From Inside Germany: Tidal Dwarf Galaxies ‘met’ in Bad Honnef!
Juni 10, 2009 von admin
In 1956 an eccentric Swiss astronomer, Fritz Zwicky, proposed that some galaxies could have formed from the debris expelled during the interaction between larger galaxies. When two galaxies get close to each other the mutual gravitational force is strong enough to trail out some stellar and gaseous material, from which new, smaller and fainter galaxies may form.
He called such objects â€śtidal galaxiesâ€ť and suggested that they should populate the neighbourhood of our galaxy.Â But at that time the astronomical community was not ready to accept his innovative ideas, and as for the dark matter that he postulated already in 1933, most judged his ideas as â€śextravagantâ€ť or even â€ścrazyâ€ť.
But surprisingly, in the last years the technological development in observational astronomy led to the discovery of extremely faint dwarf galaxies populating the Milky Way halo at distances of 100000 to 300000 light years. The research of Manuel Metz and Pavel Kroupa of Bonn University, of Gerhardt Hensler and Christian Theis of Vienna University, and of Helmut Jerjen of The Australian National University has furthermore shown that these faint dwarf galaxies are distributed about the Milky Way such that they cannot be the long-sought dark-matter dominated dwarf galaxies that are predicted to be present abundantly in standard cold-dark matter cosmology.
With this discovery, the idea that tidal galaxies may be of importance not only for the dwarf-galaxy population, but also for testing fundamental physics, came to life with renewed energies. Tidal Dwarf Galaxy (TDG) are now among the most interesting and strongly debated topics in astrophysics and have recently been the subject of a large international conference in Germany.
Bad Honnef - a nice village nearby Bonn
Between the 24 and 29th of May the very first international conference on Tidal Dwarf Galaxies (hereafter TDGs) â€śTidal Dwarf Galaxies: Ghosts from structure formationâ€ť took place in Bad Honnef, a nice village nearby Bonn along the river Rhine. Some of the most eminent experts in the field met to discuss the possible formation scenarios for dwarf galaxies. Dwarf galaxies are faint objects, composed of a few thousand to tens of millions of stars, that often lie in the neighbourhood of larger galaxies like our own and Andromeda.
Image: Famous river Rhine near Bad Honnef - “A unique historical event in Bad Honnef”
Change of gravitation away from Newtonian dynamics
In the mainstream scenario, dwarf galaxies should contain a huge quantity of dark matter in order to remain stable for several billion years. However, it was already noted above that the dark-matter theory fails to account for the properties of the Milky Way satellite galaxies. Therefore, to explain their internal structure,Â Newtonian dynamics needs to be modified. For this reasons dwarf galaxies represent a very important test-bench for the Cold Dark Matter paradigm (CDM) and for alternative gravitation theories like MOND (Modified Newtonian Dynamics). A change of gravitation away from Newtonian dynamics is equivalent to saying that Einsteinâ€™s theory ofÂ general relativity needs to be amended in the weak-field limit, and the Milky Way and Andromeda satellite galaxies appear to be holding the essential clues towards this effect.
Was Einstein wrong? - “not so much agreement”
The debate at the coference was intense and stimulating. The experts from all over the world (Australia, Korea, Japan, Iran, Europe, USA, Chile, Argenina, Brazil, Russia, Ukraine) debated on the truly very serious problem on what actually governs our universe. Does dark matter exist? Or is Einstein wrong in the weak field limit?Â If the latter were true, then most of the past theoretical research on galaxies and their origin and on cosmology would be wrong.
Despite the severity of these problems, the atmosphere was very friendly indeed, the weather wonderful and the German beer excellent. Perhaps for the first time for such a large conference, much time was left in the programme for â€śalternativeâ€ť theories, such as Modified Newtonian Dynamics (MOND), showing the interest of Bonn’s Stellar Population and Dynamics Group and of the meeting’s organizer, Prof. Pavel Kroupa,Â to keep an open mind to alternative explanations.
At the conference, there was not so much agreement about the nature of the dwarf galaxies. This does not wonder, since, if the community had agreed that the Milky Way satellite galaxies were tidal dwarfs, then this would be the same as agreeing that all of the previous theoretical cosmological research was wrong, i.e. that a large fraction of astronomers had been working on the wrong picture for many years.
The missing satellites problem
Foto: The conference took place in Bad Honnef in the DPG headquarter in Bad Honnef
The mainstream theory about the formation of dwarf galaxies suggests that they formed from the primordial collapse of dark-matter subhaloes. This scenario however has a severe problem to explain the fact that we observe a hundred times fewer dwarfs around the Milky Way than we should expect from the structure formation theory in a cosmological framework. This problem is known as â€śthe missing satellites problemâ€ť and several solutions have been proposed in order to solve it. Indeed, Prof. Hans-Walter Rix of the Max-Planck Institut fuer Astronomie suggested that the new population of even fainter dwarf galaxies, the ultrafaint dwarf galaxies, discovered by the Sloan Digital Sky Survey (SDSS) could help astronomers to fill the gap between the predicted and the observed number of dwarf galaxies around the Milky Way.
The hard life of innovators
Prof. Pavel Kroupa, from the Argelander Institut fuer Astronomie, suggested instead that the dwarf satellite galaxies are ancient tidal dwarfs that formed from the debris of galaxy encounters during a time when the Milky Way was being born. This would solve in a simple way the anisotropic distribution of the dwarf galaxies aroundÂ the Milky Way (that can’t be easily explained in the dark-matter paradigm) and it would explain several observational findings, like the fact that star formation regions are effectively observed along tidal streams. The life of innovators is hard, and fighting a main stream theory is a difficult task: but the scenario proposed by Prof. Kroupa is progressively raising the interest of the community, and in the next years we will hopefully gain more observational evidence in order to understand which of the two formation paradigms for satellite galaxies is right.
That alternative gravity is, in fact, a very attractive solution to the dwarf galaxy problem was demonstrated convincingly by Garry Angus of the University of Torino. He showed that MOND fits the stellar velocities in the dwarf satellites quite nicelyÂ (and with less than half the free parameters) than assuming dark matter were true. Prof. Hong-Sheng Zhao from the University of St. Andrews proposed a â€śbridge theoryâ€ť between MOND and CDM, saying that the Universe is permeated by a sort of â€śDarkFluidâ€ť represented by a vector field that, depending on the circumstances, can behave as a dark matter particle or as a gravity-modifying field. The idea of Prof. Zhao is very interesting, but it still lacks precise testable observational constraints.
In summary, MOND seems to explain better than dark matter the properties of dwarfÂ galaxies, and accounts much better for the rotational curves of normal galaxies. CDM instead explains better than MOND some observations of galaxy clusters and some gravitational lensing phenomena.
Born of a new theory - a unique historical event in Bad Honnef
When such contradictory situations happen in science, it usually means that scientists are making some mistake somewhere in their theories, and the probability that some new and more accurate theory about our universe will be developed increases dramatically. The hope that such a theory exists and will be developed in the next years is shared among all participants of the meeting. This conference in Bad Honnef has been an important stepping stone towards this goal. Indeed, this conference was perhaps a unique historical event because it had ensured a gathering of experts on the various theories for the very first time on this scale. - Pasquale Galianni, Dipartimento di Fisica dell’UniversitĂ del Salento, Via Per Arnesano 1, 73100, Lecce, Italia, Sub-headlines by Christian PreuĂź
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