APOD: Peculiar Galaxies of Arp 273 (2011 Apr 21)

[Ann]

Looking at the APOD (not the original photo) can anyone tell me what that blue structure is in the lower left corner?

I had the same question as well. Looking closely, there is faint yellow halo around the dense blue (ultraviolet?) core of the object in the lower left. I was guessing that it was an orbiting irregular galaxy. If it is a huge star cluster that has been cast out, is it a possible method of formation of irregular satellite galaxies that they represent a population of stars that were flung from a mother galaxy while in the process of interacting with a third?

Dwarf galaxies can definitely form that way. Take a look at this Hubble image of the Tadpole Galaxy, Arp 188. The blue clumps you can see in the Tadpole's tail are dwarf galaxies in the making:

Click to view full size image

However, I think that the stars in the blue clumps were not so much flung out of the parent galaxy as they were formed in situ out of gas that had been flung out of the parent galaxy. That's what I think about the two prominent blue clumps in today's APOD, too. Particularly the lower left structure is clumpy and irregular enough that I think it is made up of a large gas cloud that has given birth to a lot of stars, but there is still a lot of glowing gas left. So why would this gas cloud look blue and not pink from hydrogen emission? My answer would be that the filters that were chosen for this Hubble image does a relatively poor job at showing the pink emission nebula, so that, in fact, none are visible. But there can be no doubt that the larger galaxy, UGC 1810, does indeed contain pink dots of glowing nebulae. I think that if the Hubble people had imaged UGC 1810 through a hydrogen alpha filter, we would undoubtedly have seen very many pink knots in this galaxy, and I would guess that parts of the lower left blue clump would have been intensely pink.

I'm less certain of the "upper left blue clump". It is smoother and fainter than the other one, and I would guess that it is made up of slightly older and not quite so hot stars. I think it contains vary many stars, compared with the other blue clumps, but the brightest stars are somewhat fainter. This clump could perhaps be regarded as an established and slightly evolved dwarf galaxy.

Ann

[Chris Peterson]

I think I agree with Ann on this issue: At least for the "very faint light" sensitivity part of it. If we could imagine ourselves somehow transported much closer to these astonomical objects, the light they emit would be naturally amplified by the inverse square rule, and so become less faint, to the point where our eyes could actualy sense it. And in that case what we would see is more like Adam's images than Hubble's.

I'm afraid it doesn't work that way. These objects are not bright enough to show color to the human eye (or not much color). It doesn't matter how close you get, the brightness won't change. Why is that? Because as you get closer, the objects don't simply get brighter because of the inverse square law, they also get larger. Get half as far away and you'll get four times more light, but the object will have four times the area. Net result: no change in surface brightness. There is no difference at all between getting closer and observing the object at higher magnification through a telescope. In either case, virtually all extended astronomical objects are the same color: gray.

In that sense Adam's images are more "true". Just because we cannot see an object with our eyes due to it's extreme distance does not mean that wavelength shifting is called for. Of course shifting is useful when viewing emissions that our eyes could not see no matter how intense.

In the case of imaging non-visible wavelengths, shifting isn't merely useful, it is essential! But as I said before, producing an image in "natural" colors is usually inappropriate for science purposes, because the color gamut of our eyes is poorly suited to observing most astronomical objects. For instance, there is a wide range of blue "colors" produced by stellar objects, but our eyes have very poor color resolution in blue. So by using a false color palette- shifting blues to greens or reds, for instance- we can discriminate much finer variations in the actual color. In other words, we can see physical differences between objects that would be invisible in a "natural" color palette. So unless the intent is purely aesthetic (which is fine, but is often not the case for astronomical images) there is good reason to use a false color palette in presenting images.

[Chris Peterson]

I thought that most who visit this site understand the use of synthetic palettes for images, Chris. I did not object to them or request that only the visible light spectrum be used.

Nor should my comment be taken as any sort of criticism. It was just an observation about the inherent value of false color palettes, even when the source data consists only of visible light images.

[rstevenson]

... No matter how breathtaking and highly detailed an image is, kids and adults alike ask Is that the real color? and they will continue to ask that question. ...

I rotate my desktop through hundreds of astro images. Occasionally a visitor will see some fantastic galaxy image and ask, "Is that what it really looks like?" At which point I open the image in a graphics program, pop up the colour controls, then dial the saturation down to where the image is essentially just black and white and gray. Then I say, "That's what it really looks like -- to our eyes." They're always disappointed. But then I explain about filters, and 10 hour exposures, and post-processing and such, and by the time they leave they appreciate the wonders of our technology as well as the wonders of the cosmos.

By the way, I almost always use that same saturation slider to partly desaturate (in the range of -20% to -30%, usually) the images I view on my desktop; I really don't like neon galaxies.

Rob

[Guest]

....the blue cluster is where i actually come from...we call it the draycon delta complex

[geckzilla]

Geckzilla, nobody on this planet should ever have to apologize for bringing science down to any human level. It sure hit a nerve.

A nerve? I just prefer the innate complexity of science just the way it is. Actually, you might even say it's already "brought down" when it's posted to APOD every day. A paragraph of explanation isn't that much to chew on, though the embedded links do provide a lot of extra substance in most cases. I'm not even sure what you mean by "human level" ... at some point does science become inhuman?

....the blue cluster is where i actually come from...we call it the draycon delta complex

You speak English and have an English word for it, huh?

[Sam]

You speak English and have an English word for it, huh?

delta, n.

Etymology: < Greek δέλτα ( < Phoenician daleth), name of the fourth letter of the Greek alphabet; also the land at the mouth of the Nile (Herod.), the Indus (Strabo), etc.
1.a. The name of the fourth letter of the Greek alphabet, having the form of a triangle (Δ), and the power of D.

http://asterisk.apod.com/viewtopic.php?f=9&t=22764


---
Sam

[geckzilla]

Ah, so you mean the Greeks have a settlement in that Peculiar Galaxy.

[kshiarella]

I think I agree with Ann on this issue: At least for the "very faint light" sensitivity part of it. If we could imagine ourselves somehow transported much closer to these astonomical objects, the light they emit would be naturally amplified by the inverse square rule, and so become less faint, to the point where our eyes could actualy sense it. And in that case what we would see is more like Adam's images than Hubble's.

I'm afraid it doesn't work that way. These objects are not bright enough to show color to the human eye (or not much color). It doesn't matter how close you get, the brightness won't change. Why is that? Because as you get closer, the objects don't simply get brighter because of the inverse square law, they also get larger. Get half as far away and you'll get four times more light, but the object will have four times the area. Net result: no change in surface brightness. There is no difference at all between getting closer and observing the object at higher magnification through a telescope. In either case, virtually all extended astronomical objects are the same color: gray.

Another way to think about it is considering our own Milky Way. Although the stars of our own Galaxy are orders of magnitude closer from our vantage point, it is still a little more than a diffuse haze to the naked eye because the stars are correspondingly so much farther apart. The closer we would get to another galaxy, the less distinguishable from our own Milky Way it would become, at least to the naked eye.

It is more tantalizing and fantastic to think that we could get "close" to a fantastic celestial body like the "celestial event" imaged in the movie "Contact." It is sad to think that if we traveled to near the center of our own galaxy, that we would only see a particularly rich star field.

Imaging these galaxies in this way might be as spectacular as it gets.

[Hornet]

The small inserted galaxy upper right does appear to be within the larger galaxy and appears to be interacting with the larger galaxy. It is possible that it may be a background galaxy but there are star bursts at suspicious locations suggesting interaction. could the smaller inserted galaxy be interacting but behind the larger galaxy???

I like thinking about their 'ultimate interaction' - suppose that the inserted galaxy is interacting: the inserted G spiral arms closest to the core of the larger G are moving with the larger G. The outside arms of the inserted G are moving opposite of the larger G - we have colliding gasses. new stars and much material falling into the larger G's Hole. Very interesting... Oh, I better stop, I might have a star-burst of brain activity. You don't want to see that.

That black hole is going to get fat.

[visitor]

A nerve? I just prefer the innate complexity of science just the way it is. Actually, you might even say it's already "brought down" when it's posted to APOD every day. A paragraph of explanation isn't that much to chew on, though the embedded links do provide a lot of extra substance in most cases. I'm not even sure what you mean by "human level" ... at some point does science become inhuman?

Just saw your second post. Geckzilla, I get it. Anyone pathetic enough to get a kick out of seeing a rare image of the universe in in the colors of the visible light spectrum is a joke and beneath your contempt. But why are still picking apart my posts when you learned after the clarification in my second post that your presumption of my position was erroneous, that I am not ignorant of nor campaigning against the need for false-color imaging? What is it that are objecting to now that you know better? You pick apart my words like a partisan politician. What did I mean by "human level"? You really want to know? I meant the lack of human color perception in low light and the momentary desire to see the universe in a way the human eye cannot. Will this kind of image expand the knowledge base of science? Nope. It exists only for the pure, pointless enjoyment of those interested. I assume that this is the only reason APOD and the Hubble site occasionally show these images. The "human level" phrase was also going to be a set up for a crack and I thought better of it. Instead I went for truth and a mollyfying tone and said that it sure hit a nerve. Then you went after that phrase. Is ridicule your form of pure, pointless enjoyment? I will never object to clarifying my position.
Let's look at your second post in anything but a congenial way.
You say that you prefer the innate complexity of science "just the way it is". Does "just the way is is" mean you want it in grayscale? In raw data? In "visible light colors by travelling to dark enough place on a clear, cloudless night"? Do you mean that you are against assigning false colors to images? I'm new here, Is this what I am supposed to do on this forum? It's a cheap, lousy way to look smarter.

I hope the moderators allow me to fight my own battles. If joining this forum is like being on trial then I want the right to cross examine.

[bystander]

I hope the moderators allow me to fight my own battles. If joining this forum is like being on trial then I want the right to cross examine.

You first have to join and become a member, rather than an anonymous troll.

FYI: the person you chose to pick a fight with (geckzilla) is the administrator of this board.

[geckzilla]

However, I am employing an "I'm too tired for fighting" attitude rather than a big stick policy. I never meant to start a fight in the first place. heh

[bystander]

However, I am employing an "I'm too tired for fighting" attitude rather than a big stick policy.

I just figured I should point out that it's best not to go to war wielding a butter knife, when your opponent might be armed with tanks and stealth bombers.
If the visitor went back and actually read all of the posts (and the rules), he would find he was without ammunition (an apparent supply chain breakdown).

I never meant to start a fight in the first place. heh

I know, I never could figure out what the visitor was complaining about. I have a feeling he was trying to start a fight.

[Guest]

"From our perspective, the bright cores of the Arp 273 galaxies are separated by only a little over 100,000 light-years."

Is this a reference to einsteinian/lorentzian contraction?

(Apologies if you notice I'm being insulting: ) What is the separation from other perspectives?