A dusty compact object bridging galaxies and quasars at cosmic dawn

TL;DR

GNz7q at $z=7.1899\pm0.0005$ represents a UV-compact, dust-enshrouded system in a vigorously star-forming host, possibly capturing a transitional red quasar in the early growth of a SMBH. Multi-wavelength data reveal a luminous, compact UV core embedded in a massive, dusty ISM with $M_{gas}\sim2\times10^{10}M_\odot$ and $SFR\sim1600\,M_\odot\,{ m yr}^{-1}$, yet an extremely faint X-ray signature suggesting either a high Eddington ratio with a low-mass BH ($M_{BH}\sim10^{7}M_\odot$) or Compton-thick inner-disk obscuration consistent with a slim-disk accretion scenario. The source’s IR/UV properties, [C II] emission, and SED decomposition favor a quasar interpretation, positioning GNz7q as an antecedent to later unobscured quasars and aligning with simulated progenitors of high-$z$ quasars. Comparative analyses indicate GNz7q is among the most vigorously star-forming systems at $z>7$ and may imply a higher incidence of transitioning red quasars at cosmic dawn than previously recognized. JWST follow-up will be decisive for confirming broad Balmer lines and refining the SMBH growth pathway in the early universe.

Abstract

Understanding how super-massive black holes form and grow in the early Universe has become a major challenge since the discovery of luminous quasars only 700 million years after the Big Bang. Simulations indicate an evolutionary sequence of dust-reddened quasars emerging from heavily dust-obscured starbursts that then transition to unobscured luminous quasars by expelling gas and dust. Although the last phase has been identified out to a redshift of 7.6, a transitioning quasar has not been found at similar redshifts owing to their faintness at optical and near-infrared wavelengths. Here we report observations of an ultraviolet compact object, GNz7q, associated with a dust-enshrouded starburst at a redshift of z=7.1899+/-0.0005. The host galaxy is more luminous in dust emission than any other known object at this epoch, forming 1,600 solar masses of stars per year within a central radius of 480 parsec. A red point source in the far-ultraviolet is identified in deep, high-resolution imaging and slitless spectroscopy. GNz7q is extremely faint in X-rays, which indicates the emergence of a uniquely ultraviolet compact star-forming region or a Compton-thick super-Eddington black-hole accretion disk at the dusty starburst core. In the latter case, the observed properties are consistent with predictions from cosmological simulations and suggest that GNz7q is an antecedent to unobscured luminous quasars at later epochs.

Figures (15)

• Figure 1: $|$Hubble Space Telescope near-infrared images and spectrum of GNz7q. The spectrum and photometry show a strong Lyman break at $\lambda_\mathrm{obs}\sim1.0~\mu\mathrm{m}$. The top panels show the HST image cutouts ($5"\times5"$). The source is unresolved in all deep HST images up to the reddest filter available at $1.6~\mu\mathrm{m}$ (WFC3/IR F160W). In the bottom panel, the black squares and gray dots respectively show the broadband photometry and the slitless spectrum binned by a factor of 4 relative to the nominal pixel scale. The error bars denote 1$\sigma$ uncertainties. The labeled black bars indicate the expected wavelengths for the main emission lines based on the [C ii] 158-$\mu$m line redshift of GNz7q at $z=7.1899$. The blue curve represents a composite spectrum of SDSS optically luminous blue quasarsselsing2016 at $1<z<2$, while the red curve shows a red quasar at $z=3.11$ (SDSS spec-6839-56425-146)sdss_dr12 whose FUV spectrum resembles that of GNz7q. Both of the lower-$z$ quasar spectra are shifted to $z=7.1899$, normalized at 1.2 $\mu$m, and binned to the same spectral resolution as the GNz7q spectrum. The large open circles show the quasar templates integrated through the HST filter passbands. The bandpasses of the ACS/F850LP and WFC3/F105W filters shown at the bottom straddle the spectral break, explaining the faint detection in the former and the suppressed flux density relative to the continuum in the latter.
• Figure 2: $|$ The spectral energy distribution of GNz7q from optical to radio wavelengths. Photometry is shown for data from HST (0.8--1.6$\mu$m), Spitzer (3.6--24$\mu$m), Herschel (80--500$\mu$m), JCMT (450 and 850$\mu$m), NOEMA (1 and 3 mm) and VLA (3 and 20 cm) in the GOODS-North field (Extended Data Table 1). Triangles indicate 3$\sigma$ upper limits. The sum of the best-fit quasar/AGN (black solid) and galaxy (black dashed) templates is shown as a red curve. The radio detection at 20 cm is consistent with the enormous implied SFR of the host galaxy (see Methods). For comparison, we also show the SEDs of other source populations at similar redshifts: optically-luminous blue quasars at $z=7.54$ (J1342+0928banados2018; blue squares) and $z=7.08$ (J1120+0641mortlock2011; blue stars), and a dusty starburst at $z=6.90$ (SPT0311-58Wmarrone2018; orange circles). The blue curve is drawn with the quasar/AGN template normalized to J1120+6410's rest-frame UV emission. The orange curve is the best-fit SED for SPT0311-58W, taken from the literaturemarrone2018. The SED of GNz7q falls between these two categories of the dusty starburst and the blue quasar, representing a transient phase between them.
• Figure 3: $|$ The unique X-ray faintness of GNz7q. The 2 Ms deep Chandra dataxue2016 provide a stringent upper limit (red dashed line) for the X-ray luminosity, suggestive of a very high accretion rate ($\lambda_{\rm\,Edd}\gtrsim1$) onto a less massive black hole ($M_{\rm\,BH}\lesssim10^{8}\,M_{\odot}$). a. Optical to X-ray spectral index $\alpha_{\rm\,ox}$ as a function of Eddington ratio ($\lambda_{\rm\,Edd}$). The dust-corrected optical luminosity $L_{\rm 2,500}'$ is used in the upper limit estimate of $\alpha_{\rm\,ox}$ for GNz7q. SDSS quasar measurements (black squares) and the best-fit relation (black line) with its $1\sigma$ confidence level (gray shaded region) are taken from the literature (see Methods). The upper horizontal axis shows the equivalent black hole mass for GNz7q as a function of $\lambda_{\rm\,Edd}$ based on its AGN bolometric luminosity from the UV to mm SED fitting (see Methods). The red shaded region shows the $\lambda_{\rm\,Edd}$ regime of GNz7q extrapolated from the best-fit relation, where the shading becomes darker with increasing $\lambda_{\rm\,Edd}$. b. X-ray luminosity ($L_{\rm\,X}$) normalized by $L_{\rm\,IR}$. We show other populations for comparison: blue quasars at $z\sim6$ (blue squares), red quasars at $z\sim2$, and dusty starbursts at $z\sim0$ (orange squares) taken from the literature (see Methods). Gray circles are plotted for simulated galaxies (see Methods) with AGN bolometric luminosity of $L_{\rm\,bol}>10^{42}$ erg s$^{-1}$. The colour scale and the horizontal range of each red shaded region of $M_{\rm\,BH}$ corresponds to those of panel a.
• Figure 4: $|$ SFR and $M_{\rm\,BH}$ relations for progenitors of luminous quasars in a cosmological simulation. a. SFR and BH masses predicted by the semi-analytical model GQdvaliante2016 for selected $z = 7.2$ progenitors of a luminous quasar at $z=6.4$. Each system is represented by a circle colour-coded by the dark matter halo mass ($M_{\rm DM}$). The four black circles mark systems that have X-ray, optical, and host galaxy properties similar to GNz7q (See Methods). b. BH mass assembly history for the systems marked with the black circles in panel a. At $z = 7.2$, one of them has already grown to $4.0\times10^{7}\,M_{\odot}$ (red circle in panel a) and thereafter it continues to grow by gas accretion and mergers with other BHs to form a SMBH of $M_{\rm\,BH}=2.5\times10^{8}\,M_{\odot}$ at $z=6.4$ (red line). The other systems also have relatively low-mass BHs, down to $\sim10^{6.5-7.5}\,M_{\odot}$ at $z=7.2$ (grey lines), but these systems undergo mergers with galaxies hosting more massive BHs. As a result, these BHs are not the most massive progenitors of the final SMBH, which grows to $\sim10^{9.3-10}$$M_{\odot}$ by $z=6.4$ through gas accretion and mergers with other BH progenitors (grey dashed line). The black vertical line indicates the redshift of GNz7q.
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