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An international team of astronomers has used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe a well-known quasar known as the Cloverleaf. As part of the observations, they serendipitously discovered a new ultra-luminous infrared galaxy. The finding was September 30 in the Monthly Notices of the Royal Astronomical Society.

With infrared luminosities of over one trillion solar luminosities and star formation rates between 100 and 1,000 solar masses per year, ultra-luminous infrared galaxies (ULIRGs) are the most intensely star-forming galaxies in the local universe. Hence, finding new galaxies of this type could be important for improving the understanding of star formation and evolution.

H1413+117, dubbed the Cloverfield, is a quadruply-lensed quasar at a redshift of approximately 2.56. Recently, a group of astronomers led by Natsuki H. Hayatsu, has investigated the Cloverfield with ALMA. Surprisingly, these observations have uncovered the presence of a ULIRG lurking behind the quasar.

"Dusty, submillimeter-selected galaxies without optical counterparts contribute a non-negligible fraction of the star formation in the early universe. However, such a population is difficult to detect through classical optical/ultraviolet-based surveys. We report the serendipitous discovery of such an optically dark galaxy, behind the quadruply-lensed z = 2.56 quasar, H1413+117," the researchers explain.

The new ULIRG was identified about 6 arcseconds from the Cloverfield quasar, at a redshift of approximately 3.39. The galaxy has a molecular mass at a level of 40–230 billion solar masses and its black hole mass was estimated to be around 0.1 billion solar masses.

The total X-ray luminosity of the newfound ULIRG was measured to be about 400 tredecillion erg/s, while its total infrared luminosity was found to be at a level of 2.8 trillion solar luminosities. The system is optically dark and appears to be highly obscured.

The study found that the molecular gas in this ULIRG has not yet settled dynamically, likely due to a gas-rich major galaxy merger in its early stages. Hence, the astronomers predict that the galaxy is likely destined to continue as a starburst, with its rich molecular budget, triggered by the merging process.

The researchers added that it is possible that the investigated ULIRG is in the progenitor stage of the hot dust obscured galaxy (DOG) phase, perhaps evolving towards the naked active galaxy nuclei (AGN) phase and then the elliptical galaxy in the local universe. However, further studies, focused on the molecular gas of the galaxy, are required in order to draw final conclusions regarding the current evolutionary state of this system.

"To infer the evolutionary stage of such an optically dark galaxy, be it a massive starburst, QSO [quasi-stellar object], or hot DOG, it is essential to clarify the physical state of the gas, especially the dense molecular gas that provides the fuel for star formation, and this will be the subject of future follow-up studies," the authors of the paper conclude.

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