PAHSPECS: Spatially Resolved PAH Spectroscopy At Cosmic Noon With JWST MIRI MRS - astrobiology.com
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The James Webb Space Telescope (JWST) has delivered a groundbreaking look into the universe's busiest era, 'Cosmic Noon,' through the PAHSPECS program, revealing unexpected characteristics of organic molecules that challenge existing models of galaxy evolution. Using JWST's MIRI MRS instrument, scientists have achieved the first spatially resolved spectroscopy of Polycyclic Aromatic Hydrocarbons (PAHs) in distant galaxies at redshift z ~ 1.1, finding that these crucial organic tracers behave distinctly from their counterparts in the local universe. This unprecedented detail suggests a significantly different interstellar medium environment during the peak of cosmic star formation. The PAHSPECS team's initial findings indicate that PAH emission at Cosmic Noon is shaped by more intense radiation fields than previously understood, with most observed galaxies exhibiting higher 6.2/7.7 and lower 11.3/7.7 PAH ratios, pointing to a prevalence of smaller, ionized PAH grains. Critically, these distant galaxies show PAHs becoming larger and more neutral with increasing galactocentric radius, a trend opposite to what is typically observed in nearby galaxies. This reversal in radial gradients underscores the dramatic physical differences in star-forming regions during an epoch when galaxies were forming stars at rates 10 to 100 times higher than today. These revelations are poised to redefine our understanding of how galaxies assembled their stellar mass and processed dust and gas in the early universe. While the 7.7 µm PAH feature remains a reliable tracer for star formation across cosmic time, the contrasting PAH properties demand a re-evaluation of interstellar medium physics at high redshift. Future, deeper observations with JWST and complementary data from surveys like ALMA will be crucial for confirming these trends and building comprehensive models that accurately describe galaxy formation during this pivotal cosmic period.