10.09.2008 —

Only 4.5 percent of the Universe consists of matter, constitution of which is known to us. Apart from this type of matter that is designated as “baryonic” matter, there is Dark Energy as well as Dark Matter as further components. Dark Matter constitutes around 23 percent of the universe. In the astronomy, the Dark Matter indeed plays a significant role in formation of galaxies; however, the scientists are puzzled about the nature of the form of this matter till this day.

Beside particles like the so-called WIMPs (Weakly Interacting Massive Particles), objects named MACHOs (MAssive Compact Halo Objects), whose properties are under intensive research, also come into consideration as candidates for the invisible Dark Matter. A working team at the Excellence Cluster Universe has now detected one of these celestial bodies in our neighboring Galaxy of Andromeda and has introduced a new improved method (Astrophysical Journal, 10. September 2008, Vol. 684, p. 1093).

The impact of the Dark Matter appears in the rotational behavior of the spiral galaxies. Without these components, the outer regions of these galaxies would circle more slowly around the center than the more interior areas. As this is not the case, the scientists assume that the disk and the concentrated central area of spiral galaxies are embedded in an invisible halo made of Dark Matter. For this reason, researchers comb through the primary halo-regions for possible MACHOs. The goal is to classify these objects more precisely and thus to clarify, if the wanted Dark Matter is hidden in them. As for the MACHOs, it can very trivially be a matter of white dwarfs or neutron stars – that is, stellar remains of baryonic origin.

As MACHOs shine very poorly or do not shine at all, the scientists use an indirect verification procedure. This is based on a phenomenon termed “Gravitational Lensing“: massive objects can deflect light beams, for example, light from a star, so that the size and distance of the object can be inferred from the angle of deflection. The effect of the deflection of light due to gravitation can be also observed on our sun. The angle of deflection caused by the MACHOs is, however, conspicuously smaller than the resolution capacity of the best telescope. Therefore, MACHOs can only be detected by an amplified brightness of the background object – an effect called microlensing.

Arno Riffeser, Stella Seitz and Ralf Bender of the University Observatory of Munich evaluated a microlensing-event named WeCAPP-GL1 in the Andromeda Galaxy (M31). In contrast to the earlier works, the researchers took the size of the background object into consideration for the first time. With this procedure, the scientists can distinguish the bright microlensing-events from gravitational lenses, which are generated by the stars. Due to their great mass, stars can also deflect light – an effect called self-lensing.

However, why do the scientists look for MACHOs in the M31-Galaxy 2.5 light-years away, and not in the Milky Way? The advantage of the observation of the Andromeda-Galaxy lies in the fact that the scientists can simultaneously measure significantly more brightness-events per photograph. However, due to the little apparent brightness and the high stellar density, the requirements of the data analysis are disparately greater. The procedure tried and tested here should clearly improve the quality of future surveys.

In their current research work, Riffeser and his colleagues analyzed the data, which were already recorded in year 2000 by the Observatories on Mount Wendelstein and Calar Alto in Spain. Then, the scientists had researched on 4 million brightness curves in total. The results so far indicate that the matter in Halo of the Andromeda-Galaxy are partially constructed of MACHOs – future observations will show, if it is really a question of Dark Matter there.

In the truest sense of the word „illuminative“ knowledge, the researchers of University Observatory of Munich wishes for a series of research planned for three years at Haleakala-Observatory in Hawaii. With the Pan-STARRS-Telescope, 1.4 million variable sources of light can be simultaneously studied. Moreover, the scientists expect further opening of new 2-Meter-Telescop on Wendelstein, which will presumably be brought on line in 2010. Here, gravitational lenses with very short disruptions in brightness and thus small masses can be more intensively studied.

Contact:
Barbara Wankerl
Excellence Cluster Universe
Public Outreach Coordinator
Technische Universität München
Tel: +49.89.35831-7105
Mobil: +49.162.2944801
Fax: +49.89.3922-4002
E-Mail: barbara.wankerl(a)universe-cluster.de
http://www.universe-cluster.de

Dr. Arno Riffeser
Universitäts-Sternwarte München
Wissenschaftlicher Mitarbeiter
Ludwig-Maximilians-Universität München
Tel: +49.89.2180-5973
Mobil: +49.179.7039348
Fax: +49.89.2180-6003
E-Mail: arri(a)usm.uni-muenchen.de
http://www.usm.lmu.de/