Nearby Large Magellanic Cloud Galaxy Passed through Milky Way's Halo | Hubble

Nearby Large Magellanic Cloud Galaxy Passed through Milky Way's Halo | Hubble

This artist’s concept shows the Large Magellanic Cloud, or LMC, in the foreground as it passes through the gaseous halo of the much more massive Milky Way galaxy. The encounter has blown away most of the spherical halo of gas that surrounds the LMC, as illustrated by the trailing gas stream reminiscent of a comet’s tail. Still, a compact halo remains, and scientists do not expect this residual halo to be lost. 

This artist’s concept shows a closeup of the Large Magellanic Cloud (LMC), a dwarf galaxy that is one of the Milky Way galaxy’s nearest neighbours. Scientists think that the LMC has just completed its closest approach to the much more massive Milky Way. This encounter has blown away most of the spherical halo of gas that surrounds the LMC. The bright purple bow shocks represent the leading edge of the LMC’s halo that is being compressed as the Milky Way’s halo pushes back against the incoming LMC. The pressure is stripping much of the LMC’s halo and blowing it backward into a streaming tail of gas. The dwarf galaxy is cocooned within its remaining halo. An actual science image of the LMC is combined with an artist’s rendering of the galaxy’s halo.

This artist’s concept illustrates the Large Magellanic Cloud’s (LMC’s) encounter with the Milky Way galaxy’s gaseous halo. In the top panel, at the middle of the right side, the LMC begins crashing through our galaxy’s much more massive halo. The bright purple bow shock represents the leading edge of the LMC’s halo that is being compressed as the Milky Way’s halo pushes back against the incoming LMC. In the middle panel, part of the halo is being stripped and blown back into a streaming tail of gas that eventually will rain into the Milky Way. The bottom panel shows the progression of this interaction, as the LMC’s comet-like tail becomes more defined.

The NASA/European Space Agency Hubble Space Telescope, one of our nearest galactic neighbors has passed through the Milky Way galaxy’s gaseous halo. However, in the process, this dwarf galaxy, called the Large Magellanic Cloud (LMC), was stripped of most of its own surrounding halo of gas. Researchers were surprised to find such an extremely small gaseous halo remaining—one around 10 times smaller than halos of other galaxies of similar mass. Still, the LMC has held onto enough of its gas to keep forming new stars. A smaller galaxy would not have survived such an encounter. This is the first time astronomers have been able to measure the size of the LMC’s halo thanks to Hubble.  The LMC is 10 percent the mass of the Milky Way.

The Large Magellanic Cloud is one of the Milky Way galaxy’s nearest neighbors. This dwarf galaxy looms large in the southern nighttime sky at twenty times the apparent diameter of the full Moon.

Many researchers theorise that the LMC is not in orbit around our galaxy, but is just passing by. Those scientists think that the LMC has just completed its closest approach to the much more massive Milky Way. This passage has blown away most of the spherical halo of gas that surrounds the LMC.

“The LMC is a survivor,” said Andrew Fox of AURA/Space Telescope Science Institute for the European Space Agency in Baltimore, who was principal investigator on the observations. “Even though it’s lost a lot of its gas, it’s got enough left to keep forming new stars. So new star-forming regions can still be created. A smaller galaxy wouldn’t have lasted — there would be no gas left, just a collection of aging red stars.”

“Because of the Milky Way’s own giant halo, the LMC’s gas is getting truncated, or quenched,” explained STScI’s Sapna Mishra, the lead author of the paper chronicling this discovery. “But even with this catastrophic interaction with the Milky Way, the LMC is able to retain 10 percent of its halo because of its high mass.”

Most of the LMC’s halo was blown away by a phenomenon called ram-pressure stripping. The dense environment of the Milky Way pushes back against the incoming LMC and creates a wake of gas trailing the dwarf galaxy—like the tail of a comet.

“I like to think of the Milky Way as this giant hairdryer, and it’s blowing gas off the LMC as it comes into us,” said Fox. “The Milky Way is pushing back so forcefully that the ram pressure has stripped off most of the original mass of the LMC’s halo. There’s only a little bit left, and it’s this small, compact leftover that we’re seeing now.”

As the ram pressure pushes away much of the LMC’s halo, the gas slows down and eventually will be captured by the Milky Way. Nevertheless, the LMC has just passed its closest approach to the Milky Way and is moving outward into deep space again. Thus, scientists do not expect the whole halo will be lost.

Only with Hubble

To conduct this study, the research team analyzed ultraviolet observations from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute (STScI). Most ultraviolet light is blocked by Earth’s atmosphere, so it cannot be observed with ground-based telescopes. Hubble is currently the only space telescope that is tuned to detect these wavelengths of light, so this study was only possible with Hubble.

The team surveyed the halo by using the background light of 28 bright quasars. The brightest type of active galactic nucleus, quasars are believed to be powered by supermassive black holes. Shining like lighthouse beacons, they allow scientists to ‘see’ the intervening halo gas indirectly through the absorption of the background light. Quasars reside throughout the Universe at extreme distances from our galaxy.

The scientists used data from Hubble’s Cosmic Origins Spectrograph (COS) to detect the presence of the halo gas by the way it absorbs certain colors of light from background quasars. A spectrograph breaks light into its component wavelengths to reveal clues to the object’s state, temperature, speed, quantity, distance, and composition. With COS, they measured the velocity of the gas around the LMC. This allowed them to determine the size of the halo.

Due to its mass and proximity to the Milky Way, the LMC is a unique astrophysics laboratory. Seeing the LMC’s interplay with our galaxy helps scientists understand what happened in the early Universe, when galaxies were closer together. It also shows just how messy and complicated the process of galaxy interaction is.

Looking to the future

The team will next study the front side of the LMC’s halo, an area that has not yet been explored.

“In this new programme, we are going to probe five sightlines in the region where the LMC’s halo and the Milky Way’s halo are colliding,” said co-author Scott Lucchini of the Center for Astrophysics | Harvard & Smithsonian. “This is the location where the halos are compressed, like two balloons pushing against each other.”

The Hubble Space Telescope is a project of international cooperation between the European Space Agency and NASA.

Read the Science paper "The Truncated Circumgalactic Medium of the Large Magellanic Cloud" here:
https://arxiv.org/abs/2410.11960

Image Credit: NASA, ESA, R. Crawford (STScI)

Release Date: Nov. 14, 2024

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