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 ($z=3.1$) observed as part of the RUBIES program. This LRD exhibits broad emission lines (FWHM $\sim 4000$km/s), a blue UV continuum, a clear Balmer break and a red continuum sampled out to rest 4 $\mu$m with MIRI. We develop a new joint galaxy and AGN model within the Prospector Bayesian inference framework and perform spectrophotometric modeling using NIRCam, MIRI, and NIRSpec/Prism observations. Our fiducial model reveals a $M_{\ast }\sim 10^9{M}_{\odot }$ galaxy alongside a dust-reddened AGN driving the optical emission. Explaining the rest-frame optical color as a reddened AGN requires $A_{\mathrm{v}}\gtrsim 3$, suggesting that a great majority of the accretion disk energy is re-radiated 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$\alpha$ EW $\sim 800$Å). The widths and luminosities of Pa$\beta$, Pa$\delta$, Pa$\gamma$, and H$\alpha$ imply a modest black hole mass of $M_{\mathrm{BH}}\sim 10^8{M}_{\odot }$. Additionally, we identify a narrow blue-shifted HeI absorption in G395M spectra, signaling an ionized outflow with kinetic energy up to $\sim 1$\% 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 provide 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.

Accepted for publication in ApJ; 24 pages, 9 figures, 5 tables. Code implementing the joint galaxy and AGN model is made publicly available online as a part of Prospector; the model used in this work corresponds to the state of the Git repository at commit 1cce164 https://github.com/bd-j/prospector/pull/366

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