galactic mystery

Where does the Milky Way get its energy? Scientists may have found the answer

A new study provides clues to how much energy flows through the center of our galaxy.

Natapong Supalertsophon/Moment/Getty Images

Bob Benjamin was observing the Milky Way when he noticed something odd.

The professor in the University of Wisconsin-Madison's astronomy department, and co-author of a new study, saw a red, tilted structure that pierced through a hole in the dust at the dark center of the galaxy.

The Tilted Disk — This odd structure is made up of ionized hydrogen gas, or gas with enough energy to separate its electrons from atoms or molecules, evident through the red glow of light. The phenomenon became known as the Tilted Disk, because of how it appears tilted compared to the rest of the Milky Way, but scientists could not explain how it was formed. The scientific community got its answer when a team of astronomers had the rare opportunity to peer at the Tilted Disk by using optical light, which allowed them to observe its mysterious ionized gas.

Their research is in an article published last week in the journal Science Advances, and holds clues as to what powers the Milky Way.

An artist's impression of the Tilted Disk in the dusty center of the Milky Way.DHANESH KRISHNARAO / UNIVERSITY OF WISCONSIN-MADISON

The reason why astronomers are baffled by the Tilted Disk is the amount of ionized hydrogen gas requires an ongoing source of energy to keep the electrons separated, and they have not identified the source of energy.

The study found that 48 percent of hydrogen in the center of the Milky Way has been ionized by an unknown source.

Matthew Haffner, assistant professor of physics & astronomy at Embry-Riddle and co-author of the study, explains that when it comes to ionized gas, seeing is knowing.

"Without an ongoing source of energy, free electrons usually find each other and recombine to return to a neutral state in a relatively short amount of time," Haffner said in a statement released with the research. "Being able to see ionized gas in new ways should help us discover the kinds of sources that could be responsible for keeping all that gas energized."

In order to identify the source of energy, the astronomers looked at the Tilted Disk through one of the empty patches of dust and gas at the center of the Milky Way. They used the Wisconsin H-Alpha Mapper (WHAM) telescope located at the Cerro Tololo Inter-American Observatory in Chile:

Wisconsin H-Alpha Mapper (WHAM) at the Cerro Tololo Inter-American Observatory in Chile.L.M. Haffner/news.wisc.edu

"Being able to make these measurements in optical light allowed us to compare the nucleus of the Milky Way to other galaxies much more easily," Haffner said.

Near the nucleus of the Milky Way, the gas is ionized from the energy emitted by newly forming stars being birthed in the galaxy. However, further away from the galaxy's center, the gas is similar to that found in a class of galaxies called LINERs, or low ionization (nuclear) emission regions.

An aerial view of the Tilted Disk at the center of the Milky Way.Dhanesh Krishnarao / University of Wisconsin-Madison

LINER galaxies make up around a third of galaxies in the universe, and their centers have more radiation than galaxies that are only forming new stars, yet less radiation than those with active supermassive black holes.

"Before this discovery by WHAM, the Andromeda Galaxy was the closest LINER spiral to us but it's still millions of light-years away," Haffner said. "With the nucleus of the Milky Way only tens of thousands of light-years away, we can now study a LINER region in more detail."

"Studying this extended ionized gas should help us learn more about the current and past environment in the center of our Galaxy."

Now that the researchers have characterized the Milky Way galaxy based on the level of radiation it emits in its center, the next step is for them to figure out the mysterious source of energy that is ionizing the gas. For that, they will wait for the WHAM telescope's successor, which would allow them to peer at the gas in more detail.

Abstract: Optical emission lines are used to categorize galaxies into three groups according to their dominant central radiation source: active galactic nuclei, star formation, or low-ionization (nuclear) emission regions [LI(N)ERs] that may trace ionizing radiation from older stellar populations. Using the Wisconsin H-Alpha Mapper, we detect optical line emission in low-extinction windows within eight degrees of Galactic Center. The emission is associated with the 1.5-kiloparsec-radius “Tilted Disk” of neutral gas. We modify a model of this disk and find that the hydrogen gas observed is at least 48% ionized. The ratio [NII] λ6584 angstroms/Hα λ6563 angstroms increases from 0.3 to 2.5 with Galactocentric radius; [OIII] λ5007 angstroms and Hβ λ4861 angstroms are also sometimes detected. The line ratios for most Tilted Disk sightlines are characteristic of LI(N)ER galaxies.
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