Traversing Space on a Bridge of Stars

A Bridge of Stars between the Magellanic Clouds

The Magellanic Clouds.

The Magellanic Clouds in the night sky. The Large and the Small Magellanic Clouds are visible. The Clouds are moving towards the bottom left corner. Credit: V. Belokurov and A. Mellinger

If you’ve ever been south of the equator, you’ve probably seen the Magellanic Clouds in the southern hemisphere sky. These two little galaxies  look like puffy clouds separated by a whole lot of space. It turns out that the light-years between them might not be so empty as astronomers once thought. Researchers at University of Cambridge in England have found what looks like a 43,000 light-year-long bridge of stars stretching from one galaxy to the other.  Their work, based on a huge census of stars that the Gaia satellite is doing, is giving a new look at what happens when dwarf galaxies interact. The result of its mission, when completed, will be a 3D map of our galaxy, and apparently of our neighboring satellite galaxies.

Using Old Stars to Trace a Bridge

bridge of stars

The Magellanic Clouds, their stellar halos and the RR Lyrae bridge. Pale white veils and the narrow bridge pf stars between the Clouds represent the distribtuion of the RR Lyrae stars detected with the data from the Gaia satellite. Credit: V. Belokurov, D. Erkal and A. Mellinger

The team of astronomers focused their attention on data about stars called RR Lyraes. These are pulsating variables that are quite old stars. They’ve been around for a long time — at least as long as the Magellanic Clouds have existed. So, their very existence tells us something about the history of these two nearby dwarf satellite galaxies. Theastronomers used the RR Lyraes to measure the extent of the Large Magellanic Cloud first. It turns out there’s a sort of fuzzy halo of these stars stretching away from the LMC that’s being stretched out into a evanescent bridge of stars.

The big question now is why this stream exists. Normally streams of stars aren’t stretching away from a galaxy unless there’s been something to tear them away. In this case, it’s likely that the tidal pull of the e Small Magellanic Cloud has steadily lured away stars from the LMC.  As it orbits, the LMC is leaving a tracer of its stars as it goes.  There could also be stars in the stream that are being attracted by the gravity of the Milky Way, too.

A Bridge of Stars During Interactions

Interactions between galaxies often warp and reshape the participants in the galactic dances. Such interactions are also an integral part of the galaxy assembly process: big galaxies get built from the collisions of smaller ones. We’ve seen streams of stars in other interacting galaxies, so this lovely bridge between the Magellanic Clouds fits right into the idea that cosmic dances can do more than warp galaxies. They can strip stars away, too.

This is a pretty cool story of galaxy evolution taking place in our own galactic back yard. If you want more information on the work the Cambridge astronomers are doing, check out their press release here.




Peeking into Galaxy Clusters

Gravitationally Bound Galaxy Collections Have Much to Reveal

Galaxies at the heart of the Virgo Cluster. This formation is called Markarian’s Chain. Courtesy Samuel Oschin Telescope/CalTech.

Some years ago, I had the privilege of being the science writer for the Griffith Observatory exhibit project. Griffith, in case you don’t know, is located in the Hollywood Hills of Los Angeles, and is one of the best-known observatories and landmarks in the U.S. It was a rare honor, and in the years since then, I’ve found out how unusual it is for ONE person to be responsible for undertaking such a project. I didn’t know how rare it was. I was just thrilled to be on the project!

Standing by one part of the Big Picture exhibit at Griffith Observatory.

One of the exhibits that really caught my imagination (and offered me incredible writing challenges) was a giant wall containing an image taken by the Samuel Oschin telescope called The Big Picture. It features a look at the Virgo Cluster of galaxies, a collection of that lies some 53 million light-years away from us, and contains somewhere between 1,000 and 2,000 galaxies. If you visit Griffith, you can sit in front of the picture for as long as you like admiring the galaxies, quasars, stars, and other objects captured in an area of sky that you can cover with your finger, held out at arm’s length. It’s an incredible experience, and it was my great pleasure to study that image for many weeks as I wrote the descriptive text (and a video script) to go with it. In the end, it was all about what we can SEE in that image, and there’s a lot to study.

Checking Into the Coma Cluster

There are many galaxy clusters in the universe, and as astronomers get more sophisticated instruments and telescopes online, they’re digging into these galactic families to understand what makes them tick. And, it turns out that in at least one cluster (and probably many more), understanding them hinges not just on what we see, but what we don’t see.  Dark matter likely plays a huge role in what we detect going on in these galaxies and clusters. Take the Coma Cluster, for example. It is about 321 million light-years away from us and contains at least a thousand galaxies. Probably more, but many of them are very difficult to see. Why would that be?

Some of the more than 800 “dark galaxies” detected by Subaru Telescope in the Coma Cluster. Courtesy Subaru Telescope/NAOJ.

The Subaru Telescope, located on the Big Island of Hawai’i, has been studying this cluster. Recently astronomers took a look at all the archival data and found something rather interesting: it contains mysterious “dark” galaxies. More than 800 of these faint, diffuse galaxies exist, situated in the heart of a busy galactic city. Many are similar in size to our home galaxy, so imagine our immense Milky Way, but darker and dimmer. Yet, there are still stars in these galaxies. In fact, there are scads of old stellar systems, which is interesting since gravitational interactions between members of the cluster can really disrupt star systems. By all rights, those older systems should be flung around by tidal forces.

So, what’s protecting them? The best answer is: dark matter. In these galaxies, dark matter outnumbers the amount of visible matter by 99 to 1.

However, the protection of dark matter haloes isn’t the only thing going on here. Astronomers saw few “new” stars in these galaxies, which means that they lost the gas needed to form new generations of stars. This occurred fairly early in these galaxies’ lives, not long after they formed. So, the evolutionary story of these galaxies is a mystery needing further insight. And the detection of dark matter raises a LOT of questions.

Why is there so much dark matter in them? What role did it play early in the galaxies’ history? And, when did the galaxies lose the ability to form new stars? Does the dark matter than plays a role in protecting the older stellar populations have anything to do with mass loss in their home galaxies? Lots of questions, and the answers will come from additional observations of these distant galaxies.

I love looking at pictures of galaxy clusters. I’ve seen many images of the Coma Cluster, and of course, revelled in the study of the Virgo Cluster. Are all the things happening in those galaxies also happening in the many other clusters astronomers study? What’s the REALLY big picture? Stay tuned!