ALMA Telescope Unveils Complex Chemical Cosmos Of Starburst Galaxy

Galaxies come in all shapes and sizes. Some are sedate and tranquil, quietly forming stars at a leisurely pace. Others, known as starburst galaxies, are altogether different beasts — cosmic cauldrons where stars are born in a chaotic frenzy. Now, an international team of astronomers has taken an unprecedented look into the chemical heart of one of these starburst galaxies, NGC 253, using the power of the ALMA telescope in Chile.

Their findings, published in the Astrophysical Journal Supplement Series, reveal a complex interplay of molecules and physical processes that could help us understand the origins of stars and planets.

NGC 253, also known as the Sculptor Galaxy, is a mere 11.5 million light-years from Earth, making it one of our closest starburst neighbors. In its central region, stars are forming at a rate hundreds of times higher than in our own Milky Way. This cosmic nursery is a perfect laboratory for studying the raw ingredients of star formation — vast clouds of gas and dust, according to a media release.

The team, led by astronomer Nanase Harada of the National Astronomical Observatory of Japan, used ALMA to conduct an imaging spectral scan of NGC 253’s central region. This involved observing the galaxy at a wide range of radio frequencies, allowing the astronomers to identify the unique chemical fingerprints of various molecules.

The result is the most comprehensive molecular inventory ever taken of a galaxy outside our own. Researchers detected a whopping 148 distinct molecular lines associated with 44 different molecular species, some never before seen outside the Milky Way. This included the first extragalactic detection of HCNH+, a molecule related to the more familiar hydrogen cyanide, and the first extragalactic images of NO, C3H+, and HCS+.

The different colors represent the distribution of molecular gas (blue), shocked regions (red), relatively high-density regions (orange), young starbursts (yellow), developed starbursts (magenta), and molecular gas affected by cosmic-ray ionization (cyan)
The different colors represent the distribution of molecular gas (blue), shocked regions (red), relatively high-density regions (orange), young starbursts (yellow), developed starbursts (magenta), and molecular gas affected by cosmic-ray ionization (cyan). (CREDIT: ALMA (ESO/NAOJ/NRAO), N. Harada et al.)

But detecting these molecules was just the beginning. The real challenge was making sense of how their distribution and intensity varied across the chaotic landscape of the starburst region. To tackle this, the team turned to a statistical technique called principal component analysis, or PCA.

PCA is a way of finding patterns in complex datasets. It’s like taking a high-dimensional dataset and squashing it down into a more manageable number of dimensions, while preserving the most important variations. By applying PCA to their molecular maps, astronomers were able to identify key groups of molecules that tended to appear together and relate them to different physical environments and processes in the galaxy.

For example, they found that high-excitation transitions of molecules like HC3N and CH3OH were associated with the youngest, most deeply embedded sites of star formation. On the other hand, somewhat older star-forming regions showed strong signatures of PDR (photon-dominated region) tracers like CN and CCH. PCA also revealed distinctions between molecules tracing large-scale shocks, possibly from cloud-cloud collisions, and those tracing more localized shocks from stellar winds or outflows.

The analysis suggested that even in this violently star-forming environment, low-excitation dense gas tracers like HCN do not exclusively trace the actual sites of star formation. Instead, their emission seems to come from both collapsing dense cores and more diffuse, non-star-forming gas. This echoes recent findings from our own Milky Way and challenges the common assumption that these molecules are straightforward indicators of star formation.

Beyond these broad categories, the PCA also allowed researchers to identify specific outliers and peculiarities. For instance, they found enhanced emission from the ion HOC+ and the rare molecule H3O+ in regions associated with molecular outflows, hinting at unusual chemical conditions.

By painting a comprehensive picture of the chemistry in this extreme environment, the study provides a new window into the complex interplay of physical processes that shape the evolution of galaxies and the formation of stars and planets. It showcases the power of combining cutting-edge telescopes like ALMA with sophisticated statistical techniques to untangle the cosmic web of molecules. As we continue to map the molecular content of more galaxies, both near and far, studies like this will be essential stepping stones towards a deeper understanding of the universe we call home.


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