A team of international astronomers has made a significant accidental discovery, locating an intensely bright galaxy that was concealed by the light of a more proximate and well-studied quasar. The serendipitous find occurred during routine observations and provides a rare glimpse into the turbulent, star-forming nurseries of the early universe. The object is an ultra-luminous infrared galaxy, or ULIRG, a class of galaxies characterized by extreme rates of star formation that are largely shrouded in dust, making them faint in visible light but incredibly bright in the infrared spectrum.
The discovery, made using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, reveals a galaxy teeming with activity behind the Cloverleaf quasar, officially known as H1413+117. While the quasar has been a subject of astronomical study since its discovery in 1984, this background galaxy remained entirely undetected until ALMA’s sensitive instruments peered into the region. Its location, almost perfectly aligned with the foreground quasar from Earth’s perspective, allowed its faint light to be masked for decades. The finding offers a crucial new laboratory for studying the processes that governed the formation of massive galaxies when the universe was in its infancy.
An Unexpected Cosmic Revelation
The team, led by Natsuki H. Hayatsu, was conducting observations of the Cloverleaf quasar, a famous quadruply-lensed object located approximately 11 billion light-years from Earth. A quadruply-lensed quasar is one whose light is bent by the gravity of a massive foreground object, creating four distinct images of the same quasar. During this investigation, astronomers noticed a distinct and powerful emission source separate from the quasar itself. This emission was not visible in archival data from other powerful telescopes, including the Hubble Space Telescope and the Spitzer Space Telescope, because it is an “optically dark galaxy.”
Such galaxies are so enshrouded in thick clouds of cosmic dust that their visible and near-infrared light is almost completely absorbed before it can reach Earth. This dust makes them invisible to traditional optical surveys. However, the very dust that obscures them is heated by the frenetic pace of star birth within, causing it to glow brightly at the far-infrared and submillimeter wavelengths that ALMA is designed to detect. The discovery was therefore only possible due to ALMA’s unique capabilities, which allow it to penetrate these dense dust cocoons and map the underlying galactic structures.
Portrait of a Star-Forming Giant
The newly identified galaxy is a powerhouse of cosmic activity. Classified as a ULIRG, it shines with an infrared luminosity equivalent to 2.8 trillion suns. This immense energy output is driven by an astonishing rate of star formation, estimated to be between 100 and 1,000 times that of our own Milky Way galaxy. ULIRGs are considered some of the most intense star-forming regions in the universe, and this example provides a direct look at the extreme conditions that were more common in the early cosmos.
Key Galactic Characteristics
Further analysis of the ALMA data allowed researchers to measure several key properties of the galaxy. It possesses a vast reservoir of molecular gas—the raw fuel for star formation—estimated to be between 40 and 230 billion times the mass of the sun. The galaxy is located at a redshift of approximately 3.39, meaning its light has traveled for well over 11.5 billion years to reach us, placing it at a time when the universe was only about 1.8 billion years old. Additionally, astronomers estimated that the supermassive black hole at its center has a mass of about 100 million solar masses. The combination of a massive gas supply and a significant central black hole points to a system undergoing rapid and violent growth.
A System in Turmoil
The data suggest that the galaxy is in a highly unstable and dynamic phase of its evolution. The molecular gas within it has not yet settled into an orderly, rotating disk, which is typical of mature spiral galaxies like the Milky Way. Instead, the gas appears turbulent and unsettled, a likely consequence of a recent or ongoing merger between two smaller, gas-rich galaxies. Such mergers are a primary trigger for the intense starburst activity seen in ULIRGs, as the collision of gas clouds sparks a galaxy-wide frenzy of star birth.
Astronomers theorize that this galaxy is in a progenitor stage that could lead to several different evolutionary paths. With its vast fuel reserves, it will likely continue its starburst phase for some time. It may eventually evolve into what is known as a “hot dust-obscured galaxy” before potentially shedding its dusty veil to become a galaxy with an exposed, active galactic nucleus, and finally settling down as a massive elliptical galaxy. However, more detailed observations will be needed to confirm its ultimate fate.
Implications for Early Universe Studies
This serendipitous discovery underscores the vital role that dust-obscured galaxies play in the story of cosmic evolution. A significant fraction of all star formation in the early universe is thought to have occurred within such dusty systems, yet their optically dark nature means they are often excluded from conventional surveys of the distant cosmos. This can lead to an incomplete census of galactic activity and an underestimation of how rapidly massive galaxies assembled after the Big Bang.
The finding highlights the power of submillimeter telescopes like ALMA to uncover these hidden populations. By detecting the faint glow of cold dust, astronomers can identify and characterize these elusive galaxies, providing a more complete picture of the universe’s formative years. Each new discovery of an object like this one, lurking behind the glare of a foreground quasar, adds another crucial piece to the puzzle of how the first generations of massive galaxies were born and grew into the structures we see today.
