Carl Grillmair was shot and killed at his home a few days ago. I knew Carl through his foundational work on stellar streams, but also as a colleague and collaborator at conferences and other events. He was a creative, generous, and kind person, and he will be missed.
Carl was one of the first (and certainly the most prolific) to apply matched filters to search for stellar streams in catalog data from large photometric surveys (e.g., some early papers) like the Sloan Digital Sky Survey (SDSS). Stellar streams around the Milky Way are the remnants of globular clusters and dwarf galaxies that have been pulled apart by the tidal forces of the Galaxy. They are important tracers of the Milky Way’s dark matter distribution, and they are also interesting in their own right as a record of the Galaxy’s accretion history. Carl is responsible for discovering many of the first thin stellar streams (likely formed from fully-disrupted star clusters), which are also very sensitive tools for constraining the presence of (otherwise undetectable) dark matter substructures in the Milky Way.
One of the first streams Carl discovered was the GD-1 stream (named after Grillmair & Dionatos, though they didn’t go on to find streams 2, 3, etc. together!). GD-1 is my favorite stream and has been an important focus of my research for several years. It is one of the longest (in angular extent) and most-studied streams around the Milky Way because it is relatively nearby and seems to show evidence of perturbations from dark matter subhalos. But Carl’s impact on the field is much broader than just GD-1. He laid the foundation for future searches for thin streams, and his discoveries have been hugely important for understanding the population of streams around the Milky Way — he appears throughout the bibliography of our recent review of Stellar Streams in the Gaia Era.
Carl’s method for finding streams was to apply a series of matched filters to select stars in color-magnitude space that are consistent with an old, metal-poor stellar population, and to scan through a range of distances. He would turn this sequence of filtered images into a movie, watch it, stretch the video, and rewatch. It was very hands-on, but he was incredibly effective at identifying faint patterns in the filtered data. Carl was also colorblind — we sometimes joked about whether this gave him an unfair advantage in finding patterns in his black-and-white filtered images. He tried many automated (early machine-vision) methods for finding streams (e.g., using the Hough transform), but he always came back to his original method (his eyes) because it was so effective.
I later learned that his work on streams was just one part of his scientifically diverse and atypical career, having also worked on Spitzer, instrumentation at Palomar, among other things.
He was also full of completely insane stories from his other life as an avid glider pilot. I remember one about how he spent his time in his 20s. I believe he was living in Australia at the time, and he was part of an amateur paragliding group that built their own gliders and flew them in the Australian Outback. They would tow the gliders from the back of a pickup truck to take off and then fly for hours over the desert. One time, he took his (then) girlfriend up for a flight, and as they were gliding at several thousand feet, the knot that supported the seat came loose — they had to hold on to the glider with their hands and steer it back to the ground!
Carl was always a pleasure to talk to at conferences and was very supportive of younger researchers who built upon his work. I will miss seeing him around.
End gun violence.