APOD: Edge-On NGC 891 (2017 Jan 12)

[APOD Robot]

Edge-On NGC 891

Explanation: Large spiral galaxy NGC 891 spans about 100 thousand light-years and is seen almost exactly edge-on from our perspective. In fact, about 30 million light-years distant in the constellation Andromeda, NGC 891 looks a lot like our Milky Way. At first glance, it has a flat, thin, galactic disk of stars and a central bulge cut along the middle by regions of dark obscuring dust. But remarkably apparent in NGC 891's edge-on presentation are filaments of dust that extend hundreds of light-years above and below the center line. The dust has likely been blown out of the disk by supernova explosions or intense star formation activity. Fainter galaxies can also be seen near the edge-on disk in this deep portrait of NGC 891. (Editor's Note: The NGC 891 image used in today's APOD posting has been replaced and the credit corrected to indicate the author of the original work.)

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[Ann]

Nice picture!

I've always liked NGC 891, because it is so perfectly edge-on. We can't see the core at all, not even a suggestion of a central brightening.

The "chimneys" of dust and smoke rising from the thin disk are interesting. They do indeed seem to be remnants of large numbers of supernovas.

Nevertheless, NGC 891 is a pretty red galaxy. We can tell that it is, even without checking out its color indexes, which is something I am unable to do right now. But do note how big the yellow inner disk is, and how far it extends from the core.

That dust lane that runs along the length of the galaxy means serious business. It is thick and opaque indeed. The far infrared magnitude of NGC 891 is two and a half magnitudes brighter than its B magnitude, which means that this galaxy definitely contains a lot of dust. Yes, but note that half the dust lane is blue from star formation.

Today's APOD also shows off the thick disk of NGC 891 beautifully. The thick disk is the sort of non-descript beige bleeding into bluish fuzz that seems to envelope the dust lane and the bright inner disk of NGC 891. The thick disk is made up of old stars that have begun to disperse from the thin disk, where they were born a long time ago.

What a nice picture!

Ann

[csp]

Incredible details, nice pict. How it is possible to have diffraction spikes with a SCT? (It was taken with a C11@f10, f6,3 according to the spec of the creator)

[ngc1535]

Very good question.
-Adam Block

[yerfandsam]

Is there an explanation of the sphere or bubble like structures seen along the edge. The most obvious being about 30% in from the pictured right side?

[atothey]

Trillions of miles away, indeed a number of the magnitude that most people would have a brain meltdown trying to comprehend, and the detail displayed in this image is simply unbelievable. Why, then, can't we see similar detail on say the moon landing sites or indeed a straight out of the camera shot of our own homeland, Earth? I simply cannot believe that what is presented to us as fact is indeed fact, it's more fiction and make believe than anything remotely factual. We are being completely deceived.

[DL MARTIN]

The description of this galaxy as being a distance away is an astronomical (pun intended)l conceit in that it can only be qualified as an entity visualized as it existed in the past. To suggest otherwise is to award temporal context equivalent to evaluating an archeological specimen as currently viable. This deludes the public into believing that galaxies, for example, are just over there when we have no evidence that they still even exist.

[Guest]

Looking at this image, I assume the orbits of the stars in the spiral disk are fairly stable. But towards the central bulge, there must be forces that cause erratic or chaotic orbits that fling stars about unpredictably and with great energy. So I am wondering if the dimensions of the central bulge can be used to 'age' a galaxy. Any ideas?

[rstevenson]

The description of this galaxy as being a distance away is an astronomical (pun intended)l conceit in that it can only be qualified as an entity visualized as it existed in the past.

A truism.

To suggest otherwise is to award temporal context equivalent to evaluating an archeological specimen as currently viable. This deludes the public into believing that galaxies, for example, are just over there when we have no evidence that they still even exist.

Technically true, but... Why would they not still exist?

Rob

[neufer]

Lisa Randall of Harvard University promotes the idea of a thin
dark matter disk dominating the gravitational field of spiral galaxies:

http://www.sciencealert.com/there-could ... our-galaxy
https://www.quantamagazine.org/20160412 ... sk-theory/

NGC 891 would seem to be an ideal test bed for validating such an hypothesis.

[ignacio_db]

Amazingly sharp and detailed image for a C11! Congrats!

[Chris Peterson]

Incredible details, nice pict. How it is possible to have diffraction spikes with a SCT? (It was taken with a C11@f10, f6,3 according to the spec of the creator)

They're not diffraction spikes. Look at them closely and they look nothing like diffraction spikes. They're some kind of effort at an aesthetic effect created by post processing. (People who like the look of diffraction spikes but use SCTs or refractors sometimes create them by placing crosshairs across their apertures... but in this case, the uniform hue of the spikes reveals they were created after the image was made.)

[Visual_Astronomer]

They don't look like real diffraction spikes to me, either, but it is still a pretty picture.

I've looked at NGC891 many times, it is spectacular through the eyepiece.

[csp]

They're not diffraction spikes.

I still think they are "real" diffrac. spikes. Check the aligned images, the other one is from a completely different source, I guess an RC. Spikes cannot be reproduced artificially so perfect.

http://g.recordit.co/l8WMbobs6s.gif

[Chris Peterson]

They're not diffraction spikes.

I still think they are "real" diffrac. spikes. Check the aligned images, the other one is from a completely different source, I guess an RC. Spikes cannot be reproduced artificially so perfect.

http://g.recordit.co/l8WMbobs6s.gif

I agree (based on other evidence) that the diffraction spikes are present in at least one channel of the source data. Nevertheless, they look a lot more like synthesized spikes than real ones, given the total lack of dispersion.

[MarkBour]

... but note that half the dust lane is blue from star formation. ...

Interesting point. Would you say that the most active star formation visible in this galaxy is occurring in outer arms? I see what you are saying about the blue all along the left side, and I think I see a smaller, but sizable, active region at the far right. Imagining that this is a spiral galaxy with some arms, I'm guessing that the left half of what we can see is an arm, much like our own arm of the Milky Way.

I'm wondering what rotation this galaxy has (e.g. is the left half approaching us and the right half retreating, or the other way around). Are there visual characteristics of spiral arms that are typical that might give a clue of that? I don't know if a doppler shift difference would be sufficiently large to be used to answer that.

[csp]

Nevertheless, they look a lot more like synthesized spikes than real ones, given the total lack of dispersion.

Actually even the reflections around the stars are completely identical on the 2 pictures. The other one is I think a 24 inch RC.
I have a Celestron EdgeHD as well, I know how stars looks like normally. Completely different than here. This must be an enhanced SCT or so...

[geckzilla]

Today's APOD image was plagiarized. It's known. APOD editor is working on it. People do stupid things for the rat race. We move on once again.

[RJN]

This APOD has now been updated. Please see the Editor's Note now appended to the APOD text. The note states "The NGC 891 image used in today's APOD posting has been replaced and the credit corrected to indicate the author of the original work." Thanks to everyone who alerted us about the situation. We apologize for the inconvenience. - RJN

[ignacio_db]

I suspected this, the angles of the spikes were identical, as well as the whole look of the image. So lame...

Oh well, I guess we still need an 0.88m scope to get there.

[BobGillette]

Stupid me. I glanced at the image this morning and said, hm, looks like an Adam Block. Then I went to the website of the claimant and thought, hm. How'd he do that with a C11?

Sure enough, it was an Adam Block.

Never tumbled to the diffraction spikes.

Bob Gillette

[Ann]

Adam, I'm sorry for not spotting the plagiarism!

Ann

[Ann]

... but note that half the dust lane is blue from star formation. ...

Interesting point. Would you say that the most active star formation visible in this galaxy is occurring in outer arms? I see what you are saying about the blue all along the left side, and I think I see a smaller, but sizable, active region at the far right. Imagining that this is a spiral galaxy with some arms, I'm guessing that the left half of what we can see is an arm, much like our own arm of the Milky Way.

I'm wondering what rotation this galaxy has (e.g. is the left half approaching us and the right half retreating, or the other way around). Are there visual characteristics of spiral arms that are typical that might give a clue of that? I don't know if a doppler shift difference would be sufficiently large to be used to answer that.

Mark, I'm still ashamed for missing the fact that the APOD was plagiarized, but I'll try to answer your question as best I can.

Click to view full size image

NGC 1532. Copyright:
Volker Wendel and Bernd Flach-Wilken.

Star formation often happens asymmetrically. NGC 1532 is a good example. You can see that a part of one arm, at upper right, is extremely active in star formation, as is obvious from the pink color of its bright emission nebulas. The rest of the galaxy isn't forming very many new stars.

There are some important differences between NGC 891 and NGC 1532, most notably that NGC 1532 is interacting with another galaxy and is tidally disturbed by this interaction. NGC 891, by contrast, looks very serene, at least when it comes to its overall shape.

But I think that NGC 891 and NGC 1532 are similar in that they form stars asymmetrically, so that star formation is more active in some parts of their arms than in others. So I don't think that the difference in color of the dust lane of NGC 891 has anything to do with Doppler shift. But I agree that it should certainly be possible to use Doppler shift measurements to find out how NGC 891 is rotating. Probably such measurements already exist, but unfortunately I don't know the results of them.

Finally, few edge-on spiral galaxies provide us with visual clues about which way they are rotating. NGC 7331, which isn't really edge-on, is one exception to the rule. You can clearly see, just by looking at it, which way it is rotating.

Ann

[Ann]

The description of this galaxy as being a distance away is an astronomical (pun intended)l conceit in that it can only be qualified as an entity visualized as it existed in the past. To suggest otherwise is to award temporal context equivalent to evaluating an archeological specimen as currently viable. This deludes the public into believing that galaxies, for example, are just over there when we have no evidence that they still even exist.

You can be sure that NGC 891 still exists.

Galaxies, unlike individual massive stars, can't "explode as supernovas" and disappear. They can, indeed, merge with other galaxies and be incorporated into the resulting, larger galaxy. It is also possible that some lightweight, fluffy dwarf galaxies might gradually lose mass until they become to "weak" to hold on to their individual stars, so that they disperse and disappear that way.

But NGC 981 has a bright yellow population and is therefore relatively massive. It certainly has enough self-gravity to keep itself together. NGC 891 is also rather isolated in space. It doesn't have any large neighbors at all.

Our own galaxy, the Milky Way, does indeed have a quite massive neighbor galaxy, the Andromeda galaxy. The Andromeda galaxy is only about 2 million light-years away, and the Milky Way is indeed destined to merge with Andromeda. But in spite of the fact that these two galaxies are so close, the collision isn't supposed to happen for a few billion years yet.

We see NGC 891 as it was only 30 million years ago. That's nothing in the life of a non-interacting galaxy. You can be sure that NGC 891 still looks much the same way "today" as it did 30 million years ago, when the photons that carry the information necessary for earthly astronomers to make pictures of it here on Earth, left the galaxy.

Ann

[MarkBour]

Finally, few edge-on spiral galaxies provide us with visual clues about which way they are rotating. NGC 7331, which isn't really edge-on, is one exception to the rule. You can clearly see, just by looking at it, which way it is rotating.

Ann

M101.jpg (27.21 KiB) Viewed 7229 times

Ann --
Just to double-check ... I assume you're saying that when one sees a grand spiral galaxy with arms, such as at the right, we know without measuring that it must be rotating counter-clockwise (from our view). Nobody has ever come across a spiral galaxy and found out it is not rotating, or even rotating "against" the apparent sweep of the spiral arms, right?