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>Approved-By: "H. Alan Montgomery" <fhd@LCC.NET>
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>Date: Mon, 16 Aug 1999 06:13:03 -0700
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>From: "H. Alan Montgomery" <fhd@lcc.net>
>Organization: Master CraftsMon
>Subject: ISO detects signal from dark matter in a galaxy similar to the
> Milky Way
>To: SEDSNEWS@listserv.tamu.edu
>
>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 <ayee@nova.astro.utoronto.ca>
>Organization: via Internet Direct
>To: SEDSNEWS@listserv.tamu.edu
>
>ESA Science News
>http://sci.esa.int
>
>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
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)1.53.69.71.55, Fax: +33(0)1.53.69.76.90
>
>Martin F. Kessler (ISO Project Scientist)
>Tel: +34 918131254, mkessler@iso.vilspa.esa.es
>
>OTHER SCIENCE CONTACTS:
>
>Edwin Valentijn
>Kapteyn Institute
>Groningen (The Netherlands)
>valentyn@astro.rug.nl, Tel: +31 50 3634011
>
>Thijs de Graauw
>ISO SWS Principal Investigator
>Tel: +31-50-3634074, thijsdg@sron.rug.nl, Th.de.Graauw@sron.rug.nl
>
>USEFUL LINKS FOR THIS STORY
>
>ISO science web site
>http://isowww.estec.esa.nl
>
>NOAO images of NGC891
>http://www.noao.edu/image_gallery/html/im0002.html
>
>More about ISO
>http://sci.esa.int/iso
>
>ISO SWS page
>http://www.sron.rug.nl/iso/sws.html
>
>[NOTE: An image supporting this article is available at
>http://sci.esa.int/story.cfm?TypeID=1&ContentID=6311&Storytype=18]
>
>--
>Andrew Yee
>ayee@nova.astro.utoronto.ca
>