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

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, J0647₁045 at z = 4. 5319 ± 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. J0647₁045 is unresolved (r e ≲ 0. 17 kpc) in the three NIRCam long-wavelength filters but significantly extended (r e = 0. 45 ± 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 rest-frame optical/NIR, which is consistent with two distinct components fit by power laws with different attenuations: A V = 0. 38 ± 0. 01 (UV) and A V = 5. 61 ± 0. 04 (optical/NIR). Fitting the H α line requires both broad (full width at half maximum of ∼4300 ± 100 km s −1) and narrow components, but none of the other emission lines, including H β, show evidence of broadness. We calculated A V = 0. 9 ± 0. 4 from the Balmer decrement using narrow H α and H β and A V > 4. 1 ± 0. 1 from broad H α and an upper limit on broad H β, which is consistent with blue and red continuum attenuation, respectively. Based on a single-epoch H α line width, the mass of the central black hole is 8 −0. 4 +0. 5 × 10 8 M ⊙. Our findings are consistent with a multi-component model, in which 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.