Some of the largest, most intense regions of star formation are found in the smallest galaxies. Scientists think this is because stars that reach the end of their lives in so-called dwarf galaxies are more likely to turn into black holes rather than explode. supernovas. The contrast is so stark, the team says, that dwarf galaxies experience a 10-million-year delay in blowing away all their star-forming material, a process that usually depends on the forces of supernovae.
In other words, dwarf galaxies can hold on to their precious wealth of star-forming molecular gas for longer, allowing star-forming regions to grow in size and intensity and produce more stars.
Examples of such massive star-forming regions in local dwarf galaxies include 30 Doradus (de Tarantula Nebula) in the Large Magellanic Cloudabout 160,000 light-years away, and Markarian 71 in the galaxy NGC 2366, about 10 million light-years away. light years away.
Star-forming regions can produce stars of all masses; they usually produce smaller stars, but create a handful massive stars, at. When these massive stars reach the end of their lives after a few million years, their cores collapse and form one neutron star or a black hole with stellar mass. In the first scenario, a star’s outer layers bounce off the neutron star and explode as a neutron star supernova. In the latter case, however, almost an entire star falls into the resulting black hole without any whimper.
“When stars go supernova, they pollute their environment by producing and releasing metals,” said Michelle Jecmen, a researcher at the University of Michigan and lead author of the study. rack.
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When the universe began, the Big bang produced only the elements hydrogen and helium (with a touch of lithium). All other elements came later, forged in the bowels of stars or in the furnaces of their explosions. Astronomers call all these later elements ‘metals’. These metals are now spread all over the world interstellar medium, which make their way into new star-forming regions and are incorporated into the next generation of stars. Although the details are not yet clear, the presence of specific metals in a star can subtly change the way that star evolves. For example, scientists think that stars with high metal content are more likely to produce a neutron star and a powerful supernova.
Importantly, the blasts from multiple supernova explosions create a ‘wind’ that can blow away any remaining molecular gas – gas that is fertile for star formation.
More massive and developed galaxies, like ours Milky Way, have produced a greater abundance of metals during the eons of churning through countless generations of stars. However, smaller dwarf galaxies have historically shown less star formation and therefore have a more primitive composition with fewer metals. But once a star-forming region gets underway in a dwarf galaxy, it appears that the stars’ lower metallic properties mean they are more likely to produce black holes than powerful supernova explosions. Therefore, it will likely take longer for the region to become enriched in metals and start producing stars that go supernova with powerful winds that blow out all the gas.
“We argue that at low metallicity… there is a 10-million-year delay in the formation of strong superwinds, which in turn results in higher star formation,” says Jecmen.
“Michelle’s finding offers a very nice explanation,” Jecmen’s supervisor and co-author of the study, Michigan astronomer Sally Oey, said in the statement. “These galaxies have difficulty stopping their star formation because they haven’t blown away their gas.”
Oey has led observations with the Hubble Space Telescope, finding corroborating evidence for Jecmen’s model. Oey’s team reported on this in the November 21 issue of The Astrophysical Journal Letters and focused on Markarian 71. Oey was specifically looking for triple-ionized carbon. Atoms become ionized when they are struck by high-energy photons that can knock out an electron, leaving the atoms with a net positive charge. Triple ionized means that an atom has lost three electrons.
The Hubble observations found an abundance of triple-ionized carbon near the center of Markarian 71. Such triple-ionized carbon is formed when gas cools and outflowing radiation that removes energy from the gas interacts with warmer gas. But this cooling should not exist if a hot superwind is blowing, as with the winds of multiple supernovae, and those winds appear absent in Markarian 71.
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The findings also provide insight into star formation conditions in the first galaxies of the early Universe, regions that existed only a few hundred million years after the Big Bang. Galaxies during this period, sometimes called ‘Cosmic Dawn’, were also small, but with intense star formation and low metallicity. When observed, they often show signs of clumping gas clouds and ultraviolet light shining through the gaps between the clumps. Astronomers describe this as the ‘picket-fence’ model, like the light of a setting sun shining through holes in a garden fence.
A ten-million-year delay in the emergence of supernova winds would explain why the gas in early galaxies has time to form such large clumps. “Looking at dwarf galaxies with low metal content and high ultraviolet radiation is somewhat like looking all the way back to the cosmic dawn,” says Jecmen.
It’s nice to think that to learn things about the first galaxies, we don’t always need a $10 billion space telescope, but rather just look at some of our tiny neighbors.
An article was published describing these findings published on November 21 in The Astrophysical Journal