Euclid spectroscopy of quasars. 1.

We obtained an empirical spectroscopic depth of JE 21.5 and HE 21.3 at the sensitivity of the Wide Field Survey, beyond which the number of securely identified quasars declines sharply. Accordingly, the sample presented in this paper comprises spectroscopically confirmed quasars brighter than these limits. We analysed morphological parameters from the Visible Camera (VIS) using Sersic and model-independent (CAS) metrics, and a deep-learning point spread function fraction to track nuclear dominance.

The VIS morphologies show a clear redshift dependence: at low redshift (z<0.5), obvious host structures are common and a single Sersic model fits about half of the sources; at intermediate redshift (0.5<z<2), the nuclear component dominates, with 90% of the Sersic fits saturating at the upper index limit. In this intermediate redshift regime, fPSF is available, and we use it as a more reliable compactness measure than the single-Sersic and CAS parameters to quantify nuclear versus host emission.

We also explore the novel Euclid NIR colour space and discuss the role of these quasars in refining active galactic nucleus selection techniques for future Euclid data releases.