Galaxies are mass concentrations in which the gas from the Universe condenses under the action of gravity forming thousands of millions of stars. As the bricks of a house, the galaxies are the fundamental building blocks that conform the large-scale structure of the Universe. These galaxies are distributed in a sponge-like web characterized by the different large scale structure environments: dense clusters, elongated filaments, sheet-like walls and low density regions called ‘cosmic voids’. Approximately 80% of the volume of the Universe is occupied by these cosmic voids, that contain around 10% of the total mass. This makes the cosmic voids the least dense regions of the Universe. In comparison, filaments, walls and clusters are much denser, occupying around 90% of the mass of the Universe in only 20% of the total volume.
Galaxies recede from each other as the Universe expands. According to the current cosmological paradigm, at the beginning, the Universe was extremely hot, dense, small, and uniform. From these early times the Universe has continued to expand, lowering its density and temperature, and creating the sponge-like structure that characterizes the present Universe. Therefore, it is expected that the properties of galaxies will be affected by the large scale environment in which they live. Indeed, previous studies have shown that the galaxies that reside in cosmic voids tend to hold properties more characteristic of younger, less evolved systems. However, there has not been an observational proof of the fact that indeed the galaxies living in cosmic voids evolve differently to those living in denser environments.
The CAVITY project (https://cavity.caha.es/) was born with the objective of exploring the properties of galaxies in cosmic voids, aiming to determine whether there are significant differences in the properties of these galaxies as compared to those living in denser environments, such as filaments, walls and clusters.
CAVITY, led by Isabel Pérez from the University of Granada, is a collaboration of more than 40 researchers from international institutions and has as a key part the Legacy observations of cosmic void galaxies using the PMAS-PPAK integral field spectrograph located at the 3.5 m telescope at Calar Alto Observatory.
Jesús Domínguez-Gómez, a PhD student from the CAVITY collaboration, supervised at UGR by Isabel Pérez and Ute Lisenfeld, in close collaboration with Tomás Ruiz-Lara and with the support of the CAVITY team, have managed to estimate, for the first time, the speed at which galaxies in cosmic voids create their stars, and to estimate the role of the environment in their evolution. Their results are published today in the Nature journal.
The researchers have carried out, for the first time, a statistically significant study on the evolution of galaxies living in the different large scale structures of the Universe analysing the central spectra from the Sloan Digital Sky Survey (SDSS) of 10,000 galaxies living in voids, filaments and walls, and clusters. Using spectral fitting algorithms, the researchers have been able to estimate the ages and masses of the stars that form those galaxies, and they have been able to describe how they form stars throughout their lives. It has been shown that galaxies in cosmic voids evolve more slowly than those in denser environments.
"These results are based on the analysis of the integrated spectra of the central part of the galaxies, an area of great relevance although small in size. In Calar Alto, we are collecting data with a spatial resolution that will allow us to explore both the global and the local properties of the galaxies that reside in these cosmic voids”, emphasizes Rubén García-Benito, a researcher at the IAA-CSIC who participated in the work and in the CAVITY project.
Furthermore, they found that the first galaxies evolved very similarly independent of their large scale environment until 11 thousand million years ago. From that time, the evolution of the galaxies is marked by their location within the large scale structure.
This large scale structure is the result of the evolution of the Universe after the Big-Bang, and, therefore, the analysis of the current distribution of galaxies and their properties allow us to rewind in time and obtain information on the initial conditions of the Universe. This study is the first work that shows, statistically, that galaxies in cosmic voids evolve differently to those living in different environments. The higher density found in filaments, walls and clusters speed up, and alter their properties, washing out the imprints left on the galaxy properties by the initial conditions of the Universe. However, the low density regions of the Universe offer quieter environments in which galaxies can slowly evolve, making these regions excellent probes for the conditions of the early Universe.
This study highlights the importance of studying galaxies in cosmic voids and ensures the future success of the ambitious international project CAVITY led by the University of Granada and the Calar Alto Observatory.
Domínguez et al. (2023), Nature. DOI: 10.1038/s41586-023-06109-1
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