ISO detects signal from dark matter in a galaxy similar to the Milky Way
17 Aug 1999Galaxies are known to have much more matter than telescopes can currently see. Up to 90% of the total mass of the galaxies is simply missing: it has to be there, astronomers know, but it remains undetected. Is this so-called 'dark matter' made up of exotic, virtually undetectable particles, or is it merely ordinary matter hidden to instruments for some reason? A new result obtained by a Dutch team with the European Space Agency's infrared space telescope, ISO, favours the last idea.
They have detected in the disk of a galaxy the molecule of hydrogen, considered an important component of the dark matter if it is of the normal, ordinary type. Moreover, the molecular hydrogen is found precisely in the amount needed to fill the missing-mass gap.
"0ur results give a much stronger footing for the 'ordinary matter' simple solution of the dark matter problem, in the form of massive clouds in the disks of galaxies", says the main author of the finding Edwin A. Valentijn, from the Kapteyn Institute in Groningen (The Netherlands).
Astronomers discovered the existence of the dark matter long ago, by measuring the motion of the stars and gas in spiral galaxies: the velocity of this motion corresponds to a certain amount of mass in the galaxy, and measurements showed that the stars and the gas are moving too fast for the amount of mass detected. Several hypotheses have been made to explain the nature of this missing mass. While some consider it to be made of 'exotic' particles very difficult to detect, such as neutrinos, others point to ordinary matter --'baryonic matter', in scientific terms-- which for some reason remains hidden.
One of the findings helping to build the 'normal matter' explanation was obtained a decade ago by Valentijn himself. In 1989 he measured the brightness of 2,500 spiral galaxies, to determine whether these objects were transparent or opaque. Until then, most astronomers had assumed that spiral galaxies were basically transparent, this meaning that most light coming from the normal matter present would be freely emitted --thus, the matter would be bright. On the contrary, Valentijn found that spiral galaxies are heavily obscured by their own interstellar dust. Could the dark matter, or at least part of it, simply be the gas frequently associated with this interstellar dust?
If that was the case, the gas had to be made up mostly of hydrogen in the molecular form. Hence,Valentijn and his co-author van der Werf started a search for molecular hydrogen, which is extremely difficult to observe. Emission from this molecule can only be detected with highly sensitive infrared telescopes, and ESA's ISO has allowed the study of molecular hydrogen in unprecedented detail.
Using the spectrometer SWS on board ISO the Dutch team focused on a spiral galaxy called NGC 891, 30 million light-years away and very similar to our own galaxy, the Milky Way. Measurements were made in 8 different positions in the disk of NGC 891.
"The surprise is that we detect molecular hydrogen everywhere where we looked! Our team was the only one who thought the measurement was feasible, as no other ISO-observations of this kind were programmed", Valentijn says.
Moreover, the team found molecular hydrogen in the amount needed to account for the missing mass. This is established as a value relative to the amount of hydrogen in the atomic form (one molecule of hydrogen is made of two atoms of hydrogen). In NGC 891, Valentijn shows that there is 5 to 15 times more molecular than atomic hydrogen.
As the Dutch astronomer explains, "it is well established that if there is about 10 times as much molecular hydrogen as atomic hydrogen in the disks of spiral galaxies, then the missing mass problem is resolved. In the case of NGC 891we find about 5-15 times as much molecular hydrogen as atomic hydrogen".
Since NGC 891 is a rather common galaxy, it is "reasonable" -- the authors say-- to expect the result to hold for other normal galaxies too. However, the team warns that no other alternative dark matter model can be ruled out.
According to Valentijn, "the problem is complex enough to avoid drawing quick conclusions, and of course more observations should be made".
The discovery has been accepted for publication in the Astrophysical Journal Letters.
FOOTNOTE ON ISO
The European Space Agency's infrared space telescope, ISO, operated from November 1995 till May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO successfully made nearly 30 000 scientific observations.
FOR MORE INFORMATION AND ISO PICTURES
-ESA Public Relations Division: Tel: +33(0)126.96.36.199.55 Fax: +33(0)188.8.131.52.90
-Martin F. Kessler (ISO Project Scientist): Tel: +34 918131254 mkessleriso.vilspa.esa.es
OTHER SCIENCE CONTACTS:
-Edwin Valentijn, Kapteyn Institute, Groningen (The Netherlands). valentynastro.rug.nl Tel: +31 50 3634011
-Thijs de Graauw, ISO SWS Principal Investigator Tel: +31-50-3634074 thijsdgsron.rug.nl, Th.de.Graauwsron.rug.nl