Infrared emission from $z \sim 6.5$ quasar host galaxies: a direct estimate of dust physical properties

TL;DR

This work targets the poorly constrained FIR emission of quasar host galaxies at $z\sim6.5$ by combining new ALMA Band 8 data with archival measurements across multiple bands to directly infer dust properties without assuming a fixed $T_d$. Using a single-temperature, optically depth-aware modified blackbody model fitted via MCMC, the authors derive $T_d$, $M_d$, $\beta$, $L_{IR}$, and SFR for ten QSOs, finding a mean $T_d$ of $48\pm3$ K, $\log(M_d/M_\odot)\approx8.3$, $\beta\approx2.1$, $L_{IR}\approx7.9\times10^{12}\,L_\odot$, and SFRs near $10^3\,M_\odot\,\text{yr}^{-1}$; the analysis shows the FIR emission is typically optically thick and that $T_d$ is comparable to IR-luminous galaxies across cosmic time. The study also links higher $T_d$ to elevated star formation efficiency and to the [CII] deficit, and finds no strong evidence that the central AGN significantly biases the derived infrared properties, though higher-resolution Band 9/10 data are needed to fully assess AGN-related heating. Overall, the results imply that massive, dust-rich QSO hosts at $z>6$ are in intense, compact star-forming states with dust temperatures similar to local and high-redshift IR-luminous systems, underscoring the role of SF activity in early galaxy evolution and the importance of high-frequency FIR observations for dust diagnostics.

Abstract

Quasars at the dawn of Cosmic Time ($z>6$) are fundamental probes to investigate the early co-evolution of supermassive black holes and their host galaxy. Nevertheless, their infrared spectral energy distribution remains at the present time poorly constrained, due to the limited photometric coverage probing the far-infrared wavelength range where the dust modified black-body is expected to peak ($\sim80$ $\mathrm{μm}$). Here we present a study of the high-frequency dust emission via a dedicated ALMA Band 8 ($\sim$400 GHz) campaign targeting 11 quasar host galaxies at $6<z<7$. Combined with archival observations in other ALMA bands, this program enables a detailed characterization of their infrared emission, allowing for the derivation of dust masses ($M_{d}$), dust emissivity indexes ($β$), dust temperatures ($T_{d}$), infrared luminosities ($L_{IR}$), and associated star formation rates (SFRs). Our analysis confirms that dust temperature is on average higher in this sample (34-65 K) if compared to local main-sequence galaxies' values, and that this finding can be linked to the increased star formation efficiency we derive in our work, as also suggested by the [CII]$_{158μm}$ deficit. Most remarkably, we note that the average value of $T_d$ of this sample doesn't differ from the one that is observed in luminous, ultra-luminous and hyper-luminous infrared galaxies at different redshifts that show no signs of hosting a quasar. Finally, our findings suggest that the presence of a bright AGN does not significantly bias the derived infrared properties, although further high-frequency, high-spatial resolution observations might reveal more subtle impacts on sub-kiloparsec scales.

Figures (13)

• Figure 1: 407 GHz dust continuum map of the QSO J0305-3150 (contours at 3, 6 and 9 $\sigma$).
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• Figure 2: Top panel: fitted dust SED of the QSO J0305-3150. Bottom panel: corner plot of the posterior probability distribution of the fit parameters.