Where are they now? -- An Interview with KIPAC alum Marusa Bradac

In the series, "Where are they now?" we check in with KIPAC alumni: where they are now, how they've fared since their days exploring particle astrophysics and cosmology at the Institute, and how their KIPAC experiences have shaped their journeys.

Maruša Bradač, a KIPAC post-doctoral researcher from 2004-2007, is currently an associate professor of physics at UC Davis, where she continues her groundbreaking work in dark matter research – work that first gained notice at while she was at KIPAC.

In 2006 Maruša and her colleagues (including KIPAC scientist Phil Marshall) published a paper containing a now famous image of two subclusters of galaxies that were passing through each other, with the whole system being called the "Bullet Cluster."  The image was created by taking very precise measurements of the cluster's gravitational effect on light from even more distant galaxies and reconstructing the total mass distribution within the cluster from these subtle effects. 

Their results revealed, for the first time in such an incontrovertible way, the presence of dark matter in a colliding system and how it was distributed within the overall structure of the two interacting galaxy clusters.

Image of the Bullet Cluster where magenta indicates hot x-ray emitting gas (i.e. normal matter)  and blue indicates dark matter ("seen" indirectly through gravitational lensing reconstruction).

LW: Can you tell us a little bit about your academic background before you came to KIPAC?

MB: I did my PhD and graduate research at University of Bonn, Germany. My undergraduate years I spent partly in Bonn and partly in my home country Slovenia.

LW: How did you choose KIPAC?

MB: As I was finishing my PhD I was also applying for postdocs. I specifically applied to places with excellent groups doing research in dark matter and gravitational lensing, and that led me to KIPAC. However, while I had no doubt KIPAC was one of the best places in the world scientifically, I was very reserved in moving to the US from Europe. I visited Baltimore before, and I was more than a bit worried and scared in a "The Wire" sort of way. But then my friend and colleague Phil Marshall sent me a photo of his recent ski trip to Tahoe. And the rest is history.

LW: How so?

MB: I love to ski. I ski whenever I can. Since I came to California I've gone to Tahoe many more times than Phil – orders of magnitude more! (Laughs.) But occasionally we still go together.

LW: And when you weren't skiing you found some of the best evidence yet for dark matter. Can you tell us about your Bullet Cluster paper?

Maruša Bradač stands below two of the Keck telescopes.  She's holding two metal sheets called "spectroscopic masks" that were made for her observations.  The masks have tiny holes punched in them so only the light from her targeted objects reaches the camera.

MB: I would love to. It is still my favorite paper and the main reason for my successful career. In that paper for the first time we saw dark matter separately from regular matter rather than superimposed upon it. This enabled us to unambiguously detect its presence, as well as study its properties for the first time. We now have many more such systems that we are studying with the Merging Cluster Collaboration (MC^2) now at UC Davis and all the systems give a coherent story. I will be forever grateful for KIPAC for enabling this discovery, (former KIPAC Director) Roger Blandford and Phil Marshall for helping me push this project (and ask Phil if you ever wonder why NASA puts dark matter in blue color on all their press release images now). I'm also grateful for the support of the wonderful SLAC press release people Neil Calder and Kelen Tuttle. I know this all sounds a bit like Oscars thank you speech, but to be honest for me it definitely felt like that at the time.

LW: Can you give us a brief overview of your career after KIPAC?

MB: After KIPAC I went to Santa Barbara as a Hubble Fellow, where I shifted my gears and started to work on the first galaxies that formed in the Universe, the main topic of my research nowadays. We are seeing galaxies right at their birth and at the time when universe was only a few percent of its current age. It was very exciting time with new technologies coming online, like a new camera on Hubble Space Telescope. Also it was the time when I fell in love with the Keck Telescopes on Hawaii. I moved to Davis in 2009 where I continue doing my research on both dark matter and the first galaxies. In Davis I also started SURFSUP, which stands for Spitzer UltRa Faint SUrvey Program. With SURFSUP we take extremely deep images with the Spitzer Space Telescope to image first galaxies. It is exciting, as we are beginning to see that galaxies might have started forming stars earlier than we first thought. In a way the acronym is another KIPAC contribution – I learned how to surf when I was there. (Laughs.)

LW: So that's why your Physics for Nonscientists class includes lectures on the physics of surfing as well as the physics of skiing.

MB: Yes, the main attraction of Davis was a good department and access to Keck, but when I moved to Davis I also told everyone that it is halfway between skiing and surfing. I love to scuba dive, too. And recently I started mountain biking, which is why my course now also has a lecture on cycling. There's a lot of neat physics in cycling.

LW: Do you have any remembrances of KIPAC you'd like to share? Do you still keep in touch with your colleagues?

MB: Yes, and yes. KIPAC is simply an amazing place, not just because you are surrounded by people who do amazing science, but also because it felt like the place is one big family. I very fondly remember the many Thanksgiving dinners that Roger and Liz Blandford prepared for all the foreigners among us, whose families were far away. They really became my replacement family. And then there were Ziba (Mahdavi), Martha (Siegel), and Christine (Aguilar) from KIPAC Administration who always took such good care of us (sometimes you need to file a travel report, but sometimes you also need a shoulder to cry on, sometimes you want someone to laugh with). I can go on and on. Which is the reason why I still visit often, both for science and to remember good old times.

------------ Useful Links:

Maruša Bradač (personal web site)

Baby Bullet Cluster work (with KIPAC faculty member Steve Allen)

Dark Matter Observed: Most Direct Measurement of Dark Matter Allows Study of its Nature (original press release from SLAC regarding Bullet Cluster findings) 

2006 Cosmic Variance Explanation of the Bullet Cluster


The dark side of the universe: Maruša Bradac at TEDxUCDavis


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