Is the Large Magellanic Cloud (LMC) just passing by?

If you've ever looked towards the southern parts of the night sky away from the city lights and on a Moonless night you'll notice two misty patches that look like detached parts of the Milky Way. These are the Small and Large Magellanic Clouds, or SMC and LMC, and are galaxies outside the Milky Way.

They were well known in ancient times and the LMC was described by Ferdinand Magellan in 1519 during a voyage and his letters made Western astronomers aware of them.

The LMC photographed by Chi Chan and posted to the BINTEL Society Facebook page . This image shows rich star forming regions glowing in the reddish colour of hydrogen alpha emissions.

Both the SMC and the LMC will start to resolve into a star clouds and nebulae with even binoculars and are wonderful astro objects to explore and image with more powerful telescopes.

A chart from via Stellarium showing where you can find the LMC and SMC in the southern sky in Sydney on the 17th of November 2024 around 9.30pm local time.

The Magellanic Clouds were long thought to be largest satellite galaxies of the Milky Way. (Currently there's dozens of smaller galaxies surrounding the Milky Way and only the SMC and LMC can be seen visually.)

However, in recent years astronomers have thought that the LMC might just be passing by the Milky Way and not gravitationally bound into an orbit around our home galaxy.

This passing by has removed just about all of vast shell of gas surrounding the LMC and reduced it to a fraction of other galaxies of the LMC's size. This smaller gas shell has now been observed and measured by the Hubble Space Telescope (HST).

"The LMC is a survivor," said Andrew Fox of AURA/STScI 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."

An illustration showing how the HST used UV light from 28 distant quasars to measure the size of the diminished size of the LMC's gas halo. As UV light is blocked by the Earth's atmosphere, this study needed to be made by a space telescope. Image via NASA, ESA, Ralf Crawford (STScI)

How did the Milky Way strip the LMC of its gas shell?

As the LMC approached the Milky Way, its own gas halo pushed back the incoming galaxy and stripped it off behind it.

"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."

After surviving its encounter with the Milky Way, albeit bruised and battered, the LMC is now heading away from us and will take most of its remaining gas halo with it - along with its ability to form new stars.

The same team now plans to use the HST to look at the regions around the front side of the LMC to study the where the Milky Way and the LMC meet.

"In this new program, 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."

Does this change how we think of the LMC?

It might for a few people. We've traditionally regarded the SMC and LMC as satellite galaxies orbiting the Milky Way like moons around the planets in the Solar System. We've come to understand that our galactic region is complex, with numerous small galaxies both in orbit around the Milky Way and on their own paths through the intergalactic space, although heavily influenced by the larger Milky Way.

The LMC now seems to part of our local group of galaxies that will be in the sky for many millions of years to come, gradually fading over the eons. Definitely something to ponder as you look up towards our southern skies.

Cheers,

Earl White

BINTEL

15th November 2024