A UV-to-Near-infrared QSO Composite Spectrum from the SPHEREx All-Sky Survey

We present a composite spectrum of about 61,000 type 1 SDSS QSOs (median z about 1.26), constructed using SPHEREx spectrophotometric data and covering a rest-frame wavelength range of 0.14-4.5 mu m. The SPHEREx mission surveys the entire sky in 102 near-infrared spectral channels spanning 0.75-5.0 mu m with a spectral resolution of R about 35-130, providing a unique dataset for building a statistically robust QSO composite. We find that the UV and optical continuum of the resulting composite can be described by a power law, f nu proportional to nu alpha nu, with a best-fit spectral index of alpha nu = -0.10, while the near-infrared continuum is well-fit with a spectral index of-1.46. The power-law indices in both the optical and near-infrared regimes strongly depend on properties of QSOs, such that more luminous QSOs tend to exhibit flatter UV/optical and steeper near-infrared continua compared to those of less luminous ones. The IR-to-optical flux ratio decreases with increasing AGN luminosity, consistent with the predictions of the receding torus model. The line ratios of broad emission lines, including H alpha, Pa beta, and Pa alpha, are in good agreement with predictions from Case B recombination, suggesting that internal extinction is almost negligible. The equivalent widths of these emission lines are proportional to AGN luminosity, contrary to the trend expected from the Baldwin effect. Finally, the shape of the composite is sensitive to host-galaxy contamination, which must be considered when utilizing this QSO composite for subsequent scientific applications.