This artist’s conception shows the Large Magellanic Cloud, or LMC, in the foreground as it passes through the gaseous halo of the much more massive galaxy, the Milky Way. – NASA, ESA, RALF CRAWFORD (STSCI)
Nov. 15 () –
The Hubble Space Telescope has observed in the confines of our galaxy how the neighboring Large Magellanic Cloud (LMC) survives interaction with the Milky Way.
Many researchers theorize that the LMC is not in orbit around our galaxy, but is simply passing through. These scientists believe that the LMC has just completed its closest approach to the much more massive Milky Way. This step has made most of the spherical halo of gas surrounding the LMC disappear.
Now, for the first time, astronomers have been able to measure the size of the LMC halo, something they could only do with Hubble. In a new study, available on the arXiv preprint server and to be published in The Astrophysical Journal Lettersresearchers were surprised to discover that it is extremely small, about 50,000 light years in diameter. That’s about 10 times smaller than the halos of other galaxies that have the mass of the LMC. Its compactness tells the story of its encounter with the Milky Way.
“The LMC is a survivor,” he said in a statement Andrew Fox of AURA/STScI (Space Telescope Science Institute) for the European Space Agency in Baltimore, who was the principal investigator of the observations. “Although it has lost a lot of its gas, it has enough left to continue forming new stars. Therefore, new regions of star formation can still be created. “A smaller galaxy wouldn’t have lasted: there would be no gas left, just a collection of aging red stars.”
Although quite deteriorated, the LMC still retains a compact, plump halo of gas, something it would not have been able to retain gravitationally if it had been less massive. The LMC has 10 percent of the mass of the Milky Way, making it heavier than most dwarf galaxies.
“Due to the giant halo of the Milky Way itself, the LMC gas is being truncated or quenched,” explained Sapna Mishra of STScI, lead author of the paper reporting this discovery. “But even with this catastrophic interaction with the Milky Way, the LMC can retain 10 percent of its halo due to its high mass.”
Most of the LMC’s halo faded away due to a phenomenon called ram pressure detachment. The dense environment of the Milky Way pushes the approaching LMC and creates a trail of gas that follows the dwarf galaxy, like the tail of a comet.
“I like to think of the Milky Way as a giant hair dryer that blows gas out of the LMC as it approaches us,” Fox said. “The Milky Way is pushing back so hard that the impact pressure has removed most of the original mass of the LMC halo. “There’s only a little bit left, and it’s this small, compact remnant that we’re seeing now.”
As the impact pressure pushes away much of the LMC halo, the gas slows and will eventually fall as rain onto the Milky Way. But since the LMC has just passed its closest approach to the Milky Way and is moving into deep space again, Scientists do not expect the entire halo to be lost.
ONLY WITH HUBBLE
To conduct this study, the research team analyzed ultraviolet observations from the Mikulski Archive for Space Telescopes at STScI. Most ultraviolet light is blocked by Earth’s atmosphere, so it cannot be observed with ground-based telescopes. Hubble is the only current space telescope tuned to detect these wavelengths of light, so this study was only possible with Hubble.
The team examined the halo using the backlight of 28 bright quasars. Quasars, the brightest type of active galactic nucleus, are thought to be powered by supermassive black holes. Shining like beacons, they allow scientists to “see” the halo gas indirectly through the absorption of background light. Quasars reside throughout the universe at extreme distances from our galaxy.
Scientists used data from Hubble’s Cosmic Origins Spectrograph (COS) to detect the presence of halo gas by the way it absorbs certain colors of light from background quasars. A spectrograph breaks light down into its component wavelengths to reveal clues about the object’s state, temperature, speed, quantity, distance, and composition. With the COS, they measured the velocity of the gas around the LMC, which allowed them to determine the size of the halo.
Due to its mass and proximity to the Milky Way, the LMC is a unique astrophysical laboratory. Seeing the LMC’s interaction with our galaxy helps scientists understand what happened in the early universe, when galaxies were closer to each other. It also shows how messy and complicated the process of interaction between galaxies is.
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