Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected a powerful ‘wind’ in SPT-S J231921-5557.9 (SPT2319-55 for short), a dusty star-forming galaxy located 12 billion light-years away. By tracing the outflow of hydroxyl molecules, they show how some galaxies in the early Universe quenched an ongoing wildfire of starbirth.
An artist’s impression of an outflow of molecular gas from an active star-forming galaxy. Image credit: NRAO / AUI / NSF / D. Berry.
Some galaxies, like the Milky Way and Andromeda, have relatively slow and measured rates of starbirth, with about one new star igniting each year.
Other galaxies, known as starburst galaxies, forge 100s or even 1,000s of stars each year. This furious pace, however, cannot be maintained indefinitely.
To avoid burning out in a short-lived blaze of glory, some galaxies throttle back their runaway starbirth by ejecting vast stores of gas into their expansive halos, where the gas either escapes entirely or slowly rains back in on the galaxy, triggering future bursts of star formation.
Up to now, however, astronomers have been unable to directly observe these powerful outflows in the very early Universe, where such mechanisms are essential to prevent galaxies from growing too big, too fast.
New ALMA observations show a powerful galactic ‘wind’ of molecules in a galaxy seen when the Universe was only one billion years old.
“Galaxies are complicated, messy beasts, and we think outflows and winds are critical pieces to how they form and evolve, regulating their ability to grow,” said Dr. Justin Spilker, an astronomer at the University of Texas at Austin.
ALMA imaged the outflow from SPT2319-55, a galaxy seen when the Universe was only one billion years old. This image (circle call out) shows the location of hydroxyl molecules. These molecules trace the location of star-forming gas as it is fleeing the galaxy, driven by either supernovas or a black-hole powered ‘wind.’ The background star field from the Blanco Telescope Dark Energy Survey shows the location of SPT2319-55. The circular, double-lobe shape of the distant galaxy is due to the distortion caused by the cosmic magnifying effect of an intervening galaxy. Image credit: ALMA / ESO / NAOJ / NRAO / J. Spilker / AUI / NSF / S. Dagnello / AURA.
ALMA was able to observe SPT2319-55 at such tremendous distance with the aid of a gravitational lens provided by a different galaxy that sits almost exactly along the line of sight between Earth and SPT2319-55.
This lens-aided view revealed a powerful ‘wind’ of star-forming gas exiting the galaxy at nearly 800 km per second.
Rather than a constant, gentle breeze, this wind is hurtling away in discrete clumps, removing the star-forming gas just as quickly as SPT2319-55 can turn that gas into new stars.
The outflow was detected by the millimeter-wavelength signature of a molecule called hydroxyl, which appeared as an absorption line: essentially, the shadow of a hydroxyl fingerprint in the galaxy’s bright infrared light.
“That’s the absorption signature that we detected, and from that we can also tell how fast the wind is moving, and get a rough idea of how much material is contained in the outflow,” Dr. Spilker said.
“Molecular winds are an efficient way for galaxies to self-regulate their growth,” the astronomers explained.
“These winds are likely triggered by either the combined effect of all the supernova explosions that go along with rapid, massive star formation, or by a powerful release of energy as some of the gas in the galaxy falls down onto the supermassive black hole at its center.”
The findings appear in the journal Science (arXiv.org preprint).
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J.S. Spilker et al. 2018. Fast molecular outflow from a dusty star-forming galaxy in the early Universe. Science 361 (6406): 1016-1019; doi: 10.1126/science.aap8900
