Legacy Science

From asteroids to the most distant galaxies – Euclid’s breadth of science

The Euclid surveys will provide an imaging and spectra atlas of a large fraction of the sky (about ⅓ of the total) which will likely remain the standard data set for many decades. This treasure trove of data will enable a very wide range of science beyond the cosmological constraints the mission is designed to provide. Euclid will make major advances in nearly all areas of astronomy from nearby solar system objects to extrasolar planets and star formation as well as galaxies at all epochs of the universe.

For example, we expect to characterise at least 100,000 solar system bodies – many of which will be moving so quickly that they will leave streaks on our images. When we combine the data from Euclid with data from other facilities across the Earth, Euclid will determine their rotation and we expect to find more binary asteroids and Kuiper-belt objects. This will be a significant number of the total number of solar system bodies known.

Beyond the solar system Euclid will be able to use the microlensing effect to find extrasolar planets through brief increases in the light of background stars as planetary systems pass by them on the sky. This is the same gravitational lensing effect that will power our cosmological studies, but operating on much smaller scales. We will also use multiple observations of the same part of the sky to look for exploding stars, supernovae and study their properties.

Although not focused on time-domain astronomy, the multiple visits of Deep fields, but also Calibration fields, will allow Euclid to perform, for the first time, a wide-field optical/infrared transient search from space. We predict the discovery of thousands of supernovae of all types up to redshift two, and even higher redshift superluminous supernovae (SNe). SN Ia or super-luminous SN can be used to extend the ground-based Hubble diagram whereas core-collapse SNe will be used as an independent probe of the cosmic star formation history. We will also be able to test for the actual existence of the long searched Pair Instability explosions. The Euclid transient data will gain unique value when used in synergy with the incoming RUBIN/LSST survey and will constitute a preview of the future dedicated space mission.

Euclid’s ability to separate stars from distant galaxies will enable deep studies of the halo of own Milky Way galaxy, as well as ultra-cool dwarf stars. Euclid will also be a fantastic resource for mapping nearby, large, galaxies which HST could only image parts of. Hundreds of these galaxies will be close enough to be resolved into individual stars, at least in their outer parts. At the end of the Euclid survey we will have unprecedented views of a large number of bright galaxies, which we can use to study how the galaxies were assembled by looking for streams of stars being accreted into them, including globular clusters which we can also use to constrain models of dark matter.

Example of galaxy science possibilities with Euclid: from studies of the morphology of galaxies, to identifying and characterising streams of stars from past galaxy mergers, galaxies’ globular cluster systems, to studies of companions. Several hundred galaxies can be examined in this much detail with the Euclid Wide Survey. (Credit: D. Mackey et al., https://arxiv.org/abs/1910.00808)

In the more distant Universe, Euclid will give us an incredible map of the sky at near-infrared wavelengths. This is crucial for studies of galaxies because it allows us to better determine their distances, stellar masses and measurements of their star formation rates. At the same time, this multi-colour map and spectroscopy will allow us to determine the locations of galaxies and thereby reconstruct the 3D structure of the distant Universe. Cosmologists will use a version of this structure to study the cosmological properties of the Universe, but this will also allow us to study how the properties of galaxies change with location in this cosmic web, including the massive galaxy clusters.

Finally, the depth and large area of the Euclid images will allow us to find the rarest and most interesting distant objects in the Universe out to a redshift beyond 7 when the Universe was only 5% of the age it is today. These rare galaxies would be impossible to find without a survey such as Euclid, and by studying their properties we will learn more about how and when the first generations of galaxies formed and interacted with each other. These are critical observations for understanding the origin of all structure in the universe and one in which Euclid is nearly uniquely able to provide.

EuclidBefore Euclid
[1] Deep near-IR imaging: area of sky mapped to 24 mag AB in the near-IR>14,500 deg2(~10 deg2 per day)13.5 deg2
[2] Massive galaxy clusters at z>1~500,000~1000
[3] High-redshift quasars, z>7~10010
[3] High-redshift quasars, z>8~80
[4] Strong gravitational lenses (galaxy scale)170,00010,000
[5] High-redshift galaxies (z>6)~390,000~400
[6] H-alpha emitters at z>1~30 million<10,000
[7] Massive galaxies spectroscopically confirmed at z>1.5few ×100,000few × 100
[1] Pre-Euclid number entails publicly released VIDEO (12 deg²) and UltraVISTA (1.5 deg²) surveys.  Public release of an additional 9 deg² from VEIL is expected soon, but after the Euclid launch.   [Pre-Euclid number from Karina Caputi and Matt Jarvis]
[2] Euclid prediction from Sartoris et al. (2016; MNRAS, 459, 1764), using their 3-sigma threshold.  Pre-Euclid number from MaDCoWS-1 (Gonzalez et al. 2019; ApJS, 240, 33). Alternatively, the DES/unWISE sample implies ~30,000 (Wen & Han 2022; MNRAS, 513, 3946).  [Numbers from Anthony Gonzalez.]
[3] Euclid prediction from Euclid Collaboration, Barnett et al. (2019; A&A, 631, 85).  [Numbers from Daniel Mortlock.]
[4] [Numbers from Leonidas Moustakas.]
[5] Numbers based on extrapolation of COSMOS results,  from van Mierlo et al. (2022; A&A, 666, 200).  [Numbers from Karina Caputi.]
[6] [Numbers from Yun Wang, Will Percival, Gigi Guzzo, and Claudia Scarlata.]
[7] Number of H<22 galaxies at z>1.5 based on UltraVISTA catalog of the COSMOS field (Ilbert et al. 2013; A&A, 556, 55), extrapolated over 40 deg² Euclid Deep Surveys for Euclid.  [Numbers from Michele Moresco and Lucia Pozzetti.]
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