Tag Archives: plots

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!  Continue reading Stream fanning


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.  Continue reading Tidal streams in an evolving dark matter halo

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.  Continue reading Signs of hidden matter

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]).  Continue reading How Mass Segregation affects the Expansion of Star Clusters

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.  Continue reading Mass segregation in Palomar 14

Specific Frequency of Globular Clusters

Absolute magnitude (M_V), corresponding to the total mass of galaxies, plotted against the specific frequency (S_N), which gives the observed number of globular clusters divided by the total mass of the respective galaxy. Also shown are expected fractions of surviving globular clusters (f_S) for these galaxies (blue and green points). The two quantities follow a strikingly similar trend with galaxy mass.

During Steffen’s time at Columbia in February this year, we were talking a lot about erosion of globular clusters by the gravitational field of their host galaxy. Steffen came up with the great idea that this erosion process may be responsible for the observed variations of the number of globular clusters around different kinds of galaxies. In general, you see that galaxies with a small total mass will have a small number of globular clusters. But if you divide the number of a galaxy’s globular clusters by the mass of the galaxy, you will not get a flat distribution as naively expected. Instead, you get a U-shaped distribution, telling you that very low-mass galaxies and very high-mass galaxies have more globular clusters than average, whereas intermediate-mass galaxies have suspiciously few clusters. Why should those galaxies be any special??  Continue reading Specific Frequency of Globular Clusters

How Galaxies treat their Globular Clusters

After almost 2 years of development and testing, Michael Brockamp (University of Bonn) and I have finally published our MUESLI code for the simulation of globular star clusters orbiting in elliptical galaxies. Together with Holger Baumgardt from Brisbane (Australia), Ingo Thies, and Pavel Kroupa (both from the University of Bonn), we have put together a really exciting study of systems of globular clusters and how they erode over time in the gravitational field of their host galaxies.

This figure shows the number of globular star clusters at a given radius of the elliptical galaxy they were simulated in. The black line gives their initial distribution at the beginning of the simulations, the blue dashed line shows their distribution after just one billion years of evolution. As you can see, the distribution develops a core, as clusters in the center of the galaxy get destroyed more quickly. These cores are observed for giant elliptical galaxies, hence we suggest that they formed via ‘tidal erosion’, that is, the clusters in the center got disrupted by the galaxy.

Continue reading How Galaxies treat their Globular Clusters