Three Hundred Quasars from the Couch: A first look at high-redshift quasar discovery with SPHEREx

TL;DR

This study demonstrates that SPHEREx survey data can effectively identify high-redshift quasars without ground-based follow-up, discovering 87 new quasars at z=4-5.7 and validating the method with 100% success in spectroscopic confirmation.

Contribution

The paper introduces a novel, ground-based follow-up-free method for high-redshift quasar identification using SPHEREx data, improving efficiency and discovery rate.

Findings

Discovered 87 new quasars at z=4-5.7 using SPHEREx data.

Achieved 100% confirmation rate in spectroscopic follow-up of selected quasars.

Detected key spectral features at redshifts up to 6.5 with early SPHEREx data.

Abstract

Photometric selection of luminous high-redshift ($z\gtrsim4$) quasars is plagued by contamination from numerous low-mass Galactic stars, reddened lower-redshift quasars, as well as compact luminous red galaxies. Confirmation of these rare objects thus requires extensive spectroscopic campaigns on 4 and 8-meter-class telescopes with relatively low success rates. Here we demonstrate the utility of SPHEREx spectrophotometric survey data for quasar confirmation with no ground-based follow-up required, "from the couch," applied to candidates from a purposefully simplistic photometric and astrometric Gaia+WISE selection down to low Galactic latitudes ($|b|\geq8^\circ$). Primarily from the detection of their strong broad H$\alpha$ emission lines, we discover 87 new luminous $4.0 < z < 5.7$ quasars with median $M_\text{1450} = -27.5$, including 19 quasars at $z>5$, and recover 219 previously published quasars at $z>4$. We validate our SPHEREx selection with a 100% confirmation rate in ground-based spectroscopic follow-up of 29 of our new $z>4$ quasars, including 11 unpublished archival spectra. We also discover 203 additional lower-redshift quasars at $0.3 < z < 4$, consisting primarily of relatively rare highly-reddened and strong broad-absorption-line objects that are likely missed by traditional quasar surveys. Finally, we show that the Ly$\alpha$ absorption breaks and H$\alpha$ lines of luminous quasars are already detectable at redshifts $5.7\lesssim z\lesssim6.5$ after the completion of only the first of four all-sky surveys to be performed by SPHEREx during its planned two-year mission.