Tag Archives: columbia

Big in Japan

I get to be on Japanese television!! NHK, Japan’s national public broadcasting organization, asked me for an interview about the evolution of globular clusters and the formation of tidal streams. I’m still excited about it, it was so much fun!

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NHK produces a show called Cosmic Front Next, which covers one aspect of astronomy per episode. Each episode is one hour long and features several scientists. Since 2011, they have aired about one episode per week. That’s a lot of astronomy! Surprisingly, they haven’t covered globular clusters yet. So, I’m more than happy that I get to be on this episode of Season 5. Continue reading Big in Japan

How to weigh the Milky Way

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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.  Continue reading How to weigh the Milky Way

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

Juggling with black holes

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Last Friday I gave a public lecture on gravitational dynamics at the Columbia Astronomy Department Stargazing & Lecture Series. The talk was really well attended, and I was surprised to see so many people interested in hearing about astronomy on a Friday night. The big auditorium in Pupin Hall felt packed to me (my perception may have been biased, though).  Continue reading Juggling with black holes

Signs of hidden matter

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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

Specific Frequency of Globular Clusters

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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

Palomar 4

This website was never intended to be a science blog, but since science is an essential part of my life – with all its ups and downs, kinks and quirks, bores and funzies – I shouldn’t neglect it here! Today, a paper of me and my dear collaborators from Iran, Elham Hasani Zonoozi and Hosein Haghi appeared on astro-ph.

Mass function slope versus cluster radius
Slope of the stellar mass function versus projected radius from the cluster center for the observations of Palomar 4 (red) and for our best-fitting model (black). The goal was to find a model that reproduces the observed trend of the massive cluster stars being more centrally concentrated, which can be seen from the slope being below its nominal value (dashed line) in the center, while being above that line in the outer parts of the cluster.

Continue reading Palomar 4