- Title:A filament of dark matter between two clusters of galaxies
- Authors:Jörg P. Dietrich, Norbert Werner, Douglas Clowe, Alexis Finoguenov, Tom Kitching, Lance Miller, Aurora Simionescu
- First Author’s Institution: Physics Department, University of Michigan
The Cosmic Web
Throughout much of the 20th century, it was an open question in astronomy as to what the universe looked like on the largest observable scales. Were galaxies and galaxy clusters distributed uniformly throughout space, or was there a pattern? Thanks to galaxy surveys like the CfA Redshift Survey and more recently the Sloan Digital Sky Survey we know that on large scales, the matter distribution of the universe is clumpy instead of smooth. Through these surveys we observe directly the distribution of luminous matter like stars, gas, and galaxies. However, luminous matter comprises only a small fraction of the matter in the universe (17%), the rest is dark matter which interacts via gravity but does not absorb and emit electromagnetic radiation like normal matter (at least not as far as we know, though several claims have challenged that notion, see Nathan and Elizabeth’s recent astrobites here and here). Theoretical simulations of dark matter cosmologies like the Millennium Simulation (Fig 1) predict that there is a dark matter backbone to the cosmic web, with filaments of dark matter stretching between clusters of galaxies. To observe this distribution of dark matter, however, we must resort to more indirect methods than those used to detect the stars and galaxies. Although the existence of dark matter filaments is a firm prediction of standard cosmological models there has not yet been a robust detection, until now.
Measuring Dark Matter
The evidence for dark matter has been established on many size scales of the universe, from the unexpected rotation velocities of galaxies to the determination of the cosmological parameters from the Cosmic Microwave Background. The presence of dark matter in galaxy clusters can be inferred by the lensing effect that mass has on passing light. Strong gravitational lensing occurs when the gravitational lens creates an image of the background source, while weak gravitational lensing distorts the shapes of background sources, stretching or compressing them in certain directions (called shear). For more about the weak lensing effect, see Elizabeth’s astrobite.
Observing A Dark Matter Filament
At the nodes of the cosmic web there lie the largest collections of luminous and dark matter. We see the luminous matter in the form of galaxy clusters, massive collections of hundreds or thousands of galaxies, while we infer the dark matter from dynamical and gravitational lensing arguments. We can see the strands, or filaments, of the cosmic web connecting these nodes via the galaxies that live there, but until now there has been no robust determination of the bridge of dark matter that should also exist. Dietrich et al. reconstruct the matter distribution of the Abell 222 and Abell 223 supercluster system (at redshift z=0.21) using weak gravitational lensing to infer the existence of a dark matter filament at a 4.1 sigma significance (for a thorough discussion of sigmas and confidence levels in the context of the Higgs Boson, see Shannon’s recent astrobite).
In Figure 2, the authors show a visible light image of the supercluster system with the surface density of the matter (both luminous and dark) overplotted in blue with contours. Such a unique discovery naturally raises questions about its plausibility.
- First, does the bridge actually contain dark matter, or can its mass be explained by luminous matter only?
- Second, is the bridge actually a filament between clusters, and not just the tips of two elliptically shaped clusters overlapping?
- And lastly, theory predicts that dark matter filaments should be extremely difficult to observe via lensing, why were the authors able to detect this one?
That the bridge of matter connecting the two clusters is a dark matter filament is supported by many of the authors’ arguments.
#1: The authors tackle this question by observing the filament in X-rays. As explained by Dan, galaxy clusters emit very strongly in the X-ray, and the X-ray luminosity is proportional to the square of the density of the medium. By assuming the distribution of the hot gas in the filament is uniform, the authors determine an upper limit on the fraction of the filament mass that is hot gas. If the same amount of gas were clumpy, the X-ray emission would be larger because denser regions would disproportionately emit more X-rays. Thus, the authors account for all luminous matter in the filament. Any extra mass inferred from gravitational lensing must be of the dark variety.
#2: To show that the dark matter between the clusters is actually a filament and not part of the two clusters, the authors fit the galaxy clusters with elliptical profiles and the filament with a simple straight bar of mass and use a Markov Chain Monte Carlo algorithm to determine the best fit parameters. Essentially, they find that regardless of ellipticity or rotation angle of the galaxy clusters, a model with a filament connecting the clusters is robustly preferred over a model with no filament for a wide range of parameters.
#3: Theoretical predictions that claim filaments should be unobservable assume that the filaments are oriented in the plane of the sky, such that they would have the maximum extent on the projected surface of the sky, but have little depth (let’s call this configuration #1). If instead the filament was aligned in a perpendicular way, such that it has a minimum extent on the projected surface of the sky but a maximal depth (configuration #2), the gravitational lensing signature would be stronger. To explain why this orientation of the filament is not so unlikely, the authors invoke a cosmological argument based on the timing of the formation of the clusters. If the redshift difference of the galaxy clusters is due solely to the expansion of the universe, then the line-of-sight separation is 18 megaparsecs. To otherwise explain the redshift difference due the peculiar motions of the galaxy clusters would require a combined cluster mass more than 10 sigma of what the authors estimate. The combination of these arguments means that it is likely that we actually are seeing the filament lengthwise (configuration #2), giving the maximal gravitational lensing signature.
The first robust detection of a dark matter filament is exciting news, and even further proof of the cosmic web of our universe. Future missions like WFIRST and Euclid will hopefully bring many more insights into the large scale structure of our galaxy.
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