APOD: Arp 240: A Bridge between Spiral... (2016 Nov 28)

[APOD Robot]

Arp 240: A Bridge between Spiral Galaxies from Hubble

Explanation: Why is there a bridge between these two spiral galaxies? Made of gas and stars, the bridge provides strong evidence that these two immense star systems have passed close to each other and experienced violent tides induced by mutual gravity. Known together as Arp 240 but individually as NGC 5257 and NGC 5258, computer modelling and the ages of star clusters indicate that the two galaxies completed a first passage near each other only about 250 million years ago. Gravitational tides not only pulled away matter, they compress gas and so caused star formation in both galaxies and the unusual bridge. Galactic mergers are thought to be common, with Arp 240 representing a snapshot of a brief stage in this inevitable process. The Arp 240 pair are about 300 million light-years distant and can be seen with a small telescope toward the constellation of Virgo. Repeated close passages should ultimately result in a merger and with the emergence of a single combined galaxy.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

[Ann]

Click to view full size image

Arp 240. Source: Sloan digital Sky Survey/
http://faculty.etsu.edu/smithbj/researc ... /sdss.html

It is my firm belief that color is a fantastic tool for extracting "large-scale" information about galaxies.

Consider the image at left, which is a part of the Sloan Digital Sky Survey, SDSS. SDSS uses filters which generally show old yellow stars as yellow-orange, dust as brown, young blue stars as blue and red Ha emission nebulas as green. With that in mind, consider all the information we can extract from the SDSS image. We can see the old yellow stars, the thick dust clouds, the young blue stars and the bright emission nebulas.

Click to view full size image

Arp 240. Source: http://hubblesite.org/newscenter/archiv ... /image/bu/.
Processing: Lynn Hillborn.

Now consider an alternative version of the Hubble image that is the source for today's APOD, which in the image at right has been processed by Lynn Hillborn. The original Hubble image was taken through filters that give no information about Ha emission nebulas. The filters only support a differentiation between old yellow stars and young blue ones. Note how Lynn Hillborn has attempted to enhance this difference, making it easier for the viewer to understand Arp 240. Nevertheless, the color information provided by the Hubble/Lynn Hillborn image is less satisfactory than the SDSS image.

Today's APOD contains very little color information at all, apart from the strikingly reddish-brown dust clouds of NGC 5258. By contrast, it is not obvious from today's APOD that the centers of these two galaxies contain a different population of stars than the spiral arms. The muted colors of today's APOD should appeal to those who consider (enhanced) color information a distraction. Certainly an image that underscores structure and plays down color provides very interesting information, too.

Ann

[Boomer12k]

To me it looks like a distorted, extended arm, that got strewn out, and warped from both galaxies...

:---[===] *

[ChrisKotsiopoulos]

It is my firm belief that color is a fantastic tool...

Hi Ann. Thanks for pointing this out. I'll keep it in mind for the next HST process. I really want to represent coIors as accurately as possible.

Kind Regards,
Chris.

[heehaw]

Every so often in looking at pictures of galaxies, it recurs to me that there might be, or have been, civilizations there, as there is one - sort of, anyway - in our galaxy, and I think of their astronomers. We have the Magellanic Clouds to look at; astronomers and everyone on either of these two galaxies have a rather spectacular Milky-Way-like object in their skies in addition to their own galaxy!

[heehaw]

Oh no! I went to The Other Place, as I always do, and ESPD presents us with "Super Moon Bow." Oh no!

[JMC]

Hi Ann. Although I do agree that the SDSS color enhancement brings out different information, I personally find the HST detail more sharp and easier to discern. Just a matter of preference I guess.

[Chris Peterson]

It is my firm belief that color is a fantastic tool...

Hi Ann. Thanks for pointing this out. I'll keep it in mind for the next HST process. I really want to represent coIors as accurately as possible.

Kind Regards,
Chris.

Color is a useful source of information. "Accurate" color, to the extent such a thing exists, often is not. The most useful way to utilize color depends on the wavelengths the original data was collected at, and the information the imager seeks to convey.

Most of what we know about the Universe comes from false color images and from data collected outside the range of human vision entirely. There's a reason that so little professional imagery is collected through RGB filter sets.

[Roneflan@gmail.com]

When finally formed into one larger galaxy will an existing planetary system remain unchanged?

[neufer]

When finally formed into one larger galaxy will an existing planetary system remain unchanged?

• Most should.

[Chris Peterson]

When finally formed into one larger galaxy will an existing planetary system remain unchanged?

For the most part, planetary systems should be unaffected. However, once stars get closer than a light year or so they can perturb planetary orbits and create instabilities. Not good for any planets with life on them. So you wouldn't want to live on a planet in a part of the galaxy where a dense region from the other galaxy passed through.

[florid_snow]

I loved viewing this image in full resolution to enjoy the tiny red background interacting galaxy pair looking so similar to Arp 240!

Anyone with some cosmology knowledge out there, if the universe were spatially closed and surprisingly small, could you have a view like this, looking out far enough to see something repeated in its past state? Say the universe was spherically closed and only like 1e9 light years in "circumference" we are 3e8 light years from Arp 240 but if we head exactly away from it, we would reach it in 7e8 light years, so we see in the background Arp 240 but at that greater distance and earlier time. Like looking out with a telescope and seeing your own butt, to put it another way. Is that even possible?

[Chris Peterson]

Anyone with some cosmology knowledge out there, if the universe were spatially closed and surprisingly small, could you have a view like this, looking out far enough to see something repeated in it's past state? Say the universe was spherically closed and only like 1e9 light years in "circumference" we are 3e8 light years from Arp 240 but if we head exactly away from it, we would reach it in 7e8 light years, so we see in the background Arp 240 but at that greater distance and earlier time. Like looking out with a telescope and seeing your own butt, to put it another way. Is that even possible?

It is possible in principle, given a different universe that follows the rules of ours. But our own universe is larger than the observable universe, which means there's a horizon beyond which we can't see (because everything on the other side is receding at greater than c), which means that we could never see "around" or "across" it in this way. Also, it appears that our universe is spatially flat.

[Catalina]

What an amazing image. I, too am more impressed with the sharper, more detailed image and find it not too difficult to view the image with the knowledge in mind that the central stars are a different population. With the vast majority of these images being "color-coded" to denote specific composition, I also appreciate structure detail when I view them.

[neufer]

[size=150]#240 : ARP ATLAS OF PECULIAR GALAXIES[/size]

---

. Only one bridge

[Ann]

It is my firm belief that color is a fantastic tool...

Hi Ann. Thanks for pointing this out. I'll keep it in mind for the next HST process. I really want to represent coIors as accurately as possible.

Kind Regards,
Chris.

Color is a useful source of information. "Accurate" color, to the extent such a thing exists, often is not. The most useful way to utilize color depends on the wavelengths the original data was collected at, and the information the imager seeks to convey.

Most of what we know about the Universe comes from false color images and from data collected outside the range of human vision entirely. There's a reason that so little professional imagery is collected through RGB filter sets.

I agree that there is no such thing as "accurate" color in images. There are filters (and other detectors, such as the human eye) that are sensitive to different wavelengths, and then there is processing, to make sense of the data that has been detected by the filters, and to color-code the data in a meaningful way.

Click to view full size image

Double Cluster in Perseus. Source: Getty images.

Take a look at the picture of the Double Cluster in Perseus at left. The red giant stars look particularly bright in this image. Is that wrong? Well, we may assume that the filters that were used for this image react more strongly to infrared light than to blue. Are the bright red stars really brighter than the white-looking stars in the Double Cluster? Possibly, but the difference would be much smaller than this image suggests. The only way we can talk about "real brightness" in a meaningful way is if we mean "bolometric luminosity", which measures how much energy a source emits across the entire electromagnetic spectrum. Red giant stars are copious emitters of infrared light, so they are indeed "really bright". But blue supergiant stars emit a lot of ultraviolet light, so that they, too, are more energetic than they would appear to be in visual light.

As for the filters used for the Hubble data that was used for today's APOD, this page tells us that the filters were F435W (B) and F 814W (I). Even if both filters are broadband ones, there is still a big difference between a filter centered on 435 nm and one centered on 814 nm. It should certainly be possible to differentiate between old yellow and young blue populations from the data that was detected by those filters.

But it is also possible to decide that the greatest color differences in the picture of Arp 240 is that between starlight and dust in Arp 240 itself on one hand, and that between light from Arp 240 and light from the background galaxies on the other.

It is all a matter of choice. But there is a choice.

Ann

[MarkBour]

When finally formed into one larger galaxy will an existing planetary system remain unchanged?

• Most should.

I'm curious about the basis of this answer. Does it come from computational simulations of mergers?

[Chris Peterson]

When finally formed into one larger galaxy will an existing planetary system remain unchanged?

• Most should.

I'm curious about the basis of this answer. Does it come from computational simulations of mergers?

That's not really necessary. Simple statistical calculations can indicate how close stars will pass to each other, and the most basic numerical orbit models show the impact of stars interfering with nearby solar systems.

[MarkBour]

• Most should.

I'm curious about the basis of this answer. Does it come from computational simulations of mergers?

That's not really necessary. Simple statistical calculations can indicate how close stars will pass to each other, and the most basic numerical orbit models show the impact of stars interfering with nearby solar systems.

Okay, either way, then, is the modelling sufficiently complete to make such a statement with any confidence? There's a whole lot more in and around galaxies than the main-sequence easily-observable stars. And I get the impression we are slowly building up our understanding of these less-visible items as time goes on. The dwarfs, planet-sized bodies, molecular clouds, clouds of dust, black holes, and most significantly, the dark matter.

So, when two galaxies merge, if you can say: "The average planetary system has only a 1-in-1000 chance of a star passing near enough to it to mess it up", then that statement may be insufficient to bring us to the conclusion that the system will most likely be undisturbed (if a lot of other things could impact it).

[geckzilla]

I'm curious about the basis of this answer. Does it come from computational simulations of mergers?

That's not really necessary. Simple statistical calculations can indicate how close stars will pass to each other, and the most basic numerical orbit models show the impact of stars interfering with nearby solar systems.

Okay, either way, then, is the modelling sufficiently complete to make such a statement with any confidence? There's a whole lot more in and around galaxies than the main-sequence easily-observable stars. And I get the impression we are slowly building up our understanding of these less-visible items as time goes on. The dwarfs, planet-sized bodies, molecular clouds, clouds of dust, black holes, and most significantly, the dark matter.

So, when two galaxies merge, if you can say: "The average planetary system has only a 1-in-1000 chance of a star passing near enough to it to mess it up", then that statement may be insufficient to bring us to the conclusion that the system will most likely be undisturbed (if a lot of other things could impact it).

One thing you can try is loading up something like Universe Sandbox to see how difficult it is to completely separate orbiting bodies. It's pretty hard to get them to collide, too. Yeah, there is a lot of stuff out there, and some systems could get perturbed by measurable amounts if something if, say, Jupiter sized mass comes close enough, but what's your bar for "unchanged" ? Jupiter can fly by the system and leave every orbiting body around the parent star. Is it enough to say that keeping all the planets around their star is unchanged? Or do you mean unchanged as in no orbits were modified even a little bit by the encounter?

[Chris Peterson]

I'm curious about the basis of this answer. Does it come from computational simulations of mergers?

That's not really necessary. Simple statistical calculations can indicate how close stars will pass to each other, and the most basic numerical orbit models show the impact of stars interfering with nearby solar systems.

Okay, either way, then, is the modelling sufficiently complete to make such a statement with any confidence? There's a whole lot more in and around galaxies than the main-sequence easily-observable stars. And I get the impression we are slowly building up our understanding of these less-visible items as time goes on. The dwarfs, planet-sized bodies, molecular clouds, clouds of dust, black holes, and most significantly, the dark matter.

So, when two galaxies merge, if you can say: "The average planetary system has only a 1-in-1000 chance of a star passing near enough to it to mess it up", then that statement may be insufficient to bring us to the conclusion that the system will most likely be undisturbed (if a lot of other things could impact it).

I'm not sure which modeling you're referring to. We do have a good idea of the mass of galaxies, and therefore of their stellar content- regardless of how easily observed all of those stars might be. Molecular clouds, dust clouds, gas clouds, dark matter- these things don't disrupt planetary systems because they're too diffuse. Disrupting stellar system requires tiny, dense bodies- stars or very large planets. Really, only stars are of concern- rogue planets would have to pass right through stellar systems, and that's unlikely. And the stellar count of galaxies is reasonably well known. So yes, we do know enough about what's going on to support the argument that most planetary systems in merging galaxies will survive largely unaltered- at least by gravity. Many could find themselves in regions of new star formation, which won't disrupt those systems mechanically, but might result in an environment that impacts life- either damaging it or enhancing its evolution.