Tag: galaxy collision

Temba, his spiral arms wide

By Phil Plait | September 3, 2012 7:02 am

Lying roughly 50 million light years from Earth is the magnificent spiral galaxy NGC 5033. Although that distance is a soul-crushing 500 quintillion kilometers, it’s actually relatively close by on the cosmic scale. Close enough that a lot of detail can be seen in the galaxy… and it also makes for a stunner of a picture:

[Click to darmokenate.]

This shot was taken by friend-of-the-BA-blog Adam Block using the 0.8 meter Schulman Telescope on Mount Lemmon in Arizona. It’s a whopping 13 hour exposure taken in near-true color.

It’s amazing what you can see in just this picture if you know what to look for. The spiral arms of the galaxy are fairly open, which is common enough, but the outer ones stick out a bit more than you might expect. The nucleus is very small and bright, more so than I’d expect for a typical spiral as well. Both of those things led me to expect this is an active galaxy, and that turns out to be the case.

Every big galaxy – ours included – has a supermassive black hole in the center. The Milky Way’s is 4 million times the mass of our Sun! In some galaxies, like ours, happily, the black hole is just sitting there. But in some there is gas actively falling into the hole. It spirals around and forms a very hot and very large disk, which glows fiercely as the matter is heated to temperatures of millions of degrees. They disk can blast out light from radio waves up to X-rays, and we say that the galaxy is "active".

A quick search of the literature didn’t turn up any measurements for the mass of black hole in NGC 5033, but it does confirm that it’s an active galaxy. Interestingly, the black hole is not located in the exact center of the galaxy! That’s very unusual, and indicates that NGC 5033 recently merged with another galaxy, probably a smaller one. It’s a cannibal! But then, most big galaxies are. It’s how they get big… and you’re living inside a big one, so there you go.

This may explain the wide arms on the galaxy as well; a collision and merger can distort the shape of the galaxy. Also, check out all the pink blobs along the arms: those are sites of furious star formation, the hot energetic massive young stars lighting up the gas around them. That also is common after a big collision.

Finally, one more nifty thing. You can see long ribbons of dark dust festooning the galaxy in the inner region. Dust absorbs light from stars behind it. But see how the dust looks like it’s only on one side of the galaxy, the half in the picture below the center? That’s an illusion, sortof. In reality there’s dust orbiting all around the center. However, there are stars above and below the disk of the galaxy, and the ones between us and the far side fill in the darkness a little bit, so the dust is less apparent. I’ve written about this before, and it does happen in quite a few spirals. Click the links in the Related Posts section below to see more gorgeous galaxies with this feature.

It’s funny how much information you can squeeze from a single picture! You have to be careful and not over-interpret it, and of course a lot of the things I’ve written here wouldn’t have been known without other observations of NGC 5033 using different telescopes and different methods in different types of light.

But even just one picture can tell you a lot. And in my opinion – and I tend to be right about these kinds of things – the wave of beauty that flows over you when looking at this picture is only enhanced by knowing more about the galaxy itself… and is boosted in no small way by the fact that we can know these things.

Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona


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CATEGORIZED UNDER: Astronomy, Pretty pictures

Hold on tight: in 4 billion years, we're due for a galactic collision!

By Phil Plait | May 31, 2012 1:01 pm

The galaxy we live in, the Milky Way, is a large spiral galaxy that lives in a small cluster of other galaxies called the Local Group. The other big member is the Andromeda galaxy, located about 2.5 million light years away. That’s a long way off, but we’ve known for a long time that Andromeda is heading more or less toward us at a speed of roughly 100 km/sec (60 miles/second).

The question is, is it headed directly at us, or does it have some "sideways" motion and will miss us? New results announced today by astronomers using Hubble show that — gulp! — Andromeda is headed right down our throats!

But don’t panic. It won’t happen for nearly 4 billion years.

This is a pretty cool result. They used Hubble to look at stars in Andromeda’s halo, the extended fuzzy region outside the main body of the galaxy. By very carefully measuring the positions of the stars over seven years, they could directly measure the motion of those stars. Extrapolating that into the future has allowed the motion of the Andromeda galaxy itself to be determined for the first time.

So what’s going to happen?

First, watch this awesome video of the collision based on the observations:

So here are the details:

Read More

The start of a long, long dance

By Phil Plait | May 11, 2012 6:30 am

A hundred million light years away, two gorgeous spiral galaxies are locked in an embrace that may end with them merging, a dance spread across a hundred thousand light years in space and a hundred million years of time.

[Click to galactinate, and yeah, just do it. The hi-res version is big and lush and lovely indeed.]

This image, taken by frequent BABlog contributor Adam Block, shows this cosmic waltz in lovely detail (another wonderful image is available via the ESO as well [UPDATE: … and from Gemini, with a diagram of the two and a nice explanation]). The two galaxies (NGC 5426 on the left, and NGC 5427 on the right) are just starting this eons-long encounter, but affects are already visible. You can see tendrils of material stretching from NGC 5426 to its companion, drawn out by the force of NGC 5427’s gravitational attraction.

Inside the galaxies, you can easily see the pink glow of gas clouds, disturbed by the interaction, starting to furiously churn out hot young stars. Actually, stars of all masses are born in these clouds, but it’s the rare massive stars that have the most impact. They blast out ultraviolet light which makes the gas glow, and will explode as supernovae, lighting things up even more.

In galactic collisions like this the outcome can be difficult to ascertain. Perhaps they’ll pass this one time and do so with sufficient velocity to make this a one-eon stand, continuing on into the night. Or, if their relative speeds aren’t enough, they’ll pull apart, only to be drawn inexorably together once again. Even then they may pass, but this time in an ever-decreasing arc, until finally they merge into one bigger galaxy. Although this plays out over far too long a timespan to watch in real time, we see so many colliding galaxies that it’s like having snapshots at all different stages of evolution (see Related Posts below for lots of collidey goodness).

The general steps here are known, but the specific outcome of this particular encounter is still to be seen.

And we’ll see something like it up close, if not for quite some time: the Andromeda Galaxy will one day collide with our Milky Way, and when that happens we’ll be able to see what a galactic collision looks like… from the inside. Buy your tickets now. The show begins in just a billion years or two.

Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona


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The delicate aftermath of cosmic violence

CATEGORIZED UNDER: Astronomy, Pretty pictures

The radiant beauty of cosmic collisions

By Phil Plait | October 13, 2009 7:00 am

What happens when two massive spiral galaxies — each with a hundred billion stars — slam into each other head-on at hundreds of kilometers per second?

Beauty.

hst_ngc2623

[Click to embiggen; or go here to get the massive 15Mb TIF image.]

This is the unusual galaxy NGC 2623, seen in this newly-released and breathtaking Hubble Space Telescope image taken in 2007. We’re seeing this vast smash-up caught in the act, a galactic collision already in progress. It appears frozen in time, but that is an illusion of distance: at a distance of a staggering 250 million light years the tremendous velocities of the collision are reduced to a motionless tableau on the human timescale.

But we see a large number of galactic collisions when we catalog the sky, and together with our knowledge of math and physics we have a good understanding of how these encounters play out.

When two massive galaxies approach each other, the gravity of each starts to affect the other. Call them Galaxy A and B. The side of Galaxy B closer to Galaxy A feels more gravity from it, so stars and gas are drawn toward it more strongly than the stars and gas on the far side of Galaxy B. The same is true in the other galaxy. As they get closer, this force strengthens, teasing out long ribbons of material — called tidal tails — that stretch in the direction of the other galaxy.

If the encounter is off-center, then the tails get curved when the galaxies pass, arcing either gently or severely depending on the speed, encounter distance, and mass of each participant. The Hubble image clearly shows the arcing tails from each galaxy in NGC 2623.

Incredibly, even though hundreds of billions of stars are involved, each individual star is far too small to suffer a physical collision. But gas and dust clouds are much bigger than stars (they can be hundreds of trillions of kilometers across, as opposed to stars which are a trifling million or so kilometers in diameter), so collisions between them are common. When clouds collide they collapse and undergo violent bouts of star formation. This too is clear in the image: the blue clumps in the tidal tails are vast regions of clusters of stars being born; over 100 such clusters have been identified in this image in the tail on the right alone.

Collisions like this blast out energy, not just in visible light, but at other wavelengths as well. In infrared alone, NGC 2623 radiates with the power of 400 billion times the Sun’s energy. This makes NGC 2623 a ULIRG: an ultraluminous infrared galaxy. Although relatively rare locally, they are so common at great distance (and therefore earlier on in the age of the Universe) that they comprise as much as half of all the infrared background glow we see in the Universe. The huge amount of infrared comes from the collision itself; star formation produces prodigious amounts of dust which absorb ultraviolet light from newly-born stars and re-radiate it in the infrared. The collision also dumps gas and dust into the central supermassive black holes in the cores of the two colliding galaxies, which piles up in a flat disk outside the black hole, heats up hugely, and again glows brightly.

Astronomers are making a comprehensive study of such ULIRGs using a fleet of telescopes including Hubble, Spitzer, Chandra, GALEX, 2MASS, VLA, and even the venerable IRAS satellite which surveyed the sky in infrared in the 1980s, and in fact first discovered the ULIRGs.

Why study them? Because galaxies as large as our Milky Way almost certainly started off small and grew to their present size by colliding and merging with other galaxies. Studying ULIRGs is a way of examining how our galaxy came to be… and it’s a glimpse of our future as well. In a billion years or more, we will suffer a massive collision with the Andromeda Galaxy. Our own clouds of gas and dust may smash into those in Andromeda, creating huge waves of star formation and blasting out light at all wavelengths. What will our fate be then? The Earth may survive — the Sun will still be around for this event — and the gravitational repercussions may toss us out of the new galaxy, or drop us down to the core.

It may seem academic, but astronomers thirst for understanding of these events. We want to know how we came to be, and where we are headed. That knowledge may have little or no practical use for our own survival (or at least not for a few million millennia), but for now, for today, we learn more about galaxies in general, more about the physics of cosmic collisions, and more about the interaction of gas and dust on a truly mind-numbing scale.

And of course, we get to gaze on lovely images, illusions of placidity and gentleness to be sure, but lovely nonetheless.

Image credit: NASA, ESA and A. Evans (Stony Brook University, New York & National Radio Astronomy Observatory, Charlottesville, USA)

CATEGORIZED UNDER: Astronomy, Pretty pictures
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