The Infrared Medium-Deep Survey. V. A New Selection Strategy for Quasars at z > 5 based on Medium-Band Observation with SQUEAN

TL;DR

This paper introduces a new medium-band filter technique using SQUEAN to improve the selection and redshift determination of quasars at z > 5, filling a previous redshift gap and enhancing identification accuracy.

Contribution

The study develops and demonstrates a medium-band filter method that outperforms broad-band techniques in selecting high-redshift quasars and accurately measuring their redshifts.

Findings

Medium-band filters better distinguish quasars from brown dwarfs.

Redshift accuracy of quasars can be improved to Δz/(1+z) = 0.002–0.026.

Technique extends potential to z ~ 7, probing re-ionization epoch.

Abstract

Multiple color selection techniques have been successful in identifying quasars from wide-field broad-band imaging survey data. Among the quasars that have been discovered so far, however, there is a redshift gap at $5 \lesssim {\rm z} \lesssim 5.7$ due to the limitations of filter sets in previous studies. In this work, we present a new selection technique of high redshift quasars using a sequence of medium-band filters: nine filters with central wavelengths from 625 to 1025 nm and bandwidths of 50 nm. Photometry with these medium-bands traces the spectral energy distribution (SED) of a source, similar to spectroscopy with resolution R $\sim$ 15. By conducting medium-band observations of high redshift quasars at 4.7 $\leq$ z $\leq$ 6.0 and brown dwarfs (the main contaminants in high redshift quasar selection) using the SED camera for QUasars in EArly uNiverse (SQUEAN) on the 2.1-m telescope at the McDonald Observatory, we show that these medium-band filters are superior to multi-color broad-band color section in separating high redshift quasars from brown dwarfs. In addition, we show that redshifts of high redshift quasars can be determined to an accuracy of $\Delta{\rm z}/(1+{\rm z}) = 0.002$ -- $0.026$. The selection technique can be extended to z $\sim$ 7, suggesting that the medium-band observation can be powerful in identifying quasars even at the re-ionization epoch.