Graduate Students Peer into Cosmic Web

Since first light in 1993, the twin Keck Telescopes at Hawaii's W. M. Keck Observatory have produced images of stars, nebulae, and massive galaxies in unprecedented detail. However, bright objects like these represent only a small fraction of the total mass in the universe. Most regular matter exists in faint, long filaments stretching between galaxies to form the cosmic web—a vast, dim structure.

The Keck Cosmic Web Imager (KCWI), designed and built at Caltech, is a highly sensitive instrument capable of imaging the cosmic web with more precision than any other cosmic web imager. It is currently being installed onto the Keck II telescope.

The goal of KCWI is to understand the origin of galaxies, in particular, the physics of how they form within the cosmic web. Two Caltech graduate students, Donal O'Sullivan (MS '15) and Prachi Parihar (MS '16), worked on the instrumentation and observational aspects of KCWI. We sat down with them to discuss what it's like to build a brand-new instrument and what they hope it will discover.

What are your specific roles in the mission?

Donal O'Sullivan: I work on both the instrumentation and observational sides of KCWI. Obviously we haven't observed with KCWI yet, but I've spent a lot of time with its less-sensitive prototype, the Palomar Cosmic Web Imager at Caltech's Palomar Observatory. I built a small but vital component of KCWI—the module that houses the arc lamps used to spectrally calibrate the instrument—and helped the engineering team run data analysis to verify its performance.

Prachi Parihar: I have helped with the instrumentation side of KCWI and have observed with its prototype at Palomar, but my main research work is on the theoretical side. KCWI is a complex instrument and one of the very few of its kind. We are observing parts of the cosmic web that have until now been invisible to astronomers. Given the uniqueness of these observations, it is critical that we understand how to analyze and interpret them. Simulations can be very helpful in this area because we can compare them with observational predictions for the same galaxies, providing both more guidance and confidence in our interpretation of the data.

What kinds of challenges have you faced during this process?

DO: One of the biggest challenges I think I faced was learning that "good enough" is "good enough." I think a lot of scientifically minded people have a natural desire to find the optimal solution to every problem, but there are some tasks where optimizing the solution becomes a big time-sink that costs more than it is worth. Identifying how good you really need a solution to be and taking the most time-effective approach instead is a really valuable skill.

What discoveries are you looking forward to?

DO: I am most excited about the potential to discover more examples of the protogalactic disks my adviser, Chris Martin [professor of physics], discovered with the Palomar Cosmic Web Imager. Currently, we believe that galaxies form when the dark matter in the universe gravitationally collapses to form dense regions that we call dark matter halos. These halos gravitationally pull "normal"—not dark—matter like hydrogen toward their centers. This gas spirals inward and forms a disk at the center due to the conservation of angular momentum, which eventually leads to disk galaxies like the Milky Way and Andromeda. Among other things, we are trying to image this happening in the early universe.

PP: KCWI will go deeper and collect higher-quality data for more galaxies. So far, we have detected interesting structures ranging from large gaseous disks to clumps of gas. Each observation is unique, and each galaxy environment is different. I am looking forward to getting a larger statistical sample so we can determine the prevalence of different kinds of objects and piece together a more complete picture of galaxy environments during early galaxy formation. 

How did you become interested in astronomy?

DO: I've always been very widely interested in science—every field has such fascinating stories and discoveries. It was genuinely hard to decide in which direction to go when I was in school. I took every subject I could and eventually became fascinated with two main areas—astronomy and neuroscience, because I thought they were the fields that contained some of the most profound questions about the universe we observe. Astronomy looks at everything we observe in the cosmos and tries to understand it. Neuroscience looks inward at how we perceive the world and breaks that down.

I get the best of both worlds with astronomical instrumentation. We are in the most exciting time ever for opening up new windows on the universe. We have ultraviolet telescopes, X-ray telescopes, global-scale radio interferometry, and now even gravitational-wave observatories! I feel insanely lucky to be right in the middle of it all at Caltech.

PP: There is something really compelling about using the modern telescopes and instruments of today to study the same skies that have fascinated both laypeople and scientists for thousands of years. As an undergraduate physics major, I realized that my physics electives happened to all be astrophysics classes. Since that time, I have had a special fondness for galaxies because I have been amazed by the way these objects seemingly self-assemble into organized structures. KCWI is particularly exciting for me because it allows us to look at how galaxies are born.