Intergalactic Roadkill: Suicidal Speeding Marginalized Dwarf Galaxies

A tragic case of intergalactic roadkill might be the solution of one of the great mysteries of the universe: where is all the matter? And that just regards the 5% of the universe consisting of matter that we can observe, while 75% is hidden dark energy, and another 20% dark matter. But we don't even observe enough stars and galaxies to reach the 5% mark, and one of the missing parts are a large number of dwarf galaxies that should exist in the vast spaces between larger galaxies like our own. To solve this mysteries, astronomers of the international CLUES collaboration, simulating the movement and evolution of our whole local group of galaxies over billions of years, have identified "cosmic web stripping" due to "speeding violations" of galaxies as a new way of explaining the famous missing dwarf problem.

High-precision observations over the last two decades and simulations on massive supercomputers indicate that our universe consists of about 75% dark energy, 20% dark matter and 5% ordinary matter, with the galaxies and matter clumping in an intricate network of filaments and voids, known as the cosmic web. In such a universe a huge number of small "dwarf" galaxies weighing just one thousandth of the Milky Way should have formed in our cosmic neighborhood. Yet only a handful of these galaxies are observed orbiting around the Milky Way. This observed scarcity of dwarf galaxies is a major challenge for the current understanding of galaxy formation.

The international team of researchers that has now studied this issue within the Constrained Local Universe Simulations project (CLUES) uses the observed positions and peculiar velocities of galaxies within 10,000,000 light-years of the Milky Way to accurately simulate the local environment of the Milky Way. "The main goal of this project is to simulate the evolution of the Local Group -- the Andromeda and Milky Way galaxies and their low-mass neighbors -- within their observed large scale environment", said Stefan Gottlöber of the Leibniz Institute for Astrophysics Potsdam.

Analyzing the CLUES simulations, the astronomers have now found that some of the far-out dwarf galaxies in the Local Group move with such high velocities with respect to the cosmic web that most of their gas can be stripped and effectively removed. They call this mechanism "cosmic web stripping", since it is the pancake and filamentary structure of the cosmos that is responsible for depleting the dwarfs' gas supply.

"These dwarfs move so fast that even the weakest membranes of the cosmic web can rip off their gas", explained Alejandro Benítez Llambay, PhD student at the Instituto de Astronomía Teórica y Experimental of the Universidad Nacional de Córdoba in Argentina, and first author of the publication of this study which is published in the February 1 issue of Astrophysical Letters. Without a large gas reservoir out of which to form stars, these dwarf galaxies should be so small and dim that they would be hardly be visible today. The missing dwarfs may simply be too faint to see.