Broadly, I am an observational cosmologist. I use telescopes to gather data that I use to test theories, make predictions, and constrain the understanding of our universe. Big questions include:

• What do our observations say about ΛCDM?
• What is the nature of dark matter and dark energy?
• How precise do our observations need to be?

Specifically, I study merging clusters of galaxies, which are the most energetic events since the Big Bang. These mergers represent the current stage of cosmic structure formation: large clusters of galaxies are merging with others and growing as we speak. Don’t hold your breath though. These mergers occur over the course of billions of years.

First, merging or not, a cluster of galaxies (or galaxy cluster or cluster — I’ll use them all interchangeably) is an incredibly large region of space with a very large over-density of mass. More than 80-90% of this mass is in the form of dark matter. Most of the rest of the mass is a diffuse hydrogen-helium plasma  (protons, deuterium, alpha particles, and electrons) with a measurable amount of metals (note that metallicity in the astronomical sense is a slightly incorrect term meaning any nuclei more massive than those of hydrogen or helium). This plasma is called the intracluster medium (ICM). Only a small fraction (5% at most) of the mass is in the form of luminous stars and galaxies.

The name for these objects is a remnant from the original discovery technique: using a big telescope and photographic plates that absorb the same wavelengths of light that our eyes perceive. They look like clusters of galaxies. Now our telescopes are much bigger, our cameras are much more sensitive, and the locations that we put them are much more ideal (Mauna Kea or even in orbit).

I  use ensembles of particularly massive (∼10¹⁵ times the mass of the sun) mergers to piece together the merging process. The observations necessary to complete this task probe a number of interesting phenomena within these systems:

• Self-interacting dark matter
• Star formation
• Particle acceleration
• Shocks
• Turbulence
• Plasma instabilities

Note that the various interesting facets of these systems occur on time scales that are much shorter than the time scale of the merger itself. Therefore, in order to constrain the astrophysics within, precise dynamical models are vital (more on this later).

Ensembles provide statistical power to make predictions. For example, if I saw one person that has brown hair and green eyes, I have no statistical power to exclaim that all people have brown hair and green eyes. I do have the statistical power to state that the nearest person to me at that instant had brown hair and green eyes. If I saw 100 consecutive people with brown hair and green eyes, I might have wondered what it is locally that attracts brown haired, green eyed people.

Taking this analogy a step further: image if I saw so many brown haired and green eyed people that I was capable of noticing finer details in those features. What if I could describe how the features correlate with each other? Perhaps how the radius of curvature in the hair’s curl varies with the hue of green or the height of the person.

This concept is why we need large numbers of these systems. Many, many minor details are observable in clusters of galaxies. I can see how bright they are and how many X-rays are leaving them.  I can infer the mass, temperature, and the spatial distribution of the dark matter, ICM, and galaxies.

As telescopes, modeling techniques, and computational abilities improve, merging galaxy clusters appear to be among the best laboratories in all of the universe to probe the points above. In general, there are two complimentary ways in which cosmologists study merging clusters: observations (and modeling) and computational simulations. Observational constraints come from ten orders of magnitude in frequency (radio waves to X-rays). Simulations vary by resolution and physical scale and can include various astrophysics including  magneto-hydrodynamics and dark matter self-interactions.

I am an observer: an optical spectroscopic and photometric expert. I have observational experience with both the Keck II and Subaru telescopes, which are located on the summit of Mauna Kea. This is widely considered among the best two or three optical astronomy locations on the surface of the Earth. I have additional experience with data from Hubble Space Telescope (HST) and Chandra X-ray Observatory (CXO). I am also in collaboration with a team of X-ray and radio astronomy experts who provide important information from these frequencies.

Remember that clusters are composed of three primary components: dark matter, ICM, and galaxies. It takes multi-wavelength observations to characterize all of these components. The optical observations locate the galaxies and dark matter (via gravitational lensing). The X-ray and radio observations offer a view of the ICM.

In this image, the total mass (white contours), X-ray (color-scale), and radio emission (yellow contours) indicate that this merger is occurring between two subclusters oriented east-west. The diffuse radio features on the periphery of the cluster are radio relics, and they trace supersonic shocks that are begin propagating near core-passage. Further proof of this cluster being a merging system comes from the cool-core clearly traveling to the west with a wake feature trailing behind.  This is indicative of the Bullet Cluster: the most famous merging cluster of galaxies.

Keep in mind, all of the above inference comes from what amounts to a single snapshot in time. Yes, these observations occurred over the course of several years, but when several years is compared to the time it takes for this system to merge and coalesce into a single cluster, i.e. Gyrs, it is evident that human time scales and merger time scales to not match up. The merger above has been occurring for approximately 1.5 Gyr. This is 1/3 of the age of the Earth.

Because we can’t watch clusters merge, it is informative to look at an ensemble of systems and study subtle differences of broadly similar systems. In this spirit, I have compiled a list of bimodal merging galaxy clusters that contain the remains of a cool-core in at least one of the two subclusters. A literature review has revealed that at least fifteen of these systems are observed sufficiently to constrain the age of the merger; i.e., the time between the first core passage and the observed state of the cluster. With ages of each system, differences in the observables may be tracked temporally. My hypothesis is that we should be able to observe something called the “ram-pressure slingshot effect.” This is an ongoing project of mine.