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!

Tidal streams in an evolving dark matter halo


2014 has been a great scientific year for me, in which I had the opportunity to contribute to many exciting projects. I’m very fond of the paper that came out of my collaboration with Ana Bonaca and Marla Geha, which we started back in 2013 when I was at Yale. Ana, who is a PhD student at Yale and who got famous for discovering the Triangulum stream in the southern part of the Sloan Digital Sky Survey, put a lot of effort into this project, and I’ve learned a lot from working with her.

The question we asked was simple: “What is the effect of an evolving galactic dark matter halo on the formation and evolution of tidal streams!?” – motivated by the fact that in all tidal stream research we simplify the galactic halo by assuming that it has a simple analytical form and that it does not evolve with time. It was obvious to everyone that this has to be an oversimplification, as dark matter halos are supposedly made of dark matter particles, which form bound substructures on all length scales. That is, a galactic dark matter halo contains hundred thousands of subhalos orbiting within the main halo, which themselves are substructured. But due to computational difficulties, investigations have either focussed on tidal streams or on the structure of dark matter halos, not on both at the same time.

Thus, answering our question was tricky, and it required us to run a >1 billion particle simulation of a live-forming dark matter halo. Together with Jürg Diemand from the University of Zurich, we re-computed the Via Lactea II simulation (one of the highest-resolution N-body simulations of a Milky Way-sized dark matter halo) from a snapshot 6 billion years in the past to the present day. While doing so, we simultaneously generated tidal streams from >10,000 “cluster particles”, which we randomly inserted into the simulation such that they covered a wide range of possible orbits within the main halo.

Over the course of 6 billion years of simulation, each of the cluster particles generated a streakline-ish tidal stream by constantly releasing stream particles into the galactic halo. The same we did for an analytic galaxy halo that did not evolve with time. Two examples of streams forming in the evolving and in the non-evolving galaxy halos are shown in the figure above. It is immediately apparent that the streams in the “lumpy & evolving” halo are much wider and more dispersed than in the “smooth & static” case.

Instead of asking what is causing these differences in detail, we asked ourselves, how this would affect our goal of measuring the weight of a galaxy by modeling the streams. Ana took 256 random streams from both data sets (evolving halo and analytic halo) and modeled each of them with our Fast-Forward method – attempting to recover the mass and shape of the dark matter halo. It turned out that, while this was easily possible for the analytic, static halo, it can be fatal for the evolving halo.

Streams have millions of encounters with the dark matter subhalos while they are orbiting through the main halo. These encounters can be weak and negligible, or they can be strong and even deflect the orbit of the whole stream. Moreover, the encounters can be disruptive, and punch holes into the dynamically cold streams, or merely puff them up a bit. All these effects together make our recovery approach prone to biases, i.e. errors in the interpretation of the underlying mass and shape of the galactic dark matter halo. We may overestimate the mass of the galaxy by up to 50%, Ana found!

However, there’s still hope for us! By combining several streams, we will be able to correct for all the mistakes we could possibly make. So there’s our main motivation for observing more streams on the sky, and finally modeling them all together. It seems to be the only way to accurately measure the weight of a galaxy like our Milky Way.

Streams, streams, streams


For my presentation at the Gaia Challenge in Heidelberg, I made a quick ADS search for publications on tidal stream observations since their first discovery in 1995 by Carl Grillmair. Although tidal streams had been theoretically predicted before 1995, Grillmair showed for the first time that some star clusters have significant amounts of stars outside their tidal radii. At about the same time, the Sagittarius (Sgr) dwarf galaxy was discovered by Rodrigo Ibata, and people started finding patches of stars that belong to the stream emanating from this galactic satellite everywhere in the halo of the Milky Way. It took 6 more years until Michael Odenkirchen showed with commissioning data of the Sloan Digital Sky Survey that the globular cluster Palomar 5 has a coherent stream emanating from its Lagrange points out into the tidal field of the Milky Way. This publication also marked the onset of survey since in astronomy. Since 2000/2001 the rate of papers presenting new observational results on streams in the Galactic halo or around other galaxies grows exponentially. While this growth was initially driven by studies on the Sgr stream (red cumulative curve above), the focus is now shifting towards fainter streams like Palomar 5, NGC 5466 or GD-1 (blue curve). It’s a really exciting time to work in this field!

Gaia Challenge 2014


End of October, I was in Heidelberg to attend the Gaia Challenge workshop at the Max Planck Institute for Astronomy (MPIA). This workshop series was initiated last year by Justin Read, Mark Gieles and Daisuke Kawata at the University of Surrey (Guilford, south of London). This year we came together again, but this time the meeting was organized by Glenn van de Ven, who is a research-group leader at the MPIA. Glenn booked the Haus der Astronomie (house of astronomy) for us, which is a public outreach building on the premises of MPIA. The cool thing about it is that it’s shaped like a spiral galaxy (see photo above). The bulge of the Haus der Astronomie is a large auditorium and the spiral arms consist of seminar rooms and a child day care center. It’s quite a unique place!

About 80 participants came together to work on modeling problems related to the Gaia mission. We formed five groups with significant overlap between each other:

  1. spherical & triaxial systems (like dwarf galaxies),
  2. disks (meaning mainly the Galactic disk),
  3. streams & halo stars (everything which is not in the Galactic bulge or the disk),
  4. collisional systems (i.e. star clusters),
  5. astrophysical parameters (covering everything non-dynamical Gaia is going to measure).

I had the fun role of being the coordinator of the streams & halo stars group. Together with Andreaa Font, I organized our daily meetings, lead the discussions in the group, focussed our modeling efforts, and summarized our progress. Our mission is to identify streams (i.e. groups of stars that are coherent in phase space) in survey data like Gaia’s first data release (expected early 2017) and then find fitting dynamical models for it. Sounds simple, but it’s absolutely not. With still 2.5 years to go until Gaia data release 1, we are still struggling with the most basic problems. But the workshop gives the stream & halo stars community a great forum to come together and figure out how to address these issues.

The concept of the workshop is to pose challenges in form of numerical simulations (which can be downloaded from the Gaia Challenge wiki). These we take as idealized survey data. The modelers apply their respective methods to this idealized data and try to recover the underlying (known) model parameters of the simulation. For the streams group this means that we make a numerical simulation of a dissolving Galactic satellite, like Palomar 5 or the Sagittarius dwarf galaxy, and let it form a stream. The modelers then get the stream stars’ positions and velocities and have to figure out what the parameters of the Galactic model are, which was used in the simulation. A first attempt of collectively solving one specific challenge is the Palomar 5 challenge that I posted. Several people are trying to model it right now, and we are collectively writing a paper about the results using github. It’s all open science, and everybody is invited to join!