes is high enough to strip apart every massive galaxy in the cosmos 25 times over, whilst the X-ray emission from them turns out to dwarf that from every other source in the Universe put together.

Asa sums up the new results: “We are left with a startling picture of the formation history of massive galaxies, where dramatic violence in the form of the torrent of radiation from matter falling into black holes leads to the death of galaxies they inhabit.”

Adapted from information issued by RAS / A. Fabian (Cambridge) / STScI / NASA.

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