Physical properties of 15 quasars at $z\gtrsim 6.5$ (1710.01251v1)

Abstract: Quasars are galaxies hosting accreting supermassive black holes; due to their brightness, they are unique probes of the early universe. To date, only few quasars have been reported at $z > 6.5$ ($<$800 Myr after the Big Bang). In this work, we present six additional $z \gtrsim 6.5$ quasars discovered using the Pan-STARRS1 survey. We use a sample of 15 $z \gtrsim 6.5$ quasars to perform a homogeneous and comprehensive analysis of this highest-redshift quasar population. We report four main results: (1) the majority of $z\gtrsim$6.5 quasars show large blueshifts of the broad CIV 1549\AA$\,$emission line compared to the systemic redshift of the quasars, with a median value $\sim$3$\times$ higher than a quasar sample at $z\sim$1; (2) we estimate the quasars' black hole masses (M$\rm_{BH}\sim$0.3$-$5 $\times$ 10$^{9}$ M${\odot}$) via modeling of the MgII 2798\AA$\,$emission line and rest-frame UV continuum; we find that quasars at high redshift accrete their material (with $\langle (L{\mathrm{bol}}/L_{\mathrm{Edd}}) \rangle = 0.39$) at a rate comparable to a luminosity-matched sample at lower$-$redshift, albeit with significant scatter ($0.4$ dex); (3) we recover no evolution of the FeII/MgII abundance ratio with cosmic time; (4) we derive near zone sizes; together with measurements for $z\sim6$ quasars from recent work, we confirm a shallow evolution of the decreasing quasar near zone sizes with redshift. Finally, we present new millimeter observations of the [CII] 158 $\mu$m emission line and underlying dust continuum from NOEMA for four quasars, and provide new accurate redshifts and [CII]/infrared luminosities estimates. The analysis presented here shows the large range of properties of the most distant quasars.

Physical Properties of 15 Quasars at $z\gtrsim 6.5$

The paper of quasars at high redshifts offers a valuable glimpse into the early universe, aiding in the understanding of the formation and growth of supermassive black holes (SMBHs) and the evolution of galaxies. This paper focuses on investigating fifteen quasars at redshifts $z \gtrsim 6.5$, an era less than 800 million years after the Big Bang. This sample includes recent discoveries from the Pan-STARRS1 survey, supplementing earlier detections by other surveys, to provide a cohesive analysis of these distant cosmological beacons.

Key Results and Analysis

1. C\,{\scriptsize IV} Blueshifts: The paper reveals that most quasars in this high-redshift sample exhibit substantial blueshifts in their C\,{\scriptsize IV} emission lines compared to their systemic redshifts, with a median value approximately three times that of quasars at $z \sim 1$. This suggests potent winds or outflows in these early quasars' broad line regions (BLRs), implying significant dynamical processes at work possibly driven by radiation pressure or accretion disk winds.
2. Black Hole Masses and Accretion Rates: The estimated black hole masses range between $3\times10^8$ and $5\times10^9\,M_{\odot}$. These quasars accrete at rates similar to a luminosity-matched sample at lower redshifts, with an average Eddington ratio of 0.39. This indicates that their growth mechanisms are comparable to lower-redshift systems, despite the nascent universe conditions.
3. Metallicity Indicators: The Fe\,{\scriptsize II}/Mg\,{\scriptsize II} abundance ratio shows no significant evolution with redshift, consistent with prior studies suggesting rapid metal enrichment in quasar environments even at these early times. This non-evolution points toward rapid star formation and subsequent supernova-driven enrichment in these host galaxies shortly after the Big Bang.
4. Near Zone Sizes: The sizes of the ionized near zones surrounding these quasars show a mild evolution with redshift, confirming a gradual decrease in size as one moves to higher redshifts. This trend is consistent with an IGM becoming increasingly neutral, albeit inconsistently with robust predictions of reionization models. The paper supports a scenario where quasar properties are a more significant determinant for near zone sizes at such high redshifts.
5. Millimeter Observations: New [CII] 158 $\mu$m line observations for select quasars reveal a shallow evolution in the infrared luminosities, hinting at substantial reservoirs of cool gas and dust in the host galaxies. These findings align with a picture of mature, chemically evolved quasar host galaxies dominating at $z \sim 6.5$.

Implications and Future Directions

The findings challenge theories of seed black hole formation, especially under standard cosmological conditions, regarding the requisite growth timescales and accretion efficiencies necessary for the SMBHs observed at these redshifts. These quasars serve as luminous laboratories, probing the epoch of reionization, early galaxy dynamics, and feedback processes in the universe's first billion years.

Future work will benefit from augmented datasets, particularly from facilities like ALMA and the upcoming James Webb Space Telescope (JWST), to probe even higher redshifts and incorporate more substantial samples. These insights will enhance understanding of the evolutionary trajectory of quasars and their host galaxies, further clarifying the role of these luminous and massive entities in the cosmos's evolution.