Astronomers suggest that radiation, not supernovae, drives superwinds in some galaxies

This image zooms in on the Mrk 71 region of the galaxy NGC 2366. The red, blue, and green colors reflect the emission of oxygen and helium ions.  The observations were made from the Hubble Space Telescope.  Credit: Sally Oey, UM.

The discovery could provide insight into how the universe became transparent

When astronomers observe superwinds traveling at extremely high speeds from superstar clusters or “star eruptions”, they previously assumed that the winds were driven by supernovae, stellar explosions.

This was the case for a stellar eruption called Mrk 71 in a nearby galaxy. Astronomers had observed incredibly fast superwinds traveling at about 1% of the speed of light emitted from the cluster, and classical reasoning suggested that the explosions from many supernovae propel the gas to such a high speed.

But University of Michigan astronomers believe supernovae are not the cause: the cluster is too young to have supernovae. They suspect that another mechanism is behind the superwind.

By studying the properties of wind and shooting stars, astronomers found that ultraviolet radiation from the compact shooting star itself drove the superwind. Their findings, published in the Astrophysical Journal Letters, may help explain a chapter in the beginning of the universe.

This picture is a closer look at the Mrk 71 region.  Credit: Sally Oey, UM

Right after the Big Bang, the universe was very dense and opaque, says lead author and graduate student Lena Komarova. The universe was so densely packed with particles that no light could pass through them.

“But when the first stars formed in the first galaxies, they produced lots of ultraviolet light. And this essentially evaporates the gas in the universe,” Komarova said. “It’s a process similar to when you have a fog in the morning that you can’t see through – but then the sunshine comes and hits the fog, it starts to break up into smaller droplets, and you start to see the light pass.”

In this analogy, neutral hydrogen atoms, which make up 92% of the cosmos, are the “nebulae” of the universe. But as light began to shine from the first stars formed in the universe, ultraviolet light from these stars began to break up the hydrogen particles.

“The universe becomes essentially transparent, and this happens at so-called cosmic dawn when the first stars appear,” Komarova said. “And that’s what we’re trying to figure out: How do you get this UV light, which is energetic enough to evaporate the universe, to get out of the galaxies without it all being absorbed by hydrogen?”

Komarova and Sally Oey, UM professor of astronomy and senior author of the paper, believe the answer lies in the superwinds they found are generated by the radiation from these compact starburst galaxies. The radiation ultraviolet light “evaporates” hydrogen atoms, which are composed of a single proton and a single electron, by removing the electrons and ionizing them.

“Only UV light is capable of doing this because the light must be above a certain threshold energy,” Oey said. “Once the hydrogen is ionized, it becomes transparent because it cannot capture more UV photons.”

In the picture, a map of gas brightness in four speed ranges is shown at the top, showing wind behavior.  Credit: Lena Komarova et al., UM

The UM astronomers hypothesized a radiation-driven wind to explain Mr 71, a star-erupting region in the galaxy NGC 2366. By studying the spectrum of this region, Komarova and Oey were able to study the gas velocity structure and found a steady wind emanating from it. brightest Mrk 71 superstar heap.

“We found that even if you had supernovae, there would still not be enough energy to accelerate the gas to the speeds we observe,” Komarova said. “We compared the force of starlight on the gas with gravity, and we found that the radiation is much stronger than gravity – so it can actually push the gas out without gravity bringing it in again. This is what we call a radiation-driven wind.”

The acceleration itself occurs when intense light irradiates dense blobs of hydrogen gas from one direction and pushes the gas together, similar to how exploding gas forces a bullet out of a gun. The flaps must be too tight to be evaporated by the ultraviolet radiation. But light also escapes through spaces between the dense blobs of hydrogen, blowing blobs further out from the star cluster.

“The reason this is associated with the very high velocities is that in order for the blobs to accelerate to such high velocities, they must constantly capture these UV rays, even at great distances from the star cluster,” Oey said.

What the researchers then suggest is that this process clears paths for ultraviolet light to pass between lumps of hydrogen gas.

“So that UV light can actually leave its cluster where it was born and go out into the rest of the universe to evaporate it,” Komarova said. “This adds another piece to the puzzle of evaporation of the universe and provides a specific physical mechanism for how you do it.”

The scientists drew their conclusion using the Hubble Space Telescope and archival data collected at the Gemini-North Observatory in Hawaii.

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