Galactic Mysteries Unlocked: Dwarf Galaxies Revealed As Unexpected Star-Forming Powerhouses

University of Michigan astronomer Sally Oey studied a star-forming region in the host galaxy, NGC 2366, which is a typical dwarf irregular galaxy. Credit: Observatorio de Calar Alto, J. van Eymeren (AIRUB, ATNF) & Á.R. López-Sánchez

When you look at vast galaxies filled with countless stars, it’s easy to think of them as star factories, churning out bright balls of gas. However, these are less evolved dwarf galaxies Dwarf galaxies have larger regions of star factories, with higher rates of star formation.

The recent findings of researchers from the University of Michigan shed light on this phenomenon: Dwarf galaxies experience a delay of about 10 million years before they release the gas that crowding their space. This delay allows the star-forming regions of these galaxies to retain their gas and dust longer, promoting the formation and growth of more stars.

Dwarf Galaxies: Cradles of Star Formation

In these relatively pure dwarf galaxies, massive stars—stars about 20 to 200 times the mass of our sun—collapse into black holes instead of exploding as supernovae. But in more evolved, dirty galaxies, like ours Milky Way, they are more likely to explode, thus creating a collective superwind. Gas and dust erupt from the galaxy, and star formation quickly ceases.

Cut to Mrk 71 A

The cutout of Mrk 71-A from the Hubble Space Telescope, which is the region that shows strong radiative cooling (and therefore lacks superwinds). Credit: Observatorio de Calar Alto, J. van Eymeren (AIRUB, ATNF) & Á.R. López-Sánchez

Their findings were published in Astrophysical Journal.

“As stars go supernova, they pollute their environment by producing and releasing metals,” said Michelle Jecmen, first author of the study and an undergraduate researcher. “We argue that at low metallicity – galactic environments that are relatively unpolluted – there is a 10 million year delay in the onset of strong superwinds, which, in turn, results in higher star formation.”

The Hubble Tuning Fork and Galactic Classification

The UM researchers pointed to the so-called Hubble tuning fork, a diagram that describes the way astronomer Edwin Hubble classified galaxies. On the handle of the tuning fork are the largest galaxies. Big, round, and full of stars, these galaxies have made all their gaseous stars. Along the tines of the tuning fork are spiral galaxies with regions of gas and star-forming along their compact arms. At the end of the tuning fork tines are the least evolved, smallest galaxies.

“But these dwarf galaxies have these star-forming regions,” said UM astronomer Sally Oey, senior author of the study. “There are a few ideas as to why that is, but Michelle’s finding offers a very nice explanation: These galaxies have trouble stopping their star formation because they can’t expel their gas.”

In addition, this 10 million year period of silence offers astronomers the opportunity to observe scenarios similar to the cosmic dawn, a period of time after the Big Bang, said Jecmen. In pristine dwarf galaxies, gas clumps together and forms gaps through which radiation can escape. This previously known phenomenon is called the “picket fence” model, with UV radiation escaping between the slats of the fence. The delay explains why the gas has time to accumulate.

Cosmic Dawn and Ultraviolet Radiation

Ultraviolet radiation is important because it ionizes hydrogen—a process that also happened after the Big Bang, causing the universe to go from opaque to transparent.

“And so looking at low-metallicity dwarf galaxies with a lot of UV radiation is a bit like looking at all the cosmic dawn returns,” Jecmen said. “Understanding the time near the Big Bang is very interesting. It’s foundational to our knowledge. It’s something that happened a long time ago—it’s very compelling that we can see similar conditions in galaxies that exist today.”

Observational Evidence

The second study, published in Astrophysical Journal Letters and led by Oey, used the Hubble Space Telescope to look at Mrk 71, a region of a nearby dwarf galaxy about 10 million light years away. At Mrk 71, the team found observational evidence of Jecmen’s scenario. Using a new technique on the Hubble Space Telescope, the team used a filter set that looked at the light of triply ionized carbon.

In more evolved galaxies with many supernova explosions, the explosions heat the gas in a star cluster to very high temperatures—up to millions of degrees Kelvin, Oey said. As this hot superwind expands, it blows away the remaining gas from the star clusters. But in low-metallicity environments like Mrk 71, where stars have not exploded, the energy inside the region dissipates. It has no chance of forming a superwind.

The team’s filters picked up a diffuse glow of ionized carbon throughout Mrk 71, indicating that the energy was radiating. Therefore, there is no hot superwind, instead allowing dense gas to remain throughout the environment.

Oey and Jecmen said there are many implications for their work.

“Our findings may also be important in explaining the properties of galaxies seen in the cosmic dawn of James Webb Space Telescope just now,” said Oey. “I think we’re still in the process of understanding the consequences.”

References: “Massive-star Mechanical Feedback Delayed at Low Metallicity” by Michelle C. Jecmen and MS Oey, 21 November 2023, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad0460

“Nebular C iv λ1550 Imaging of the Metal-poor Starburst Mrk 71: Direct Evidence of Catastrophic Cooling” by MS Oey, Amit N. Sawant, Ashkbiz Danehkar, Sergiy Silich, Linda J. Smith, Jens Melinder, Claus Leitherer, Matthew Hayes , Anne E. Jaskot, Daniela Calzetti, You-Hua Chu, Bethan L. James and Göran Östlin, 21 November 2023, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ad07dd


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Image Source : scitechdaily.com

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