Galaxy NGC 660's bizarre structure lead many to believe it holds a large quantity of dark matter.Credit: Hubble/NASA/European Space Agency

Reaching for the dark side

24 February 2016

Like many of us, Ricky Chue was fascinated by planets and stars in school. Astronomy courses in college introduced him to a veritable treasure trove of questions, particularly on the nature of dark matter and dark energy. “I was amazed that we could know that they exist, but not what they are, even though these two components make up 95% of the universe. To me, it is the ultimate puzzle.”

Chue has followed this puzzle to his current doctoral research on dark matter at the University of Illinois at Urbana-Champaign, determining assembly bias using the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), which provides a deeper view, giving clearer clues on dark matter and its spatial distribution. It is difficult to probe dark matter directly in observation, as it hardly interacts with light. Chue uses gravitational lensing, a technique based on gravity’s distortion of light, to analyze CFHTLenS data. As dark matter exhibits mass, gravitational lensing is a powerful tool in studying the gravitational interaction between ordinary matter, dark matter, and its dynamical properties. The CFHTLenS’ expanded perception includes more source galaxies further away which emit light. This light is then bent by the lens galaxies in the front through gravitational lensing, distorting the observed images of source galaxies. The level of distortion helps determine the mass distribution of lens galaxies.

“We need a clearer description of how dark matter behaves, and in science, that means you need an accurate and simple model that attempts to describe its behaviour and your expectation,” Chue says. In galaxies, mass is predominantly hosted in surrounding dark matter halos, believed to trace the clustering of the underlying matter density field. Previously, the relation between the clustering of halos and matter, the halo bias, was thought to depend only on the total mass of the system, and the dark matter halo model was established accordingly. Large halos with large mass are more strongly clustered than smaller, less massive halos. Chue compares it to how tall mountains are found mostly in high plateaus, such as the Himalayas; high density peaks in the universe are also found in regions of high background density, and are the seeds of the most high-mass halos.

Subsequent theories have proposed that halo bias is dependent on additional factors besides mass, with recent observational evidence suggesting the existence of assembly bias in high-mass halos. These massive structures dominate their environment, and younger halos with rapid mass accretion tend to occur in higher density regions, where there is more mass supply to draw from. These younger halos are therefore more spatially clustered, as opposed to older ones which grow slowly. On the other hand, the clustering of low mass halos is known to be anti-biased, as the low density fields tend to merge into a higher one when the background density is raised. However, older halos are more likely to be located in the vicinity of massive structures, whose gravitational potential slows their mass accretion, and are expected to be more spatially clustered and unbiased over time. Studying assembly bias is important, as it allows cosmologists to understand more precisely how matter clusters and yields clues on how large scale structures form. If it can be definitely observed, it will also mean a revision of the dark matter halo model.

Observations have confirmed the probability of assembly bias in high-mass halos, offering encouraging news to cosmologists, although detection in low-mass halos remain elusive. In light of this, Chue attempts to detect assembly bias for low mass halos, though there can be significant challenges in finding a subtle variation in something not directly visible. Using gravitational lensing, he selects two galaxy samples with similar halo masses but different galaxy colors, the proxy to halo assembly histories. If a difference in clustering behavior is found, it could provide observable confirmation of assembly bias. However, there are several challenges to observing this phenomenon, including ensuring a clean galaxy sample with little satellite contamination. Low-mass halos are likely to be satellites hosted by a much more massive host halo, similar to the Moon as a satellite of the Earth. Contamination from satellites can greatly influence the mass determination of galaxy samples, which might potentially lead to a false detection.

“There is little concept of finiteness in cosmology,” he explains, “we build on theory and curiosity, and being able to answer one question usually opens up ten more.” The next idea on Chue’s wishlist illustrates this point. Galaxies are not stationary, but move according to gravitational interactions with large scale structures in their surroundings. Given this attraction, galaxies’ movement through space must have some quantifiable velocity, and figuring out the pattern of this movement would in turn help in tracing the distribution of large scale structures in the universe and constraining cosmology. “Large scale structures are another big question, so by estimating their distribution, we could get some idea of how they behave and evolve,” Chue says. One is tempted to ask if déjà vu is an astrophysics question. “Well, actually…”

Ricky Chue won a Croucher Scholarship in 2014 in support of his doctoral research on dark matter halos and large scale structures. He received his undergraduate degree in Astronomy and Physics from the University of Hong Kong in 2012, and is currently a PhD candidate at the University of Illinois at Urbana-Champaign. 

To view Ricky’s personal Croucher profile, please click here.

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