Connections on the Cosmic Web

Title: The impact of the connectivity of the cosmic web on the physical properties of galaxies at its nodes

Authors: Katarina Kraljic, Christophe Pichon, Sandrine Codis, Clotilde Laigle, Romeel Davé, Yohan Dubois, Ho Seong Hwang, Dmitri Pogosyan, Stéphane Arnouts, Julien Devriendt, Marcello Musso, Sébastien Peirani, Adrianne Slyz, and Marie Treyer

First Author’s Institution: Institute for Astronomy, University of Edinburgh

Status: Submitted to MNRAS; [open access on Arxiv]

The universe was not perfectly uniform when it began, some areas had higher density than others. As the universe evolved, these areas of high density contained most of the matter and began forming galaxies where there was the highest concentration of stuff. This large-scale structure is known as the ‘cosmic web’ and connects the observed clusters of galaxies via a series of filaments. A model of what this looks can be seen in Figure 1.

Figure 1: The skeleton of a cosmic web traced out by an algorithm run on a sample of observed galaxies. The far right shows the entire web while the left images show zoomed in portions. Blue areas show points of higher density. A 3D rendering can be seen at this link. (Source: Fig. 1 in the paper)


At the Crossroads

The cosmic web is a representation of the density of space, and can be traced out with the observed distribution of galaxies, connecting those closest together. This is done with similar algorithms for both observations and simulations. The densest areas are the most massive, central galaxies that are located at the crossroads of multiple filaments (called nodes), like the blue areas in Figure 1. It had been shown by another paper from different authors that the mass of a galaxy increased with the number of connected filaments (the “connectivity”). The authors of today’s paper wanted to know whether the connectivity was related to star formation and whether it helped prevent star formation from happening.

Forming Stars, or Not

To accomplish this, the authors investigated observed and simulated galaxies that lay at the densest regions (nodes) within the cosmic web. Observed galaxies were taken from the SDSS data release and number about 7*105. Galaxies from two independent simulations were compared to the observations to see if the same general trends hold, but direct comparison is difficult because simulations have significantly smaller sample sizes. This is due to the computational cost involved in simulating large amounts of galaxies. 

These galaxies were connected with a median of three filaments. For the observed galaxies, the authors compared the connectivity to the star formation rate (measured by comparing the relative ages of the stars). Figure 2 shows that as the connectivity decreases the excess star formation rate increases for all types of galaxies. The authors found this trend also occurred for the simulated galaxies, where star formation can be chosen as an output parameter.

Figure 2: A comparison for the observed galaxies of the average connectivity and excess star formation rate relative to the average for that type. The black lines are for all galaxies. Top: The blue line is for star forming galaxies and the red line is for passive galaxies which are not actively forming a significant amount of stars. Bottom: The blue line is for spherical or irregularly shaped galaxies while the red line is for elliptical galaxies. Increasing star formation is to the right. This shows that as connectivity increases, star formation decreases. (Source: Figure 5 in the paper)

This is counter-intuitive; filaments can funnel cold gas into the galaxy, which could be expected to trigger star formation. The authors provide a few different explanations for this. It could be that the gas received from multiple filaments could be chaotic, instead of the more uniform funnelling that a single filament would provide. Instead, it could be that the filaments may not be effective at feeding the inner regions of a galaxy that would form stars, or that they are experiencing less mergers that are mixing less gas. Whatever the explanation is, the authors have shown that more connectivity means less star formation.

Transitioning to Old Age

Finally, the authors investigated what impact connectivity had on AGN activity. Feedback from AGN is important in galaxy formation as it helps transition a galaxy from being new and star forming to a passive old age. AGN give off vast amounts of energy that can shoo star-forming gas out of the galaxy and quench star formation. To link AGN activity and connectivity, the authors compared two similar simulations: one with AGN and one without. They found that AGN were more efficient at quenching star formation when they were in galaxies with high connectivity. The AGN were able to give off more energy to stop star formation as they were fed more material.

The authors of today’s paper showed that connectivity is an important factor in galaxy formation, as it affects when stars will stop forming, as well as the total mass and morphology of a galaxy in both observations and simulations. The evolution of galaxies is impossible to observe as it takes many millions of years, but any information we can glean through work like this helps us understand our universe. 

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