Signs of hidden matter

Comparison of different observational data (rows) to three different cluster models (columns) of the globular cluster NGC 6624. The observed accelerations of a low-mass X-ray binary and of three pulsars (bottom row) tell us that the density of matter (top row) in the center (small radius R) has to be higher than what we see in the form of stars.

When we look into the night sky we are fooled by the bright and sparkling stars, and we think they make up most of the matter in the Universe. Among astronomers that’s commonly believed to not be true, since observations tell us that a significantly larger amount of mass fills the Universe in the form of dark matter. What dark matter is, and whether it exists at all, is difficult to answer and no one has found a definite answer yet. However, there are other forms of matter that hide from our naked eyes, or even from our large armada of telescopes. Planets, brown dwarfs, low-mass stars, neutron stars, white dwarfs and black holes are all emitting barely any electromagnetic radiation (a.k.a. light). Thus, only in very few, and nearby cases we can actually observe these objects. For most parts of the Universe, this hidden matter remains unseen and we have to add a certain amount of dark mass to the mass we see in stars based on the best of our knowledge.

It’s important to note at this point, though, that this hidden matter is by far not enough to explain the mass discrepancy in the Universe that led us to the hypothesis of dark matter. However, in certain parts of the Universe, hidden matter in the form of black holes, neutron stars and white dwarfs can make up a large fraction of the local matter density. One of these places is the Milky Way globular cluster NGC 6624. According to a paper I published recently with Miklos Peuten, Michael Brockamp and Pavel Kroupa, the center of this cluster should be filled to the brim with these dark remnants, as we call them. One third of all matter in this star cluster is hidden from our deepest observations, and dwells in the cluster’s center. There it causes such a strong gravitational acceleration on the neighboring stars, and especially on a low-mass X-ray binary called 4U 1820-30, that we can measure how these objects got accelerated over the past 20 years.

Space telescopes and radio antennae have monitored 4 special objects (the above mentioned X-ray binary and 3 pulsars) in the cluster over decades, and they tell us what the gravitational field in the cluster is like. By comparing this inferred gravitational field with the mass distribution in form of the stars, we can deduce that we’re seeing only 2/3 of the mass of the star cluster. Our combination of observations and modeling of this cluster allows us to exclude that this hidden matter is in the form of a huge black hole with several thousand times the mass of the Sun – although this was the kind of hidden object we were originally looking for. Our new interpretation of the observational material is no less spectacular, as dark remnants are usually not believed to be dynamically important in star clusters. We show that in fact they are, and that in some parts they can even entirely dominate the dynamics of the stars we see!

Staten Island Half


I ran my first half marathon today! I’m still so excited, it was such a good race! The Staten Island half marathon was the last of this year’s NYRR Five-Borough series, and it was a cold but beautiful Sunday morning with a glorious sunrise. Seeing the Manhattan skyline on your last mile, and running into the baseball field of the Staten Island Yankees was a great motivator to finish strong. I’m glad I did it.

Carne, Pisco and a Conference in Chile


With Steffen Mieske I’m currently organizing a conference called “Satellites and Streams in Santiago”, which will take place in April 2015 in Santiago de Chile. I’m very excited about this meeting, as the topic will be (as the name suggests) streams and satellites, and there hasn’t been a similar gathering of experts in these fields in more than a decade. With about 100 people, we will focus for one week on dwarf galaxies and globular clusters, which orbit around the Milky Way and nearby galaxies, as well as tidal streams, which these satellites produce while orbiting and dissolving in their host galaxy’s gravitational potential. Aim of the meeting is to bring the two communities together to create the big picture of how large galaxies like the Milky Way assemble mass over billions of years by eating smaller satellites and globular clusters, and how the gravitational fields of the host galaxies transform satellites with time.

The conference will take place on the beautiful premises of the European Southern Observatory in Vitacura. It is generously sponsored by ESO with a large grant allowing us to invite amazing speakers from all over the world. Together with the scientific organizing committee, we came up with a list of keynote speakers that should guarantee a comprehensive and controversial representation of the current standing of the fields. I am very grateful to Steffen, who has gathered a lot of experience with this kind of organization in his years at ESO. He is supported by the amazing staff in Vitacura and in the ESO headquarters in Garching, who also took over the design of the conference poster (above). I’m sure they will help us to turn this meeting into a great experience for all participants. In the middle of the week we will have a Chilean asado in the ESO garden, with lots of Chilean beef and yummy pisco sour – my mouth is watering while I’m writing this – I really can’t wait to get to Chile!

Registration is now open – register here!

How Mass Segregation affects the Expansion of Star Clusters

Orange points show the observed radii (r_h) of Milky Way globular clusters plotted against their distance from the Galactic center (R_G). Globular clusters in the outer halo of the Galaxy tend to be significantly more extended than the ones nearby. Lines show different models for the sizes, star clusters can expand to at a given radius within the age of the Universe.

Recently, my Iranian collaborators and I published another paper on mass segregation in outer-halo globular clusters. This time we looked at the effect that primordial mass segregation can have on the size evolution of these clusters (i.e., what happens if heavy stars are preferentially born closer to a star cluster’s center). The problem is the following: if you look at the globular clusters in the outer halo of our Galaxy, you find them to be significantly more extended (i.e., with a radius larger than 5-6 pc) than their counterparts that are closer to the Galactic center (orange points in the Figure above; but see also my previous posts [1] [2]).

We are interested in the question: “Have these outer-halo cluster always been extended, or did they form as compact clusters like the majority of all clusters we observe in the Universe?” This is important for our understanding of star formation. Most of our knowledge about the formation of galaxies in the early Universe (i.e., like 10-13 billion years ago) is based on the assumption that star formation followed more or less universal laws. By looking at unusual objects like the outer-halo clusters of the Milky Way, we can test if our understanding of the laws of star formation is good enough to make such assumptions about the past.

Hosein, the lead author of our publication, made extensive N-body simulations of star clusters evolving at different distances from the Galaxy and with different conditions at birth. As shown in the above Figure, we find that star clusters can only reach the large extensions they show today in the outer halo of the Milky Way, when they either start more extended than most of the clusters we observe today in the Universe (blue line), or if they were born with very strong primordial mass segregation (red line). The latter would mean that the most massive stars were all born in the centers of these clusters. When such massive stars explode as super novae within the first few million years of a cluster’s lifetime, they release huge amounts of binding energy, which cause the cluster to expand. If these massive stars explode somewhere else in the cluster, and not in the center, the energy budget of the cluster does not change so dramatically and the cluster expands less.

Together with our previous publications on the outer-halo clusters Palomar 4 and Palomar 14, these results indicate that star formation and the assembly of star clusters in weak tidal fields, like the outer halos of galaxies, may be different from our standard picture of star formation.

ISIMA 2014 Toronto

Lonely moose guarding the gate of the University of Toronto

I spent all of August in beautiful Canada at the University of Toronto for the International Summer Institute for Modeling in Astrophysics (ISIMA).

Trinity College

ISIMA is organized by the Theoretical Astronomy and Astrophysics department of UC Santa Cruz. It takes place on a regular basis every few years, and was this time hosted by the Canadian Institute for Theoretical Astrophysics (CITA) at the University of Toronto.

View from the Canadian Institute for Theoretical Astrophysics (CITA)

The University of Toronto was Canada’s first university and was founded in 1827. I literally didn’t expect anything of this place, so I was more than pleased to find a beautiful university in the heart of this fascinating city! Both university and city made this year’s edition of ISIMA a fun adventure.

View from CITA on downtown Toronto

Each ISIMA has a different modeling theme, and this year’s topic was gravitational dynamics. Chair of the summer institute was Pascale Garaud, professor in applied mathematics at UCSC.

Interesting architecture of the University of Toronto

The first week of the six-week program she laid out more like a conference. In the morning we heard lectures, and in the afternoon we had talk sessions.

Interesting architecture of the Royal Ontario Museum

Pascale organized four renown senior lecturers, Scott Tremaine, Douglas Heggie, Doug Lin, and David Merritt, who all gave great introductions to several aspects of gravitational dynamics.

Random downtown building

Less senior people like me, and many other of my gravity friends (yes, we’re gravity friends), were given the opportunity to present their current research and advertise our student projects, which we had to design before coming to Toronto.

Downtown Toronto

The main purpose of ISIMA is bringing  students and advanced researchers from all over the world together to do research together on new and interesting projects. 15 grad students attended the summer institute, and within the first week they had to chose a project with one of the postdocs, junior or senior faculty. For the rest of the six-week program, they then worked on these projects and in the last week presented their results. Many of the projects were designed such that they could be finished within the 5 weeks. Some may need some more work back home, but all of them may end up in actual publications! A win-win for all participants.

The waterfront

Fortunately, it was not just all about work. It was summer at last, and Toronto turned out to be absolutely amazing. I shared an apartment with Shangfei Liu, Mark Gieles and Florent Renaud. Our neighbors within the student dorm were Pascale, Sukanya Chakrabarti, Allice Quillen, Elena d’Onghia and Anna-Lisa Varri.

Kensington Market: alternative/hipster/hippie part of town right next to the University

We went out together, had potluck dinners in the dorm, or joined the students on a few of their numerous excursions to the fun parts of town.

Colorful Kensington Market

Most of these excursions went to Kensington Market, a colorful neighborhood right next to the University of Toronto.

Everything is artsy in Kensington
And political/critical
Chinatown is right next door

One group excursion took us to Niagara Falls, which is about 1 hour drive from Toronto – or 4 hours if you stop at the flower clock, the ice-wine winery, Niagara by the Lake, the hydroelectric and the cable car. We even did the Maid of the Mist tour.

Niagara Falls

Anyways, it was great fun, and we all enjoyed the splendid summer weather. Toronto has presented itself as a gloriously open-minded city full of people from all over the world, with lots of recreational possibilities, tons of shopping opportunities, and a kick-ass restaurant & coffee scene.

Toronto, not only multicultural, but also super LBGTx friendly
It’s a life. Working on a Sunday afternoon in Toronto

I mainly focused on Toronto’s coffee, food and park infrastructure, which I thoroughly explored with the help of bike sharing and my best friend yelp.

It’s a life. Shopping on a Sunday afternoon in Toronto
Random encounter with weird looking Canadian
Boat trip with Florent & sunset over the Toronto skyline

Towards the end of our stay, Florent and I were bold enough to go for a dinner in the 360 Restaurant on top of the CN tower. We were absolutely stoked by the food, the wine and the view from the rotating restaurant 351 m (1150 ft) above the ground!

Symbol of Toronto’s virility, CN Tower
…where they actually serve most delicious food!
…with a view (and a prize)!
Boston Red Sox visiting (and winning)
ISIMA people standing in line for ramen!

Before heading out, Pascale and I initiated a trip to Toronto’s best ramen place: Sansotei Ramen. Look at us, casually standing in line like some New York hipsters waiting for a Ramen Burger at Smorgasburg. It’s been said that Toronto is New York in the movies – not just there, I’d say, also in our hearts. It’s been a great ISIMA 2014. I’d do it again, anytime.

Cambridge, MA


Cambridge is really beautiful in summer – but it’s also full of tourists! Cohorts of eager parents are walking around MIT and Harvard, many with a sparkle in their eyes, fantasizing that their children may one day be one of the very few kids that actually make it there. Nevertheless, I enjoy spending time in this quirky university town: Kayaking on the Charles River, coffee and sticky buns at Flour Bakery, or Asian-food shopping at H-Mart, always make it worth a trip!

Mass segregation in Palomar 14

The mass function slope is shown as it changes with radius of Palomar 14 (measured from the cluster center). A low value of this slope means that we detect more massive stars at this radius than further out, where the slope is larger. Such a signature is called mass segregation, and is usually a consequence of dynamical evolution of a star cluster.

Matthias Frank, Eva Grebel (both in Heidelberg), and I have recently published another paper on one of our Milky Way’s outer-halo globular clusters. Using archival data from the Hubble Space Telescope and Matthias’ sophisticated photometry tools, we measured the masses of stars within one of the most controversial globular clusters known to us: Palomar 14. With a distance of about 70 kpc (or 230,000 light years) from the Galactic Center, it is one of the most distant star clusters of our Galaxy. It is also one of the least massive clusters, and, rather surprisingly, it is the most extended stellar system without dark matter in the nearby Universe. As such, it has been the subject of numerous observational and theoretical studies since its discovery half a century ago by Sidney van den Bergh. According to Sidney, it was nothing more than a faint smudge on one of the photographic plates of the Palomar Observatory Sky Survey. This unusual fuzziness is due to its large extent: compared to regular globular clusters of the same mass, which show a characteristic radius of 3 pc (10 light years), Palomar 14 has a radius of 46 pc (150 light years)! This was also the motivation for our investigation: in such a loosely bound system you don’t expect the stars to interact much – they stay together as a system, but they don’t exchange energy through fly-bys like all these NASA satellites do with planets when they swing by an inner planet to gain velocity in order to reach a more distant planet/comet/whatever. However, consequences of this energy-exchange process are clearly visible in Palomar 14: the most massive stars tend to sit in the center of the cluster, whereas the least massive ones preferentially orbit further out (summarized in the plot above). This means that either Palomar 14 must have been more than a factor of 10 denser in the past, or it was born with this very unusual configuration. Both scenarios give us a headache, since we haven’t been able to reproduce them in simulations. Even more so, since its distant companion, Palomar 4, has exactly the same issues!