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

Abstract

Photometric selection of luminous high-redshift ($z\gtrsim 4$) 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 ($|b|\ge 8^{\circ }$). Primarily from the detection of their strong broad H$\hat{I}\pm$ 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4$ quasars, including 11 unpublished archival spectra.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$\hat{I}\pm$ absorption breaks and H$\hat{I}\pm$ lines of luminous quasars are already detectable at redshifts $5.7\lesssim z\lesssim 6.5$ after the completion of only the first of four all-sky surveys to be performed by SPHEREx during its planned two-year mission.

12+4 pages, 11+2 figures, submitted to A&A