Just as two great songbirds work side by side and inspire each other to do their best, the Magellanic Clouds produce new stars every time two galaxies meet.
Visible to the naked eye but best viewed from the Southern Hemisphere, the Large and Small Magellanic Clouds are the brightest of the many galaxies orbiting the Milky Way. New observations show that on several occasions the two bright galaxies have formed a cluster of stars together, researchers report March 25. Monthly Notices of the Royal Astronomical Society: Letters,
Astronomer Pol Massana at the University of Surrey in England and his colleagues examined the Small Magellanic Cloud. The five peaks in the galaxy’s star formation rate – 3 billion, 2 billion, 1.1 billion and 450 million years ago and present – correspond to peaks at the same time in the Large Magellanic Cloud. This is a sign that one galaxy triggers star formation in the other whenever the two dance together.
“This is the most detailed star formation history ever [Magellanic] Clouds,” says Paul Ziwick, an astronomer at Texas A&M University in College Station who was not involved in the new work. “It’s painting a very compelling picture that there’s been a very intense interaction between these two over the past two to three giga-years.”
Even when the two galaxies orbit the Milky Way at a distance of between 160,000 and 200,000 light-years from Earth, they also orbit each other (SN: 1/9/20) their orbits are elliptical, which means they pass each other from time to time. As study co-author Gurtina Besala, an astrophysicist at the University of Arizona in Tucson, says, just as the moon’s gravity shakes the ocean, the tides from one galaxy’s gravity swirl around the gas of another, spurring star births. We do.
During the last encounter, which occurred 100 million to 200 million years ago, the smaller galaxy probably broke right through to the larger one, Besla says, which led to the current outbreak of star birth. The last star formation peak in the Large Magellanic Cloud occurred only in its northern part, so she says this is probably where the collision occurred.
Based on star formation peaks, the period between Magellanic encounters has decreased from one billion to half a billion years. Besla attributes this to a process known as kinetic friction. As the Small Magellanic Cloud orbits its companion, it passes through the larger galaxy’s dark halo, attracting the dark matter behind it. The gravitational pull of this dark matter wake slows the smaller galaxy, shrinks its orbit, and shortens the time it takes for it to travel around the Large Magellanic Cloud.
However, the future of the two galaxies may not be so starry. They have come closest to the Milky Way recently, and its tides, Besla says, may have separated the pair. If so, the Magellanic Clouds, now separated by 75,000 light-years, may never come near each other again, ending their most productive episodes of star formation, just as musicians have ever had. Sometimes the bandmates leave to start a solo career.
