Euclid Q1 spectroscopy of bright quasars

Our paper “Euclid Quick Data Release (Q1). XXXVIII. Euclid spectroscopy of quasars. 1. Identification and redshift determination of 3500 bright quasars” has been accepted by Astronomy & Astrophysics.

ADS: https://ui.adsabs.harvard.edu/abs/2025arXiv251208803E
arXiv: https://arxiv.org/abs/2512.08803
DOI: 10.1051/0004-6361/202558490

This post is the first entry in a new series, AGN Sciences with Euclid, where I’ll keep notes, figures, and data products related to my Euclid-based AGN and quasar work.

Overview

In this paper we present a visually inspected sample of nearly 3500 bright quasars identified with Euclid/NISP red-grism spectroscopy in the Q1 fields. One of the main takeaways is that Euclid slitless spectroscopy already delivers reliable quasar redshifts over a broad redshift range, including not only Gaia-detected quasars but also a redder non-Gaia population that is harder to recover with optical selection alone.

The paper includes:

a catalogue of Euclid Q1 bright quasars with spectroscopic redshifts;
visual-inspection classifications and redshift-quality information;
an analysis of Gaia and non-Gaia quasar subsets;
a rest-frame near-UV to near-IR Euclid quasar composite spectrum;
examples and diagnostics for NISP-based quasar redshift determination.

The CDS tables will become available after the A&A publication process is finished. Until then, I’m hosting the accepted-paper version of the tables here.

Euclid Q1 quasar spectroscopy in a nutshell

What Euclid/NISP quasar spectra look like

**Figure 1. Representative Euclid/NISP quasar spectra.** The examples illustrate the range of spectral features used for quasar identification and redshift determination in the Q1 sample. Depending on redshift, different rest-frame UV, optical, and near-infrared emission lines enter the NISP red-grism wavelength range.

Figure 1 shows a few representative Euclid/NISP spectra from the visual-inspection sample. In practice, the redshift constraints come from broad and narrow emission features such as Mg II, Hβ, [O III], Hα, and near-infrared hydrogen lines when they fall inside the NISP wavelength range.

Gaia and non-Gaia quasars in Euclid colour space

An important part of the Q1 sample is the comparison between Gaia-detected and non-Gaia quasars. The non-Gaia subset is generally redder and optically fainter, which makes it a nice demonstration of how Euclid can extend quasar selection beyond Gaia-based and purely optical samples.

**Figure 2. Colour distributions of Gaia and non-Gaia Euclid quasars.** The Euclid colours show that the non-Gaia subset occupies a redder region of colour space, consistent with Euclid’s ability to recover quasars that are faint, reddened, or missed by optical/Gaia-based selections.

Redshift determination with slitless spectroscopy

Figure 3 is a convenient way to see how the main quasar emission line features shift with redshift. In the observed wavelength versus redshift plane, the strongest emission lines trace diagonal tracks. This makes it easy to see which lines drive the redshift determination in different intervals, and where single-line ambiguities can become a problem.

**Figure 3. Emission-line visibility in Euclid/NISP.** The stacked spectra show how major quasar emission lines move through the NISP wavelength range as a function of redshift. This diagram is useful for understanding both the strengths and the limitations of redshift determination with Euclid slitless spectra.

A telluric-free Euclid quasar composite spectrum

The Q1 sample also makes it possible to construct a Euclid quasar composite spectrum covering rest-frame near-UV to near-IR wavelengths. Because Euclid observes from space, the composite is free of telluric absorption, which makes it a useful reference for future near-infrared quasar and AGN work.

**Figure 4. Euclid Q1 quasar composite spectrum.** The mean and median composites provide a telluric-free rest-frame near-UV to near-IR reference spectrum constructed from visually confirmed Euclid quasars. The comparison with previous composites highlights the advantages of observing from space.

Individual examples

Besides the population-level figures, it is worth showing a few individual low-redshift quasars simply because the Euclid images are beautiful. They also give a more concrete sense of the sample and highlight Euclid’s excellent spatial resolution and image quality, reaching about 0.16 arcsec in visible light. In Figure 5, the upper panels show the Euclid images and the lower panels show the NISP spectra used for redshift measurement.

EUCL J034954.84−490146.8 — clean NISP redshift example with strong He I and Paβ.
EUCL J175441.90+681906.9 — resolved host with spirals and possible nearby companion.
EUCL J180934.25+645054.5 — active nucleus in an edge-on galaxy.

EUCL J034954.84−490146.8 — **Figure 5. Example Euclid Q1 quasars.** The upper panels show Euclid images of the sources and their environments; the lower panels show the NISP spectra. Dashed lines mark the expected positions of emission lines at the measured redshifts.

EUCL J175441.90+681906.9 — **Figure 5. Example Euclid Q1 quasars.** The upper panels show Euclid images of the sources and their environments; the lower panels show the NISP spectra. Dashed lines mark the expected positions of emission lines at the measured redshifts.

Data products

The files below are the accepted-paper versions and are hosted here temporarily before the CDS tables go live:

Euclid Q1 bright-quasar catalogue (Table B1): CDS data file; also available as a FITS table.
Euclid Q1 quasar composite spectrum (Table 2): CDS data file.
README and column descriptions: ReadMe.

Please cite the accepted A&A paper if you use these data. Once the CDS version is available, that should be treated as the archival reference.

And stay tuned: Euclid DR1 should bring a much larger sample of spectroscopically identified quasars. More about the release is available on the Euclid DR1 page.

Suggested citation

Euclid Collaboration: Fu, Y., Bouwens, R., Caputi, K. I., et al. 2025, A&A, in press, https://doi.org/10.1051/0004-6361/202558490, arXiv:2512.08803

BibTeX entry:

@ARTICLE{Q1-SP068,
       author = {{Euclid Collaboration: Fu}, Y. and {Bouwens}, R. and {Caputi}, K.~I. and others},
       title = "{Euclid Quick Data Release (Q1). XXXVIII. Euclid spectroscopy of quasars. 1. Identification and redshift determination of 3500 bright quasars}",
      journal = {A\&A, in press, \url{https://doi.org/10.1051/0004-6361/202558490}},
     keywords = {Astrophysics of Galaxies, Instrumentation and Methods for Astrophysics},
         year = 2025,
        month = dec,
          eid = {arXiv:2512.08803},
        pages = {arXiv:2512.08803},
          doi = {10.48550/arXiv.2512.08803},
archivePrefix = {arXiv},
       eprint = {2512.08803},
 primaryClass = {astro-ph.GA},
       adsurl = {https://ui.adsabs.harvard.edu/abs/2025arXiv251208803E},
      adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

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