A dusty compact object bridging galaxies and quasars at cosmic dawn (2204.06393v1)

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.

• The paper identifies GNz7q, a dusty UV compact object at redshift 7.19 that signifies a transitional phase between galaxies and quasars.
• It demonstrates extreme starburst activity with a formation rate of about 1,600 solar masses per year and a high infrared luminosity.
• The study highlights GNz7q’s unusual X-ray faintness, suggesting either UV-dominated star formation or a Compton-thick, super-Eddington accretion scenario.

A Dusty Compact Object Bridging Galaxies and Quasars at Cosmic Dawn

The paper by Fujimoto et al. presents a detailed investigation of a distinctive ultraviolet (UV) compact object, GNz7q, which is associated with a dust-enshrouded starburst at a high redshift of approximately $z=7.1899$. This discovery is significant as it provides insight into the transitional phases of quasars, especially during the cosmic dawn period, roughly 700 million years after the Big Bang.

Key Observations and Results

• Identification and Redshift: GNz7q was identified through the re-analysis of archival Hubble Space Telescope (HST) imaging and spectroscopy in the GOODS-North field. The object was spectroscopically confirmed at a redshift of $z=7.1899 \pm 0.0005$, characterized by a distinct Lyman break at around 1.0 $\mu$m.
• Luminosity and UV Characteristics: The object displays a rest-frame 1450 \AA{} luminosity with an absolute magnitude of $M_{1450}=-23.2$ mag, placing its brightness between typical quasars and galaxies known at comparable redshifts. Notably, GNz7q has the reddest UV continuum slope ($\alpha_{\lambda} = 0.1 \pm 0.3$) among such populations at these redshifts.
• X-ray Faintness: Despite expectations based on its UV properties, GNz7q is unusually faint in X-rays, with an upper limit on the 2–10 keV X-ray luminosity, indicating a potential case for either a UV-biased view of star formation or a Compton-thick, super-Eddington accretion scenario.
• Star Formation and Infrared Emission: The host galaxy of GNz7q forms stars at a prodigious rate of approximately 1,600 solar masses per year, confined within a compact region with an effective radius of about 480 parsecs. Its infrared (IR) luminosity, $L_{\rm\,IR} = (1.2\pm0.6)\times10^{13}$ solar luminosities, suggests a highly active period of star formation possibly nearing the Eddington limit for star-forming galaxies.
• Morphology and Associated Structures: The NOEMA observations reveal a spatially resolved $[CII]$ line with an effective radius of $r_e \approx 1.4$ kpc, although the 1~mm continuum remains unresolved with an upper limit $r_e \leq 0.5$ kpc. The detection of a nearby sub-mm source, potentially a dusty companion galaxy, underscores an environment hosting complex interactions and possibly mergers.

Implications and Theoretical Context

The unique characteristics of GNz7q, especially its transitionary nature and X-ray faintness, position it as a candidate for understanding the evolutionary stages of early quasars. The findings align with predictions from cosmological simulations indicating that dusty, starbursting environments could give rise to obscured, super-Eddington accreting black holes—an antecedent to optically bright quasars.

Future Directions

The paper opens new avenues for exploring the early universe's quasar population. Systematic deep surveys, potentially utilizing upcoming tools such as the James Webb Space Telescope, could reveal more objects like GNz7q, providing a clearer picture of quasar formation and evolution mechanics at such early epochs. Moreover, follow-up studies could focus on the interplay between massive starbursts and emergent quasar activity, examining how these processes contribute to cosmic reionization and large-scale structure formation.