Spatial decomposition of Little Red Dots with JWST/NIRSpec IFU into broad-line red cores and narrow-line blue host galaxies

Yuzo Ishikawa, Anna-Christina Eilers, Rohan P. Naidu, Jorryt Matthee, Rongmon Bordoloi, John Chisholm, Jenny E. Greene, Yilun Ma, Pascal A. Oesch, Wendy Q. Sun, Alberto Torralba, John R. Weaver, Stijn Wuyts, Mengyuan Xiao

First listed 2026-07-13 | Last updated 2026-07-10

Abstract

Little Red Dots (LRDs) are a population of compact red sources discovered by the James Webb Space Telescope (JWST). Imaging and spectroscopy have shown that LRDs exhibit a complex spectrum with a ``V-shaped" continuum, broad Balmer emission lines, and in some cases Balmer absorption. While the physical origin of these components remains debated, recent studies propose that they arise from a compact central engine likely hosting a rapidly growing black hole embedded within a more extended host galaxy. We test this central engine + host galaxy model using JWST/NIRSpec integral field unit (IFU) spectroscopy to spectrally decompose the observed continuum, narrow and broad emission lines, and absorption. We spatially map each component for five broad Ha-selected LRDs at z~5 observed with both the prism and high-resolution G395H grating. We find that the blue continuum emission is co-spatial with the narrow emission line region, while the red continuum arises from a compact core co-spatial with the broad Balmer emission and absorption. Spatial maps of the [OIII] equivalent width reveal a pronounced decrease in the central core. Our work provides further evidence that the LRD emission is produced by at least two distinct physical components arising from a red central engine embedded within a blue host galaxy.

Short digest

This paper uses JWST/NIRSpec IFU prism plus G395H data to spatially and spectrally decompose five broad-Hα-selected little red dots at z≈5 into continuum, narrow-line, broad-line, and absorption components. The key result is a clean spatial split: the blue continuum tracks the narrow-line region, while the red continuum is unresolved and coincident with the broad Balmer emission and absorption in a compact core. Radial profiles and [O III] equivalent-width maps show that the core is continuum-dominated, with a pronounced central [O III] EW dip and more extended narrow-line emission than the broad component. That directly strengthens the picture that LRDs are composite systems, with a red central engine embedded in a bluer host galaxy.

Key figures to inspect

• Figure 2. This figure is the best overview of the paper’s decomposition framework, showing how the PRISM spectra are split into blue and red continuum components and how the G395H data isolate narrow forbidden lines, broad Balmer emission, and Balmer absorption. It lets readers see exactly what observables are being mapped spatially in the rest of the paper and how the two-component interpretation is built from the spectroscopy.
• Figure 3. Use this representative IFU map to visualize the paper’s central claim in a single object. The contrast between spatially broader blue continuum and narrow-line emission versus the compact red continuum, broad Balmer emission, and absorption makes the red-core plus blue-host picture immediately tangible.
• Figure 4. This figure turns the continuum decomposition into a size measurement by comparing wavelength-dependent spatial profiles against the modeled JWST PSF. It is important because it quantitatively shows that the red continuum is unresolved while the blue continuum is extended, which is one of the paper’s cleanest tests of the central-engine-plus-host scenario.
• Figure 5. This is the line-emission counterpart to the continuum-size analysis, demonstrating that the broad Balmer component stays compact while the narrow-line emission, especially [O III], can be more extended. It matters because it ties the broad lines to the compact core and the narrow forbidden lines to the larger-scale host environment.
• Figure 6. The [O III] equivalent-width maps provide the sharpest physical diagnostic in the paper by revealing a central dip or plateau in EW where the red continuum is strongest. That non-monotonic structure is key evidence that the core is continuum-dominated rather than simply the brightest part of the narrow-line region, reinforcing the interpretation of a compact red central engine embedded in a blue host galaxy.