Eight Minutes Old

There it is finally. Our article about dwarf galaxies and dark matter has been published in the current issue of the German popular science magazine “Spektrum der Wissenschaft“, which is the German version of the Scientific American. The text  is about testing the current cosmological standard model using the satellite galaxies of the Milky Way. In it, my professor, Pavel Kroupa, and I not only describe how predictions of the Cold Dark Matter model can be compared to the Local Group of neighboring galaxies, but also how new doubts in the existence of dark matter arise from the failed tests. The article is even mentioned on the front page and in the editorial and got 10 full pages, which is a lot.

New food for your feed-reader!

While the article is in German, we also have to offer something to our English readers. Spektrum der Wissenschaft has convinced us to start a new blog on the topic to allow a direct discussion. We decided that this blog should be in Englisch. It got the title “The Dark Matter Crisis - The rise and fall of a cosmological hypothesis“ and can be found at the SciLogs. Besides Professor Kroupa and me, also Anton Ippendorf, a student in our research group also working on dwarf and satellite galaxies will write posts there. Currently I am preparing a blogpost for the Dark Matter Blog on the Bullet Cluster, one of the most often heard arguments against alternative gravity theories like MOND or MOG.

Article as free PDF

If you read German, you can buy the current version of Spektrum der Wissenschaft from today on, it costs 7.40 Euro in Germany and Austria. But if you are only interested in our article, you can also download it as a free PDF.

So, now I am waiting for the reactions. Have fun with the text and our new blog!

Tags: Blog, free online-article, magazine, MOG, MOND, Pavel Kroupa, Scientific American, Spektrum der Wissenschaft, Astronomers, Astronomy, Blog Specific, Cosmology, Dark Matter, Galaxies, Gravity, popular science, Satellite Galaxies, Science, Tidal Dwarf Galaxies

UPDATE: It is online in the Mediathek.

Apparently, the documentary for which a German television team visited the institute last week will be aired tomorrow it now seems to be scheduled for Friday 23rd evening on “3Sat” in a “Nano” show at 6:30 pm. Unfortunetely it is in German, of course, but it will be available online after the show. I will post a link once it is online.

Tags: 3Sat, Newton, Astronomy, Dark Matter, Effelsberg, Gravity, IYA, Law of gravity, Nano, Newtonian Dynamics, Radio Astronomy, Radio Telescope, Star Gazer, Tips, TV series, TV, Universe, University of Bonn

Friday, as it was the last day of the conference, included a final discussion. Here, as in the shorter discussions before and during the breaks, it again became clear that there is not yet a consensus. That’s actually a good and natural thing, as right now there is no theory able to explain everything. I’m happy that most people, as convinced as they might be by the theory they prefer, still enjoy a discussion, dialog and are open minded to attend such a controversial meeting.

There was quite some media coverage of the conference, the following is a short selection of links:

The official press release about the conference: Can not be found anymore because the Bonn University changed it’s website.

Three press releases about the problems for dark matter emerging out of the strange distribution of the Milky Way satellite galaxies:

• One from Bonn University (Germany), Prof. Pavel Kroupa and Dr. Manuel Metz, unfortunately it has the same problem as mentioned above.
• Another from the University of Vienna (Austria), Prof. Gerhard Hensler.
• And number three from the Australian National University, Dr. Helmut Jerjen.

The press releases were covered by many different media, for example here, here and here, also by one of Germany’s leading online newspapers, Spiegel Online. … and some more links.

Furthermore, Prof. Kroupa was interviewed by the German web-radio show “Computer Club 2” (cczwei.de), on the topic of modified gravity laws.

A report (English) about the meeting by Pasquale Galliani, who also was a guest at the Institute during the week after the conference.

An interview (German) with Prof. Pavel Kroupa, organizer of the conference.

And there are two (German) posts in the blog of a colleague who also attended the conference: “TDG Bonn 2009” and “And yet another conference goes by…“.

Tags: Bad Honnef, MOND, Newton, TDGBonn, Antennae Galaxies, Astronomy, Bonn, Conference, Dark Matter, Gravity, Law of gravity, Newtonian Dynamics, Rhine, Simulation, Structure Formation, Tidal Dwarf Galaxies, Tidal Streams, University of Bonn

What are Tidal Dwarf Galaxies?

Tidal dwarf galaxies (TDGs) are, as the name implies, small. Dwarf galaxies as such (tidal or not) are, compared to the ”usual” galaxies, discs and ellipticals consisting of several 100 billion stars, really dwarfs: TDGs have about ten million stars, only one 10,000th of the Milky Way (but there are smaller ones with as few as several thousand stars, too). Dwarf galaxies are categorized  according to their shape (morphology), there are dwarf Ellipticals (dE, these include dwarf Spheroidals, dSph), irregular galaxies (dI, which can have deformed shapes). One example for the latter is the Small Magellanic Cloud (SMC), a dwarf galaxy which orbits the Milky Way and is deformed by the gravitational field of our galaxy and another neighbouring galaxy, the Large Magellanic Cloud. Both dwarf satellite galaxies can be seen in the southern hemisphere with the bare eye. Maybe dwarf versions of spiral galaxies exist, too, but there is debate on this issue. As they are small and have few stars, dwarf galaxies are very dim. They have few gas and most are rather diffuse structures.

Now, these are dwarf galaxies, but what makes a dwarf a tidal one? For this one has to consider how they formed. While most galaxies are assumed to have formed early in the universe (see Section “Classical” galaxy formation below), there are exceptions.
Galaxies are not  fixed in space, they move around and sometimes can approach to relatively short distances. They might even merge to form a bigger one. In these interactions, the galaxies swing around each other, the gravity of the galaxies pull them into new directions. This results in “tidal forces” acting on the material of the galaxies: stars and gas. This is similar to the tides on earth, lifting the water of the oceans by some meters.

While the earth can keep it’s water, galaxies interactions are much more violent. The different forces acting on both sides of the galaxies result in matter thrown out of them into the intergalactic space. This usually happens along so-called “tidal tails”. One beautiful example are the antenna galaxies, a pair of merging galaxies with two very pronounced tidal tails. I think you will notice them at once in the picture on the left, taken with the Hubble Space Telescope (created by Brad Whitmore (STScI) and NASA).

In contrast to the tides on earth, these interactions take millions of years. Thus we can only see one snapshot of these interactions. But we can make models of galaxies in the computer and calculate what happens to them when these, for example, pass another galaxy.

In this movie of one of my simulations, you see the yellow disc galaxy passes by another galaxy. As it is dragged around, material of the disc is torn out in two directions, towards the other galaxy and into the opposite direction (like in earth, where there is always a high tide on two sides. That’s why we have high tides about twice a day). BTW: It’s so cool to make movie!

Now, within these tidal tails, matter can a clump under it’s own gravity. When there is enough matter collected, stars an mostly gas, new stars can form out of the latter. That way, within the tidal tails a new galaxy is born. Of course it is a small galaxy, as only a fraction of the original galaxy’s material finds itself within the tidal tail. Thus, these galaxies are referred to as Tidal Dwarf Galaxies. The first one has been discovered in the tidal tails of  above mentioned Antennea galaxies by Mirabel, Dottori & Lutz in 1992. So you can see this still is a relatively new topic. But it could have strong effects on our understanding of the universe. To explain this, we first need to know how galaxies form usually …

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