See our galaxy as it was billions of years ago

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Apr 5, 2017

A telescope may be the closest thing we have to a TARDIS, because the deeper we look into the universe, the further back in time we can see—all the way back to what the Milky Way could have looked like billions and billions of years ago.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have observed nascent galaxies that they believe to be not unlike spiral galaxies such as our own, when it first came into being. 12 billion light-years from Earth, these galaxies appear to us as they were when they were just born, that many years ago (think 1 billion years after the Big Bang). Astronomers peered backwards into an epoch when galactic rotation had just begun to take off, and star formation was starting to accelerate inside the hydrogen-haloed discs that are a probable mirror for the early Milky Way.

These galaxies appear to be massive, dusty, and rapidly star-forming systems, with large, extended layers of gas,” said Universtiy of Califiornia astrophysicist and team leader, J. Xavier Prochaska, lead author of a paper recently published in the journal Science. He likens the observations to “an adolescent growth spurt.”

Prochaska’s team focused on two particular galaxies, ALMA J081740.86+135138.2 and ALMA J120110.26+211756.2, which they observed in the infrared. They used the light from two even more distant quasars to illuminate objects in the foreground. Quasar light shines so intensely that it usually obscures the light of a galaxy (or really anything) in front of it, but Prochaska used these spectra to his advantage. Some of the light can actually be absorbed by material floating ahead of it, if it passes through that material en route to Earth. If that light runs into a galaxy, the spectral signature of the galaxy’s gas will become imprinted on it. Shooting through a galaxy, the light then becomes a damped Lyman-alpha system (DLA) because of the neutral hydrogen concentration that can now be identified in it. Both quasars used in the study had known DLAs.

ALMA composite image of a quasar behind an emerging Milky Way-like galaxy.

Hydrogen density associates DLAs with embryonic galaxies due to “super-haloes” of the gas that surround them. The observation of DLAs in absorption, rather than as direct light, is what allows scientists to study how gases—and ultimately galaxies—behaved and evolved billions of years back in time. ALMA’s hypersensitive instruments enabled the astronomers to pickup far-infrared emission signatures from both galaxies that were distinguishable from the light emitted from the quasars. This is how the glow emitted by ionized carbon, abundant in the regions of the galaxy where stars emerge out of gas and dust, was made visible to them. The glow of these emissions can be used to determine the structure of a distant galaxy.

“Imagine a tiny firefly next to a high-power search light. That’s what astronomers are up against when it comes to observing these youthful versions of our home galaxy,” said UC Santa Cruz postdoctoral fellow Marcel Neeleman, who co-authored the paper with Prochaska. “We can now see the galaxies themselves, which gives us an amazing opportunity to learn about the earliest history of our own galaxy and others like it.”

(via Seeker)