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Red Nuggets Show Glimpse Into How Supermassive Black Holes Grow So Big

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A rare breed of galaxy shows how black holes smothered the birth of new stars and used up all the unused energy to grow themselves.

What Is A Red Nugget?

In 2005, astronomers first detected the existence of an unusual type of galaxy far off in the distant, early years of the universe. Showing up just 3 to 4 billion years after the Big Bang, these small, compact galaxies appear red to observers on Earth because of the proliferation of red stars.

Called red nuggets, these galaxies are said to be the progenitors of the giant elliptical galaxies, which are sometimes called red-and-dead galaxies because they harbor only old, low-mass red stars. Although only about one-fifth the size of a medium-sized elliptical galaxy, a red nugget has about the same mass. Over time, red nuggets collided with other galaxies. However, a few of them remain alone in the wilderness.

Astronomers working at NASA Chandra X-ray Observatory have decided to study a couple of the few isolated red nuggets to see how galaxies and the supermassive black holes at their center behave when left to themselves.

Studying Red Nuggets

A group of astronomers headed by Norbert Werner of the MTA-Eotvos University Lendulet Hot Universe and Astrophysics Research Group in Hungary has looked into the behavior of two red nuggets called MRK 1216 and PGC 032673. Both galaxies are younger than the first red nuggets observed, each of them located just 295 million and 344 million light-years from Earth, respectively.

The researchers looked into the behavior of the hot interstellar gas near the center of each red nugget. The X-ray emissions produced by the hot gas can reflect back signs of activity of the nearby black hole in each galaxy. From these observations, Werner's team found that the black hole prevents the gas from cooling and forming into stars.

"We are finding that the black holes in these galaxies take over and the result is not good for new stars trying to form," says Werner.

Black Hole Activity

The super-strong gravitational and magnetic fields of black holes can deflect material falling into them and stream them outward as jets traveling close to the speed of light. These black hole jets produce massive amounts of heat, which affects the surrounding interstellar gas and keeps it from cooling into new stars.

MRK 1216, in particular, has a hotter center than the other galaxy. This suggests that the red nugget was only recently heated by the jet from its black hole. PGC 032673, which is slightly older, has hot gas 10 times fainter, which may owe to the more relentless bursts from its own black hole that blew the gases away.

The scientists also detected radio emission from both black holes, which is typical of black holes with jets. The X-ray signals from both black holes are also a hundred million times below the Eddington limit, a limit on how fast a celestial body can grow when the inward pull of gravity is balanced by the outward force of radiation. This is also characteristic of jet-emitting black holes.

Black Holes Thwarting Star Birth

Werner and his team also propose that supermassive black holes may have grown to size by eating up the hot gas near the core of their galaxies.

The black holes in MRK 1216 and PGC 032673 are two of the most massive black holes known to man. By measuring the speed of stars near the center of each red nugget, the researchers peg the mass of each black hole at around 5 billion times of the Sun's mass.

Also, the mass of each black hole comprises a small percentage of the combined mass of the stars near the galactic center. More typical galaxies have black holes that have a mass 10 times less.

"Not only do they prevent new stars from forming, they may also take some of that galactic material and use it to feed themselves," says coauthor Massimo Gaspari of Princeton University.

The findings are published in the Monthly Notices of the Royal Astronomical Society.

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