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New Astronomy Finding Uncovers the Mystery of Star Formation at the Edge of Galaxies

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International research team led by SBU’s Jin Koda to present “molecular cloud” research at national meeting

The mystery of star formation in galaxies continues to intrigue astronomers worldwide. Yet, a key question remains just how and why and where do stars form in the Universe? A new discovery from an international team of astronomers provides a significant clue to star formation.

Led by Jin Koda, a professor in the Department of Physics and Astronomy, the research team used the Atacama Large Millimeter/submillimeter Array (ALMA) and investigated the far edge of the spiral galaxy M83, at a distance of 15 million light years from Earth. They uncovered 23 concentrations of dense molecular gas called “molecular clouds,” which are evidence of the birthing region of stars.

Molecular clouds are a typical site for star formation in the inner parts of galaxies. When it comes to the far edges of many galaxies, scientists had yet to understand how and why stars form because they could not pinpoint their formation sites. Yet, a surprising number of very young stars are known to exist at the far edges of many galaxies. The discovered 23 molecular clouds appear different from their counterparts in typical star-forming sites in galaxies. The large bodies of these clouds were not visible like “normal” molecular clouds — only their star-forming dense cores, the “hearts” of the clouds, were observed.

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Research on the far edge of galaxy M83 reveals unusual star formation in an extreme environment. This area, outlined in yellow, is shown in data from several different instruments. From left to right: optical image from CTIO, ultraviolet image from GALEX, HI 21cm image from VLA and GBT, and CO(3-2) image from ALMA. In this last image, the star-forming “hearts” of molecular clouds, circled with white, are shown.  Credit: Jin Koda

This new research finding opens the door to a better understanding of the process of star formation in the Universe in general.

Additionally, the molecular cloud discovery uncovered a key link to the large reservoir of the diffuse atomic gas within the clouds. Normally, the atomic gas condenses into dense molecular clouds, where even denser cores develop and form stars. This process of conversion from atomic to molecular gas occurs even at the galaxy edges, but the conversion turned out to be very inefficient.

Koda and colleagues are presenting their findings at the 243rd meeting of the American Astronomical Society (AAS) in New Orleans on January 8. The AAS presentation includes a confirmation of the hypothesis presented in a 2022 paper, titled “First Detection of the Molecular Cloud Population in the Extended Ultraviolet Disk of M83,” published in The Astrophysical Journal, along with new findings by way of the Jansky Very Large Array (VLA) and Green Bank Telescope (GBT).

“The star formations at galaxy edges have been a nagging mystery since their discovery by the NASA’s GALEX satellite 18 years ago,” said Koda, lead researcher. “Astronomers are eager to understand how stars form, and our discovery provides a clue to star formation processes.”

Koda and co-authors write that “these molecular clouds are likely the main drivers of the star formation activity in galaxy edges.” They further explain: “We hypothesized that these clouds share, on average, the same common structure (mass distribution) as molecular clouds in the Milky Way, such as Orion, and have star-forming dense cores embedded in thick layers of bulk molecular gas. However, their envelopes are invisible.”

Amanda Lee, who was an undergraduate student on Koda’s research team at Stony Brook University, processed telescopic data from the GBT and VLA for these findings. Through this, she discovered the atomic gas reservoir at the galaxy edge.

“We still do not understand why this atomic gas does not efficiently become dense molecular clouds and form stars,” adds Lee, who is now pursuing her PhD in astronomy at UMass Amherst. “As often is the case in astronomy, pursuing answers to one mystery can often lead to another. That’s why research in astronomy is exciting.”

Koda and colleagues came to their conclusions by using data collected using several instruments from the National Science Foundation’s National Radio Astronomy Observatory (NRAO), including the Atacama Large Millimeter/submillimeter Array (ALMA), the Karl G. Jansky VLA, and the GBT, as well as with the National Astronomical Observatory of Japan’s (NAOJ) Subaru Telescope and the NASA Galaxy Evolution Explorer (GALEX).

The research that led to team’s findings was supported in part by a National Science Foundation grant by a NASA grant.

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