Hubble Sees a Bridge of Stars Connecting Two Galaxies

The galaxy NGC 5427 shines in this new NASA Hubble Space Telescope image. Image Credits: NASA, ESA, and R. Foley (University of California – Santa Cruz); Processing: Gladys Kober (NASA/Catholic University of America)

The poetic-minded among us like to point out how Nature is a dance. If they’re right, then galaxies sometimes form unwieldy pairs. With the Hubble Space Telescope, we can spot some of these galactic pairs as they approach one another.

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An Incredible View Into the Heart of the Small Magellanic Cloud

A radio-telescope image of the Small Magellanic Cloud reveals more detail than ever seen before. Image Credit: N. Pingel et al.

The Small Magellanic Cloud (SMC) is over 200,000 light-years away, yet it’s still one of our galaxy’s closest neighbours in space. Ancient astronomers knew of it, and modern astronomers have studied it intensely. But the SMC still holds secrets.

By studying it and revealing its structure in more detail, astronomers at The Australian National University hope to grow our understanding of the SMC and galaxies in general.

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The Milky Way is Already Starting to Digest the Magellanic Clouds, Starting With Their Protective Halos of Hot Gas

A view of the gas in the Magellanic System as it would appear in the night sky. The Magellanic Corona covers the entire sky while the Magellanic Stream is seen as gas flowing away from the two dwarf galaxies, the Large and the Small Magellanic Clouds. This image, taken directly from the numerical simulations, has been modified slightly for aesthetics. Image Credit: COLIN LEGG / SCOTT LUCCHINI

Massive galaxies like our Milky Way gain mass by absorbing smaller galaxies. The Large Magellanic Cloud and the Small Magellanic Cloud are irregular dwarf galaxies that are gravitationally bound to the Milky Way. Both the clouds are distorted by the Milky Way’s gravity, and astronomers think that the Milky Way is in the process of digesting both galaxies.

A new study says that process is already happening, and that the Milky Way is enjoying the Magellanic Clouds’ halos of gas as an appetizer, creating a feature called the Magellanic Stream as it eats. It also explains a 50 year old mystery: Why is the Magellanic Stream so massive?

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About 3.5 Million Years Ago, a Stream of Gas Outside the Milky Way Would Have Lit Up the Night Sky

An illustration of our hominid ancestors, likely Australopithecus, walking at night, under the lit up stream of gas about 3.5 million years ago. Image Credit: NASA, ESA, G. Cecil (UNC, Chapel Hill), and J. DePasquale (STScI)

It’s a truism to point out that modern humans have only been around for the blink of an eye, relative to the age of the Universe. But the Universe was an active place long before we were around to observe all of that activity. And about 3.5 million years ago, it’s possible—if only remotely—that our ancient ancestors noticed something change in the night sky.

Would it have stirred something inside them? Impossible to know.

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Astronomy Without A Telescope – Situation Cloudy

In 2010 Nidever et al found the Magellanic Stream was much longer than previously realised - maybe 2.5 billion years old. The stream trails behind the Small and Large Magellanic Clouds - visible below the Milky Ways galactic disk, to the right. Ahead of the Clouds is another structure called the Leading Arm. This is a false colour image - the Stream, Arm and Magellanic Bridge between the two Clouds are only visible in radio light.


Most people agree that the Magellanic Clouds are in orbit around the Milky Way. What’s not clear is whether it is a bound orbit or just a temporary ‘ships passing in the night’ arrangement. Something which could clarify the relationship is the Magellanic Stream, a 600,000 light year long string of gas dragged through and beyond the Small and Large Magellanic Clouds.

For the complete picture, note that there is also a shorter trail of gas drawn out ahead of the Clouds, known as the Leading Arm – and the gas flow between the Clouds is known as the Magellanic Bridge. The Bridge is an indication that the Clouds are gravitationally bound in a binary pair – at least for now. The Large Magellanic Cloud may dragging the Small Magellanic Cloud behind it, since the Magellanic Stream ‘skid mark’ is most chemically similar to the contents of the Small Magellanic Cloud.

What remains unresolved is whether the Clouds are in a bound orbit around the Milky Way – or are they just passing by? The level of uncertainty about the dynamics of objects that are relatively close to us, and are easily visible to the naked eye, may seem surprising.

Firstly, it is tricky to gain an accurate estimation of each Cloud’s velocity relative to the Milky Way – partly because we, the observers, have our own independent movement and we need to find a reference frame that we can reliably measure the Clouds’ velocity against.

Estimates derived from Hubble Space Telescope observations by Kallivayalil and colleagues in 2006, measured the Clouds’ velocities against a background of distant quasars, which are visible through the Clouds. These data were then used by Besla and colleagues to propose that the Clouds’ velocities were too fast to be in bound orbits around the Milky Way and so must be just passing by.

But there is another area of uncertainty, where – even with the Clouds’ velocity determined – you still need to decide what escape velocity they need to avoid being caught in a bound orbit of the Milky Way. While we can estimate the Milky Way’s mass, there is the issue of dark matter – which we can’t see and hence can’t locate accurately – so there is some uncertainty about how the combined mass of the Milky Way’s visible and dark matter is distributed.

If, like the visible matter, the dark matter is centralized around the galactic hub, the Clouds won’t need so much velocity to escape. But if the dark matter is more evenly distributed with the galactic disk of visible matter being surrounded by a spherical halo of dark matter, then it’s less clear as to whether the Cloud’s could escape (a scenario that was acknowledged by Besla et al).

A spherical halo of dark matter is the generally preferred model for the Milky Way’s total mass distribution – since, without it, the outer edges of the Milky Way’s visible disk are rotating so fast that they should fly off into space.

Diaz and Bekki have run with this idea by computer-modeling a Milky Way with a circular velocity of 250 kilometres a second (a recent new estimate), which hence requires a more substantial dark matter halo than was assumed by Besla et al. Otherwise, they still use the same Cloud velocities determined from the 2006 Hubble Space Telescope observations.

Left: The neutral hydrogen Magellanic Stream stretching upwards past the Large (red) and Small (green) Magellanic Clouds Right: A computer-modeled scenario in which both clouds are in bound orbits around the Milky Way. Most of the Stream, Bridge and Leading Arm structures are replicated - and are found to originate from the substance of the Small Magellanic Cloud. Credit: Diaz and Bekki.

Their model, when wound back in time, suggests the Clouds have been locked in bound orbits around the Milky Way for more than 5 billion years – with the Magellanic Stream and Leading Arm arising more recently, following a close encounter between the two Clouds (an idea also proposed in Besla et al’s unbound orbit model).

Diaz and Bekki suggest that the Clouds began separate orbits, but passed close to each other around 1.25 billion years ago and then became the binary pair we observe today. The Leading Arm is freed gas being drawn into the Milky Way’s halo – an indication that both Clouds may eventually be assimilated.

Further reading: Diaz and Bekki. Constraining the orbital history of the Magellanic Clouds: A new bound scenario suggested by the tidal origin of the Magellanic Stream.