A groundbreaking new survey that offers the most complete view yet of how galaxies formed and evolved during a critical period in the history of our universe has been unveiled by astronomers.
The newly released results, made possible using NASA’s James Webb Space Telescope, are part of the MIRI EGS Galaxy and AGN (MEGA) survey, showcasing the power of Webb’s Mid-Infrared Instrument (MIRI).
Capable of peering through cosmic dust that once obscured astronomical features from view by past space observatories, the new data is revealing more profound insights into the hidden processes of star formation and black hole growth at “Cosmic Noon,” the era from roughly 10 to 12 billion years ago when galaxies built up much of their stellar mass.
With high-resolution imagery that spans 70 arcminutes of the Extended Groth Strip, MEGA allows researchers to study fainter galaxies than past observations would have allowed. The new findings reveal details of these galaxies and improve our understanding of how they form and co-evolve alongside supermassive black holes over time.
A New Window into the Dusty Universe
In the past, space telescopes like Spitzer and WISE could not obtain imagery at a resolution that was high enough to provide data on galaxies at high redshifts obscured by cosmic dust.
Webb’s MIRI instrument, by contrast, cuts through the cosmic haze and delivers imagery with unprecedented clarity, which allows researchers to resolve galaxies up to redshift z~3 (a value denoting a very distant galaxy, with its light significantly shifted towards longer wavelengths due to the expansion of the universe).
“While Cosmic Noon has had unobscured star formation measurements from UV/Optical emission lines,” the team writes in a new paper detailing their findings, adding that “MIRI now allows the obscured star formation rate to be measured by polycyclic aromatic hydrocarbon (PAH) emission features,” which are good markers of star-forming regions. This allows astronomers an effective tool for measuring star formation in these once-obscured regions.
Stellar Birth at “Cosmic Noon”
About half of all stellar mass is believed to have been formed at “Cosmic Noon.” Understanding how dust obscures this star formation is crucial to expanding our understanding of how astronomers can reconstruct the complete history of galactic growth.
With MIRI’s sensitivity, dusty environments surrounding active galactic nuclei (AGN), the energetic regions surrounding supermassive black holes at the heart of galaxies, can be studied.
In the recent study, the MEGA survey combined 67 hours of observations made by Webb across four filters—F770W, F1000W, and F1500W, three of the reddest bands, and F2100W, a blue one—using sophisticated data reduction techniques to correct for background noise and other artifacts. The researchers employed a super background subtraction method, which helped improve overall uniformity in the imagery they obtained, allowing the details of even some of the faintest previously unseen galaxies to be revealed.
The team also applied rigorous methodology to help maintain a quality catalog, which resulted in a false positive rate of only 2–4%, depending on the testing method used. This successfully expanded the number of detectable galaxies overall, and MEGA’s catalog now represents one of the deepest and most complete mid-infrared galaxy surveys to date.
Galactic Growth and Black Hole Accretion
The rich MEGA dataset allows scientists to simultaneously trace the rates of star formation, and those of black hole accretion rates within the same galaxies. This offers the potential for otherwise rare insights into the co-evolution of these cosmic phenomena, and early results are already pointing to how MIRI observations could extend the boundaries of past estimates, offering a more expansive and complete assemblage of galaxies at Cosmic Noon.
Additionally, the survey reveals some significant differences between the morphology of galaxies seen at mid-infrared wavelengths, as compared to near-infrared imaging, which reveals features that include more merger activity than past surveys by the Hubble Space Telescope, or even earlier NIRCam data obtained by Webb, managed to reveal.
Hunting Hidden Black Holes
With its mid-infrared coverage, the MEGA data is also a formidable tool for identifying low-AGN with low luminosity and those heavily obscured by cosmic dust. Based on early findings, it is already estimated that around 10% of galaxies observed during Cosmic Noon are likely to possess active black holes, which may reshape existing models about their growth and relationship to early galaxies and their formation.
Additionally, the longer-wavelength sensitivity of the MEGA data also allows astronomers to investigate cosmic mysteries like “little red dots,” which are compact galaxies with hot dust signatures that are believed to indicate black hole activity.
Overall, the MEGA survey results clearly demonstrate how mid-infrared astronomy, made possible by the Webb telescope, is fundamentally reshaping our understanding of some of the most formative periods in the history of our universe. The full photometric catalog, accompanying images, and data products have been made available online for further scientific exploration.
With the telescope’s unprecedented ability to unveil the previously “hidden” side of star formation and black hole evolution, MEGA helps to fill a longstanding gap in current astrophysical models, while paving the way towards even deeper and more detailed studies in the future.
The new study, “MEGA Mass Assembly with JWST: The MIRI EGS Galaxy and AGN Survey,” was uploaded to the preprint server arXiv.org and can be read online.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.