The way some galaxies rotate today still seems to bear the mark of the conditions that prevailed shortly after the Big Bang. An international team detected this imprint in the orientation of the gas of nearby massive elliptical galaxies, with a statistically significant signal. This measurement provides a direct test of an idea put forward several decades ago about the origin of galactic rotation.
To understand this result, one has to go back to the birth of galaxies. Before their formation, matter was not uniformly distributed in the Universe. Density differences produced uneven gravitational forces, called tidal forces. According to the tidal torque theory, these forces gave future galaxies a rotational motion even before they took their current shape.
Galactic rotations bear the imprint of the primordial Universe.
The problem is that billions of years of evolution then blurred this initial signal. Galaxies merged, attracted gas and interacted with their environment. One could therefore wonder whether their current orientation still retained measurable information about these very ancient conditions.
The authors compared observations of nearby galaxies with a reconstruction of the initial distribution of matter in the same region of the Universe. This reconstruction, from the ELUCID project, makes it possible to estimate the primordial gravitational field that acted on the structures that would become the galaxies observed today.
The clearest signal appears in the gas of massive central elliptical galaxies. Their rotation axis shows a directional correlation with the one predicted from the primordial field, with a significance of about 7 sigma, meaning that the probability of obtaining such an agreement by random statistical fluctuation is extremely low according to the model used.
Not all galaxies show the same level of correlation. The main analysis notably covers 781 elliptical galaxies and 815 spiral galaxies with reliable measurements of gas rotation. The spirals show a less marked signal, partly because a larger fraction belongs to mass halos where the reconstruction becomes less precise.
In other words, the initial information has not completely disappeared despite cosmic evolution. This persistence reinforces the tidal torque theory, which directly links galaxy rotation to the gravitational irregularities present in the young Universe. It also turns galaxy orientation into a new exploitable piece of data for testing certain cosmological models.
The authors notably plan to use these correlations to constrain parameters such as neutrino mass. This avenue still requires larger samples and more precise reconstructions. The data and several codes used in the study have been made public, making it possible to reproduce the measurements and test the method on other surveys.