The giant
galaxy Messier 87 has finally been sized up, but outer parts that should have
been there are missing, scientists now find. The galaxy is smaller than expected.
Messier 87
belongs to the Virgo Cluster of galaxies, the nearest galaxy cluster to our own
Milky Way. The relatively young cluster is located about 50 million light-years
away in the constellation Virgo and contains many hundreds of galaxies.
A team of
astronomers used the super-efficient FLAMES spectrograph at the European
Southern Observatory's (ESO) Very
Large Telescope in Chile to make ultra-precise measurements of a host of
planetary nebulae in the outskirts of Messier 87. (Planetary nebulae are the
final phase in the life of sun-like stars, when stars eject their outer layers
into space.)
Observing
the light of the planetary nebulae in the Virgo Cluster from Earth is akin to
looking at a 30-Watt light bulb from a distance of about 3.7 million miles (6
million kilometers), or about 15 times the distance between the Earth and the
moon.
"It is
a little bit like looking for a needle in a haystack, but in the dark,"
said team member Magda Arnaboldi of the ESO.
The new
observations, detailed in an upcoming issue of the journal Astronomy and
Astrophysics, show that Messier
87's halo of stars is missing some members. The team estimates that it has
a diameter of about a million light-years, which is significantly smaller than
expected (though still three times the extent of the Milky Way's halo).
"This
is an unexpected result," said team member Ortwin Gerhard of the
Max-Planck Institute for Extraterrestrial Physics in Germany. "Numerical
models predict that the halo around Messier 87 should be several times larger
than our observations have revealed. Clearly, something must have cut the halo
off early on."
Just what
that something is, scientists aren't yet sure, though they have a few ideas.
The
collapse of dark matter nearby in the galaxy cluster could account for the
"cut-off." Another possibility is that another galaxy in the
cluster, Messier 84, came much closer to Messier 87 in the past (about a
billion years ago) and dramatically perturbed it.
"At
this stage, we can't confirm any of these scenarios," Arnaboldi said.
"We will need observations of many more planetary nebulae around Messier
87."
One thing
the observations showed for sure though is that Messier 87 is on
a collision course with another nearby neighbor, Messier 86.
"We
may be observing them in the phase just before the first close pass,"
Gerhard said. "The Virgo Cluster is still a very dynamic place and many
things will continue to shape its galaxies over the next billion years."
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