APOD: Galaxy NGC 474: Shells and Star Streams (2014 Jan 05)

[APOD Robot]

Galaxy NGC 474: Shells and Star Streams

Explanation: What's happening to galaxy NGC 474? The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant. Regardless of the actual cause, the above image dramatically highlights the increasing consensus that at least some elliptical galaxies have formed in the recent past, and that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with -- and accretions of -- smaller nearby galaxies. The halo of our own Milky Way Galaxy is one example of such unexpected complexity. NGC 474 spans about 250,000 light years and lies about 100 million light years distant toward the constellation of the Fish (Pisces).

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[Wer]

Just wondering if the many background objects are stars of our galaxy? Seems pretty clear that the big foreground star-like objects are ours but I am impressed by the density of the background. Thanks, Wer

[geckzilla]

You can easily tell which are background galaxies by looking at the large picture and noting any subtle shape to the individual objects. If they look smudge-like or blurry, they are very likely galaxies. It's hard to tell on some of them but the vast majority of those look like galaxies to me.

[BMAONE23]

I count what looks to be some 42 or 43 stars in the field. The remainder should be galaxies

[Ann]

This is a very interesting galaxy indeed. But shells around elliptical galaxies are not uncommon, although they are rarely as complex as the shells and streams around NGC 474.

In his book A View of the Universe (1993), David Malin wrote about how he discovered shell-like features in the outer envelope of galaxy Centaurus A during a meeting with astronomer John Graham:

John offered this plate as a suitable object for demonstrating unsharp masking and we very quickly discovered that the faint outer envelope of the galaxy embraces a series of incomplete arcs or shells, illustrated in Fig. 8.30.
...
The improved photographic emulsions of the last two decades and the photographic techniques they have triggered have shown that a surprisingly large number of otherwise normal-looking ellipticals had shell-like structures of the kind found in Cen A.

Obviously some of the features seen in NGC 474 are tidal tails and stellar streams similar to the ones found around the Milky Way, but personally I believe, in my complete amateur way, that ripples have a lot to do with the shells:

APOD Robot wrote:
Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant.

Ripples are like splashes when a rock falls into a pond. When it comes to a galaxy, it is easy to imagine that a density wave ripples through the galaxy, reaches a maximum size, and withdraws again. But it might leave arcs and shells of stars behind in a way similar to how a wave may wash pebbles onto a beach.

The ultimate ripple galaxy may be the Cartwheel galaxy. The Cartwheel galaxy is of course not an elliptical galaxy, but it might become one in the future. And then it might sport some magnificent shells. Maybe it will look like NGC 1344, an elliptical galaxy that just might have been a Cartwheel Galaxy in the past.

Ann

[petsie]

Wow! This is the most exciting APOD I've seen for months. My first thought led me to gravitational waves. Does anyone out there know which mass two super-mega-hyper-massive black holes should have to create these waves by their collision? At least by Occam's principle it is more probable that this phenomenon is due to less exciting interferences of density, but it woud be interesting to calculate the required masses of black holes that would have the power to generate such a fascinating view

[hohoho]

What a photo! It reminds me again that "what you see is what you see, not what is there." New ways of looking reveal startling additional phenomena. And remember, you are seeing only the baryonic matter; most of what is really there you can't see at all, because it is dark matter. And we don't know what the dark matter is: what kind of particles. It is an exciting time for astronomy!

[rekesle]

Looking at this image as a processed image I would like to see the image without the added star "spikes" and the round "clouds" on the foreground stars. I wonder if during processing if some of the larger central galactic "round clouds" were installed. The processed spike/cloud appearance on the stars looks very similar to the concentric clouds formed around the galaxy,

[Nitpicker]

Bravo! Up until now I thought elliptical galaxies were unphotogenic. Turns out one just needs to look a little deeper.

[geckzilla]

Looking at this image as a processed image I would like to see the image without the added star "spikes" and the round "clouds" on the foreground stars. I wonder if during processing if some of the larger central galactic "round clouds" were installed. The processed spike/cloud appearance on the stars looks very similar to the concentric clouds formed around the galaxy,

Those spikes and the circular halo were not added. They are simply part of the telescope's optics. Getting rid of could potentially require a lot of extra work. The round clouds around the big galaxy certainly aren't the same. It has to be a bright point source for it to happen. The bright star in the upper right is unfortunate (and probably an unseen one at the upper left) but as long as you understand what's going on then you won't be confused.
Wiki article: https://en.wikipedia.org/wiki/Diffraction_spike
The extra bright white one on the upper right star is a charge bleed. I can't find a simple article explaining that. The star was so bright that the pixel filled up with electrons which bled into the columns adjacent to it... best I can explain that.

Bravo! Up until now I thought elliptical galaxies were unphotogenic. Turns out one just needs to look a little deeper.

I think most of them actually are unphotogenic. But the ones that aren't have a mystique to them that the vastly more popular grand design spirals can't touch. I think we could use more APODs on lenticular galaxies, too. For a long time I thought the only two categories were spirals and ellipticals.

[Boomer12k]

Reminds me of a merger video animation....it had streams of stars being sent out, and even though they were moving, the streams were constant, as far as "shape" was concerned...a warpage of space time, that the stars are riding on...in..??? It showed much of the same features...layered areas of greater density, and far flung areas....

As for being young....well....the age of elliptical galaxies can be 1 to 15 gyr....Gigayears...Billion....so...they can actually be some of the oldest galaxies in the universe. Kind of like Globular clusters in the Halo of the Milky Way....10-ish billion years....this means that some Ellipticals formed close to the Beginning....

Of course I do see the point that "YOUNG" may refer to "RECENT" mergers....

Why "MERGER" over "Tidal Pull"?

When you look at galaxies with tidal activity, you don't see the "Shock" and disruption as much...to my observation....you see warp and distortion....but not the circular waves of shock lines....unless they are very close, and then you have a "dance"....you can see star streams, and the like, but I don't see this concentric circular appearance like this.....although warped and strung out...unless they are very close and in the process of merging...but then that would be MERGER!!! And you would expect the shock and disruption, and not just a little warpage...

There appears to have been a merger with a much smaller galaxy on the right side....it got totally disrupted, and the larger got shocked....like a small car hitting a big truck....the small car gets decimated, and the truck has a dent....and shock points in various places.

Of course...it could "just be me"....

:---[===] *

[Chris Peterson]

Wow! This is the most exciting APOD I've seen for months. My first thought led me to gravitational waves. Does anyone out there know which mass two super-mega-hyper-massive black holes should have to create these waves by their collision? At least by Occam's principle it is more probable that this phenomenon is due to less exciting interferences of density, but it woud be interesting to calculate the required masses of black holes that would have the power to generate such a fascinating view :)

Gravitational waves from even close supermassive black holes are far too weak to create visible structure in a galaxy. Indeed, it's unclear whether gravitational waves could create structure under any circumstances. Also, the energy produced is strongly related to the distance between the objects. To release large amounts of gravitational energy, the bodies would have to be extremely close, which means they would have very rapid orbits- hours or less. We wouldn't expect waves propagating with such a high frequency to create stable structures thousands of light years across.

[Llewellyn]

NGC474 is a collapsing galaxy. The black hole is swallowing stars at a tremendous rate. The waves you see are the results of the disintegration of the stars...... Try that on for a theory. Seemed like as good an idea as some I've read.

[Chris Peterson]

NGC474 is a collapsing galaxy. The black hole is swallowing stars at a tremendous rate. The waves you see are the results of the disintegration of the stars...... Try that on for a theory. Seemed like as good an idea as some I've read.

Black holes have almost no effect on the structure of a galaxy. Black holes don't suck things in. The stars in the galaxy are in orbit around the common center of mass, of which any central supermassive black hole is only a small part.

[starsurfer]

This is a very interesting galaxy indeed. But shells around elliptical galaxies are not uncommon, although they are rarely as complex as the shells and streams around NGC 474.

In his book A View of the Universe (1993), David Malin wrote about how he discovered shell-like features in the outer envelope of galaxy Centaurus A during a meeting with astronomer John Graham:

John offered this plate as a suitable object for demonstrating unsharp masking and we very quickly discovered that the faint outer envelope of the galaxy embraces a series of incomplete arcs or shells, illustrated in Fig. 8.30.
...
The improved photographic emulsions of the last two decades and the photographic techniques they have triggered have shown that a surprisingly large number of otherwise normal-looking ellipticals had shell-like structures of the kind found in Cen A.

Obviously some of the features seen in NGC 474 are tidal tails and stellar streams similar to the ones found around the Milky Way, but personally I believe, in my complete amateur way, that ripples have a lot to do with the shells:

APOD Robot wrote:
Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant.

Ripples are like splashes when a rock falls into a pond. When it comes to a galaxy, it is easy to imagine that a density wave ripples through the galaxy, reaches a maximum size, and withdraws again. But it might leave arcs and shells of stars behind in a way similar to how a wave may wash pebbles onto a beach.

The ultimate ripple galaxy may be the Cartwheel galaxy. The Cartwheel galaxy is of course not an elliptical galaxy, but it might become one in the future. And then it might sport some magnificent shells. Maybe it will look like NGC 1344, an elliptical galaxy that just might have been a Cartwheel Galaxy in the past.

Ann

NGC 474 is one of my favourite shell ellipticals and you are right that they are not as rare as some people think. There are at least hundreds of examples of them, the majority in the southern hemisphere. David Malin discovered shells around many elliptical galaxies in the early 1980's, most notably NGC 1344 like you mentioned as well as NGC 3923. He published a catalogue of them with John Carter and many more have been discovered since then.

I have never seen the Cartwheel Galaxy described as a ripple galaxy before, its common type is a collisional ring galaxy.

Also this image is surprisingly colourless, the various galaxies are actually silvery blue. There is a more recent one by the same team: http://www.cfht.hawaii.edu/hawaiianstar ... l2013.html

Another really cool shell elliptical is NGC 7600, there is an amazing simulation of its formation that can be seen here: http://vimeo.com/32176781

Finally on the topic of outer tidal features, here is an interesting paper that describes very faint outer tidal features in M31 and M33: http://www.physics.usyd.edu.au/~gfl/Arx ... /paper.pdf

[neufer]

Another really cool shell elliptical is NGC 7600, there is an amazing simulation
of its formation that can be seen here: http://vimeo.com/32176781

Ancient "Bullet Cluster" reverberations

[BDanielMayfield]

Explanation: What's happening to galaxy NGC 474? The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images.

That demonstrates the fact that the more deeply we look at even ordinary looking things, the more extraordinary they often become.

The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant.

At “about 250,000 light years” NGC 474 is about 2.5 times bigger than our Milky Way galaxy, I assume that that 250,000 ly size includes the wispy outer parts too.

I was wondering about the ordinary looking spiral galaxy above NGC 474, which is NGC 470, and should be about the same size as our galaxy. It doesn’t appear to be distorted or have tidal tails or such evidence of interaction, and yet apparently NGC 470’s presence is what is ringing NGC 474 like a bell and causing it to look like it’s having a bad hair day. (And remember, a “day” to a galaxy is as a few hundred million years to us.) To me it seems odd that NGC 470 looks so undisturbed, when the supposedly more massive NGC 474 is the one that is all shook up. Are we sure which galaxy is dominant here? Just who’s the cannibal and who’s about (in universal time terms) to be consumed here?

Bruce

[Ann]

BDanielMayfield wrote:
I was wondering about the ordinary looking spiral galaxy above NGC 747, which is NGC 740, and should be about the same size as our galaxy. It doesn’t appear to be distorted or have tidal tails or such evidence of interaction, and yet apparently NGC 740’s presence is what is ringing NGC 747 like a bell and causing it to look like it’s having a bad hair day. (And remember, a “day” to a galaxy is as a few hundred million years to us.) To me it seems odd that NGC 740 looks so undisturbed, when the supposedly more massive NGC 747 is the one that is all shook up. Are we sure which galaxy is dominant here? Just who’s the cannibal and who’s about (in universal time terms) to be consumed here?

Not all galaxies look disturbed even if they are obviously interacting. An interesting example is the "Siamese Twins" galactic pair, NGC 4567 and NGC 4568. Even if they are so close that they appear to be overlapping, they are both still perfectly formed.

I don't think that elliptical galaxy NGC 474 looks so dishevelled because it has been interacting with spiral galaxy NGC 470. Rather, I believe that NGC 474 has merged with other, smaller galaxies in the relatively recent past. When I talked about "ripples" in NGC 474 in my previous post, I meant ripples somewhat similar to those you get when you throw a pebble in a pond. In other words, I think that at least one other galaxy has fallen like a pebble into the "pond" of NGC 474, stirring it up and causing all kinds of ripples and disturbances.

As for NGC 470, I think it has just been waiting in the wings until it, too, gets the chance to enter the stage and take part in the merging and colliding cosmic funfair fun.

Ann

[Chris Peterson]

I was wondering about the ordinary looking spiral galaxy above NGC 474, which is NGC 470, and should be about the same size as our galaxy. It doesn’t appear to be distorted or have tidal tails or such evidence of interaction, and yet apparently NGC 470’s presence is what is ringing NGC 474 like a bell and causing it to look like it’s having a bad hair day. (And remember, a “day” to a galaxy is as a few hundred million years to us.) To me it seems odd that NGC 470 looks so undisturbed, when the supposedly more massive NGC 474 is the one that is all shook up. Are we sure which galaxy is dominant here? Just who’s the cannibal and who’s about (in universal time terms) to be consumed here?

Tidal effects are sensitive to orientation, which can be hard to discern.

The only paper I've found compares two theories about the formation of shells in NGC 474. One theory is that the galaxy had an earlier collision with a now lost galaxy. The other is that it is gravitationally interacting with NGC 470. The conclusion of the paper, based on spectroscopic observations, is that the shells formed from interaction, not collision. But the paper isn't very detailed, and is fairly old (1999).

[BDanielMayfield]

Thank you both Ann and Chris. (Ann, you were so quick on the draw with your answer that you copied my transposition mistakes on the NGC numbers before I had corrected them!) Both of your answers are clear and logical.

Apparently then, as shown by the example Ann provided of the “Siamese Twin” Galaxies, large spirals are somewhat resistant to being tidally distorted. They seem to need to have actually collided (past tense) before they show much distortion.

Bruce

[Ann]

I don't think that spiral galaxies are ever resistant to the forces of gravity. But like Chris said, tidal effects are sensitive to orientation and can be hard to detect.

Also, galaxies are not "frozen in time". Merging galaxies change shape all the time. Consider "The Mice", NGC 4676. I found a youtube video showing the collision and eventual merger of these galaxies. Although we have no way of knowing if the details of the process as shown in this video are correct, the larger picture is true. Galactic interactions take a long time, and the galaxies change shape all the time.

Click to play embedded YouTube video.

Ann

[BDanielMayfield]

I need to clarify what I meant by “large spirals are somewhat resistant to being tidally distorted.” Ann was right to express doubt “that spiral galaxies are ever resistant to the forces of gravity.” All matter in the universe is constantly affected by the pull of gravity.

But consider that what we see is only the proverbial “tip of the iceberg.” What we see is only stars and reflected starlight; we can’t see what must be an extensive region of dark matter enveloping spiral galaxies. This dark matter is much more massive than the small percentage of visible matter we can see. One of the reasons we know this dark matter must be there is due to the large velocities of stars out toward the rims of spirals.

So as a spiral galaxy approaches a collision with another galaxy tidal effects of course are there, but the overall relative speed of the colliding galaxies, plus the dampening effects from the dark matter attenuates the apparent disruption until the point of actual collision. Note that in the simulation (and yes of course, it is only a computer simulation, but it’s based on how both matter and dark matter are expected to behave in these situations) the colliding galaxies maintain their shapes right up until the collision actually begins, which matches what we see in photos of impending collision.

Note that in the simulation it is only as the centers pass that things really start getting gnarly, with the outer reaches of the galaxies really getting whipped around. But prior to the collision, there isn’t much distortion. I don't think that it's only a mater of pre-colision distortion being hard to see. I think there really is little distortion prior to the actual collision.

Bruce