This is Our Home Galaxy, and a Couple of Neighbors

As the Milky Way rises over the horizon at the European Southern Observatory, its companion galaxies also come into view. The Large Magellanic Cloud (LMC) at far left lies about 160,000 light-years away, while the Small Magellanic Cloud (SMC, above and to the right of the LMC) lies about 200,000 light-years away. New simulations show that the LMC stole stars from the SMC when the two galaxies collided 300 million years ago. Microlensing events that have been observed are due to LMC stars passing in front of a stream of stars pulled from the SMC.
Credit: ESO/Y. Beletsky

When you look out at the night sky, you’re looking at our ancient home. Yes, Earth is our current home. But, in the grand scheme of things, the galaxy — and all the elements that make it are also our home.  The elements that make up our bodies, our planets, and our star all were either created in the Big Bang (hydrogen, for example), or inside other stars (carbon, oxygen, nitrogen, etc.).  Multiple generations of stars have lived and died in the galaxy, and we are the resulting “star stuff”.

But, there’s more than star stuff out there.  There are mysterious things that may tell astronomers more about types of matter in the cosmos and distribution of that matter throughout the universe.

Astronomers have been studying one of those two irregular-looking clouds of stars that appear just below our galaxy in this image to understand a category of objects called MACHO (Massive Compact Halo Objects). These were thought to be things about the mass of a star that were so faint they couldn’t be easily detected. Surveys of this region of our galactic neighborhood have been underway to see if MACHOs could be part of that mysterious collection of “stuff” called “dark matter” that seems to be an incredibly important part of the universe.

In order for MACHOs to make up dark matter, they must be very faint. To even decide if they’re “there”, astronomers looked for a phenomenon known as microlensing. During a microlensing event, a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten. If a MACHO does this, then they’d know a little bit more about the object.

By studying the LMC, astronomers hoped to see MACHOs within the Milky Way lensing distant LMC stars. The number of microlensing events observed by various teams was smaller than needed to account for dark matter, but much higher than expected from the known population of stars in the Milky Way. This left the origin of the observed events a puzzle and the existence of MACHOs as exotic objects a possibility.

Instead of MACHOs, a trail of stars removed from the SMC could well be responsible for the microlensing events. How do astronomers know this? They’ve done computer simulations showing that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC. Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from our point of view.

Although the evidence for the trail of lensed stars is persuasive, they haven’t been directly observed yet. That will take time, since these could be faint. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds. The computer models used to simulate the trail will point the way for astronomers to find the other “stuff” that makes up the galaxies… and intergalactic space.

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This entry was posted on Monday, October 29th, 2012 at 17:14 pm and is filed under astronomy, astronomy news, dark matter, european southern observatory. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.