Using ESO's Very Large Telescope, astronomers have stumbled across a black hole that may be building its own host galaxy, an observation that could help resolve a long-standing mystery as to why the masses of black holes are larger in galaxies that contain more stars.

The question of which came first, the supermassive black holes that frantically devour matter or the enormous galaxies where they reside, is one of the most debated questions in astronomy.

“Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies,” says lead author David Elbaz. “This link could also explain why galaxies hosting larger black holes have more stars.”

The curious object in question is a five billion light year distant quasar known as HE0450-2958, and is the only one currently known for which a host galaxy has not been detected. Previous research suggested that the quasar's galaxy was obscured by thick dust, and so the mid-infrared eyes of the VLT were called upon to seek out these supposed dust clouds.

“Observing at these wavelengths would allow us to trace dust that might hide the host galaxy,” says Knud Jahnke, who led the VLT observations. “However, we did not find any. Instead we discovered that an apparently unrelated galaxy in the quasar’s immediate neighborhood is producing stars at a frantic rate.”

The companion galaxy is forming stars at a rate equivalent to about 350 Suns per year, one hundred times more than rates for typical galaxies in the local Universe. Earlier observations had already shown that the quasar is injecting a jet of highly energetic particles into its companion, accompanied by a stream of fast-moving gas. This process suggests that the quasar itself might be inducing the formation of stars and thereby creating its own host galaxy. In this scenario, galaxies would have evolved from clouds of gas hit by the energetic jets emerging from quasars.

“The two objects are bound to merge in the future: the quasar is moving at a speed of only a few tens of thousands of kilometres per hour with respect to the companion galaxy and their separation is only about 22,000 light-years,” says Elbaz. “Although the quasar is still ‘naked’, it will eventually be ‘dressed’ when it merges with its star-rich companion. It will then finally reside inside a host galaxy like all other quasars.”

Based on this observation, the team propose that black hole jets are a possible driver of galaxy formation, which may also represent the long-sought missing link to understanding why the mass of black holes is larger in galaxies that contain more stars. In order to probe this idea further the team will search for similar objects in other systems, and in the future, instruments such as the Atacama Large Millimeter/submillimeter Array, the Extremely Large Telescope and the James Webb Space Telescope will be able to search for such objects at even greater distances.

The new research is presented in the journal Astronomy & Astrophysics.