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

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$\alpha$ line requires both broad (full width at half-maximum $\sim 4300\pm 300km{s}^{-1}$) and narrow components, but none of the other emission lines, including H$\beta$, show evidence of broadness. We calculate $A_V=1.1\pm 0.2$ from the Balmer decrement using narrow H$\alpha$ and H$\beta$, and $A_V>4.1\pm 0.2$ from broad H$\alpha$ and upper limit on broad H$\beta$, consistent with the blue and red continuum attenuation respectively. Based on single-epoch H$\alpha$ 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.

13 pages, 10 figures. Submitted to A&A. Comments welcome

References in corpus (18)

• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities
• Understanding Galaxy Evolution through Emission Lines
• A Luminous Quasar at Redshift 7.642
• A CEERS Discovery of an Accreting Supermassive Black Hole 570 Myr after the Big Bang: Identifying a Progenitor of Massive z > 6 Quasars
• GA-NIFS: A massive black hole in a low-metallicity AGN at $z\sim 5.55$ revealed by JWST/NIRSpec IFS
• The innermost region of AGN tori: implications from the HST/NICMOS Type 1 point sources and near-IR reverberation
• A 0.8-2.4 microns spectral atlas of Active Galactic Nuclei
• CEERS Spectroscopic Confirmation of NIRCam-Selected z > 8 Galaxy Candidates with JWST/NIRSpec: Initial Characterization of their Properties
• Evolution of the UV LF from z~15 to z~8 Using New JWST NIRCam Medium-Band Observations over the HUDF/XDF
• A Candidate of a Least-Massive Black Hole at the First 1.1 Billion Years of the Universe
• A dusty compact object bridging galaxies and quasars at cosmic dawn
• The MUSE Hubble Ultra Deep Field Survey: IV. Global properties of C III] emitters
• GA-NIFS: Early-stage feedback in a heavily obscured AGN at $z=4.76$
• On inferring extinction laws in z~6 quasars as signatures of supernova dust
• Metal enrichment and evolution in four z > 6.5 quasar sightlines observed with JWST/NIRSpec
• The most massive Population III stars
• The ALPINE-ALMA [CII] Survey: Investigation of 10 Galaxies at $z\sim 4.5$ with [OII] and [CII] Line Emission $-$ ISM Properties and [OII]$-$SFR Relation
• Rayleigh and Raman scattering cross-sections and phase matrices of the ground-state hydrogen atom, and their astrophysical implications

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