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

TL;DR

The study analyzes RUBIES-BLAGN-1, a luminous infrared little red dot at $z_{ m spec}=3.1034\pm0.0002$, using JWST/NIRSpec Prism and G395M spectroscopy alongside NIRCam/MIRI imaging. A composite galaxy+AGN model in Prospector (including a dust-reddened accretion-disk continuum and a CLUMPY torus) yields a host mass of $M_\*\sim 10^9\,M_\odot$ and a dust-attenuated AGN with $A_V \gtrsim 3$, while broad Balmer lines imply a relatively modest black hole mass of $M_{\rm BH}\sim 10^8\,M_\odot$ and an ionized outflow with kinetic energy up to $\sim1\%$ of the AGN luminosity. The results reveal tension between a red rest-frame optical continuum and the apparent lack (or coldness) of hot torus emission, suggesting either very cold dust or a substantial evolved stellar component. This work demonstrates the power of joint SED modeling with JWST data to disentangle AGN and host galaxy contributions in extreme, compact high-redshift systems and provides a framework for understanding LRDs in the context of early SMBH-galaxy coevolution.

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 $\sim4000$km/s), a blue UV continuum, a clear Balmer break and a red continuum sampled out to rest 4 $μ$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 $M_*\sim 10^9M_\odot$ galaxy alongside a dust-reddened AGN driving the optical emission. Explaining the rest-frame optical color as a reddened AGN requires $A_{\rm v}\gtrsim3$, 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$α$ EW $\sim800$Å). The widths and luminosities of Pa$β$, Pa$δ$, Pa$γ$, and H$α$ imply a modest black hole mass of $M_{\rm BH}\sim10^8M_\odot$. Additionally, we identify a narrow blue-shifted HeI absorption in G395M spectra, signaling an ionized outflow with kinetic energy up to $\sim1$\% 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.

Figures (2)

• Figure 1: Photometric observations of RUBIES-BLAGN-1. (a) Cutouts are shown in selected filter bands, including HST/WCS, JWST/NIRCam, and MIRI detections. A color image, composited from NIRCam/F115W, F277W, and F444W, is included as the last panel. Each panel is 5$^{\prime\prime}$ in width. (b) RUBIES-BLAGN-1 is spectroscopically confirmed to be among the lowest redshift LRDs. The spectroscopic redshifts of the samples from Greene2023Matthee2023:lrdWang2024:bb are plotted as light purple, purple, and pink triangles, respectively. RUBIES-BLAGN-1 is bright in F444W ($\sim$ 22 AB mag) and exhibits an extreme red color with F150W-F444W $\sim$ 3.8 mag. The red objects at higher redshifts are included as pink dots for reference Labbe2023:massiveLabbe2023:agn. The parent sample, including all sources identified from PRIMER having F444W $<$ 27.5 AB mag, is shown in gray. To facilitate a better comparison to the high-$z$ LRDs, we show a track tracing the colors of RUBIES-BLAGN-1 as it moves to higher redshifts. Its color at $z=7$ is explicitly included as a red plus sign. RUBIES-BLAGN-1 is clearly an outlier in the color space, and possibly can be seen as an analog of the high-$z$ LRDs.
• Figure 2: JWST/NIRSpec data of RUBIES-BLAGN-1. (a) The first row displays the 2D Prism spectrum, with the rest-frame wavelength shown along the $x$-axis; the histogram indicates the extracted region. The left panel in the second row shows the 1D spectrum in $F_\lambda$ as a function of the observed wavelength. Data and uncertainties are plotted in black and blue, respectively. The beige horizontal line is at $y=0$ to guide the eye. The cutout in the F444W band is shown to the right, with the slitlets overlaid. (b) Same as the above figure set, but for the G395M visits. Only the 2D spectrum of the second visit is included, which has a higher signal-to-noise ratio.