Black hole may construct its own galactic home

Observations of a 'homeless' quasar suggest new ideas for galaxy formation

10:19am, December 4, 2009
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A homeless black hole might be building a new galaxy to live in, new observations suggest.

The find could be the latest clue in a longstanding mystery in astronomy: whether supermassive black holes or the galaxies that house them form first.

“If it is true, it is very surprising, as it gives a whole new role to the black holes in the universe — not as devourers but as creators of galaxies,” comments Padelis Papadopoulos of the University of Bonn in Germany. The new observations, which were published online and in the December 1 issue of Astronomy & Astrophysics, could represent a shift in galaxy formation theories — but some think it’s too soon to rewrite the textbooks.

A team of astronomers, led by Knud Jahnke of the Max Planck Institute for Astronomy in Heidelberg, Germany, observed an apparently homeless quasar called HE0450-2958 about 5 billion light-years away. Most quasars are active galaxies with supermassive black holes at their centers. Gas swirling around the black hole heats up and emits massive amounts of energy, making the region around the black hole glow like a star. But according to previous observations in the visible spectrum, HE0450-2958 and its black hole lack a galaxy.

In the visible spectrum, stars show up and dust is opaque. Some astronomers thought the host galaxy was there, but that its stars were concealed by its own dust. Jahnke and colleagues used the European Southern Observatory’s Very Large Telescope in Chile to take images in the mid-infrared part of the electromagnetic spectrum, where the galaxy’s dust would glow brightly. But the observations still failed to find dust that would signal a host galaxy. Instead, the telescope caught a nearby galaxy that appears to be forming stars like mad.

The galaxy is building the equivalent of 350 suns per year, about 100 times the typical star formation rate for local galaxies. It also lies just 22,000 light-years, or about one-fifth the diameter of the Milky Way, from the quasar. The velocities of the quasar and galaxy are such that the two are almost guaranteed to collide within the next half-billion years, Jahnke says.

Suggestively, previous observations also revealed a jet of radio waves and fast-moving gas extending from the quasar to the companion galaxy. The new paper proposes that the galaxy could have lurched to life with a zap of energy from the quasar, like Frankenstein’s monster, and will soon make a comfortable home for the nearby black hole.

“If that’s the case, then it suggests that maybe quasars are the first objects to form in the universe, and then you form galaxies,” says lead author David Elbaz of the French Atomic Energy Commission in Saclay, France.

This idea could explain the mysterious link between the masses of galaxies and their black holes. All galaxies in the nearby universe have about 700 times more mass in their stellar bulges than in their black holes, regardless of their sizes. But when the universe was younger, black holes were much more massive with respect to their galaxies, observations show. A black hole that formed first and built its host galaxy from scratch provides a neat explanation (SN Online: 1/7/09).

But the new idea runs counter to the accepted ideas about radio jets and galaxies, which say that jets from black holes heat gas until it’s too hot for star formation and thus would halt galaxy-building in its tracks.

“We’re saying in the early stages of galaxy evolution, it might be exactly the reverse,” Elbaz says. “This thing that you say kills, it might kill later on. But there is evidence that it might give birth.”

Jet-induced star formation is not unheard of. A cloud of gas called Minkowski’s Object is thought to form stars at a rapid rate because radio jets from a nearby galaxy compress the gas until it’s dense enough to ignite.

“We are going only one step further saying whole galaxies could be formed like that,” Elbaz says.

That step might be too large, says George Djorgovski of Caltech. “Going from inducing some star formation to making entire galaxies is a stretch,” he says. “Their observations are nice, but this is only a single system, and thus hardly enough to justify a new paradigm, even if it was right.”

There are other explanations for the apparently homeless black hole. The quasar could have been ejected from a galaxy after interacting with a third object. Alternatively, the black hole could reside in a nearly gasless galaxy in the process of colliding with the starbusting companion. The interactions between two such galaxies could look just like this object, Papadopoulos says.

The next generation of telescopes, like the ALMA telescope array in Chile and the James Webb Space Telescope due to launch in 2014, could help resolve the issue by looking for similar objects in the more distant universe.

“We’re describing a really interesting option which might become very important once the new sets of instrumentation come online,” Jahnke says. “At that point we’ll come back to this scenario and test how strong this could possibly be.”

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