Tag Archives: satellites

University of Virginia


Last week I visited Jennifer Sobeck and my former Yale-colleague Nitya Kallivayalil at the University of Virginia. Jen is the deputy project manager of APOGEE-2, which is a large spectroscopic survey collecting 300,000 high-resolution spectra of Milky Way stars (and other bright targets). Hence, her head is full of stellar atmospheres and element abundances. Nitya is the go-to person for proper motions of Galactic satellites, which is why there’s tons of overlap between my work and both of theirs. Frequently distracted by great food and stunning weather, we scienced around for a few days. Our brainstorming for low hanging fruits in the APOGEE-2 dataset brought us a few great ideas, which we will hopefully pursue within the next weeks and months. Steve Majewski, PI of APOGEE and also based at UVa, outsourced the same task to all his undergrads. Let’s see who’s first to have some results!

Chaos in the Galaxy

N-body simulations of tidal streams formed by Galactic satellites on regular orbits (A & B), a weakly chaotic orbit (C), and a strongly chaotic (D). The orbits of the four satellites are quite similar in terms of eccentricity and apo/pericenters, but the resulting streams show (and amplify) the underlying chaos.

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

ESO Messenger


An article I just wrote with Steffen Mieske (ESO) about our Satellites and Streams in Santiago meeting earlier this year appeared today in the ESO Messenger. Go, check it out!

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!

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

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.  Continue reading Carne, Pisco and a Conference in Chile