Astrometry, mapping and distance scales in the Universe

1️⃣ Where is Gaia ?

For the Gaia mission to achieve its astrometric precision objectives, the satellite’s orbit must be perfectly known. To achieve this, its position relative to the stars is analyzed using telescopes 1m to 2m in diameter distributed around the globe, as part of the GBOT (Ground-Based Optical Tracking) program.

The speed of the satellite, located 1.5 million km from the Earth at Lagrange point L2, must be determined to the nearest 2.5 mm/s, as calculations of the position of objects in the Gaia catalog must take into account the speed of the observer (rotation and revolution of the Earth) and that of the satellite.

Similarly, Gaia’s position on its orbit must be known to within 150 m in order to best measure the position of nearby objects such as asteroids.

By 2018, 20,000 of these had been observed by the GBOT team, of which 9,000 were unknown.

Since 2020, GBOT’s tracking of Gaia has been officially used for the reconstruction of Gaia’s orbit, which will be used to produce the fourth and fifth catalogs (scheduled for publication in 2025 and 2030 respectively).

2️⃣ Mapping the Milky Way disk : gas and dust

Gaia’s knowledge of the distances to stars, combined with measurements of their luminosities and colors, also produced by Gaia or measured on the ground, has made it possible to reconstruct in three dimensions the clouds of interstellar dust grains that obscure and redden their light [Vergely et al., 2022 ; Lallement et al., 2022].

Here we show a cross-section of this three-dimensional distribution, in the vicinity of the Sun. The cut plane is parallel to the galactic plane, the Sun (black circle) is at the center of the image and the Galactic Center, about twice the width of the map, is to the right. Areas of high extinction, dense with grains, are shown in black.

The accuracy of the maps decreases with distance from the Sun, due to the reduced density of stars used as input to the reconstruction calculation. This map highlights the presence of dust-empty cavities of various sizes, but these cavities make it difficult to identify the spiral arms deduced from the stars.

3️⃣ Building the distance scale using Cepheids

Cepheids, pulsating stars, are at the heart of extragalactic distance measurement. Their easily measurable pulsation period is correlated with their luminous power. This relationship (discovered by astronomer Henrietta S. Leavitt) makes it possible to determine the luminous power of a Cepheid by measuring its period of brightness variation (between 2 and 200 days). As the amount of light received decreases with the square of the distance, we can accurately estimate its distance. Cepheids are intrinsically very bright (on the order of 10,000 times the luminosity of our Sun). They are therefore observable in very distant galaxies. But Leavitt’s relation has to be calibrated by directly measuring (without using the relation itself !) the distance of a Cepheid sample. The second catalog of the Gaia mission has greatly improved the accuracy of distances to Cepheids close to us, and hence the Leavitt relation [Breuval et al. 2020].

In the diagram above, Cepheid members of stellar pairs are shown in blue, and those in star clusters in red.

But why look at binary Cepheids and Cepheids in clusters, rather than isolated stars ?

Because Cepheids are variable, Gaia’s measurement of their distance is more delicate than for stars whose luminosity is constant. Cepheid companions are identified by Gaia’s measurements of their speed across the sky (Figure below) [Kervella et al. 2019].

Cepheids present in clusters enable us to better estimate their distance by calculating the average distance value of the many stars in the cluster, which is more accurate than that of the Cepheid alone.

The period-luminosity relationship of Cepheids, calibrated by Gaia, was further improved with DR3 in 2022. This confirmed that the current expansion rate of the Universe, known as the Hubble constant, does not correspond to that predicted by the standard cosmological model.

This question is still open today !

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