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Gaining Deeper Insights Into the Invisible Galaxy

A puzzling and very far away object in the universe, as it was “just” two billion years following the Big Bang, which hides from even the highly sophisticated instruments. At last, its features have been explained by a research group from Scuola Internazionale Superiore di Studi Avanzati (SISSA) in a recently published study reported in The Astrophysical Journal.

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

A highly remote celestial body is present in a still-young Universe which is one-sixth the size of our own. It is an object so dark that it is nearly invisible, even to highly advanced instruments.

For a long period of time, the nature of the celestial body has been under debate. However, through surveys that have been made with the ALMA interferometer, the SISSA research team headed by Professor Andrea Lapi, which conducts research into the formation and evolution of galaxies, has finally been successful in determining its primary properties.

It is compact and contains huge quantities of interstellar dust; as a young galaxy, it forms stars at nearly 1000 times the speed of the Milky Way.

The galaxy’s description will be beneficial for disclosing more about this highly distant object and describing new methods for the study of other “dark” celestial bodies. Also, the study reported in The Astrophysical Journal will offer better insights into developing sophisticated models of galaxy formation and evolution.

Remote, Dark, and Invaluable: The Most Distant Galaxies

Very distant galaxies are real mines of information about the past and future evolution of our Universe. However, studying them is very challenging. They are very compact and therefore difficult to observe. Also, because of distance, we receive very weak light from them.

Marika Giulietti, Study First Author, Astrophysics and Cosmology, Scuola Internazionale Superiore di Studi Avanzati

Giulietti added, “The cause of this obscuration is the massive presence of interstellar dust, which intercepts visible light from young stars, and makes it difficult to detect with optical instruments, and re-emits it at greater wavelengths where it can be observed only with powerful interferometers in the (sub-)millimeter and radio wavebands”.

Such dark bodies are not specifically rare:

Giulietti stated, “In recent years several distant galaxies have been discovered that are particularly obscured, appearing completely invisible even to the most powerful optical instruments, such as the Hubble Space Telescope.”

Gravitational Lensing

A tool utilized in such cases is gravitational lensing, known to be a solution of huge scientific potential. The principle is simple: general relativity implies that space objects that are closer exhibit a great mass that distorts light coming from highly distant sources that are ideally aligned with them.

In this way, large celestial bodies act as a kind of enormous cosmic lens that makes the ‘background’ galaxies appear larger and brighter, allowing them to be identified and studied. About a hundred have been discovered so far, but there could be many more.

Marika Giulietti, Study First Author, Astrophysics and Cosmology, Scuola Internazionale Superiore di Studi Avanzati

In the last decade, several observation programs have been performed with this method.

A Truly Special Object

It was in one of such investigations, states Giulietti, that the primary object of this present study was determined.

This was a very special celestial body. It is very bright and potentially subject to lensing, but this occurs only at certain precise wavelengths, probably due to the presence of large quantities of interstellar dust. Studying it is consequently very complex. Observations made with ALMA, a very modern sub-millimeter interferometer located in the Atacama desert of Chile, enabled us to determine its features.

Marika Giulietti, Study First Author, Astrophysics and Cosmology, Scuola Internazionale Superiore di Studi Avanzati

Giulietti continued, “We studied this peculiar object by adopting particular codes that enabled us to reconstruct the original shape of the background source and also to understand certain properties of the lens itself. The observations also provided valuable information about the gas content of this source, and we were able to determine how it is distributed.

Our analysis showed that this object is very compact, presumably young, and forming stars at an extremely high rate. In the future, the James Webb Space Telescope will reveal much more about this galaxy, something that only it can do at the moment,” added Giulietti.

Professor Lapi, co-author of the research, concludes by stressing the importance of this study: “Distant galaxies that are young, compact, characterized by vigorous star formation, and largely obscured by dust, and that possess a very rich reservoir of molecular gas, are forerunners of the massive quiescent galaxies that we see in the local Universe, and therefore provide very valuable insights into the processes leading to the formation and evolution of these structures during the history of the Cosmos.”

Lapi added, “I would like to emphasize that the success of this research was achieved through synergy between the SISSA Astrophysics and Cosmology group and the ALMA Regional Centre based at the INAF - Institute of Radio Astronomy in Bologna (in particular 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 research today.”

Journal Reference

Giulietti, M., et al. (2023) ALMA Resolves the First Strongly Lensed Optical/Near-IR-dark Galaxy. The Astrophysical Journal.

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