A luminous quasar at a redshift of z = 7.085

Abstract: The intergalactic medium was not completely reionized until approximately a billion years after the Big Bang, as revealed by observations of quasars with redshifts of less than 6.5. It has been difficult to probe to higher redshifts, however, because quasars have historically been identified in optical surveys, which are insensitive to sources at redshifts exceeding 6.5. Here we report observations of a quasar (ULAS J112001.48+064124.3) at a redshift of 7.085, which is 0.77 billion years after the Big Bang. ULAS J1120+0461 had a luminosity of 6.3x10^13 L_Sun and hosted a black hole with a mass of 2x10^9 M_Sun (where L_Sun and M_Sun are the luminosity and mass of the Sun). The measured radius of the ionized near zone around ULAS J1120+0641 is 1.9 megaparsecs, a factor of three smaller than typical for quasars at redshifts between 6.0 and 6.4. The near zone transmission profile is consistent with a Ly alpha damping wing, suggesting that the neutral fraction of the intergalactic medium in front of ULAS J1120+0641 exceeded 0.1.

• The paper reveals that ULAS J1120+0641 hosts a massive black hole (~2×10^9 M☉) with a luminosity of 6.3×10^13 L☉ just 0.77 billion years after the Big Bang.
• The paper finds an ionized near zone measuring 1.9 Mpc, indicating unique intergalactic medium conditions at this extreme redshift.
• The paper provides evidence of a Lyα damping wing consistent with an IGM neutral fraction above 0.1, offering new insights into the reionization epoch.

Observations of a Luminous Quasar at Redshift z = 7.085

The paper, authored by D. J. Mortlock et al., reports the discovery and preliminary analysis of a quasar at an extraordinary redshift of $z = 7.085$, which corresponds to a time just 0.77 billion years after the Big Bang. The quasar, designated ULAS J112001.48+064124.3 (ULAS J1120+0641), was identified using data from the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey and followed up with spectroscopic observations from a suite of instruments.

Key Findings

1. Luminosity and Black Hole Mass: ULAS J1120+0641 was found to have a luminosity of $6.3 \times 10^{13} L_{\sun}$ and hosts a central black hole estimated to have a mass of $2 \times 10^9 M_{\sun}$. Such values are notable for being both luminous and massive within the current understanding of early universe cosmology.
2. Ionized Near Zone: The quasar's ionized near zone radius was measured to be 1.9 Mpc, significantly smaller than what is typical for quasars with redshifts between 6.0 and 6.4. This has implications for the study of the intergalactic medium (IGM) at high redshifts, suggesting different environmental conditions than those observed at slightly lower redshifts.
3. Neutral Fraction of IGM: The transmission profile around the quasar is consistent with a Ly$\alpha$ damping wing, which indicates that the neutral hydrogen fraction in the surrounding IGM exceeded 0.1, pointing towards substantial residual neutrality in the early universe.

Theoretical and Observational Implications

• Reionization Epoch: The presence of this quasar at such a high redshift provides a rare glimpse into the era of reionization and contributes valuable data toward modeling the history and characteristics of this epoch. The effective optical depth measurements and the implication of a non-negligible neutral fraction in the IGM at $z > 6.5$ provide evidence against a fully reionized universe at this time.
• Black Hole Formation and Growth: The discovery of a massive black hole so shortly after the Big Bang challenges existing theories of black hole seed formation and growth mechanisms. It necessitates a re-evaluation of models that describe the early growth rates and accretion processes, considering factors such as Eddington-limited accretion with an efficiency rate and compounded growth through mergers.

Future Directions

This work underscores the necessity for more observations of high-redshift quasars to advance understanding of the conditions and processes occurring in the early universe. It also suggests that additional spectroscopic observations of ULAS J1120+0641 could refine the specific contributions of Ly$\alpha$ damping wings and other absorption features to the assessment of IGM neutrality. Moreover, the discovery suggests a potentially larger population of similarly bright, high-redshift quasars that remain undetected, necessitating broader and deeper observational campaigns.

The aforementioned findings indicate substantial advances in our understanding of the early universe, challenging prevailing notions about the reionization epoch and cosmic structure growth. These observations serve as a foundational step for further investigations into the intricate interplay between early black hole growth, quasar formation, and IGM evolution.