Profile 03 Alessandro Sonnenfeld

Hometown: Lucca, Tuscany, Italy
Position: Postdoc/Project Researcher
Recommended reading: Dialogue Concerning the Two Chief World Systems, by Galileo Galilei. “The book gives you an opportunity to understand the sky using very limited, every day tools. It is a good introduction to science,” says Sonnenfeld.
Favorite activity in Japan: Going to karaoke. “I try to go to karaoke every other week. I like 1980’s classics.”

What can you tell us about your current research at Kavli IPMU?
I’m basically involved in two projects. The first involves data from the Hyper Suprime-Cam (HSC) digital camera. HSC is undergoing a large survey of the sky, called the Subaru Strategic Program, which is covering 1,400 degrees of sky. Within this data, I’m trying to discover as many gravitational lenses as possible. As you know, gravitational lensing is a relatively rare phenomenon. It is also very simple. It happens when you have a massive object, which is typically a galaxy, in front of a light source, which is another galaxy in the background.

Now, if the alignment is good enough and the foreground galaxy is massive enough, then the light from the background object gets distorted, and you can get multiple images of the same background galaxy. You can use these images to measure the mass of the galaxy that is in front; the more massive it is, the larger the separation between the images.

Why is this important?

Gravitational lensing allows us to make one of the most precise measurements in astronomy; remember that astronomical measurements have in the past been famous for being very uncertain. With gravitational lensing, we can get precisions of 1 or 2 percent, which is pretty robust.

What questions do you seek to answer?

The main questions here are: how much dark matter is there? What are galaxies made of? How do galaxies grow in time? As far as we know, galaxies grow by merging with smaller galaxies, but the details of how that happens are not obvious. And while we can model these questions, we have not been able to test them using experiments, until now. Gravitational lensing gives us the opportunity to do that, and HSC is a great instrument for answering these questions.

Can you say something about the second part of your work at Kavli IPMU? 

The other aspect of my work is collaborating with people like Alexie Leauthaud, who is now at the University of California, Santa Cruz, but was based at Kavli IPMU. We are trying to develop a more statistically robust way to analyze weak lensing data. With weak lensing, you have a massive object in the foreground that is lensing background images, but the lensing or distortion is small or weak—you don’t get multiple images of objects. Rather, there are only tiny distortions that can only be measured statistically by summing over large numbers of background sources. We are trying to develop a more statistically advanced way of analyzing this data that does not rely on some of the assumptions—such as “stacking” —that are typically made.

As weak lensing requires summing over thousands of galaxies and measurements, we usually “stack” the measurements by assuming that all of the galaxies look the same, and then we repeat the measurement again and again until we reach high precision and small uncertainty. What we are trying to do is weak lensing without resorting to this “trick” while taking advantage of all the data that there is.

What questions are being tackled via weak lensing data? 

In this case, the main questions are such things as: what does the dark matter halos of galaxies look like? As you may know, dark matter has mass, and while we cannot see it with the naked eye, we can see it with gravitational lensing—that is what makes gravitational lensing so powerful. We are trying to link the properties of the dark matter halo—that is the dark matter distribution around a galaxy (which we measure from weak lensing) —with properties of the galaxies that we can actually see. So far, our understanding is that more massive galaxies live in more massive halos, but there might be other correlations that we don’t know of. For example, it could be that, for the same mass, galaxies that are more extended live in more massive halos. To measure this robustly, we need to develop new statistical analysis methods.

Can you say something about your experience of life at Kavli IPMU?

The thing I enjoy the most is the number and frequency of visitors. Even though Japan is geographically far away from where most of the latest science is done globally, in places like Europe and the USA, we still have lots of opportunities to interact with smart people that visit here. And these people come from around the world.

What can you tell us about your academic background? 

I did my undergrad and masters in physics in Pisa, Italy. It was during my masters that I decided to focus on astronomy and gravitational lensing. I then joined the Phd. program at the University of California, Santa Barbara, followed by a short postdoc at the University of California, Los Angeles, before coming to Kavli IPMU.

What drew you to science in the first place?

I remember enjoying looking at astronomy books as a kid, especially images of the planets of the solar system. I think that feeling of enjoyment stayed with me until I realized that I could do this as a profession. I also had very good teachers. My elementary school teacher, for example, was very good at making us curious about science. I remember in the first day of school he said to us, “‘Alright guys, by 5th grade, we are going to go to the Moon!’ My reaction was: “‘Alright. Let’s do that!’ But it didn’t happen, so I figured I might have to work a little harder for it to happen.”

What can you tell us about future projects?

I would like to keep working on HSC data. In the very first chunk of data released, we found 50 new strong gravitational lenses, which was special because it showed two background sources. So there may be other cool objects in there waiting to be discovered. I’m also looking to work more with the Prime Focus Spectograph, which is to be installed on the Subaru Telescope as part of the Subaru Measurement of Images and Redshifts (SuMIRe) Project.

Where can we see or read about your research?

You can find news of my publications by visiting my webpage at: