Astronomers solve cosmic “whodunit” with interstellar forensics

Summary: By comparing new Hubble observations with data from the Green Bank Telescope, astronomers have discovered the origin of a huge cloud of gas bridging the Large Magellanic Cloud and the Small Magellanic Cloud, two dwarf galaxies that orbit the Milky Way. In a cosmic version of tug-of-war, the Large Magellanic Cloud is using its added heft to rip away gas from its smaller companion, creating an enormous bow-like feature — known as the Leading Arm – which is crashing onto the outer parts of our galaxy.

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The Green Bank Telescope Discovering Interstellar Organic Molecules

aromatic molecule benzonitrile was detected by the GBT in the Taurus Molecular Cloud 1 (TMC-1). Credit: B. McGuire, B. Saxton (NRAO/AUI/NSF)
The aromatic molecule benzonitrile was detected by the GBT in the Taurus Molecular Cloud 1 (TMC-1). Credit: B. McGuire, B. Saxton (NRAO/AUI/NSF)

Summary: Astronomers using the Green Bank Telescope have made the first definitive interstellar detection of benzonitrile, an intriguing organic molecule that helps to chemically link simple carbon-based molecules and truly massive ones known as polycyclic aromatic hydrocarbons. This discovery is a vital clue in a 30-year-old mystery: identifying the source of a faint infrared glow that permeates the Milky Way and other galaxies.

Astronomers had a mystery on their hands. No matter where they looked, from inside the Milky Way to distant galaxies, they observed a puzzling glow of infrared light. This faint cosmic light, which presents itself as a series of spikes in the infrared spectrum, had no easily identifiable source. It seemed unrelated to any recognizable cosmic feature, like giant interstellar clouds, star-forming regions, or supernova remnants. It was ubiquitous and a bit baffling.

The likely culprit, scientists eventually deduced, was the intrinsic infrared emission from a class of organic molecules known as polycyclic aromatic hydrocarbons (PAHs), which, scientists would later discover, are amazingly plentiful; nearly 10 percent of all the carbon in the universe is tied up in PAHs.

Even though, as a group, PAHs seemed to be the answer to this mystery, none of the hundreds of PAH molecules known to exist had ever been conclusively detected in interstellar space.

New data from the National Science Foundation’s Green Bank Telescope (GBT) show, for the first time, the convincing radio fingerprints of a close cousin and chemical precursor to PAHs, the molecule benzonitrile (C₆H₅CN). This detection may finally provide the “smoking gun” that PAHs are indeed spread throughout interstellar space and account for the mysterious infrared light astronomers had been observing.

The results of this study are presented today at the 231st meeting of the American Astronomical Society (AAS) in Washington, D.C., and published in the journal Science.

The science team, led by chemist Brett McGuire at the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, detected this molecule’s telltale radio signature coming from a nearby star-forming nebula known as the Taurus Molecular Cloud 1 (TCM-1), which is about 430 light-years from Earth.

“These new radio observations have given us more insights than infrared observations can provide,” said McGuire. “Though we haven’t yet observed polycyclic aromatic hydrocarbons directly, we understand their chemistry quite well. We can now follow the chemical breadcrumbs from simple molecules like benzonitrile to these larger PAHs.”

Though benzonitrile is one of the simplest so-called aromatic molecules, it is in fact the largest molecule ever seen by radio astronomy. It also is the first 13-atom molecule with a 6-atom aromatic carbon ring (a hexagonal array of carbon atoms bristling with hydrogen atoms) molecule ever detected with a radio telescope.

While aromatic rings are commonplace in molecules seen here on Earth (they are found in everything from food to medicine), this is the first such ring molecule ever seen in space with radio astronomy. Its unique structure enabled the scientists to tease out its distinctive radio signature, which is the “gold standard” when confirming the presence of molecules in space.

As molecules tumble in the near vacuum of interstellar space, they give off a distinctive signature, a series of telltale spikes that appear in the radio spectrum. Larger and more complex molecules have a correspondingly more-complex signature, making them harder to detect. PAHs and other aromatic molecules are even more difficult to detect because they typically form with very symmetrical structures.

To produce a clear radio fingerprint, molecules must be somewhat asymmetrical. Molecules with more uniform structures, like many PAHs, can have very weak signatures or no signature at all..

Benzonitrile’s lopsided chemical arrangement allowed McGuire and his team to identify nine distinct spikes in the radio spectrum that correspond to the molecule. They also could observe the additional effects of nitrogen atom nuclei on the radio signature.

“The evidence that the GBT allowed us to amass for this detection is incredible,” said McGuire. “As we look for yet larger and more interesting molecules, we will need the sensitivity of the GBT, which has unique capabilities as a cosmic molecule detector.”

Videos

About benzonitrile – Vimeo video

Benzonitrile discovered in Taurus molecular cloud – Vimeo video

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Reference: “Detection of the Aromatic Molecule Benzonitrile (c-C6H5CN) in the Interstellar Medium,” B. McGuire, et al., Science, Jan. 2018. [http://science.sciencemag.org/]

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Swarm of Hydrogen Clouds Flying Away from Center of our Galaxy

Photo Source: S. Brunier; Design & Illustration: P. Vosteen (CC BY-ND)

A team of astronomers has discovered what appears to be a grand exodus of more than 100 hydrogen clouds streaming away from the center of the Milky Way and heading into intergalactic space. This observation, made with the National Science Foundation’s Green Bank Telescope (GBT), may give astronomers a clearer picture of the so-called Fermi Bubbles, giant balloons of superheated gas billowing out above and below the disk of our galaxy.

The results are presented today at the 231st meeting of the American Astronomical Society in Washington, D.C.

“The center of the Milky Way is a special place,” notes Jay Lockman, an astronomer at the Green Bank Observatory in West Virginia. “At its heart is a black hole several million times more massive than the Sun and there are regions of intense star birth and explosive star destruction.”

These energetic processes, perhaps individually or together, have generated a powerful cosmic “wind” that has blown two enormous bubbles above and below the disk of the Milky Way that are filled with gas at tens-of-millions of degrees. This superheated gas, however, shines feebly at radio, X-ray and gamma-ray wavelengths.

The bubbles appear prominently in observations made by NASA’s Fermi Gamma-ray Space Telescope, which is why astronomers refer to them as the Fermi Bubbles.

“One problem that hinders study of this hot cosmic wind is that the gas has such low density that its emission is very faint, so there is no practical way to track its motion,” notes Lockman. “This is where the hydrogen clouds come in.”

Just like a handful of dust thrown into the air can show the motion of wind on Earth, the hydrogen clouds can act as test particles revealing the flow of the hotter, invisible wind from the center of the Milky Way.

Neutral hydrogen gas, the principal component of these clouds, shines brightly at the radio wavelength of 21 centimeters. These hydrogen clouds were first discovered by a team led by Naomi McClure-Griffiths of the Australian National University using a radio telescope array in Australia. However, that survey was confined to a region just a few degrees around the galactic center, so it gave only limited information on the number and extent of these clouds.

New research with the 100-meter GBT greatly extends these observations.

A group led by Lockman, McClure-Griffiths, and Enrico DiTeodoro, who is also with the Australian National University, mapped a much larger area around the galactic center in search of additional hydrogen clouds that might be entrained in the nuclear wind. They found a gigantic swarm of more than 100 high-velocity gas clouds. The properties of these clouds allow the scientists to learn about the shape of the wind-blown region and the enormous energies that are involved.

“The signature of these clouds being blown out of the Milky Way is that their velocities are crazy,” said Lockman. “Gas motions in the Milky Way are usually quite regular and are dominated by the orderly rotation of the Galaxy. In the Fermi Bubbles we see clouds right next to each other on the sky that have velocities differing by as much as 400 kilometers per second.”

According to the researchers, the most likely explanation for these wildly differing velocities is that they’re traveling within a cone of material that is expanding upward and away from the galactic center, so the front portion is coming toward us and the back part is flying away.

By modeling the distribution and velocities of the clouds, the astronomers found that they would fill a cone stretching above and below the galaxy to a distance of at least 5,000 light-years from the center. The clouds have an average speed of about 330 kilometers per second.

Di Teodoro notes: “What is especially puzzling is that we have not yet found the edge of the swarm of clouds. Somewhere above the galactic center, the hydrogen clouds have to dissipate or become ionized. But we have not found that edge yet, so there’s still a lot to learn.”

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