by Some of the largest and most intense star-forming regions are found in the smallest galaxies, and scientists believe this is because stars that reach the end of their lives in so-called dwarf galaxies are more likely to become holes. blacks than to explode. supernovas. The contrast is large enough, the team says, that dwarf galaxies experience a 10-million-year delay in ejecting all of their star-forming material, a process that typically depends on the forces of supernovae.
In other words, dwarf galaxies can retain their precious treasure of star-forming molecular gas for longer, allowing star-forming regions to grow in size and intensity and produce more stars.
Examples of such huge star-forming regions in local dwarf galaxies include 30 Doradus (the Tarantula Nebula) in it Large Magellanic Cloudlocated about 160,000 light years away, and Markarian 71 in the galaxy NGC 2366, located about 10 million Light years far.
Star-forming regions can produce stars of all masses; They mostly produce smaller stars, but create a handful of massive stars, also. When these massive stars reach the end of their lives after a few million years, their cores collapse to form a neutron star or a stellar mass black hole. In the first scenario, the outer layers of a star bounce off the neutron star and explode like a supernova. In the latter case, however, almost an entire star falls into the resulting black hole with barely a whimper.
“As stars go supernova, they pollute their environment by producing and releasing metals,” Michelle Jecmen, an undergraduate researcher at the University of Michigan and lead author of the study, said in a paper. statement.
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When the universe began, the big Bang produced only the elements hydrogen and helium (with a pinch of lithium). All the other elements came later, forged in the bowels of the stars or in the furnaces of their explosions. Astronomers call all of these later elements “metals.” These metals are now scattered throughout the interstellar medium, finding its way to new regions of star formation and joining the next generation of stars. Although the details are still unclear, the presence of specific metals within a star can subtly alter how that star evolves. For example, scientists believe that high metallicity stars are more likely to produce a neutron star and a powerful supernova.
Importantly, multiple supernova explosions create a “wind” that can blow out any remaining molecular gas, gas that is fertile for forming stars.
More massive and more evolved galaxies, like ours. Milky Way, have produced a greater abundance of metals over the eons during which they have churned up countless generations of stars. However, smaller dwarf galaxies have historically shown less star formation and therefore have more primitive compositions with fewer metals. But once a star-forming region in a dwarf galaxy gets going, it would appear that the lower metallicity of its stars means they are more likely to produce black holes rather than powerful supernova explosions. Therefore, it is likely that it will take longer for the region to become enriched with metals and start producing stars that become supernovae with powerful winds that expel all the gas.
“We argue that at low metallicity… there is a 10 million year delay in the onset of strong superwinds, which in turn results in increased star formation,” Jecmen said.
“Michelle’s finding offers a very good explanation,” Jecmen’s supervisor and study co-author, Michigan astronomer Sally Oey, said in the statement. “These galaxies have trouble stopping their star formation because they didn’t expel their gas.”
Oey has conducted observations with the Hubble Space Telescope that found evidence corroborating Jecmen’s model. Reporting on them in the Nov. 21 issue of The Astrophysical Journal Letters, Oey’s team targeted Markarian 71. Specifically, Oey was looking for triply ionized carbon. Atoms become ionized when they are hit by high-energy photons that can remove an electron, leaving the atoms with a net positive charge. Triple ionized means that an atom has lost three electrons.
Hubble observations found an abundance of triply ionized carbon near the center of Markarian 71. Such triply ionized carbon forms when the gas cools and radiative outflows that remove energy from the gas interact with the hotter gas. But these cooling flows should not exist if a hot superwind were blowing, as is the case with winds from multiple supernovae, and such winds appear absent in Markarian 71.
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The findings also provide insight into star-forming conditions in the first galaxies of the early universe, realms that existed just a few hundred million years after the Big Bang. Galaxies during this period, known as “Cosmic Dawn,” were also small, but with intense star formation and low metallicity. When observed, they often show evidence of gas clouds clumping together and ultraviolet light shining through the spaces between the groups. Astronomers describe this as the “fence” model, like light from a setting sun shining through gaps in a garden fence.
A 10 million year delay in the rise of supernova winds would explain why gas from early galaxies has time to form such large clumps. “Looking at low-metallicity dwarf galaxies with lots of ultraviolet radiation is akin to watching the cosmic dawn,” Jecmen said.
It’s interesting to consider that to learn things about the first galaxies, we don’t always need a $10 billion space telescope, but can simply look at some of our tiny neighbors.
An article describing these findings was published November 21 in The Astrophysical Journal
