ALMA detects more than one hundred molecular species in the nearby Starburst galaxy| Trending Viral hub

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected more than 100 molecular species at the center of the starburst galaxy NGC 253, many more than previously observed in galaxies beyond the Milky Way.

Artist's impression of the center of the starburst galaxy NGC 253. Image credit: NRAO/AUI/NSF.

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. These galaxies are called starburst galaxies.

How exactly such prolific star formation can occur and how it ends remains a mystery.

The possibility of stars forming depends on the properties of the raw material from which they are born, such as molecular gas, a gaseous material of several molecules.

For example, stars form in dense regions within molecular clouds where gravity can act more effectively.

Some time after active star formation, 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 on the intensity of the molecules’ signals.

Because each molecule emits at specific frequencies, observations over a wide range of frequencies allow us to analyze physical properties and provide insight into the mechanism of starbursts.

As part of the ALMA High-Resolution Comprehensive Extragalactic Molecular Inventory (ALCHEMI), Dr. Nanase Harada of the National Astronomical Observatory of Japan observed NGC 253a starburst galaxy 11.5 million light years far away in the constellation of the Sculptor.

They were able to detect more than one hundred molecular species in the central molecular zone of the galaxy.

This chemical feedstock is the richest found outside the Milky Way and includes molecules that have been detected for the first time beyond the Milky Way, such as ethanol and the phosphorus-containing PN species.

First, astronomers found that high-density molecular gas will likely promote active star formation in this galaxy.

Each molecule emits at multiple frequencies and the intensity of its relative and absolute signal changes depending on density and temperature.

By analyzing numerous signals from some molecular species, the amount of dense gas in the center of NGC 253 turned out to be more than 10 times greater than that in the center of the Milky Way, which could explain why NGC 253 is forming stars about 30 years ago. times more efficiently even with the same amount of molecular gas.

One mechanism that could help compress molecular clouds into denser ones is a collision between these clouds.

At the center of NGC 253, cloud collisions are likely to occur where streams of gas and stars intersect, generating shock waves that travel at supersonic speeds.

These shock waves evaporate molecules such as methanol and HNCO, freezing into icy dust particles.

When the molecules evaporate as a gas, they become observable using radio telescopes such as ALMA.

Certain molecules also track ongoing star formation. It is known that complex organic molecules abound around young stars.

A schematic image of the center of NGC 253, depicting the locations where several species of tracer molecules are enhanced and the spectra from the ALCHEMI study.  Image credit: ALMA/ESO/NAOJ/NRAO/Harada et al.

A schematic image of the center of NGC 253, depicting the locations where several species of tracer molecules are enhanced and the spectra from the ALCHEMI study. Image credit: ALMA/ESO/NAOJ/NRAO/Harada et al.

In NGC 253, this study suggests that active star formation creates a warm, dense environment similar to those observed around individual protostars in the Milky Way.

The number of complex organic molecules at the center of NGC 253 is similar to that around the galaxy’s protostars.

In addition to the physical conditions that could promote star formation, the study also revealed the harsh environment left by previous generations of stars, which could slow 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 H3oh+ and H.O.C.+ that cosmic rays have stripped some of their electrons from the molecules in this region at a rate at least 1,000 times greater than near the Solar System.

This suggests a considerable input of energy from supernovae, making it difficult for gas to condense to form stars.

Finally, the ALCHEMI study provided an atlas of 44 molecular species, doubling the number available in previous studies outside the Milky Way.

By applying a machine learning technique to this atlas, the researchers were able to identify which molecules can most effectively trace the history of star formation mentioned above, from beginning to end.

As described above with some examples, certain molecular species track phenomena such as shock waves or dense gas, which could help star formation.

Young star-forming regions host rich chemistry, including complex organic molecules.

Meanwhile, the developed starburst shows an enhancement of the cyano radical indicating the production of energy from massive stars in the form of UV photons, which could also hinder future star formation.

“Finding these tracers can help plan future observations using the broadband sensitivity improvement expected this decade as part of the ALMA 2030 development roadmap, with which simultaneous observations of multiple molecular transitions will be much more manageable,” the scientists said.

His paper appears in the Astrophysical Journal Supplement Series.


Nanase Harada et al. 2024. ALCHEMI Atlas: Principal Component Analysis Reveals Starburst Evolution in NGC 253. apjs 271, 38; doi:10.3847/1538-4365/ad1937

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