On the last Monday of April we had our first meeting of star cluster aficionados in New York: GONYC – Globular, Open, Nuclear and Young massive star Clusters. Together with Nathan Leigh from the American Museum of Natural History I initiated this monthly get-together, because we both felt a bit detached from the rest of the star cluster community. This is mostly due to the fact that star cluster research is significantly underrepresented in the U.S., so we have to connect more actively across institutions. We decided to meet at the AMNH since it is the birthplace of one of the most important conference series in the star cluster community – the MODEST meetings. The long-term goal is to host a MODEST meeting at the AMNH again after the first one in 2002. It’s a great time for GONYC – the rapidly growing fields of nuclear star clusters, young massive star clusters, and super-massive star clusters like UCDs has boosted interest in basic gravitational dynamics and star cluster physics. If you want to join the meetings (next one is on May 20) contact me or Nathan!

How to weigh the Milky Way

The Northern Hemisphere of the sky as seen by the Sloan Digital Sky Survey. Stellar streams stick out from the vast number of stars in this view, of which most lie within the Milky Way disk. The Palomar 5 stream is the densest of the stellar streams discovered so far and turned out to be a perfect scale and yardstick for our understanding of the Milky Way. (Credit: Ana Bonaca, Marla Geha and Nitya Kallivayalil with data from the Sloan Digital Sky Survey.)

The Milky Way consists of roughly 100 billion stars like our Sun, which form a huge stellar disk with a diameter of 100-200 thousand light years. The Sun is also part of this structure, hence, when we look into the sky, we look right into this gigantic disk of stars. The vast number of stars and the huge extent on the sky make it hard to measure fundamental quantities for the Milky Way – such as its weight.

Together with my esteemed collaborators I used stars outside this disk, which orbit around the Milky Way in a stream-like structure, to weigh the Milky Way to high precision. In a study published in The Astrophysical Journal, we demonstrated that such streams, which are produced by dissolving globular clusters, can be used to measure not only the weight of our Galaxy, but can also be exploited as yardsticks to determine the location of the Sun within the Milky Way.

Globular clusters are compact groups of thousands to several millions of stars that were born together when the Universe was still very young. They orbit around the Milky Way and slowly disintegrate over the course of billions of years, leaving a unique trace behind. Such star streams stick out from the rest of the stars on the sky as they are dense and coherent, much like contrails from airplanes easily stick out from regular clouds.

We used data from the Sloan Digital Sky Survey, which scanned the sky of the Northern Hemisphere for about 10 years to create a comprehensive catalog of stars on the sky. The stream we tested the new technique on was produced by a globular cluster named Palomar 5, and had already been discovered in 2001 high above the Galactic disk. Coauthor Eduardo Balbinot from the University of Surrey in England revisited the Sloan data and detected density wiggles in the stream of Palomar 5. We found the wiggles to be very pronounced and regularly spaced along the stream. Such variations cannot be random.

It is in fact these wiggles that allowed us to gain the unprecedented precision of our measurements. Using the Yeti supercomputer of Columbia University, we created several million models of the stream in different realizations of the Milky Way. From these models and from comparing the wiggle pattern of the models to the observations, we were able to infer the mass of the Milky Way within a radius of 60,000 light years to be 210 billion times the mass of the Sun with an uncertainty of only 20%. The unique pattern of the density wiggles helped significantly to rule out models of the Milky Way, which were either too heavy or too skinny.

An important advance in this work was using robust statistical tools – the same ones used to study changes in the genome and employed by internet search engines to rank websites. Graduate student Ana Bonaca from Yale University was key in taking such a rigorous approach, which helped significantly in achieving the high precision in weighing the Milky Way.

Such measurements have been tried before with different streams, but the results were always quite ambiguous. Our new measurement breaks these ambiguities by exploiting the unique density pattern that Palomar 5 created as it orbited around the Milky Way for the past 11 billion years.

In the future, we aim at using more structures like the Palomar 5 stream to gain an even higher precision and to create the most realistic model of the Milky Way to date. From the improved precision we hope to learn about the formation and composition of our home galaxy, and to understand how the Milky Way compares with other galaxies in the Universe. So far, the results indicate that the Milky Way is a healthy patient – neither too skinny nor too heavy for its size.

Update: Our press release was picked up by the German press – BILD online, astronewspro-physik, Deutschlandfunk and Frankfurter Allgemeine.

Satellites and Streams in Santiago 2015


What a week! The ESO workshop ‘Satellites and Streams in Santiago’ is over now, and I am still amazed by how flawless this meeting was. As announced here a while ago, I’ve been organizing this conference with Steffen Mieske (ESO) for the last few months. In the end we were 107 registered participants plus some 10 guests. The line-up was absolutely amazing, ESO Director General Tim de Zeeuw gave an opening address, and we heard keynote presentations from the who-is-who in the fields of satellites (Jorge Peñarrubia, Michelle Collins, Pavel Kroupa, Rodrigo Ibata, Vasily Belokurov, Gurtina Besla), streams Steven Majewski, Amina Helmi, Aaron Romanowski) and the star-cluster/dwarf-galaxy interface (Oleg Gnedin, Jay Strader, Dougal Mackey, Michael Hilker, Anil Seth).

I have learned so many things, and made so many new friends and acquaintances, it will take a while to let all of that sink in. For our all convenience, the conference was heavily tweeted by several people, and Coral Wheeler was so kind to make a storify out of the tweets. Interestingly, the official #SSS15 hashtag of the meeting was also occasionally used by beauty & nail ads. For those who want more than 140 character summaries of the talks, we will upload the slides as pdfs within the next days on the ESO conference website.

Being chair of this conference was definitely one of the highlights of my astronomy career so far, and it’ll be hard to improve next time! But I realized that the administrative aspect of science is something I wholeheartedly enjoy!! I guess that’s a good thing?!

Hubble Fellows Symposium 2015

2015 Hubble_Fellows_Symposium_GroupPhoto 2

Right after getting back from Chile I headed straight to Baltimore for this year’s Hubble Fellows Symposium at the Space Telescope Science Institute. Every year most of the 51 Hubble fellows (three years with each 17 fellows) get together at these meetings to present their work to each other. It is quite a fun meeting, since you get exciting updates on nearly all major fields of astronomy within three days. Last year we criticized, though, that the schedule left no time for chatting, schmoozing and collaborating. This year the organizers relaxed the schedule  a bit, so every talk was only 20 minutes long. It made the meeting much more enjoyable and put the fun back into the symposium.

MODEST15 in Concepción

One of my favorite conference series is “Modeling and Observing Dense Stellar Systems” (MODEST). This year, MODEST15 took me to Concepción in the beautiful south of Chile. Flying into Santiago over the Andes is already a spectacle.


But once landed in the south, the beauty of the landscape is breathtaking, and a few hours drive brings you to the most amazing places in the Andes, full of outdoor activities and gorgeous views.


With astronomer Florent Renaud, I flew in a few days before the conference to enjoy the late summer of Chile and explore the area around the majestic Villarrica volcano.


The Villarrica lies in the Araucanía Region of Chile, famous for its thick forests of araucaria trees and its volcanic activity – in fact, Florent and I missed the first eruption of the volcano in thirty years by only two days!


Even though we were not allowed to climb the volcano (yellow alert!), the trip was very enjoyable, which may be due to the termas geometricas that are fed by volcanic hot springs.


But there’s so much more to do than just boiling in 45 degree Celsius hot water. Staying in the touristy town of Pucón offers countless opportunities, of which the best may be the day trips to the national parks in the area.


The trip would have already been worth it just for the hikes with their serious elevation gains and rewarding views (both very breathtaking).


But there’s just so much more to do! Bazillions of waterfalls and mountain lakes with gorgeous beaches make Pucón one of the most popular travel spots in Chile.


After all this outdoor fun, the conference at the Universidad de Concepción was very enjoyable. The theory group at the astronomy department in Conce has organized a great meeting with a very interesting scientific program – which was even featured on Chilean television.


This week of being back in Chile and seeing all my friends and colleagues again made me realize how long I’ve been in the US for already! It’s so valuable for me to keep the connections to this amazing community alive, since it is so completely underrepresented in the US. To foster our relationship and increase our visibility, the East Coast MODEST’lers Nathan Leigh, Steve McMillan and I agreed on having a dense stellar system get-together in New York City every now and so often. We are even considering to have a big MODEST meeting at the Museum of Natural History again next year – the place where the MODEST series was born in 2001!

Stream fanning

streamfanning In my first year at Columbia I worked with grad student Sarah Pearson on an idea that Kathryn Johnston had while trying to find an orbit for Palomar 5 in a Law & Majewski potential. Wait what? Who’s Sarah, who’s Palomar 5, and what is a Law & Majewski potential?

Palomar 5 is a globular cluster in the halo of our Galaxy, the Milky Way. It is about 12 billion years old and consists of roughly 30,000 stars. The star cluster can be seen within the footprint of the Sloan Digital Sky Survey. But even more fascinating is that we can also see a stream, consisting of at least as many stars, stretching out from the cluster along its orbit. This stream – there are actually two, one in the leading direction and one in the trailing direction – spans about 23 degrees on the sky, while being on average half a degree wide. That’s about the size of 50 full moons!

In the above figure, Palomar 5 can be seen in the blue density contours as a blob at roughly RA = 229 deg and Dec = 0 deg. The stream stretches from the upper left to the lower right. There’s lots of random fluctuations all over this density map, but we made sure that the stream is really only along this diagonal, and that the rest is just noise. Red points in the figure show the N-body model that comes closest to the observed shape of the stream. That’s not close AT ALL you may say, and you’re right. What Sarah used to generate this model is a very specific form of the mass distribution within the Milky Way and its dark matter halo. This form was proposed by David Law and Steve Majewski as they modeled the Sagittarius stream – a similar stream to the one of Palomar 5, but much further out in the halo and from a different kind of progenitor, a dwarf galaxy.

The Law & Majewski potential, as we call it, looks like an American football, and its oriented such that it causes all stars, star clusters and dwarf galaxies to be on weird orbits within the Galaxy halo. Sarah found that the orbits are in fact so weird that it’s impossible to find a thin and curved stream like the Palomar 5 stream within this potential. The streams don’t stay thin and long, but instead they fan out. From this we concluded that a) the Law & Majewski potential must be wrong within the inner part of the halo of the Galaxy, and b) that whenever we see a thin and curved stream in the sky, we can rule out certain classes of (weirdly shaped) potentials. As a consistency test, Sarah tried the same with a simple spherical dark matter halo – and it was super easy to get perfect models of Palomar 5.

Given the fact that this was Sarah’s first-year project, and that she was still taking lots of classes at the time, we’re all stoked that she got this substantial paper out so quickly. Go check out her website, she’s up to great things.

With the AAS in Seattle


The year 2015 started off with a trip to Seattle to attend the 225th meeting of the American Astronomical Society (AAS). I just became a member of the AAS (for which I’m grateful to my sponsors Kathryn Johnston and Jerry Ostriker!), and so this was the first time for me to attend one of these ginormous, annual meetings. When I first looked at the program, I was impressed (not to say overwhelmed) by the number of attendees, the variety of splinter meetings, and by the quality & quantity of career opportunities. This impression didn’t change much throughout the week.

About 3000 astronomers from all over the world (but mostly the US) come to the traditional winter meeting. All fields, subfields, and niches of astronomy are present. So whatever you’re looking for you’ll find it there. On top of that, many companies, institutions and media representatives show up at this meeting. It’s the one time of the year where astronomy makes big news and big business.

Now, usually a conference has about 100 participants from one discipline of which maybe 50 will give a talk of about 20-30 min. At the AAS meeting, talks are only 5 minutes, and the sessions are very mixed. Hence, it’s much more favorable to present a poster – which I did (see above). I haven’t presented a poster in years, so I was skeptical in the beginning. But now after this meeting I’m a big fan of posters! The interactions you have while standing in front of your poster are much deeper and more fruitful than what you get from a talk. Without the big audience listening, people can and will ask dumb questions, which turn out to be not dumb at all. They make you realize what you can’t realize during a talk – how many people in the audience don’t get what you’re doing because you didn’t bother to explain.

Besides the poster, I enjoyed many helpful career sessions, a packed (!) impostor-syndrome session, a memorable astronomer party, and lots of great food and coffee with amazing colleagues and friends. The whole event is really more of a big party than a conference. Thus, I’m looking forward to the next winter meeting in Florida – even though beautiful and coffee-loving Seattle is hard to beat as a venue!