Giant galaxy M87 finally sized up
Astronomers have probed the edge of M87 for the first time, and they found it to be about three times as large as our own Milky Way.
Provided by ESO, Garching, Germany
May 20, 2009
Using European Space Observatory's (ESO) Very Large Telescope, astronomers have measured the giant galaxy Messier 87 (M87). They were surprised to find that its outer parts have been stripped away by still unknown effects. The galaxy also appears to be on a collision course with another giant galaxy in this dynamic cluster.
The new observations reveal that M87's halo of stars has been cut short, with a diameter of about a million light-years, despite being about three times the extent of the halo surrounding our Milky Way. Beyond this zone only few intergalactic stars are seen.
"This is an unexpected result," said coauthor Ortwin Gerhard. "Numerical models predict that the halo around M87 should be several times larger than our observations have revealed. Clearly, something must have cut the halo off early on."
The team used the Fibre Large Array Multi Element Spectrograph (FLAMES), the super-efficient spectrograph at ESO's Very Large Telescope at the Paranal Observatory in Chile, to make ultra-precise measurements of a host of planetary nebulae in the outskirts of M87 and in the intergalactic space within the Virgo Cluster of galaxies, to which M87 belongs. FLAMES can simultaneously take spectra of many sources, spread over an area of the sky about the size of the Moon.
The new result is quite an achievement. The observed light from a planetary nebula in the Virgo Cluster is as faint as that from a 30-Watt light bulb at a distance of about 3.7 million miles (6 million kilometers) — about 15 times the Earth-Moon distance. Furthermore, planetary nebulae are thinly spread throughout the cluster so even FLAMES' wide field of view could only capture a few tens of nebulae at a time.
"It is a little bit like looking for a needle in a haystack, but in the dark," said team member Magda Arnaboldi. "The FLAMES spectrograph on the VLT was the best instrument for the job".
At a distance of approximately 50 million light-years, the Virgo Cluster is the nearest galaxy cluster. It is located in the constellation Virgo and is a relatively young and sparse cluster. The cluster contains many hundreds of galaxies, including giant and massive elliptical galaxies, as well as more spirals like our own Milky Way.
Astronomers have proposed several explanations for the discovered "cut-off" of M87's halo, such as collapse of dark matter nearby in the galaxy cluster. It might also be that another galaxy in the cluster, M84, came much closer to M87 in the past and dramatically perturbed it about a billion years ago. "At this stage, we can't confirm any of these scenarios," said Arnaboldi. "We will need observations of many more planetary nebulae around M87."
One thing the astronomers are sure about, however, is that M87 and its neighbor M86 are falling towards each other. "We may be observing them in the phase just before the first close pass," said Gerhard. "The Virgo Cluster is still a very dynamic place and many things will continue to shape its galaxies over the next billion years."