Tidal Dwarf Galaxies Bonn 2009: Ghosts from structure formation
Mysteriöser Sternenkitt [Link]
Forscher diskutieren über die rätselhafte Kraft, die die Galaxien zusammenhält, 19.05.2009
Die Eigenrotation von Galaxien sollte eigentlich dafür sorgen, dass die Sterne aufgrund der Fliehkraft auseinander getrieben werden. Eine rätselhafte Kraft scheint das jedoch zu verhindern. Viele Physiker vermuten daher, dass die so genannte Dunkle Materie aufgrund ihrer Masseanziehung die Galaxien zusammenhält. Bislang hat jedoch niemand diesen mysteriösen Sternenkitt tatsächlich nachweisen können. Auf einer internationalen Tagung im Physik-Zentrum Bad Honnef diskutieren Forscher aus aller Welt über alternative Erklärungsmöglichkeiten. Die Veranstaltung läuft vom 24.05. bis 29.05.; sie wird von Physikern der Universität Bonn organisiert. Journalisten sind herzlich eingeladen, daran teilzunehmen.
Die Veranstalter erwarten 95 Wissenschaftler aus der ganzen Welt. Eine Woche lang werden sie in Bad Honnef die schwierigen Probleme diskutieren, mit denen sich die heutige Kosmologie konfrontiert sieht. Es geht unter anderem darum, woher die kleinsten Galaxien stammen: Sind sie voll von dunkler Materie, wie es manche Beobachtungen andeuten? Oder sind sie durch Kollisionen anderer Galaxien entstanden? Dann könnten sie jedoch keine dunkle Materie enthalten. In letzter Konsequenz würde das eine Modifizierung der Newtonschen Gravitationstheorie erfordern. "Falls diese Vorstellung zutrifft, müssten wir die gesamte Dynamik des Universums neu überdenken", erklärt Professor Dr. Pavel Kroupa vom Argelander-Institut für Astronomie an der Universität Bonn. "Möglicherweise müssten sogar die Einsteinschen Feldgleichungen neu formuliert werden."
Journalisten sind herzlich eingeladen, an der Tagung teilzunehmen. Um Anmeldung bei der E-Mail-Adresse firstname.lastname@example.org wird gebeten. Die Tagungssprache ist English; das Programm kann unter http://www.astro.uni-bonn.de/~tdgbonn/index.html heruntergeladen werden.
Study plunges standard Theory of Cosmology into Crisis [Link]
New insights into Milky Way satellite galaxies raise awkward questions for cosmologists, 03.06.2009
Do we have to modify Newton's theory of gravitation as it fails to explain so many observations? Voices are increasingly being heard that support this heretical hypothesis. Two new studies conducted by physicists at the University of Bonn, in collaboration with scientists from Austria and Australia, are likely to provide yet more grist for the mill. Their latest results about so-called "satellite galaxies" at the periphery of the Milky Way could rock the theoretical foundations of standard physics.
As modern cosmologists rely more and more on the ominous "dark matter" to explain otherwise inexplicable observations, much effort has gone into the detection of this mysterious substance in the last two decades, yet no direct proof could be found that it actually exists. Even if it does exist, dark matter would be unable to reconcile all the current discrepancies between actual measurements and predictions based on theoretical models. Hence the number of physicists questioning the existence of dark matter has been increasing for some time now. Competing theories of gravitation have already been developed which are independent of this construction. Their only problem is that they conflict with Newton's theory of gravitation. "Maybe Newton was indeed wrong", declares Professor Dr. Pavel Kroupa of Bonn University´s Argelander-Institut für Astronomie (AIfA). "Although his theory does, in fact, describe the everyday effects of gravity on Earth, things we can see and measure, it is conceivable that we have completely failed to comprehend the actual physics underlying the force of gravity".
This is a problematical hypothesis that has nevertheless gained increasing ground in recent years, especially in Europe. Two new studies could well lend further support to it. In these studies, Professor Kroupa and his former colleague Dr. Manuel Metz, working in collaboration with Professor Dr. Gerhard Hensler and Dr. Christian Theis from the University of Vienna, and Dr. Helmut Jerjen from the Australian National University, Canberra, have examined so-called "satellite galaxies". This term is used for dwarf galaxy companions of the Milky Way, some of which contain only a few thousand stars. According to the best cosmological models, they exist presumably in hundreds around most of the major galaxies. Up to now, however, only 30 such satellites have been observed around the Milky Way, a discrepancy in numbers which is commonly attributed to the fact that the light emitted from the majority of satellite galaxies is so faint they remain invisible.
A detailed study of these stellar agglomerates has revealed some astonishing phenomena: "First of all, there is something unusual about their distribution", Professor Kroupa explains, "the satellites should be uniformly arranged around their mother galaxy, but this is not what we found". More precisely, all classical satellites of the Milky Way - the eleven brightest dwarf galaxies - lie more or less in the same plane, they are forming some sort of a disc in the sky. The research team has also been able to show that most of these satellite galaxies rotate in the same direction around the Milky Way - like the planets revolve around the Sun.
Contradiction upon Contradiction
The physicists do belief that this phenomenon can only be explained if the satellites were created a long time ago through collisions between younger galaxies. "The fragments produced by such an event can form rotating dwarf galaxies", explains Dr. Metz, who has recently moved across to the Deutsches Zentrum für Luft- und Raumfahrt (German Aero-space Center). But there is an interesting catch to this crash theory, "theoretical calculations tell us that the satellites created cannot contain any dark matter". This assumption, however, stands in contradiction to another observation. "The stars in the satellites we have observed are moving much faster than predicted by the Gravitational Law. If classical physics holds this can only be attributed to the presence of dark matter", Manuel Metz states.
Or one must assume that some basic fundamental principles of physics have hitherto been incorrectly understood. "The only solution would be to reject Newton´s classical theory of gravitation", says Pavel Kroupa. "We probably live in a non-Newton universe. If this is true, then our observations could be explained without dark matter". Such approaches are finding support amongst other research teams in Europe, too.
It would not be the first time that Newton's theory of gravitation had to be modified over the past hundred years. This became necessary in three special cases: when high velocities are involved (through the Special Theory of Relativity), in the proximity of large masses (through the theory of General Relativity), and on sub-atomic scales (through quantum mechanics). The deviations detected in the satellite galaxy data support the hypothesis that in space where extremely weak accelerations predominate, a "modified Newton dynamic" must be adopted. This conclusion has far-reaching consequences for fundamental physics in general, and also for cosmological theories. Famous astrophysicist Bob Sanders from the University of Groningen declares: "The authors of this paper make a strong argument. Their result is entirely consistent with the expectations of modified Newtonian dynamics (MOND), but completely opposite to the predictions of the dark matter hypothesis. Rarely is an observational test so definite."
Metz, Manuel; Kroupa, Pavel; Theis, Christian; Hensler, Gerhard; Jerjen, Helmut: Did the Milky Way dwarf satellites enter the halo as a group? (The Astrophysical Journal 2009; doi: 10.1088/0004-637X/697/1/269)
Metz, Manuel; Kroupa, Pavel; Jerjen, Helmut: Discs of Satellites: the new dwarf spheroidals (Monthly Notices of the Royal Astronomical Society 2009; doi: 10.1111/j.1365-2966.2009.14489.x)
Dr. Manuel Metz
Tidal Dwarf Galaxies "met" in Bad Honnef! [Link]
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 "tidal galaxies" and suggested that they should populate the neighbourhood of our galaxy. 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 "extravagant" or even "crazy".
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) "Tidal Dwarf Galaxies: Ghosts from structure formation" 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.
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, 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 Einstein’s theory of 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? 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 "alternative" 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, 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
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 "the missing satellites problem" 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 around 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 nicely (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 "bridge theory" between MOND and CDM, saying that the Universe is permeated by a sort of "DarkFluid" 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 dwarf 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. -
by Pasquale Galianni, Dipartimento di Fisica dell’Universita del Salento, Via Per Arnesano 1, 73100, Lecce, Italia,