RUBIES: JWST/NIRSpec Confirmation of an Infrared-luminous, Broad-line Little Red Dot with an Ionized Outflow

Abstract

The JWST discovery of “little red dots” (LRDs) is reshaping our picture of the early Universe, yet the physical mechanisms driving their compact size and UV-optical colors remain elusive. Here, we report an unusually bright LRD ( z _spec = 3.1) observed as part of the RUBIES program. This LRD exhibits broad emission lines (FWHM ∼ 4000 km s ^−1 ), a blue UV continuum, a clear Balmer break, and a red continuum sampled out to rest-frame 4 μ m with MIRI. We develop a new joint galaxy and active galactic nucleus (AGN) model within the Prospector Bayesian inference framework and perform spectrophotometric modeling using NIRCam, MIRI, and NIRSpec/Prism observations. Our fiducial model reveals a M _* ∼ 10 ^9 M _⊙ galaxy alongside a dust-reddened AGN driving the optical emission. Explaining the rest-frame optical color as a reddened AGN requires A _V ≳ 3, suggesting that a great majority of the accretion disk energy is reradiated as dust emission. Yet, despite clear AGN signatures, we find a surprising lack of hot torus emission, which implies that either the dust emission in this object must be cold, or the red continuum must instead be driven by a massive, evolved stellar population of the host galaxy—seemingly inconsistent with the high-EW broad lines (H α rest-frame EW ∼ 800 Å). The widths and luminosities of Pa- β , Pa- δ , Pa- γ , and H α imply a modest black hole mass of M _BH ∼ 10 ^8 M _⊙ . Additionally, we identify a narrow blueshifted He i λ 1.083 μ m absorption feature in NIRSpec/G395M spectra, signaling an ionized outflow with kinetic energy up to ∼1% the luminosity of the AGN. The low redshift of RUBIES-BLAGN-1, combined with the depth and richness of the JWST imaging and spectroscopic observations, provides a unique opportunity to build a physical model for these so-far mysterious LRDs, which may prove to be a crucial phase in the early formation of massive galaxies and their supermassive black holes.

Keywords

• AGN host galaxies
• Active galactic nuclei
• Galaxy kinematics
• Galaxy formation
• Photoionization
• Spectral energy distribution