In an unexpected discovery, an international team of astronomers has found an ultra-luminous infrared galaxy hiding behind a well-known quasar. The serendipitous finding, made with the Atacama Large Millimeter/submillimeter Array (ALMA), provides a rare glimpse into the frenetic pace of star formation in the early universe. The newly identified galaxy is a prime example of a dusty, star-forming powerhouse that has been largely invisible to previous observations, offering new insights into the evolution of massive galaxies.
The discovery underscores the importance of such objects, known as ultra-luminous infrared galaxies (ULIRGs), in the cosmic history of star formation. These galaxies are characterized by their extreme luminosity, which is powered by intense bursts of star birth, and they are thought to be the progenitors of the massive elliptical galaxies we see today. The location of this particular ULIRG, concealed behind the glare of a bright quasar, suggests that there may be a hidden population of such galaxies that have so far gone undetected. This finding challenges astronomers to reconsider their methods for surveying the early universe and opens up new avenues for understanding the processes that governed the formation of the first large galaxies.
A Fortuitous Find
The astronomers who made the discovery were not initially searching for a new galaxy. Their target was a well-studied object known as the Cloverleaf quasar, or H1413+117. This quasar, discovered in 1984, is a type of active galactic nucleus, a supermassive black hole at the center of a distant galaxy that is actively accreting matter and shining brightly across the electromagnetic spectrum. The Cloverleaf is a particularly interesting example because its light is gravitationally lensed by a foreground galaxy, creating four distinct images of the quasar. It was during routine observations of this lensed quasar with ALMA that the team noticed a faint, extended emission that was not associated with the Cloverleaf.
Further analysis of the ALMA data revealed the presence of a massive, gas-rich galaxy located just six arcseconds away from the quasar on the sky. The galaxy’s redshift was measured to be 3.39, indicating that it is even more distant than the Cloverleaf quasar, which has a redshift of 2.56. The near-perfect alignment of the two objects explains why the galaxy had never been seen before; it was effectively hidden in the glare of the much brighter quasar. This serendipitous discovery highlights the power of ALMA to unveil previously unknown objects and phenomena in the distant universe.
An Invisible Giant
The newly discovered galaxy is a true giant, with a molecular mass estimated to be between 40 and 230 billion times the mass of our sun. Its total infrared luminosity is a staggering 2.8 trillion times that of the sun, placing it firmly in the category of ultra-luminous infrared galaxies. This immense luminosity is not due to the light of stars, but rather to the thermal radiation emitted by vast clouds of dust that are heated by the intense star formation within them. In fact, the galaxy is so enshrouded in dust that it is completely invisible in optical and near-infrared light, earning it the designation of an “optically dark galaxy”.
A Dusty Veil
The dust in this galaxy is so thick that it absorbs virtually all of the visible light from its young, hot stars. This absorbed energy heats the dust to a few tens of degrees above absolute zero, causing it to glow brightly in the far-infrared and submillimeter wavelengths that are detectable by ALMA. This is why the galaxy was able to remain hidden for so long; telescopes that operate at visible wavelengths, such as the Hubble Space Telescope, are blind to this type of object. The discovery of this optically dark galaxy confirms the existence of a significant population of massive, dusty, star-forming galaxies in the early universe that have been largely missed by traditional galaxy surveys.
A Window into the Early Universe
The discovery of this ULIRG provides a unique opportunity to study the processes of star formation and galaxy evolution at a time when the universe was only a few billion years old. The galaxy is forming stars at a prodigious rate, likely hundreds or even thousands of times faster than our own Milky Way galaxy. This intense starburst is thought to have been triggered by a major merger between two gas-rich galaxies, a common occurrence in the early universe.
The molecular gas in the galaxy has not yet settled into a stable, rotating disk, which is further evidence that a merger has recently taken place. This chaotic, turbulent environment is the perfect breeding ground for new stars. By studying the properties of the gas and dust in this galaxy, astronomers can learn more about the physical conditions that prevailed in the early universe and how these conditions influenced the formation of the first massive galaxies.
The Power of ALMA
The Atacama Large Millimeter/submillimeter Array, or ALMA, was the key to this discovery. ALMA is a powerful interferometer located in the high-altitude desert of northern Chile. It consists of 66 high-precision antennas that work together as a single telescope, allowing it to observe the universe with unprecedented sensitivity and resolution at millimeter and submillimeter wavelengths. It is this capability that allowed astronomers to peer through the dust and glare of the Cloverleaf quasar and detect the faint emission from the hidden galaxy behind it.
ALMA is particularly well-suited for studying the cold, dusty universe, where the processes of star formation and galaxy evolution are most active. By observing the emission from molecules such as carbon monoxide, astronomers can map the distribution and kinematics of the molecular gas that fuels star formation. This information is crucial for understanding the physical processes that govern the formation of stars and the growth of galaxies over cosmic time.
Future Prospects
The discovery of this hidden galaxy has important implications for our understanding of the early universe. It suggests that there may be a large population of optically dark, dusty, star-forming galaxies that have been missed by previous surveys. Finding and studying these galaxies is a major goal for astronomers, as they hold the key to understanding how the massive galaxies we see today came to be.
The Search Continues
Future observations with ALMA and other telescopes will be needed to determine the true nature of this newly discovered galaxy and to search for other, similar objects. Astronomers will be keen to measure the galaxy’s star formation rate more precisely, as well as to determine the mass of its central black hole. These observations will provide crucial constraints on models of galaxy formation and evolution and will help to complete our picture of the cosmic history of star formation. The serendipitous discovery of this ultra-luminous infrared galaxy behind the Cloverleaf quasar is a testament to the power of new observational facilities and the importance of keeping an open mind when exploring the universe.