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

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$α$ 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4$ quasars, including 11 unpublished archival spectra.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$α$ absorption breaks and H$α$ 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.

Figures (15)

• Figure 1: SPHEREx spectrophotometry of the luminous ($M_{1450} = -28.9$) previously-known quasar SDSS J0306$+$1853 Wang15 at $z=5.37$Brazzini25 is shown by the colored points, and the blue curve shows the Selsing16 quasar template for comparison. The labels D1 through D6 indicate the six detectors of SPHEREx probing consecutive spectrophotometric bands. The most prominent features enabling the quasar's idenfication in SPHEREx data are the broad and strong H$\alpha$ line and the Ly$\alpha$ continuum break.
• Figure 2: Illustration of our fiducial target selection in color space. The initial parent sample selected from Gaia photometry is shown by the cloud of small brown points. Sources lying above and to the right of the dashed line were chosen for extraction of SPHEREx photometry. The black points show the newly discovered quasars (filled circles) and known quasars recovered by our search (open circles).
• Figure 3: Mollweide projection map of new quasars (solid circles) and recovered known quasars (open circles) identified with SPHEREx spectrophotometry. The exclusion region above and below the Galactic plane ($|b|\leq8^\circ$) is shown with a grey band.
• Figure 4: Distribution of the $G_{\rm RP}$ (left) and $W1$ (middle) magnitudes for new (filled circles) and previously known (open circles) $z>4$ quasars in our search, as well as the $M_{1450}$ values derived from template fitting (right). Black points were found by the fiducial selection, while the green and blue points were found by the $G_{\rm RP}$ faint and $G_{\rm BP}$ flux selections, respectively. Star-shaped points denote quasars with optical confirmation spectra in the ESO archive or in this work (Section \ref{['sec:fup']}).
• Figure 5: Example spectrophotometry of previously known quasars recovered by our SPHEREx quasar search. The small transparent rainbow points show the raw measurements from the individual SPHEREx detectors (e.g. Figure \ref{['fig:j0306']}), while the black curve is re-binned onto a common wavelength grid. The wavelengths of common emission lines are shown by the vertical dashed lines.