An international team of astronomers has discovered that “dark matter”, the
mysterious material that seems to make up most of the mass of galaxies, is
not as all-pervasive as previously believed. Surprising new results from
studies of several elliptical galaxies show they are not surrounded by halos
of dark matter as was expected. The findings will be presented at the
UK/Ireland National Astronomy Meeting in Dublin on Wednesday April 9th by Dr
Aaron Romanowsky of the University of Nottingham.

Dark matter was first discovered in galaxies in the 1970s using studies of
gas in the outer parts of these systems. The high speeds at which this gas
was found to be travelling implied a large gravitational pull, and hence
that there must be large amounts of unexplained mass far from the centres of
galaxies. Unfortunately, only the beautiful spiral galaxies contain the gas
that allows such measurements to be made; the other main class of galaxy,
the elliptical systems, cannot be studied in this way. It has, however, long
been assumed that these galaxies are also enveloped by similar “dark halos”.

Now, though, the new study casts serious doubts on this seemingly-reasonable
assumption. A team of astronomers from Australia, Italy, the Netherlands and
the UK has developed and built a new instrument, the Planetary Nebula
Spectrograph, which is capable of detecting and measuring the velocities of
planetary nebulae in the outer parts of elliptical galaxies. Planetary
nebulae are stars in the final stages of their lives. They are bright enough
to be detected even in quite distant galaxies, and their motions can be used
to infer the amount of mass in the previously unexplored outer parts of
ordinary elliptical galaxies.

With this instrument the team have made the first systematic study of
velocities in the outer parts of ordinary elliptical galaxies. They have
clear results from three galaxies and supporting data from several others.

“We were expecting to find the same kinds of high velocities that are found
in the outer parts of spiral galaxies,” said Dr Romanowsky. “Instead, the
relatively low speeds of planetary nebulae we actually observed are what we
would expect if there were little or no dark matter around these galaxies.”

“We were certainly surprised by the result, but there are some clues as to
what might be going on,” commented team member Professor Michael Merrifield.
“Elliptical galaxies are mostly found in dense galaxy clusters, and this
makes for a pretty rough environment with frequent collisions between
galaxies. This kind of violent interaction might well also be responsible
for stripping away these galaxies’ dark halos. However, this is just
speculation, and as yet we have no detailed picture as to how these naked
systems of stars might have formed.”

IMAGE

An image showing the data obtained using the Planetary Nebula Spectrograph
for one of the project’s target galaxies is available at

http://www.nottingham.ac.uk/~ppzmrm/M105PR.jpg

The caption for the figure reads: Kinematics of Planetary Nebula in Messier
105. The underlying image shows the starlight from this round elliptical
galaxy (and its near neighbour, NGC3384). The dots show the positions of
planetary nebulae located in this system: the colour of each dot shows
whether the nebula is receding or approaching, while its size indicates its
speed. Note how planetary nebulae can be detected well beyond the apparent
edge of the galaxy, and that the dots tend to get smaller far from the
galaxy, indicative of slow speeds and hence a lack of dark matter.

CONTACTS

Dr Aaron J. Romanowsky
University of Nottingham, United Kingdom
E-mail: aaron.romanowsky@nottingham.ac.uk
Phone: (+44) (0)115 951 5130 (+44) (0)7766 293523 (mobile)

Prof Michael R. Merrifield
University of Nottingham, United Kingdom
E-mail: michael.merrifield@nottingham.ac.uk
Phone: (+44) (0)115 951 5186 (+44) (0)7711 382612 (mobile)

Dr Magda Arnaboldi
Observatory of Turin, Italy
E-mail: arnaboldi@to.astro.it

Dr Nigel G. Douglas
Kapteyn Institute, The Netherlands
E-mail: ndouglas@astro.rug.nl
Phone: (+31) 50 363 4088

Prof Ken C. Freeman
Research School of Astronomy & Astrophysics, Mt Stromlo Observatory,
Australia
E-mail: kcf@mso.anu.edu.au
Phone: (+61) 2 6125 0264

Prof Konrad Kuijken
University of Leiden, The Netherlands
E-mail: kuijken@strw.leidenuniv.nl
Phone: (+31) 71 527 5848

NOTE

The research team gratefully acknowledges the support of the Isaac Newton
Group of telescopes in La Palma, in the Canary Islands. Their technical
assistance in commissioning the Planetary Nebula Spectrograph on the 4.2-m
William Herschel Telescope has been invaluable.