Euclid Quick Data Release (Q1) -- Characteristics and limitations of the spectroscopic measurements

Abstract

The SPE processing function (PF) of the \Euclid pipeline is dedicated to the automatic analysis of one-dimensional spectra to determine redshifts, line fluxes, and spectral classifications. The first \Euclid Quick Data Release (Q1) delivers these measurements for all $H_\mathrm{E}<22.5$ objects identified in the photometric survey. In this paper, we present an overview of the SPE PF algorithm and assess its performance by comparing its results with high-quality spectroscopic redshifts from the Dark Energy Spectroscopic Instrument (DESI) survey in the Euclid Deep Field North. Our findings highlight remarkable accuracy in successful redshift measurements, with a bias of less than $3 \times 10^{-5}$ in $(z_{\rm SPE}-z_{\rm DESI})/(1+z_{\rm DESI})$ and a high precision of approximately $10^{-3}$. The majority of spectra have only a single spectral feature or none at all. To avoid spurious detections, where noise features are misinterpreted as lines or lines are misidentified, it is therefore essential to apply well-defined criteria on quantities such as the redshift probability or the \ha\ flux and signal-to-noise ratio. Using a well-tuned quality selection, we achieve an 89\% redshift success rate in the target redshift range for cosmology ($0.9<z<1.8$), which is well covered by DESI for $z<1.6$. Outside this range where the \ha\ line is observable, redshift measurements are less reliable, except for sources showing specific spectral features (e.g., two bright lines or strong continuum). Ongoing refinements along the entire chain of PFs are expected to enhance both the redshift measurements and the spectral classification, allowing us to define the large and reliable sample required for cosmological analyses. Overall, the Q1 SPE results are promising, demonstrating encouraging potential for cosmology.

Figures (9)

• Figure 1: Top: Map of the mean number of dithers in the EDF-N field. Bottom: Map of valid pixels.
• Figure 2: Typical spectrum (black line, uncertainties in green) of a galaxy from the EWS target sample, showing no continuum and a single emission line. From external spectroscopy, we know that this is at $z=1.2711$; however, as is discussed in the text, SPE interprets the line as [Oii] in this case and assigns an erroneous redshift.
• Figure 3: Comparison between SPE and DESI redshifts for a selection corresponding to a baseline cosmology sample of emitters with SPE probability larger than 0.99. This plot is discussed extensively in Sect. \ref{['sc:purity_cosmo']}.
• Figure 4: Comparison of SPE redshift measurements with those for all objects in common (including non-emitting galaxies and stars) from the DESI Early Data Release. Top: All Q1 objects with at least three observed dithers. This panel deliberately does not include any quality selection, so as to evidence the impact of the prior (Sect. \ref{['sc:Haprior']} and \ref{['sc:comp']}) and the limitations of spectroscopic data in the Q1 release beyond the cosmology selection of Fig. \ref{['fig:z_vs_z_cosmo']} (see Sect. \ref{['sc:z_failures']}). Bottom: Same as top panel, but now selecting only high-quality objects classified as galaxies (i.e. with a SPE redshift probability $> 0.99$ and line width strictly narrower than the prior limit of $680\,\kms$). The colour coding indicates the local density of points, with black corresponding to the highest density.
• Figure 5: Normalised distribution of $(z_{\rm SPE} - z_{\rm DESI}) / (1+z_{\rm DESI})$ for all the objects classified by SPE as galaxies (blue) and only in the target redshift range ($0.9<z<1.8$, orange).