Blackbody Quasar and Radio Source (BBQSORS): A Candidate of Transitional Little Red Dots with a $T\sim10^4\ K$ Blackbody Spectrum

TL;DR

This study presents spectroscopic evidence of a quasar with a blackbody spectrum around 10,000 K, indicating a transitional phase from a Little Red Dot (LRD) to a typical quasar.

Contribution

First detailed spectroscopic analysis of a candidate transitional LRD with blackbody features, supporting the evolutionary link to quasars.

Findings

Spectrum shows a broad MgII emission line with FWHM > 4000 km/s.

UV continuum is best described by a blackbody at ~10,000 K.

Photometry fits three blackbody components representing SMBH, dust torus, and host galaxy.

Abstract

We report Subaru/PFS spectroscopic follow-up of a radio-loud quasar at $z=1.715$ from the UNVEIL radio AGN catalog and with X-ray detections. The PFS spectrum displays a broad MgII emission line with an $\mathrm{FWHM}\gtrsim4000\ km/s$, accompanied by a narrow absorption feature. The spectrum reveals a characteristic $\Lambda$-shape over the rest-frame wavelength ranging $\sim1500-3500\ \r{A}$. This underlying UV continuum is too curved to be reproduced by simply applying dust extinction to the spectrum of typical unobscured quasars. Alternatively, it is well described by a blackbody spectrum with a temperature of $T\approx10000\ K$. This result is in good agreement with its UV to MIR photometry that can be well modeled by three blackbody components representing the SMBH envelope ($\mathit{T}\approx9700\ K$), dust torus ($T\approx1500\ K$), and host galaxy dust ($T\approx80\ K$). The source is marginally detected in the GALEX NUV, revealing a potential V-shaped spectral energy distribution around $1400\ \r{A}$, reminiscent of the spectral feature reported for recently discussed LRDs whose V-shapes occur around $3000-4000\ \r{A}$. This wavelength shift is broadly consistent with the temperature contrast between our blackbody component, with $T\sim10^4\ K$, and the lower effective temperature of $T\sim5000\ K$ expected for an optically thick photosphere surrounding the SMBH in LRDs. These properties suggest that this source might be caught in a transient evolutionary phase in which the dense gas envelope characteristic of LRD has begun to fragment, allowing us to witness the emergence of a quasar from an LRD-like state.