Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected over 100 molecular species in the center of the starburst galaxy NGC 253 — far more than previously observed in galaxies beyond the Milky Way.
An artist’s impression of the center of the starburst galaxy NGC 253. Image credit: NRAO / AUI / NSF.
In the Universe, some galaxies form stars much faster than our Milky Way Galaxy. These galaxies are called starburst galaxies.
It is still a mystery how exactly such a highly prolific formation of stars can occur and how it ends.
The chance for stars to form depends on the properties of the raw material from which stars are born, such as molecular gas, a gaseous material of various molecules.
For example, stars form in dense regions within molecular clouds where gravity can act more effectively.
Sometime after the active formation of stars, existing stars and explosions of dead stars impart energy to the surrounding medium, which could hinder future star formation.
These physical processes impact the chemistry of the galaxy and imprint a signature in the strengths of signals from molecules.
Because each molecule emits at specific frequencies, observations over a wide frequency range enable us to analyze the physical properties and give us insights into the mechanism of starbursts.
As part of the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI), Dr. Nanase Harada of the National Astronomical Observatory of Japan observed NGC 253, a starburst galaxy 11.5 million light-years away in the constellation of Sculptor.
They were able to detect more than one hundred molecular species in the galaxy’s central molecular zone.
This chemical feedstock is the richest found outside the Milky Way, and it includes molecules that have been detected for the first time beyond the Milky Way, such as ethanol and the phosphorus-bearing species PN.
First, the astronomers found that high-density molecular gas will likely promote active star formation in this galaxy.
Each molecule emits at multiple frequencies, and their relative and absolute signal strengths change according to the density and the temperature.
By analyzing numerous signals of some molecular species, the amount of dense gas in the center of NGC 253 turned out to be more than 10 times higher than that in the center of the Milky Way, which could explain why NGC 253 is forming stars about 30 times more efficiently even with the same amount of molecular gas.
One mechanism that could aid the compression of molecular clouds into denser ones is a collision between such clouds.
At the center of NGC 253, cloud collisions likely occur where streams of gas and stars intersect, generating shock waves traveling at supersonic speeds.
These shock waves evaporate molecules such as methanol and HNCO, freezing onto icy dust particles.
When the molecules evaporate as gas, they become observable by radio telescopes such as ALMA.
Certain molecules also trace ongoing star formation. It has been known that complex organic molecules are abundant around young stars.
A schematic image of the center of NGC 253, describing locations where various tracer molecular species are enhanced, and spectra from the ALCHEMI survey. Image credit: ALMA / ESO / NAOJ / NRAO / Harada et al.
In NGC 253, this study suggests that active star formation creates a hot and dense environment similar to the ones seen around individual protostars in the Milky Way.
The amount of complex organic molecules in the center of NGC 253 is similar to that around protostars in the galaxy.
In addition to physical conditions that could promote star formation, the survey also revealed the harsh environment left by previous generations of stars, which could slow down future star formation.
When massive stars die, they cause massive explosions known as supernovae, which emit energetic particles called cosmic rays.
The molecular composition of NGC 253 revealed from the enhancement of species such as H3O+ and HOC+ that molecules in this region have had some of their electrons stripped off by cosmic rays at a rate at least 1,000 times higher than that near the Solar System.
This suggests a considerable energy input from supernovae, which makes it difficult for gas to condense to form stars.
Finally, the ALCHEMI survey provided an atlas of 44 molecular species, doubling the number available from previous studies outside the Milky Way.
By applying a machine-learning technique to this atlas, the researchers could identify which molecules can most effectively trace the story of star formation mentioned above — from the beginning to the end.
As described above with some examples, certain molecular species trace phenomena such as shock waves or dense gas, which could aid star formation.
Young star-forming regions host rich chemistry, including complex organic molecules.
Meanwhile, the developed starburst shows an enhancement of the cyano radical that indicates energy output from massive stars in the form of UV photons, which could also hinder future star formation.
“Finding these tracers may help to plan future observations using the wideband sensitivity upgrade expected this decade as a part of the ALMA 2030 Development roadmap, with which simultaneous observations of multiple molecular transitions will become much more manageable,” the scientists said.
Their paper appears in the Astrophysical Journal Supplement Series.
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Nanase Harada et al. 2024. The ALCHEMI Atlas: Principal Component Analysis Reveals Starburst Evolution in NGC 253. ApJS 271, 38; doi: 10.3847/1538-4365/ad1937
Source : Breaking Science News