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Black holes stunt dwarf galaxy growth
BY AMANDA DOYLE
ASTRONOMY NOW
Posted: 24 May 2012


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Black Hole An artist's impression of the accretion disc around a supermassive black hole and the energetic jets than emanate from the disc. If the jets are pointing directly at us then it is called a blazar. Image: ESA/NASA, the AVO project and Paolo Padovani.

Scientists have discovered that black holes may have been sneakily turning up the thermostat in the early Universe, making it difficult for dwarf galaxies to form.

Every galaxy contains a monster within; a supermassive black hole that can be up to billions of times the mass of our Sun. While no light can escape from a black hole, energy can be emitted from the disc of material that is spiralling into the depths of the black hole. The energy emitted from this accretion disc can take the form of powerful jets, and when these jets point directly towards us, the object is called a blazar.

Blazars can emit intense radiation in the form of gamma rays, and these high energy gamma rays will eventually smash into lower energy optical radiation. When this happens, two types of particles are created; electrons and positrons. A positron is the antimatter partner of an electron, as it has a positive charge as opposed to the electron's negative charge. These interactions typically take places millions of light years away from the blazar.

The electron positron pairs have extremely high energies in the region of teraelectron volts. It was originally thought that the energetic pairs would interact with the cool, less energetic cosmic microwave background (CMB) photons. The CMB is a relic of the very early Universe, marking the time when photons of light originally separated from matter particles. If the particles from the blazars interact with CMB photons, then the energy of the particles would be decreased by a factor of 1000 into the range of gigaelectron volts.

However when astronomers viewed the spectra of blazars, this gigaelectron volt energy was missing. It was thought that perhaps magnetic fields might deflect these particles away from our line of sight. That was when Christoph Pfrommer from the Heidelberg Institute for Theoretical Studies (HITS) and his colleagues had a brainwave.

"We were thrilled to read about such an exciting possibility but started to think about other physical phenomena that could have happened along this complicated chain of events," Pfrommer tells Astronomy Now. "Very soon, we started to realise that there may be another mechanism overlooked so far by all astrophysicists that would eventually cause the gigaelectron volt emission to vanish."

An alternative method of making the gigaelectron volt energy disappear is that the particles decelerate much faster than originally thought, by interacting with the surrounding diffuse gas. This rapid braking would result in the particles converting their energy into heat, thus accounting for the "missing" energy. The heat will in turn warm up the background medium that is interspersed between galaxies, known as the intergalactic medium.

The temperature of the gas can eventually rise to a few hundred thousand Kelvin, and this can greatly inhibit the formation of small galaxies. Gas of a low temperature is needed to collapse and form stars, and hot gas is not easily compressed by dwarf galaxies. "For these temperatures, the gravitational pull of small dark matter halos that host dwarf galaxies is not large enough anymore to collect the gas and compress it to the point where it can emit cooling radiation and become available for star formation," explains Volker Springel, scientific group leader at HITS. "Larger galaxies are much more massive, however, and so have no problem to overcome the gas pressure of the heated gas. Their gravitational potential is simply much deeper, so that they don't care all that much that the gas is already hotter when it is falling into these galaxies."

"This may solve a number of problems in the theory of galaxy formation, including the 'missing satellite problem' in the Milky Way," adds Pfrommer. Simulations predict that there should be more small satellite galaxies surrounding the Milky Way than are actually detected, but if their formation was suppressed by indirect heating from black holes then the simulations can finally agree with the observations.

The group are intending on running further simulations to increase the understanding of this unusual phenomenon.

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