Dark Cosmos Mysteries: Unravel the Secrets of an Invisible Galaxy

 By SISSA

April 09, 2023

                      A very distant celestial body in a still young universe, one-sixth the size of our own. an object so dark that it is almost invisible, even to highly sophisticated instruments. Its nature has long been debated, but thanks to surveys conducted with the ALMA interferometer, the SISSA research group led by Prof. Andrea Lapi, which studies the formation and evolution of galaxies, has finally identified its main properties. It is a young galaxy, forming stars at about 1000 times the rate of the Milky Way. It is compact and contains a large amount of interstellar dust. This galaxy's description will be useful for learning more about this very distant object and indicating new approaches for studying other "dark" celestial bodies. The findings, which were recently published in The Astrophysical Journal, will also aid in the development of advanced models of galaxy formation and evolution.

Image of the lensed galaxy acquired with the ALMA interferometer Credit: Giulietti et al.

The most distant galaxies are remote, dark, and invaluable.

"Very distant galaxies are real mines of information about the past and future evolution of our universe," says Marika Giulietti, the first author and a SISSA astrophysics and cosmology student. "However, studying them is extremely difficult. They are extremely compact and thus difficult to observe. We also receive very weak light from them due to the distance." The massive presence of interstellar dust, which intercepts visible light from young stars, making it difficult to detect with optical instruments, and re-emits it at longer wavelengths where it can only be observed with powerful interferometers in the (sub-)millimetre and radio wavebands, is the cause of this obscuration."

These dark bodies aren't particularly uncommon: "Several distant galaxies have been discovered in recent years that are particularly obscured, appearing completely invisible even to the most powerful optical instruments, such as the Hubble Space Telescope," Giulietti explains.

The phenomenon of gravitational lensing

The gravitational lens, a solution with great scientific potential, is a tool used in these cases. The principle is straightforward: general relativity states that objects with large masses close to us distort light from more distant sources that are perfectly aligned with them. "In this way, large celestial bodies act as a kind of enormous cosmic lens, making the "background" galaxies appear larger and brighter, allowing them to be identified and studied," Giulietti continues. Many observation programmes have been carried out using this method over the last decade. "There have been about a hundred discovered so far, but there could be many more."

A truly unique item

According to Giulietti, the main object of this current study was discovered during one of these investigations: "This was a very special celestial body." It is very bright and potentially lensed, but this occurs only at specific wavelengths, most likely due to the presence of a large amount of interstellar dust. As a result, studying it is extremely difficult. We were able to determine its features thanks to observations made with ALMA, a very modern sub-millimetre interferometer located in Chile's Atacama Desert. We investigated this strange object using specific codes that allowed us to reconstruct the original shape of the background source as well as understand certain properties of the lens itself. The observations also provided useful information about this source's gas content, and we were able to determine how it is distributed. Our analysis revealed that this object is very compact, presumably young, and rapidly forming stars. The James Webb Space Telescope will reveal much more about this galaxy in the future, which it is currently the only one capable of doing.

Prof. Lapi, a co-author of the study, concludes, "Distant galaxies that are young, compact, characterised by vigorous star formation, largely obscured by dust, and possess a very rich reservoir of molecular gas are forerunners of the massive quiescent galaxies that we see in the local universe, and thus provide very valuable insights into the processes leading to the formation and evolution of these structures during t."

"I would like to emphasise," the professor continues, "that the success of this research was achieved through collaboration between the SISSA Astrophysics and Cosmology group and the ALMA Regional Centre based at the INAF - Institute of Radio Astronomy in Bologna (particularly through collaboration with Dr. Marcella Massardi, co-author of the study), which allowed our students to access and learn how to use effectively the ALMA data archive, a real gold mine for astrophysics."

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