The case for an Astrometric Mission Extension of Euclid. Extending Gaia by 6 magnitudes with Euclid covering one-third of the sky

TL;DR

The paper argues for extending Euclid with a second Wide Survey epoch to create a ~6-year baseline that enables high-precision proper motions for sources far fainter than Gaia can reach, by repeating the main survey in near-identical fashion. It assesses the observing strategy, potential depth and calibration gains, and the feasibility given mission constraints, while also exploring the possibility of two additional epochs to access parallaxes. The approach promises transformative advances in stellar and Galactic astrophysics, including kinematics of faint populations, tidal streams, clusters, and dwarf satellites, as well as enhanced spectroscopic redshifts and weak-lensing calibration. By integrating multi-epoch Euclid data with Gaia, LSST, and the Roman Space Telescope, the proposal aims to secure a lasting, high-impact legacy in space-based astrometry and Galactic archaeology.

Abstract

The nominal duration of Euclid's main mission is six years, but current best estimates indicate that the observatory has sufficient propellant to operate for up to ~14 years in total. In this work, we advocate dedicating six of these ~8 additional years to repeating the main survey, covering approximately one-third of the sky. This repetition would not only improve the sampling, signal-to-noise, quality, and depth of the survey, but -- most importantly -- would provide a six-year time baseline between two epochs if executed in the same sequence. The availability of multiple epochs would enable the derivation of proper motions for stars as faint as V~27, i.e., more than five magnitudes fainter than those measured by the Gaia mission. Although it may seem early to propose such a mission extension, in this work we quantitatively illustrate its immense scientific potential. We therefore intend to initiate the technical and scientific discussions early to ensure optimal planning. The here proposed extension would employ only the VIS channel -- owing to its superior astrometric capability and depth -- while simultaneously using NISP in slitless-spectroscopy mode to enhance the signal-to-noise ratio of first-epoch spectra that would also benefit of proper motions to identify and reject objects within the local Universe.

Figures (4)

• Figure 1: Comparison of imaging depth and resolution from different space observatories. On top, Euclid VIS observations of the globular cluster NGC 6397 as combined by 2024AA...692A..96L. While both HST and JWST achieve substantially greater imaging depths than Euclid in these specific data sets (HST large program GO-10424 and JWST program GO-1979), the angular resolution of Euclid is comparable to that of HST and JWST, enabling high-precision morphological studies over a considerably larger survey area. The Euclid$\sim1^{\circ}\!\times\!1^{\circ}$ field of view presented in 2024AA...692A..96L is shown in green, the JWST$\sim2.5'\!\times\!6'$ field studied by 2021jwst.prop.1979B2024AN....34540039B in red, and the HST$\sim3'\!\times\!3'$ field from program GO-10424 in blue. The yellow region, of about $50"\!\times\!50"$, in the top panel is common to the three data sets, which are shown individually in the bottom panels with observatories labelled according to the same colour code. [Images and catalogues available here: https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/JWST/GO-1979/P01/ and https://web.oapd.inaf.it/bedin/files/PAPERs_eMATERIALs/Euclid/Paper01/].
• Figure 2: The published catalogue by 2024AA...692A..96L have demonstrated the 1D-positional RMS precision of VIS@Euclid imaging astrometry. Mild saturation begins at $I_{\rm E} \sim 19$ (light-gray shaded regions), while sources brighter than $\sim$18 are severely saturated (region in dark-gray). The region of the sources accessible to Gaia is indicated by a shaded region in azure. (Top:) Shows the quality-fit parameter 2024AA...692A..96L as function of the calibrated VIS-magnitude in filter $I_{\rm E}$. (Middle:) Illustration of the astrometric precision as function of the magnitude (in VIS pixels on left axis, and in mas on the right axis). (Bottom:) The precision is as good as 20 mas in a single exposure for sources at $I_{\rm E} \sim 26$, but it can reach sub-mas precision on a single image for a well-exposed unsatured point sources.
• Figure 3: Overview of the comparative astrometric analyes of Euclid and Gaia as presented by 2024AA...692A..96L. The left panel shows Gaia-$G$ magnitude as a function of 1D PM error (the sum in quadrature of the PM errors along the $\alpha\cos\delta$ and $\delta$ directions divided by $\sqrt 2$). Stars present in both the Gaia DR3 and Euclid-Gaia catalogs are shown in black and red, respectively, while blue points are sources with a PM measurement only in the Euclid-Gaia catalog (i.e., with positions but no motions available in the Gaia DR3 catalogues). The vector-point diagrams of the PMs in four magnitude bins (the thresholds of which are drawn in the left panel) are shown in the middle and right panels for the Euclid-Gaia and Gaia DR3 cases, respectively. Clearly Euclid-Gaia DR3 proper motions are ten-times more precise in this common magnitude interval, and provide PMs for sources with positions entries in Gaia DR3, but not PMs. However, the significant improvements over DR3 seen in the narrow magnitude range $19 < G < 21$ are expected to become progressively smaller with the advent of the upcoming DR4 and DR5 of far superior astrometric precision and accuracy (see text).
• Figure 4: (Top): Visualization of the Euclid survey areas over the sky. (Bottom): Survey's planned progression with the operational years. [For panels, credits by: ESA/Euclid Consortium/Planck Collaboration/A. Mellinger.]