Hyper-luminous, Yet Surprisingly Organized
The galaxy PJ0116-24 lives about 10 billion years ago and appears about 10,000 times brighter in the infrared than our Milky Way. It belongs to a rare population of so-called hyper-luminous infrared galaxies (HyLIRG), which are usually formed by the collision of several galaxies. Members of the Infrared Group at the Max Planck Institute for Extraterrestrial Physics (MPE) including Daizhong Liu and Natascha M. Förster Schreiber, together with researchers from the European Southern Observatory (ESO) and other international institutes, now showed that a HyLIRG can also arise in a massive turbulent rotating disk within a single galaxy, where the gas is organized in a structured way. This finding was made possible through new observations including from the novel ERIS instrument at the ESO Very Large Telescope, built by a consortium also led by the MPE Infrared Group, and strong magnification caused by a massive elliptical galaxy that lies between us and PJ0116-24. This galaxy acts as a gravitational lens stretching PJ0116-24 in an “Einstein ring” and making it appear 17 times brighter.
Hyper-luminous infrared galaxies (HyLIRG) belong to the most luminous persistent objects in the universe and are often characterized by an extremely high star formation rate. In the Planck All-Sky Survey to Analyze Gravitationally-lensed Extreme Starbursts Project (PASSAGES), which observed more than 10^4 square degrees of the sky, a team of researchers including MPE members, detected about 20 HyLIRGs. Among those, the galaxy PJ0116-24 has been identified as the brightest.
“PJ0116-24 is a galaxy that is about 10 billion light years away and appears 10,000 times brighter in the infrared than our Milky Way, making it ideal for deep, high-resolution observations”, explains Daizhong Liu, first author of the study and researcher in the infrared-group at MPE. In addition to being intrinsically hyperluminous, the galaxy is further highly magnified via an astronomical phenomenon known as Einstein ring. A massive elliptical galaxy, which lies in the line of sight between our Earth and PJ0116-24, acted as a gravitational lens and stretched the light of PJ0116-24 into a near-perfect circle. This increased its luminosity by the factor 17 times, making it perhaps the IR-brightest high-redshift galaxy in the southern sky. The highly stretched image also reveals the structure of the source in roughly 10 times finer detail than if it were not lensed.
This rare cosmic setup enabled the research team to study the galaxy's rich structure in great detail using the Enhanced Resolution Imager and Spectrograph (ERIS), developed by the ERIS consortium led by MPE IR and mounted on the ESO Very Large Telescope in 2022, and the Atacama Large Millimeter/submillimeter Array (ALMA). With ERIS, the team detected important hydrogen, nitrogen, oxygen and sulfur lines in the galaxy’s spectrum, that revealed extreme dust levels, much higher than typical high-redshift galaxies, while ALMA was used to map the cold gas in PJ0116-24 in 3D.
PJ0116-24 is actually a massive rotating disk galaxy
The analysis showed that the gas in this unusual galaxy was rotating in an orderly manner, contrary to the chaotic motion typically expected following a galactic collision. Therefore, the researchers concluded that – despite its intense infrared brightness and starburst activity – PJ0116-24 is actually a massive rotating disk galaxy. That was an interesting finding that challenged previously beliefs about extremely bright galaxies, according to which HyLIRGs are associated with major collisions of multiple galaxies.
"We expected to see a complex and highly disturbed system as it’s the case for the most luminous galaxies in the present-day Universe. It was quite a surprise to find instead that a large fairly regular disk could be the host to such a high level of star formation”, says Natascha Förster Schreiber, one of the co-authors of the study.
"It's very exciting to see that such an extremely IR-bright galaxy can be well studied in both ionized gas and cold gas tracers. Ionized gas emission lines are usually too obscured to be detected in dusty galaxies, but the strong lensing helps to magnify the less dusty regions in the galaxy. Thanks to ERIS/VLT and ALMA, the circular motion of both ionized gas and cold gas can be clearly revealed, shedding new light on what triggers starbursts”, says Daizhong Liu and concludes: “Our results emphasize the crucial role of sharp views from high resolution observations of the structure and the gas motions of such luminous systems to understand them. PJ0116-24 is the second such case of a high redshift HyLIRG that is not in a messy merging system but a fairly ordered rotating disk. There may be more, which would be very interesting”
