Dust-free quasars in the early Universe

Abstract: The most distant quasars known, at redshifts z=6, generally have properties indistinguishable from those of lower-redshift quasars in the rest-frame ultraviolet/optical and X-ray bands. This puzzling result suggests that these distant quasars are evolved objects even though the Universe was only seven per cent of its current age at these redshifts. Recently one z=6 quasar was shown not to have any detectable emission from hot dust, but it was unclear whether that indicated different hot-dust properties at high redshift or if it is simply an outlier. Here we report the discovery of a second quasar without hot-dust emission in a sample of 21 z=6 quasars. Such apparently hot-dust-free quasars have no counterparts at low redshift. Moreover, we demonstrate that the hot-dust abundance in the 21 quasars builds up in tandem with the growth of the central black hole, whereas at low redshift it is almost independent of the black hole mass. Thus z=6 quasars are indeed at an early evolutionary stage, with rapid mass accretion and dust formation. The two hot-dust-free quasars are likely to be first-generation quasars born in dust-free environments and are too young to have formed a detectable amount of hot dust around them.

Dust-Free Quasars at High Redshifts

The paper, "Dust-free quasars in the early Universe," presents a compelling analysis of the characteristics of high-redshift quasars, specifically those around z ≈ 6. This work contributes significantly to the understanding of quasars' evolutionary stages and the formation of dust in the early universe. Through the utilization of the Spitzer Space Telescope, a sample of 21 quasars spanning a luminosity range has been meticulously analyzed to uncover variations in infrared radiation pertinent to dust emission.

Key Findings

Ultimately, the paper reveals that most z ≈ 6 quasars display emission properties similar to those at lower redshifts in ultraviolet/optical and X-ray bands, suggesting mature accretion disks despite the youth of the universe at these redshifts. However, the discovery of two quasars, J0005−0006, and J0303−0019, with no detectable hot-dust emission is intriguing. Their absence of NIR excess positions them as potential first-generation quasars in dust-free environments, set apart by narrow emission lines and diminished black hole mass (~2–3 × 10⁸ M⊙), with rapid accretion unresolved by dust formation.

Implications for Quasar Evolution

This research indicates a significant correlation between hot-dust abundance and black hole mass at high redshifts, contrasting starkly with observations at lower redshifts where dust abundance is largely independent of black hole mass. Such findings imply an early evolutionary stage for these z ≈ 6 quasars, characterized by rapid mass accretion concurrent with dust production. Furthermore, the absence of hot dust challenges existing unification models that predict pervasive infrared emission from dust surrounding the black hole accretion disk.

Speculative Developments

Given that these quasars exist less than one billion years after the Big Bang, traditional sources of dust such as AGB stars are insufficient to explain their properties. Alternative dust production mechanisms such as quasar-induced outflows and supernovae could have played pivotal roles. This not only underscores substantial variance in dust formation processes but also invigorates further research into high-redshift environments, potentially altering predictions concerning AGN accretion and dust evolution.

Future Directions

The paper propels future investigations into the nascent stages of quasar development, revealing the necessity for more comprehensive infrared observations which could elucidate the complexities of dust generation and accretion processes at early cosmic epochs. Expansion of such studies will enhance comprehension of the primordial conditions impacting the growth of black holes and the influence of newly formed dust in shaping active galactic nuclei.