Magellanic Clouds May Be Just Passing Through

January 9, 2007
Magellanic Clouds May Be Just Passing Through
Astronomers have measured the 3-D velocities of the Large Magellanic Cloud (shown here) and the Small Magellanic Cloud. They found surprisingly high speeds, which may indicate that the Milky Way is twice as massive as previously thought, or that the Magellanic Clouds are not gravitationally bound to the Milky Way but instead are "just passing through." (Copyright Robert Gendler and Josch Hambsch 2005)

The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are two of the Milky Way's closest neighboring galaxies. Both are visible only in the southern hemisphere. By studying their orbits, astronomers can learn about both the histories of the Clouds and the structure of the Milky Way (from its influence on the Clouds' motions).

Astronomers Nitya Kallivayalil and Charles Alcock (Harvard-Smithsonian Center for Astrophysics) and Roeland van der Marel (Space Telescope Science Institute) have made the most accurate measurements to date of the three-dimensional velocities through space of the LMC and SMC. Their surprising results hold profound implications for both the Milky Way and its companions.

"We found that the velocities of the LMC and SMC are unexpectedly large - almost twice those previously thought," says Kallivayalil.

These findings were presented today in a press conference at the 209th meeting of the American Astronomical Society.

The radial velocities (motion along the line of sight) for both Clouds are well known and relatively easy to measure.

Much more difficult to measure is the proper motion (motion across the sky), requiring extraordinary precision over the course of several years. Both proper motion and line-of-sight motion must be known to calculate the true 3-d velocity.

By making two sets of observations two years apart with NASA's Hubble Space Telescope, Kallivayalil and her colleagues calculated accurate proper motions for the LMC and SMC. By combining proper motions and radial velocities, they found that the LMC speeds through space at 378 km/sec (235 miles/sec) while the SMC has a speed of 302 km/sec (188 miles/sec).

There are two possible explanations for these high speeds:

1) The mass extent of the Milky Way is larger than previously thought. If the Clouds are gravitationally bound to the Milky Way, then the Milky Way must be much more massive than previous data suggested. The excess mass would pull on the Clouds, keeping them "close at hand."

2) The Magellanic Clouds are not gravitationally bound to the Milky Way. If previous calculations of the Milky Way's mass are accurate, then the Galaxy is not massive enough to hold onto its companions. In a few billion years, they will escape from the Milky Way.

"The Magellanic Clouds may not be true companions of the Milky Way," explains Kallivayalil. "Perhaps they are travelers just passing through the neighborhood."

The velocities of the Magellanic Clouds relative to each other also are surprisingly high. This suggests that the Magellanic Clouds may be coincidental companions and are not gravitationally bound to each other. Alternatively, their high velocities may explain why these two galaxies, if bound, did not merge with each other long ago.

Future measurements of the Magellanic Stream--a long streamer of hydrogen gas trailing behind the Clouds--may clarify the previous paths of the Clouds and their relationships with each other and with the Milky Way.

"Regardless of what future work finds, our study shows that we need to reassess the orbital histories of the Clouds," says Kallivayalil.

Source: Harvard-Smithsonian Center for Astrophysics

Explore further: Image: Hubble Investigates Stellar Shrapnel

Related Stories

Image: Hubble Investigates Stellar Shrapnel

August 22, 2016

Several thousand years ago, a star some 160,000 light-years away from us exploded, scattering stellar shrapnel across the sky. The aftermath of this energetic detonation is shown here in this striking image from the NASA/ESA ...

Image: Hubble gazes at long-dead star

August 1, 2016

This NASA/ESA Hubble Space Telescope image captures the remnants of a long-dead star. These rippling wisps of ionized gas, named DEM L316A, are located some 160,000 light-years away within one of the Milky Way's closest galactic ...

NASA's Fermi mission expands its search for dark matter

August 12, 2016

Dark matter, the mysterious substance that constitutes most of the material universe, remains as elusive as ever. Although experiments on the ground and in space have yet to find a trace of dark matter, the results are helping ...

Image: Hubble gazes at stars of the Large Magellanic Cloud

June 24, 2016

This colorful and star-studded view of the Milky Way galaxy was captured when the NASA/ESA Hubble Space Telescope pointed its cameras towards the constellation of Sagittarius (The Archer). Blue stars can be seen scattered ...

Recommended for you

Milky way had a blowout bash six million years ago

August 29, 2016

The center of the Milky Way galaxy is currently a quiet place where a supermassive black hole slumbers, only occasionally slurping small sips of hydrogen gas. But it wasn't always this way. A new study shows that 6 million ...

NASA's Juno successfully completes Jupiter flyby

August 29, 2016

NASA's Juno mission successfully executed its first of 36 orbital flybys of Jupiter today. The time of closest approach with the gas-giant world was 6:44 a.m. PDT (9:44 a.m. EDT, 13:44 UTC) when Juno passed about 2,600 miles ...

The proliferation of Jupiter-like worlds

August 29, 2016

Our galaxy is home to a bewildering variety of Jupiter-like worlds: hot ones, cold ones, giant versions of our own giant, pint-sized pretenders only half as big around.

Hubble spots an irregular island in a sea of space

August 29, 2016

This image, courtesy of the NASA/ESA Hubble Space Telescope's Advanced Camera for Surveys (ACS), captures the glow of distant stars within NGC 5264, a dwarf galaxy located just over 15 million light-years away in the constellation ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.