Deciphering the JWST spectrum of a ‘little red dot’ at z ∼ 4.53: An obscured AGN and its star-forming host

¹ Cosmic Dawn Center (DAWN), Jagtvej 128, 2200 Copenhagen N, Denmark
² Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen Ø, Denmark
³ Instituto de Estudios Astrofícos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago 8370191, Chile
⁴ David A. Dunlap Department of Astronomy & Astrophysics, University of Toronto, 50 St George Street, Toronto, ON M5S 3H4, Canada
⁵ Department of Physics, Clark University, Worcester, MA 01610-1477, USA
⁶ Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Association of Universities for Research in Astronomy (AURA) for the European Space Agency (ESA), STScI, Baltimore, MD, USA
⁸ Center for Astrophysical Sciences, Department of Physics and Astronomy, The Johns Hopkins University, 3400 N Charles St., Baltimore, MD 21218, USA

^⋆ Corresponding author; meghana.killi@mail.udp.cl

Received: 5 December 2023
Accepted: 18 September 2024

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 = 4.5319 ± 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. J0647_1045 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 Hα 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 Hβ, show evidence of broadness. We calculated A_V = 0.9 ± 0.4 from the Balmer decrement using narrow Hα and Hβ and A_V > 4.1 ± 0.1 from broad Hα and an upper limit on broad Hβ, which is consistent with blue and red continuum attenuation, respectively. Based on a single-epoch Hα line width, the mass of the central black hole is 8_−0.4^+0.5 × 10⁸ 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.