ISO detects signal from dark matter in a galaxy similar to the

Larry Klaes (
Wed, 18 Aug 1999 13:02:09 -0400

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>Subject: ISO detects signal from dark matter in a galaxy similar to the
> Milky Way
>Subject: ISO detects signal from dark matter in a galaxy similar to the Milky
> Way
>Date: Tue, 17 Aug 1999 15:01:07 -0400
>From: Andrew Yee <>
>Organization: via Internet Direct
>ESA Science News
>17 Aug 1999
>ISO detects signal from dark matter in a galaxy similar to the Milky Way
>Galaxies are known to have much more matter than telescopes can currently
>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,
>an important component of the dark matter if it is of the normal, ordinary
>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
>of galaxies", says the main author of the finding Edwin A. Valentijn, from
>Kapteyn Institute in Groningen (The Netherlands).
>Astronomers discovered the existence of the dark matter long ago, by
>the motion of the stars and gas in spiral galaxies: the velocity of this
>corresponds to a certain amount of mass in the galaxy, and measurements
>that the stars and the gas are moving too fast for the amount of mass
>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
>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
>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
>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
>for molecular hydrogen, which is extremely difficult to observe. Emission
>this molecule can only be detected with highly sensitive infrared telescopes,
>and ESA's ISO has allowed the study of molecular hydrogen in unprecedented
>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
>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
>Our team was the only one who thought the measurement was feasible, as no
>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
>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
>-- 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.
>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.
>ESA Public Relations Division
>Tel: +33(0), Fax: +33(0)
>Martin F. Kessler (ISO Project Scientist)
>Tel: +34 918131254,
>Edwin Valentijn
>Kapteyn Institute
>Groningen (The Netherlands)
>, Tel: +31 50 3634011
>Thijs de Graauw
>ISO SWS Principal Investigator
>Tel: +31-50-3634074,,
>ISO science web site
>NOAO images of NGC891
>More about ISO
>ISO SWS page
>[NOTE: An image supporting this article is available at
>Andrew Yee