FOUND: Missing Matter

Galaxies seem to have less matter than they should. Has the missing matter been found at last in the form of hot, sparse gas?

The Troubling Matter of Normal Matter

image of a spiral galaxy

The space surrounding a galaxy isn’t empty. Instead, it’s filled with tenuous circumgalactic gas invisible to optical telescopes. [NASA, ESA, CXC, SSC, and STScI; CC BY 4.0]

From tiny dwarf galaxies to enormous ellipticals, all galaxies have something in common: they all seem to have less baryonic matter — the stuff that makes up stars, gas, dust, and everything we can see and touch — than we expect. Some galaxies only have a few percent of the matter they should have.

Researchers suspect that the matter isn’t really “missing” but is instead “hidden,” present in a form that’s hard to observe. Recently, a major breakthrough happened when researchers zoomed out from the luminous, starry disks of galaxies to study the tenuous gas of the circumgalactic medium. There, they found immense reservoirs of cool (~104K) gas accounting for as much as 50% of the lost matter. In a recent article, researchers have turned up the heat on the search, seeking out a second, hotter (~106K) component of the circumgalactic medium.

cartoon of the observing setup

A diagram (not to scale!) of the observing setup. X-rays emitted by the quasar pass through the circumgalactic medium of a foreground galaxy. [Kerry Hensley/AAS Nova]

A Critical Alignment

A team led by Fabrizio Nicastro (Italian National Institute for Astrophysics) aimed to track down the remaining missing matter by observing the light from quasars as it passes through galaxies in the foreground. Quasars are the luminous centers of distant galaxies that are powered by supermassive black holes consuming matter. Superheated disks surrounding these active black holes emit enormous amounts of X-rays. As these X-rays travel from the quasar to our telescopes, they can be intercepted by galaxies along their path. If the sought-after hot gas is present in the surroundings of these galaxies, it’ll make itself known by absorbing some of the quasars’ X-ray light.

Nicastro and collaborators analyzed X-ray spectra of three quasars whose lines of sight pass through the circumgalactic medium of a foreground galaxy. None of the individual quasar spectra showed definitive signs of the elusive hot circumgalactic gas, but a signal emerged when the team combined observations of all three quasars. Using several different fitting methods and ways of combining the data, the team detected a signal at a significance ranging from 4.2 to 6.8 sigma.

Missing Mass Found

plot of an X-ray absorption line

Example of an X-ray absorption line identified after stacking the observations from all three quasars. [Adapted from Nicastro et al. 2023]

Based on how much the quasars’ X-ray light was absorbed by the gas in its path, the team estimated the mass of the hot gas surrounding the three galaxies in their sample — and it was a lot. Nicastro and collaborators estimated that 70% of the remaining missing matter can be found in the hot circumgalactic medium, and it might even contain the full amount that is missing.

The discovery of this supply of hot circumgalactic gas has significance beyond solving the missing matter problem: the newfound detection of hot gas can also tell us about how galaxies have exchanged gas with the circumgalactic medium throughout their lives.


“X-ray Detection of the Galaxy’s Missing Baryons in the Circumgalactic Medium of L* Galaxies,” Fabrizio Nicastro et al 2023 ApJL 955 L21. doi:10.3847/2041-8213/acec70