Deciphering the JWST spectrum of a 'little red dot' at $z \sim 4.53$: An obscured AGN and its star-forming host

TL;DR

This study uses JWST NIRCam imaging and NIRSpec prism spectroscopy to dissect a little red dot at $z\sim4.5$, revealing a two-component system: a blue, star-forming host that dominates the UV and a heavily obscured AGN core that dominates the optical/NIR with a broad Hα line ($\mathrm{FWHM}\approx4300$ km s$^{-1}$) and a central SMBH mass of $\sim8\times10^{8}$ M$_\odot$. The morphology shows an extended UV component and a compact, unresolved NIR core, consistent with a spatially blended AGN+host system. Spectral modelling with two attenuated continua ($A_V^{UV}\approx0.54$, $A_V^{opt/NIR}\approx5.7$) and multi-line fits indicates high ionization, extremely low metallicity, and a hard radiation field, suggesting an obscured AGN with a star-forming host. The work discusses multiple origin scenarios for the characteristic V-shaped SED of LRDs, finding that an AGN-driven extinction curve can reproduce part of the shape, but overall supports a scenario in which LRDs are low-metallicity, dust-poor AGN viewed through their torus, providing insight into early SMBH growth and AGN–host co-evolution at $z>4$. The results add to the emerging view of LRDs as a populous, obscured AGN population with diverse host properties, and point toward future JWST surveys and high-resolution follow-up to map their demographics and evolution.

Abstract

JWST has revealed a class of numerous, extremely compact sources, with rest-frame red optical/near-infrared (NIR) and blue ultraviolet (UV) colours, nicknamed "little red dots". We present one of the highest signal-to-noise ratio JWST NIRSpec/PRISM spectra of a little red dot, J0647_1045 at $z = 4.5321 \pm 0.0001$, and examine its NIRCam morphology, to differentiate the origin of the UV and optical/NIR emission, and elucidate the nature of the little red dot phenomenon. J0647_1045 is unresolved ($r_e < 0.17$ kpc) in the three NIRCam long-wavelength filters, but significantly extended ($r_e = 0.45 \pm 0.06$ kpc) in the three short-wavelength filters, indicating a red compact source in a blue star-forming galaxy. The spectral continuum shows a clear change in slope, from blue in the optical/UV, to red in the restframe optical/NIR, consistent with two distinct components, fit by power-laws with different attenuation: $A_V = 0.54 \pm 0.01$ (UV) and $A_V = 5.7 \pm 0.2$ (optical/NIR). Fitting the H$α$ line requires both broad (full width at half-maximum $\sim 4300 \pm 300 km s^{-1}$) and narrow components, but none of the other emission lines, including H$β$, show evidence of broadness. We calculate $A_V = 1.1 \pm 0.2$ from the Balmer decrement using narrow H$α$ and H$β$, and $A_V > 4.1 \pm 0.2$ from broad H$α$ and upper limit on broad H$β$, consistent with the blue and red continuum attenuation respectively. Based on single-epoch H$α$ linewidth, the mass of the central black hole is $8 \pm 1 \times 10^8 M_\odot$. Our findings are consistent with a multi-component model, where the optical/NIR and broad lines arise from a highly obscured, spatially unresolved region, likely a relatively massive active galactic nucleus, while the less obscured UV continuum and narrow lines arise, at least partly, from a small but spatially resolved star-forming host galaxy.

Figures (10)

• Figure 1: RGB image (created using the F115W, F277W, and F444W NIRCam filters) of J0647_1045, with the NIRSpec slits (magenta rectangles) overlaid. The image is centered on the coordinates of the source at RA=101.933406, Dec=70.198268, and is 2 arcsec on a side.
• Figure 2: Top: 2D NIRSpec PRISM spectrum of J0647_1045 showing the bright H$\rm \alpha$, H$\rm \beta$, and [O iii] emission. Bottom: 1D spectrum (corrected for Galactic extinction) with several identified lines labelled. The observed wavelengths are shown in $\upmu$m on the top axis, while the rest-frame wavelengths are plotted in Å on the bottom axis. The NIRCam photometric fluxes in a 0.5 arcsec diameter aperture in the UV filters F115W, F150W, and F200W are shown as blue stars with the NIRCam bands shown as errorbars. Optical/NIR filters F277W, F356W, and F444W are similarly shown as pink stars with errorbars.
• Figure 3: GALFIT models and residuals in the six NIRCam bands using PSF, Sérsic, and combined Sérsic+PSF models. The PSF fit shows clear residuals, especially in the UV bands. The fit improves considerably with the addition of a Sérsic component. The best fit is obtained when both Sérsic and PSF components are used, with $\Delta$BIC indicating a significant improvement in all bands (Sec. \ref{['sec:bestfit_morph']}).
• Figure 4: The observed spectrum (blue step curve) with the best-fit model (black dashed curve; plotted with a finer sampling than the observed spectrum), and zoom-in cutouts of H$\rm \beta$+[O iii] and H$\rm \alpha$ regions. The residuals are shown in the bottom panel.
• Figure 5: Corner plot showing the MCMC results for heights and widths of the narrow and broad H$\rm \alpha$ components. Height is given in the same units as Fig. \ref{['fig:bestfit_spec']}, 10$^{-17}$ erg s$^{-1}$ cm$^{-2}\,\upmu$m$^{-1}$, and velocity width is given in km s$^{-1}$.