The Pan-STARRS1 distant z>5.6 quasar survey: more than 100 quasars within the first Gyr of the universe (1608.03279v1)

Abstract: Luminous quasars at z>5.6 can be studied in detail with the current generation of telescopes and provide us with unique information on the first gigayear of the universe. Thus far these studies have been statistically limited by the number of quasars known at these redshifts. Such quasars are rare and therefore wide-field surveys are required to identify them and multiwavelength data are needed to separate them efficiently from their main contaminants, the far more numerous cool dwarfs. In this paper, we update and extend the selection for z~6 quasars presented in Banados et al. (2014) using the Pan-STARRS1 (PS1) survey. We present the PS1 distant quasar sample, which currently consists of 124 quasars in the redshift range 5.6<z\<6.7 that satisfy our selection criteria. Seventy-seven of these quasars have been discovered with PS1, and 63 of them are newly identified in this paper. We present composite spectra of the PS1 distant quasar sample. This sample spans a factor of ~20 in luminosity and shows a variety of emission line properties. The number of quasars at z\>5.6 presented in this work almost double the quasars previously known at these redshifts, marking a transition phase from studies of individual sources to statistical studies of the high-redshift quasar population, which was impossible with earlier, smaller samples.

• The paper introduces a refined Pan-STARRS1 technique to identify 124 high-redshift quasars (z>5.6), including 63 new detections.
• It employs multi-wavelength selection criteria to differentiate quasars from cool dwarf star contaminants with improved efficiency.
• Composite spectral analysis reveals diverse Lyα emissions and a higher fraction of weak-line quasars, informing early universe models.

An Overview of the Pan-STARRS1 Distant Quasar Survey

The paper entitled "The Pan-STARRS1 Distant $z>5.6$ Quasar Survey" by Ba~{n}ados et al. presents a comprehensive analysis of quasars located at redshifts greater than 5.6. These objects are critical for probing the first gigayear of the universe, offering insights into the early stages of galaxy formation and the reionization era. While quasars at such high redshifts provide a unique view into the young universe, they are exceedingly rare, necessitating extensive surveys for their discovery.

Survey Methodology and Sample Selection

The primary tool in this exploration is the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1), which undertakes wide-field imaging to detect quasars in the specified redshift range. The paper builds on prior work by implementing updated selection criteria leveraging multi-wavelength datasets to improve the efficiency of identifying high-redshift quasars from cool dwarf star contaminants—significant due to the latter's prevalence.

Employing these refined criteria, the authors catalog 124 quasars ranging from $z \approx 5.6$ to $z \approx 6.7$, representing a substantial increase in known high-redshift quasars. This includes 63 new identifications from this paper, thereby doubling the size of the sample available for understanding the early universe's quasar population statistically.

Composite Spectra and Emission Line Analysis

A notable feature of the research is the construction of composite spectra from the sample, allowing for the assessment of emission line properties across a broad luminosity range. This spectral diversity is highlighted by variations in the Ly$\alpha$ line strength, indicating a mixture of quasars with both typical and weak emission line signatures. The paper uses these spectral composites to compare with lower-redshift quasar spectra, noting that while many properties remain similar, increased intergalactic medium (IGM) absorption in high-redshift events yields notable differences.

Implications for High-Redshift Quasar Research

The paper further explores the intriguing prevalence of weak-line quasars in this cosmological epoch. The analysis reveals a higher fraction of these objects than observed at lower redshifts, prompting discussions about possible evolutionary models that could account for a scant or still-forming broad-line region.

Future Directions and Implications

This research initiates a transition from individual distant quasar studies to statistical studies, significantly impacting our theoretical understanding of early black hole and galaxy formation. The dataset's expansion and the corresponding composite spectral analysis afford a deeper understanding of the mechanisms at play during the universe's reionization phase.

Future research facilitated by larger samples and improved detection methods could unlock detailed insights into the early universe's conditions. Additionally, upcoming instruments and surveys will likely drive further strides in observing fainter and more distant quasars, refining our understanding of high-redshift phenomena.

In conclusion, the Pan-STARRS1 survey marks a pivotal step in characterizing early-universe quasars, laying the groundwork for future observational strategies and theoretical frameworks to explore the universe's nascent stages with unprecedented clarity.