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arXiv · 2609.34903

Exploring the observed properties of JWST Little Red Dots with cosmological simulations and dust radiative transfer

Saksham Chandna·Fabio Di Mascia·Simona Gallerani·Stefano Carniani·Pierluigi Rinaldi·Roberta Tripodi·Milena Valentini·Alessia Tortosa·Luca Zappacosta
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Abstract

JWST has uncovered compact high-redshift (z>=4) sources, dubbed Little Red Dots (LRDs), characterised by a distinct V-shaped continuum. Their nature remains debated since broad-line AGN signatures coexist with non-AGN continua, marked by weak X-ray and mid-IR emission. We investigate which properties of LRDs naturally emerge within simulations of massive AGN host galaxies and which require additional physics. We post-process two cosmological zoom-in simulations of the same massive halo, one including black hole growth and AGN feedback (AGNfid) and the other with only star formation and stellar feedback (SFonly). We analyse snapshots spanning z=6-9 to probe different evolutionary stages. Using SKIRTv8, we develop a forward-modelling framework exploring various dust models, dust-to-metal ratios, and intrinsic AGN spectra. We generate mock spectra and images to apply JWST observational LRD selection criteria. Not designed to model LRDs, our AGNfid simulations naturally reproduce their properties: V-shaped continua, compact morphologies with rest-UV complexity, UV luminosities, attenuations, overmassive BHs, sub-Eddington accretion, and Balmer break strengths (~1-2) typical of most LRDs. Spectral decomposition reveals that the rest-UV continuum is dominated by stellar emission, whereas the optical continuum stems from a dust-attenuated AGN. Dust geometry and AGN-to-stellar contribution drive photometric differences between LRDs and Little Blue Dots. Our results suggest that LRDs do not necessarily represent a distinct class of objects. Instead, their defining properties can arise in massive AGN host galaxies from the interplay between stellar emission, an obscured central engine, and dust geometry. Super-Eddington accretion, dust-free gas columns, evolved stellar populations, and photoionisation modelling are required to test whether this framework reproduces the diversity of observed LRDs.

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Saksham Chandna, Fabio Di Mascia, Simona Gallerani, Stefano Carniani, Pierluigi Rinaldi, Roberta Tripodi, Milena Valentini, Alessia Tortosa, Luca Zappacosta. 2026-09-28. Exploring the observed properties of JWST Little Red Dots with cosmological simulations and dust radiative transfer. https://arxiv.org/abs/2609.34903

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