Gaia Image of the Week - Gaia

Image of the Week

Gaia DR4 data product introduction: the residual image

Figure 1. Gaia residual image for Gaia DR4 source ID 6243393817024157184 (2MASS J16042165-2130284 / RXJ1604.3-2130). Residual images are a new data product in Gaia Data Release 4. What is seen is the uncalibrated flux (in electrons per second) for a source, where the contribution from the point spread function of the central Gaia source has been removed, leaving the residual diffuse light. The two dashed circles show the 0.5 and 1 arcsec radial distance from the centre of the image. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

Published today is a story on how Gaia detects hints of planets in baby star systems. The astrometric information from Gaia is an essential tool to spot the tiny motions of the stars caused by their companions orbiting them. This technique is applied for the first time to find planets and companions around stars that are still forming, stars with protoplanetary disks. Wouldn't it be wonderful if Gaia could embark on this quest with its photometric information as well?

With Gaia Data Release 4 the astrometry will be again improved upon, and an exciting new data product referred to as "residual image" will also be published. It has the potential to reveal scattered light from protoplanetary disks and other sources of diffuse light emission like the debris disk or the dusty circumstellar environments around evolved stars.

The residual image is created from stacking all available individual images (aka "windows") of the astrometric field observations after removing the contribution from the point spread function of the central Gaia source. When the contribution of the star is successfully subtracted from each observation, the image produced from these stacked observations has the sensitivity to reveal scattered light around the host star.

Figure 2. Number of Astrometric Field (AF) window samples used for the creation of the residual image for Gaia DR4 source ID 6243393817024157184 (2MASS J16042165-2130284 / RXJ1604.3-2130). Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

Not all Gaia sources were processed to generate residual images as two-dimensional astrometric field data is needed to achieve the required resolution. Generally, only sources brighter than G magnitude 13 are observed in this configuration.

This new product will be published in a table in the Gaia Archive named residual_image with Gaia's Data Release 4 (expected in December 2026), and will be available for a selection of bright Gaia sources. To show the potential of this data product, an image of the face-on protoplanetary disk of RXJ1604.3-2130 constructed from the Gaia data is shown in the figure above. The object RXJ1604.3-2130 is a member of the Upper Scorpius association, located at a distance of about 150 parsec. This structure seen in the Gaia residual image is in agreement with that seen in the images obtained by SPHERE on "Variable Outer Disk Shadowing around the Dipper Star RXJ1604.3–2130" as shown in Figure 3.

In contrast to the SPHERE images, which allow to detect the variability directly, Gaia's residual images are constructed from stacking the individual images, hence contain a "mean" observation obtained from data collected during the first 5.5 years of Gaia's scientific observations (mid-2014 to early-2020).

On the other hand, Gaia observed the sky in survey mode and scanned the entire sky, covering both well-known sources as new potentially interesting sources. No dedicated request for observations is needed, the data set comes as a bonus with Gaia's Data Release 4.

Figure 3. Variable Outer Disk Shadowing around the Dipper Star RXJ1604.3–2130, images taken with SPHERE, as published in P. Pinilla et al 2018 ApJ 868 85.

Only isolated sources, i.e. those without any other source of similar brightness in their immediate vicinity, had their residual image computed by Gaia. This is because the colour of the other source will generally not be known well enough for its accurate subtraction, and without subtracting its flux it will obscure any scattered light.

Some residual images though will contain other point sources, but those will be at least 3 to 4 magnitudes fainter than the main Gaia source. This limit comes from the detection limit used in Gaia DR4 for a specific processing algorithm named SEAPipe, dedicated to the analysis of the environment of the source. The SEAPipe results are another new Gaia DR4 data product, published in a table named gaia_source_environment.

With these restrictions, residual images will be published for approximately 7 million sources in Gaia DR4, and a small subset of those will show circumstellar diffuse light emission. Gaia's Data Release 4 will contain over 130 different data products and for over half of them, December 2026 is their première. Only 12 months to go to discover this exciting new Gaia data set!

Gaia Image of the Week - Gaia

Image of the Week

Gaia DR4 data product introduction: the residual image

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