Tag Archives: tidal tails

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!


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

Stream Meeting Ringberg Castle

It’s been a while since I’ve found the time to give an update on my activities. The reason is that there are just too many these days! Let me start catching up by telling you about an amazing workshop that took place from July 19-24 in Germany.


Jo Bovy (CITA) and Hans-Walter Rix (MPIA) organized a stream meeting at Ringberg Castle, which is a conference venue owned by the Max Planck Society. It’s not just called ‘castle’, it really is one! And the conference participants are literally locked up on a mountain, giving the meetings up there a very intimate and focussed atmosphere. I was put in the tower – but with a view like that (above) it didn’t occur to me to complain!  Continue reading Stream Meeting Ringberg Castle

How to weigh the Milky Way

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

Satellites and Streams in Santiago 2015


What a week! The ESO workshop ‘Satellites and Streams in Santiago’ is over now, and I am still amazed by how flawless this meeting was. As announced here a while ago, I’ve been organizing this conference with Steffen Mieske (ESO) for the last few months. In the end we were 107 registered participants plus some 10 guests. The line-up was absolutely amazing, ESO Director General Tim de Zeeuw gave an opening address, and we heard keynote presentations from the who-is-who in the fields of satellites (Jorge Peñarrubia, Michelle Collins, Pavel Kroupa, Rodrigo Ibata, Vasily Belokurov, Gurtina Besla), streams (Steven Majewski, Amina Helmi, Aaron Romanowski) and the star-cluster/dwarf-galaxy interface (Oleg Gnedin, Jay Strader, Dougal Mackey, Michael Hilker, Anil Seth).  Continue reading Satellites and Streams in Santiago 2015

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

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!