The Hubble Space Telescope optical image of the star-forming Antennae galaxies. Ultra-lumi- nous X-ray sources are observed close to stellar clusters, seen here as bright optical sources.
expected if they were intermediate-mass black holes. Compared to the stars comprising the clusters, an intermediate- mass black hole would be massive, and consequently, due to a superior gravitational force, would fall to the centre of such a grouping. This revelation helped the team conclude
that the very bright X-ray sources were indeed stellar-mass black holes, as opposed to intermediate-mass black holes, but the question of why these normal black holes were found just outside the centre of star- forming regions still remained. It had been previously thought
that a
supernova explosion - the forming of a black hole when a star collapses - could have provided the force to effectively kick the resulting binary black hole system away
from its original position, and
Poutanen’s team wanted to put this to the test.
spectroscopy of the fields around the X-ray sources using the European Very Large Telescope (VLT), which allowed measurement of the ages of the stellar clusters
and therefore the black holes
concerned. The results placed them between two and six million years old, relatively very young in star lifetime terms, meaning that these black holes must have already been produced during the formation of the stellar clusters and ejected by gravitational interactions with other massive stars, as they would not have had
32 In order to do so, they studied
million and ten billion solar masses, and are found at the very centre of most galaxies, with unparalleled levels of radiation emitted from the surrounding region, the quasar. The brightest of all quasars, blazars, are highly compact and are known to be among the most energetic objects in the universe. A study conducted in 1995 showed that
blazars could be observed via gamma rays, and not solely through radio emissions as was previously thought. Poutanen’s team
There was no clear answer as to where it comes from,” says Poutanen. The team looked at how the energy
spectrum of these gamma-ray photons, and in particular the breaks in the spectrum, related to those of other known emissions from these super massive black holes. They noticed that the breaks in the gamma spectrum corresponded with strong emission lines in the ultraviolet spectrum, i.e. energy levels corresponding to atomic transitions, resulting in the emission of UV photons.
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time in their short lifetimes to travel such a distance away from said clusters.
Super-massive black holes The final part of the team’s research has concentrated on the most captivating of all black holes, the super-massive black hole. These titans are of a near- incomprehensible size, between a few
have been studying the gamma ray spectrum to determine their place of origin. Studying data taken from NASA’s Fermi Gamma Ray Space Telescope, the team analysed the spectrum of gamma radiation from several blazars, and saw that there were gaps at a certain energy. “You start
to think about
how you can produce such a sharp feature in the spectral distribution of those sources.
“These are big discoveries that will help to develop scientific understanding of black holes and how they operate, but we know that in Astronomy it can take several years before people are convinced”
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