Deciphering the Nature of Virgil: An Obscured AGN Lurking Within an Apparently Normal Lyman-α Emitter During Cosmic Reionization

Pierluigi Rinaldi, Pablo G. Pérez-González, George H. Rieke, Jianwei Lyu, Francesco D'Eugenio, Zihao Wu, Stefano Carniani, Tobias J. Looser, Irene Shivaei, Leindert A. Boogaard, Tanio Diaz-Santos, Luis Colina, Göran Östlin, Stacey Alberts, Javier Álvarez-Márquez, Marianna Annuziatella, Manuel Aravena, Rachana Bhatawdekar, Andrew J. Bunker, Karina I. Caputi, Stéphane Charlot, Alejandro Crespo Gómez, Mirko Curti, Andreas Eckart, Steven Gillman, Kevin Hainline, Nimisha Kumari, Jens Hjorth, Edoardo Iani, Hanae Inami, Zhiyuan Ji, Benjamin D. Johnson, Gareth C. Jones, Álvaro Labiano, Roberto Maiolino, Jens Melinder, Thibaud Moutard, Florian Peißker, Marcia Rieke, Brant Robertson, Jan Scholtz, Sandro Tacchella, Paul P. van der Werf, Fabian Walter, Christina C. Williams, Chris Willott, Joris Witstok, Hannah Übler, Yongda Zhu

First listed 2025-04-02 | Last updated 2025-04-05

Abstract

We present a comprehensive analysis of the MIRI Extremely Red Object Virgil, a Lyman-$α$ emitter at $z_{spec} = 6.6379 \pm 0.0035$ with the photometric properties of a Little Red Dot. Leveraging new JWST/MIRI imaging from the MIDIS and PAHSPECS programs, we confirm Virgil's extraordinary nature among galaxies in JADES/GOODS-South, exhibiting a strikingly red NIRCam-to-MIRI color (F444W $-$ F1500W = $2.84\pm0.04$ mag). Deep NIRSpec/PRISM spectroscopy from the OASIS program offers key insights into the host galaxy, revealing properties of an average star-forming galaxy during Cosmic Reionization, such as a subsolar metallicity, low-to-moderate dust content, and a relatively high ionization parameter and electron temperature. By estimating the star formation rate of Virgil from UV and H$α$, we find evidence that the galaxy is either entering or fading out of a bursty episode. Although line-ratio diagnostics employed at high-$z$ would classify Virgil as an Active Galactic Nucleus (AGN), this classification becomes ambiguous once redshift evolution is considered. Nonetheless, Virgil occupies the same parameter space as recently confirmed AGNs at similar redshifts. The new deep MIRI data at 15 $μ$m reinforce the AGN nature of Virgil, as inferred from multiple spectral energy distribution (SED) fitting codes. Virgil's rising infrared SED and UV excess resemble those of Dust-Obscured Galaxies (DOGs) studied with Spitzer at Cosmic Noon, particularly blue-excess HotDOGs. Our results highlight the need for a multi-wavelength approach incorporating MIRI to uncover such extreme sources at $z\gtrsim6$ and to shed light on the interplay between galaxy evolution and early black hole growth during Cosmic Reionization.

Short digest

JWST/MIRI imaging plus deep NIRSpec/PRISM place the LAE “Virgil” at z=6.6379 and reveal Little Red Dot–like photometry with an extreme color F444W−F1500W=2.84±0.04, and a strongly rising mid-IR SED. SED fits across multiple codes favor an obscured AGN whose IR slope and UV excess resemble blue-excess HotDOGs. The host shows subsolar metallicity, low–moderate dust, and high ionization/electron temperature; UV and Hα SFRs suggest Virgil is entering or fading a burst. High‑z line‑ratio diagnostics (e.g., O32–R23, Ne3O2) nominally flag AGN, though this is ambiguous after accounting for redshift evolution; a tentative broad Hα improves residuals only marginally, while the new 15 μm data strengthen the AGN case and highlight MIRI’s leverage on early black‑hole growth.

Key figures to inspect

• Figure 1 (2D PRISM + NIRCam RGB/slits): Verify the secure z=6.6379 via the stacked emission-line pattern and check for any slit contamination from the nearby foreground LAE; the slit overlays also show how Virgil was targeted relative to neighbors.
• Figure 2 (mass–metallicity): See where Virgil’s subsolar metallicity lands against JADES/CEERS and simulation tracks (Astraeus/FirstLight/FLARES/TNG50), clarifying that the host is chemically typical for z~6–7 SF galaxies despite its extreme mid‑IR color.
• Figure 3 (O32–R23 and Ne3O2 grids): Inspect Virgil’s position with and without dust correction relative to CLOUDY model grids, SDSS, local analogs, and z>6 samples to understand why classical cuts imply AGN yet become ambiguous once redshift evolution is considered.
• Figure 4 (Hα+[NII] fits): Compare fits with/without a broad Hα component; note the improved residuals but only marginal BIC gain, framing how weak BLR evidence contrasts with the strong mid‑IR AGN signal.