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

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 $\mathrm{FWHM}\gtrsim3400\ km/s$, accompanied by a narrow absorption feature. The spectrum reveals a characteristic $Λ$-shape over the rest-frame wavelength ranging $\sim1500-3500\ Å$. 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 $1400\ Å$, reminiscent of the spectral feature reported for recently discussed LRDs whose V-shapes occur around $3000-4000\ Å$. 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.

Figures (8)

• Figure 1: Overview of observations for the blackbody radio-loud quasar BBQSORS. Top: Optical images collected from Subaru HSC-SSP, IR images collected from Spitzer, and VLASS radio images at 3 GHz observed at Epoch 2 and 3. The false-color image is rendered using HSC $g+r+z$, overlaid with the white contours representing the VLASS Epoch 2 imaging. The blue circle represents the PFS fiber with a diameter of $1\hbox{$.\!\!^{\prime\prime}$}02$. Middle: Subaru/PFS spectrum of BBQSORS and fitted using $\mathtt{S^{3}Fit}$. Bottom: Zoom-in of the Subaru/PFS spectrum C iii]$\lambda1909$ and Mg ii$\lambda2800$, as well as best-fits to the underlying continuum and emission/absorption, without incorporating the Fe ii template. The spectra are Gaussian-smoothed by every 9 data points. For absorptions of Al iii$\lambda\lambda1855,1863$ and Mg ii$\lambda\lambda2796,2803$, we show their centers and FWHMs in the corresponding panels.
• Figure 2: X-ray detection of BBQSORS with XMM-Newton. The left panel presents the EPIC/pn image in the 0.4–7.2 keV (rest-frame $\sim$1–20 keV) band, detected at a significance of 3.42$\sigma$. The green circle indicates the source region with a radius of 20$"$. The right panel shows the EPIC/pn spectrum folded with the energy response (blue squares) and the best-fitting model (black curve), consisting of source and background components. Residuals between the data and the model are shown in the bottom panel.
• Figure 3: SED fitting for UV-to-MIR photometry with three blackbody components representing the SMBH envelope (blue), AGN dust torus (brown), and galactic dust (golden), plus a UV PL with fixed ${\beta}_\mathrm{\lambda}-2$, assuming $A_\mathrm{V}=0.31$ derived from the PFS spectrum. The purple solid line represents an LRD-like MBB component with $T = 5000$ K and ${\beta}_\mathrm{MBB} = 0$, with its amplitude scaled according to $L \propto T^4$. The dust-extincted UV PL continuum intersects with the blackbody component originating from the accretion disk/LRD envelope, reproducing the LRD-like V-shape.
• Figure 4: Upper: Comparison of the spectrum for BBQSORS with the median spectra of typical blue quasars (cQSOs) and of red quasars (rQSOs) suffering moderate dust reddening fawcett2022_rqso, where all spectra are normalized at 4000 Å. We de-redden the PFS spectrum assuming a PL dust extinction curve with $A_\mathrm{V}=0.7$ and $A_\mathrm{V}=1.2$. Although the de-reddened PFS spectrum have similar spectral shapes with cQSOs and rQSOs from about 2200 Å to 4000 Å, there are large discrepancies at the blue end. Lower: Comparison of PFS, SDSS, and DESI spectra for BBQSORS, where all are normalized at 3000 Å.
• Figure 5: The evolution path of high-$z$ LRDs to low-$z$ BBQSORS.