Black Hole winds pull the plug on star formation
While we have already known about the existence of fast winds produced from other super-massive black holes, this is the first time we have been able to directly see its effect on the rest of the galaxy at the same time
Astronomers have found proof that the strong winds blown by a supermassive black hole are dispersing the gas reservoir of its host galaxy. The new finding relies on the observation of two phenomena in the same galaxy: a large-scale galactic outflow, seen by ESA's Herschel space observatory, and a black-hole driven wind at the galaxy's core, detected with the JAXA/NASA’s Suzaku X-ray observatory. The discovery proves the proof of the key role played by black holes in regulating the formation of stars in their host galaxies.
Galaxies have been forming stars since the Universe was only a few hundred million years old, but this activity has declined over time. In fact, while the production of stars reached its peak a few billion years after the Big Bang, galaxies in the present Universe are no longer such prolific stellar factories, with a typical galaxy giving birth to just a few new stars every year.
Astronomers have long been wondering about the physical processes that regulate star formation in galaxies: what slowed it down over cosmic history, even quenching it entirely in some cases? They suspected that the activity driven by the supermassive black holes at the centre of massive galaxies might be responsible for triggering such a feedback mechanism, but until recently there was no direct proof that this scenario could be acted out on a global galactic scale.
The findings, reported in Nature this week by Dr James Reeves, of the astrophysics group at Keele University, UK, as part of a study led by Dr Francesco Tombesi at NASA Goddard Space Flight Centre, USA, may help us to learn more about galaxy evolution.
"While we have already known about the existence of fast winds produced from other super-massive black holes, this is the first time we have been able to directly see its effect on the rest of the galaxy at the same time" explained James Reeves. "The wind from the black hole is acting like a snowplough, sweeping the gas and dust up in its wake. This is likely to stop new stars from forming and can limit how big both the galaxy and the black hole can become."
“This is the first time that we see a supermassive black hole in action, blowing away the galaxy's reservoir of ‘star-making’ gas,” explains Francesco Tombesi from NASA's Goddard Space Flight Center and the University of Maryland, USA.
Supermassive black holes feed on the surrounding matter but also eject large amounts of material through powerful winds and jets.
“Until now we could only assume that a black hole's activity has a global impact on its host galaxy, but now we can directly prove it,” he adds.
Tombesi and his collaborators have found the first evidence that mighty winds driven by supermassive black holes are powering massive outflows in the interstellar matter around them, pushing large amounts of molecular gas away from the centre of their host galaxies. The results are reported in the journal Nature.
This new finding was made possible by combining data from ESA's Herschel, which looked at the sky in the far-infrared portion of the electromagnetic spectrum, and from the JAXA/NASA’s Suzaku X-ray observatory, which probes some of the most energetic phenomena in the cosmos through their X-ray emission. The multi-wavelength aspect of this study was crucial to linking what happens in the vicinity of the black hole and its global effect on the galaxy as a whole.
The majority of supermassive black holes are idle, but those that are actively accreting matter give rise to intense emission across the electromagnetic spectrum. In particular, X-ray observations are sensitive to what happens very close to the black hole, and have been widely used in the past decade to extract information about the speed and energy of winds driven by the accretion process.
“In a previous study, we demonstrated that winds are fairly common in galaxies hosting an active black hole: these winds blow away significant amounts of highly ionised gas up to about one third the speed of light,” explains Tombesi.
However, while X-ray observations can investigate the outflow of ionised gas in the central regions of a galaxy, they provide only a crude indication of what is going on at a galactic level. It is at the longer (far-infrared) wavelengths probed by Herschel that astronomers can keep an eye on larger-scale outflows by looking at the molecular gas – the raw material needed for stars to form.
“With Herschel, we could finally see that galactic winds are affecting also the molecular gas,” says co-author Marcio Melendez, also from the University of Maryland.
In previous studies, such outflows were detected in several galaxies scrutinised with Herschel. While these data started to offer more solid evidence that galactic winds are blowing away the gas necessary to form stars, something was still missing: one single galaxy where winds are observed both in the ionised gas around the black hole and in the molecular gas on a broader scale.
“This was an observational challenge, since galaxies that shine brightly at far-infrared wavelengths are not necessarily strong sources of X-rays,” explains Melendez.
Eventually, the astronomers found the right candidate: IRAS F11119+3257, an Ultra-Luminous InfraRed Galaxy (ULIRG) located some two billion light-years away. This galaxy, which had already been observed with Herschel, revealing the presence of large-scale outflows of molecular gas, is also a bright X-ray source, so it seemed to be the best for this study. In May 2013, the astronomers observed it with the X-ray telescope on the Suzaku satellite to look for a signature of its black hole activity.
“And it was a good choice, since the X-ray data revealed that the black hole at the centre of this galaxy is producing one of the most powerful accretion-driven winds ever detected,” comments Tombesi.
With both Herschel and Suzaku data at hand, the astronomers could finally test the feedback models quantitatively. The new observations showed that about twenty percent of the fast, black-hole driven wind impacts molecular gas in the diffuse interstellar medium that pervades the galaxy, initiating a slower but more widespread wind on galactic scales.
“While the initial wind only blows away about the equivalent of one solar mass of ionised gas every year, the outflow of molecular gas is much more substantial, affecting about 800 solar masses per year,” explains Eduardo Gonzáles-Alfonso of the Universidad de Alcalá, in Spain.
While the black-hole driven wind in this galaxy blows at about 25 percent the speed of light, the outflow of molecular gas is more extended but significantly slower, progressing at about 1000 km/s.
“These values are consistent with the black hole starting a feedback mechanism that can eventually sweep away most of the host galaxy's molecular gas reservoir, possibly bringing an end to its star-forming activity,” adds Gonzáles-Alfonso.
Moreover, the comparison between the X-ray and far-infrared data suggests that energy is conserved, not dissipated, in the interaction between the accretion-driven wind and the surrounding interstellar medium. This causes an energy boost and powers the vast outflow of molecular gas, in agreement with the predictions from black-hole feedback models.
“Herschel has been a game-changer for studying how stars formed in galaxies throughout the history of the Universe,” comments Göran Pilbratt, Herschel Project Scientist at ESA. “This joint effort between Herschel and Suzaku has now found the mechanism responsible for switching off the formation of stars: as we suspected, the black hole did it!”