During Adrian Price-Whelan’s dissertation talk today at the winter meeting (AAS227) of the American Astronomical Society in Kissimmee, Florida, I was reminded that I haven’t mentioned our publication here. Adrian went through a whole lot of effort and characterized regular and chaotic orbits in a typical galactic gravitational potential. Usually, orbits in such a potential can be broadly categorized into chaotic and non-chaotic orbits. Adrian looked at this distinction in terms of the streams that are formed by satellites on such orbits. Continue reading Chaos in the Galaxy
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
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
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
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
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  ). Continue reading How Mass Segregation affects the Expansion of Star Clusters
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