Astronomers using the Atacama Large Millimeter/Submillimeter Array (ALMA) have detected more than 100 molecular species at the center of starburst galaxy NGC 253. This is far more than anything previously observed in galaxies outside the Milky Way.
In the Universe, some galaxies form stars much faster than our Milky Way. These galaxies are called starburst galaxies.
Exactly how such extremely prolific star formation occurs and how it ends is still a mystery.
The probability of star formation is determined by the properties of the raw material from which stars are formed, such as molecular gas, which is a gaseous substance made up of various molecules.
For example, stars form in dense regions within molecular clouds where gravity can work more effectively.
Some time after a star has been actively forming, explosions from existing or dead stars can energize the surrounding material and prevent future star formation.
These physical processes affect the galaxy's chemistry and imprint signatures on the strength of the signals from its molecules.
Because each molecule emits light at a specific frequency, observations over a wide frequency range can analyze its physical properties and provide insight into the mechanism of starbursts.
It was observed by Dr. Nanase Harada of the National Astronomical Observatory of Japan as part of the ALMA Comprehensive High-Resolution Extragalactic Molecular Inventory (ALCHEMI). NGC253 a starburst galaxy located 11.5 million light-years away in the constellation Corina.
They were able to detect more than 100 molecular species in the galaxy's central molecular belt.
This chemical raw material is most abundantly found outside the Milky Way, and includes molecules such as ethanol and the phosphorus-containing species PN, which were first detected beyond the Milky Way.
First, astronomers found that the dense molecular gas likely fuels active star formation in this galaxy.
Each molecule emits at multiple frequencies, and its relative and absolute signal strength varies with density and temperature.
Analysis of numerous signals from several molecular species revealed that the amount of dense gas at the center of NGC 253 is more than 10 times greater than the amount of gas at the center of the Milky Way. This could explain why NGC 253 forms about 30 stars. With the same amount of molecular gas, you can get many times more efficiency.
One mechanism by which molecular clouds compress and become denser is through collisions between them.
At the center of NGC 253, cloud collisions occur where gas streams and stars intersect, creating shock waves that travel at supersonic speeds.
These shock waves vaporize molecules such as methanol and HNCO and freeze them onto ice dust particles.
Once the molecules evaporate as a gas, they can be observed with radio telescopes such as ALMA.
Certain molecules also track ongoing star formation. It is known that complex organic molecules exist in abundance around young stars.
Schematic image of the center of NGC 253. Spectra from the ALCHEMI survey are shown where different tracer species are enriched.Image credits: ALMA / ESO / National Astronomical Observatory of Japan / NRAO / Harada other.
The study suggests that in NGC 253, active star formation creates a hot, dense environment similar to that found around individual protostars in the Milky Way.
The amount of complex organic molecules at the center of NGC 253 is similar to that found around galactic protostars.
In addition to the physical conditions that can promote star formation, the study also uncovered harsh environments left behind by previous generations of stars that could slow the formation of future stars.
When a massive star dies, a massive explosion known as a supernova occurs, releasing energetic particles called cosmic rays.
Molecular composition of NGC 253 revealed by enhancement of species such as H3○+ and HOC+ Molecules in this region are stripped of some of their electrons by cosmic rays at least 1,000 times faster than molecules near the solar system.
This suggests that there is a significant energy input from the supernova, making it difficult for the gas to condense and form a star.
Finally, the ALCHEMI survey provided an atlas of 44 molecular species, double the number obtained in previous studies outside the Milky Way.
By applying machine learning techniques to this atlas, the researchers were able to identify which molecules can most effectively track the star formation story described above from beginning to end.
As explained above with some examples, certain molecular species track phenomena such as shock waves and dense gas that can help star formation.
Young star-forming regions are rich in chemicals, including complex organic molecules.
On the other hand, the developed starbursts show an enhancement of cyanogen radicals, which indicate an energy output in the form of ultraviolet photons from massive stars, which could also hinder future star formation.
“Finding these tracers may help plan future observations to take advantage of the broadband sensitivity improvements expected over this decade as part of the ALMA 2030 development roadmap. “Simultaneous observation of molecular transitions will become more manageable,” the scientists said.
Their paper will appear in Astrophysical Journal Appendix Series.
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Nanase Harada other. 2024. ALCHEMI Atlas: Principal component analysis reveals starburst evolution of NGC 253. APJS 271, 38; doi: 10.3847/1538-4365/ad1937
Source: www.sci.news
