# Discovery of 16 New z ~ 5.5 Quasars : Filling in the Redshift Gap of Quasar Color Selection

## TL;DR

This paper introduces a new multi-wavelength selection method to identify quasars at redshift around 5.5, filling a gap caused by color similarities with stars, and reports the discovery of 17 such quasars in a large survey area.

## Contribution

Developed a novel selection technique combining optical, near-IR, and mid-IR data to find $z\,\sim\,5.5$ quasars, and discovered 17 new quasars in this redshift range.

## Key findings

- Discovered 17 new quasars at $z\sim 5.5$ in a 4800 sq. degree area.
- Successfully applied the method to UKIRT Hemisphere Survey data.
- First UHS-selected $z\sim 5.5$ quasar discovered.

## Abstract

We present initial results from the first systematic survey of luminous $z\sim 5.5$ quasars. Quasars at $z \sim$ 5.5, the post-reionization epoch, are crucial tools to explore the evolution of intergalactic medium, quasar evolution and the early super-massive black hole growth. However, it has been very challenging to select quasars at redshifts 5.3 $\le z \le$ 5.7 using conventional color selections, due to their similar optical colors to late-type stars, especially M dwarfs, resulting in a glaring redshift gap in quasar redshift distributions. We develop a new selection technique for $z \sim$ 5.5 quasars based on optical, near-IR and mid-IR photometric data from Sloan Digital Sky Survey (SDSS), UKIRT InfraRed Deep Sky Surveys - Large Area Survey (ULAS), VISTA Hemisphere Survey (VHS) and Wide field Infrared Survey Explorer (WISE). From our pilot observations in SDSS-ULAS/VHS area, we have discovered 15 new quasars at 5.3 $\le z \le$ 5.7 and 6 new lower redshift quasars, with SDSS z band magnitude brighter than 20.5. Including other two $z \sim$ 5.5 quasars already published in our previous work, we now construct an uniform quasar sample at 5.3 $\le z \le$ 5.7 with 17 quasars in a $\sim$ 4800 square degree survey area. For further application in a larger survey area, we apply our selection pipeline to do a test selection by using the new wide field J band photometric data from a preliminary version of the UKIRT Hemisphere Survey (UHS). We successfully discover the first UHS selected $z \sim$ 5.5 quasar.