Probing Missing Baryons with Jets in Galaxy Groups

by Brian Morsony | NSF Astronomy & Astrophysics Postdoctoral Fellow, Astronomy Department, UW-Madison
Posted Jun 05, 2013

Baryons, the ordinary matter that makes up stars and galaxies, constitute only 5 percent of the total contents of the Universe – the rest is mysterious dark matter and even more esoteric dark energy. Given that this is the stuff we can actually see, touch, and interact with, it is puzzling that scientists can still only account for less than half of the baryons that exist in the cosmos. Where is the rest?

In an attempt to answer this question, NSF Astronomy & Astrophysics Postdoctoral Fellow Brian Morsony, with Professors Sebastian Heinz and Eric Wilcots and research assistants Emily Freeland and Jake Miller, carried out a series of computer simulations of bent-double radio sources to quantify how well these sources can be used to estimate the density of the material surrounding them. To do this, they injected jets from active galactic nuclei (AGN) moving through a background intergalactic medium (IGM), and then modeled the radio and X-ray emission of the resulting structures. Given the dynamic content, this is best viewed in movie form, shown below:

Movie of bent-double jets: The headwind of the gas the black hole is running into bends the jets to the right. Left panel: large scale view of the trail of gas left behind by the black hole. Right panel: Zoomed-in view of the bending.

AGNs are powered by a super-massive black hole at a galaxy's center, accreting material from its surroundings. Some produce pairs of powerful, fast-moving jets that can extend for hundreds of thousands of light-years. As these jets propagate, they push aside material surrounding the galaxy and create large lobes filled with high-energy particles. By their spiraling action around the strong magnetic field lines lacing this gas, these super-fast particles produce synchrotron radiation visible at radio wavelengths.

Bent-double radio sources are created when the galaxy producing the jets is moving relative to the IGM. The jets then feel a headwind from the IGM, and, like a curling iron, the pressure from this wind forces the jets to curve backwards. This creates a radio source with two narrow jets bent in the same direction and two large trailing radio lobes.

Galaxy groups can store a significant amount of baryonic mass in their IGM, but the exact amount is difficult to measure. In individual galaxies, most of the baryons are in stars, and the amount of mass in stars is relatively straightforward to measure. In large galaxy groups and galaxy clusters, the gas is hot enough that it emits large amounts of X-ray radiation. However, in smaller groups, the IGM is typically too cool and too faint to observe directly. Smaller groups are quite common, and their IGM could account for a large fraction—up to 70 percent—of all the baryons in the Universe.

Bent-double radio sources provide a way to measure this density. The amount of bending the jet experiences depends on the pressure and width of the jet, the velocity of the galaxy, and the density of the IGM. The properties of the jet can be measured based on the amount of radio emission. In groups of galaxies—collections of several to several dozen galaxies gravitationally bound together—the velocity of the galaxy relative to the IGM can be estimated by measuring average velocity of all the galaxies in the group. Using this information, the radius of curvature of the jet, measured from radio observations, allows the density of the IGM to be estimated. Unlike other methods, bent-double radio sources probe the total density of the IGM, not just a limited temperature range.

Freeland and Wilcots previously used radio observations to measure the IGM density in groups, and found high densities for the IGM, consistent with groups accounting for most of the missing baryons in the Universe. The computer simulations presented in the current work allowed the team to determine how uncertain the density estimates of the IGM are, accounting for the resolution of radio observations, the unknown angle of the jets towards or away from the Earth, and the unknown angle of motion of the galaxy producing the jets relative to the Earth. They found that the density estimates from radio observations are at worst within a factor of 2 of the true values. They also found that, because the jets are narrower than the resolution of the radio telescopes used to detect them, the density estimates are probably low by, on average, 50 percent. “These findings support the idea that a large fraction of baryons are in the IGM of galaxy groups," says Morsony. "Accounting for this gas is important when trying to find the missing matter in the Universe.”

Looking to the future, they modeled the X-ray emission coming from their simulated bent-double sources. Although the background IGM is too cool and too low density to be detected in X-ray emissions, a shock is created when the IGM hits the jets. This shock compresses and heats the gas, and creates a large cocoon of hot gas around the bent-double source. This gas is potentially detectable with existing X-ray telescopes, particularly for a fast moving galaxy in a fairly dense IGM. An X-ray detection, combined with radio observations, would allow measurents of the galaxy velocity directly, rather than using the average galaxy velocity. This would enable an even tighter constraint on the IGM density.

The full article, “Simulations of bent-double radio sources in galaxy groups,” by Morsony, B.; Miller, J., Heinz, S., Freeland, E., Wilcots, E., Bruggen, M., and Ruszkowski, M. was published in the 2013 Monthly Notices of the Royal Astronomical Society (MNRAS), 431, 781.

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