APOD: Elliptical M60, Spiral NGC 4647 (2016 Jan 28)

[APOD Robot]

Elliptical M60, Spiral NGC 4647

Explanation: Giant elliptical galaxy M60 and spiral galaxy NGC 4647 do look like an odd couple in this sharp cosmic portrait from the Hubble Space Telescope. But they are found in a region of space where galaxies tend to gather, on the eastern side of the nearby Virgo Galaxy Cluster. About 54 million light-years distant, bright M60's simpler egg-like shape is created by its randomly swarming older stars, while NGC 4647's young blue stars, gas and dust are organized into winding arms rotating in a flattened disk. Spiral NGC 4647 is estimated to be more distant than M60, some 63 million light-years away. Also known as Arp 116, the pair of galaxies may be on the verge of a significant gravitational encounter, though. M60 (aka NGC 4649) is about 120,000 light-years across. The smaller NGC 4647 spans around 90,000 light-years, about the size of our own Milky Way.

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[Nitpicker]

Is the term "egg-like" used because describing the shape of an elliptical galaxy as "elliptical" is self-referential?

What's wrong with "oval"?

Nobody mention footballs, please.

[Ann]

APOD Robot wrote:
About 54 million light-years distant, bright M60's simpler egg-like shape is created by its randomly swarming older stars, while NGC 4647's young blue stars, gas and dust are organized into winding arms rotating in a flattened disk. Spiral NGC 4647 is estimated to be more distant than M60, some 63 million light-years away.

Well, that explains it. I have long wondered how delicate spiral galaxy NGC 4647 could be so perfectly undisturbed even though it appears to be on the verge of plunging straight into the maw of big elliptical bully M60. But if M60 and NGC 4647 are actually so far apart that they are not (yet) interacting, then that explains NGC 4647's undisturbed shape.

I note that NGC 4647 looks bluish, as I expect it to do. But fascinatingly, M60 looks almost perfectly colorless. It is hard to argue from a picture like this one that M60 consists almost exclusively of "old red stars".

Ann

[geckzilla]

It's very typical when processing to set a white point based on an elliptical galaxy.

[Nitpicker]

Maybe we need red-blue glasses?

[tomatoherd]

A very beautiful image. Thank you.
Nitpicker: egg is 3-D. "oval" is 2-D; that's all, likely.

[tomatoherd]

So...are these old stars in the elliptical galaxy likely to have familiar carbon-based life? Or are they so old they are low-metallicity?

[Ann]

So...are these old stars in the elliptical galaxy likely to have familiar carbon-based life? Or are they so old they are low-metallicity?

One thing is certain, these massive elliptical galaxies are likely to be quite metal-rich. There will be no shortage of carbon in them.

Ann

[HunterofPhotons]

It's very typical when processing to set a white point based on an elliptical galaxy.

If you're talking about a 'white point' which is the maximum pixel value of a particular image then any entity in the image suffices, be it a galaxy, star, or any bright object.
If, on the other hand, you're talking about an object to be used as a white reference object in color calibration an elliptical galaxy would not be a suitable candidate as most of them have a reddish hue. A more suitable galaxy would be a spiral of Hubble classification of Sa, Sb, Sc, Scd, SBa, SBb, SBc or SBcd. Many consider these galaxies to have a combined white color.

dan kowall

[BMAONE23]

Is the term "egg-like" used because describing the shape of an elliptical galaxy as "elliptical" is self-referential?

What's wrong with "oval"?

Nobody mention footballs, please.

Perhaps Ellipsoid

[geckzilla]

It's very typical when processing to set a white point based on an elliptical galaxy.

If you're talking about a 'white point' which is the maximum pixel value of a particular image then any entity in the image suffices, be it a galaxy, star, or any bright object.
If, on the other hand, you're talking about an object to be used as a white reference object in color calibration an elliptical galaxy would not be a suitable candidate as most of them have a reddish hue. A more suitable galaxy would be a spiral of Hubble classification of Sa, Sb, Sc, Scd, SBa, SBb, SBc or SBcd. Many consider these galaxies to have a combined white color.

dan kowall

I'm not saying it's right, I'm just saying it's common, especially for ESA's and STScI's Hubble processors. It's very easy to end up with something blue in the middle where it's bright and yellow on the outside where it's dimmer (or the other way around) so instead it's done in a way to try to minimize the color all along the gradient. I'm not going to argue over what is correct or incorrect because most Hubble images aren't even RGB images so to simplify things ellipticals end up white or gray, or colorless, however you want to say that.

[Nitpicker]

A very beautiful image. Thank you.
Nitpicker: egg is 3-D. "oval" is 2-D; that's all, likely.

Fairy nuff. So, "ovoid" then.

By the same logic, elliptical galaxies should perhaps be known as ellipsoidal galaxies.

[Chris Peterson]

A very beautiful image. Thank you.
Nitpicker: egg is 3-D. "oval" is 2-D; that's all, likely.

Fairy nuff. So, "ovoid" then.

By the same logic, elliptical galaxies should perhaps be known as ellipsoidal galaxies.

I'd argue that both ovoid and ellipsoid are incorrect. I'm not aware of any ovoid galaxies, as the word implies an egg-like shape- larger at one end than the other. Neither is it clear that any elliptical galaxies are ellipsoidal. They are called "elliptical" because they appear, in two dimensions, similar to ellipses. In fact, most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape.

[Nitpicker]

A very beautiful image. Thank you.
Nitpicker: egg is 3-D. "oval" is 2-D; that's all, likely.

Fairy nuff. So, "ovoid" then.

By the same logic, elliptical galaxies should perhaps be known as ellipsoidal galaxies.

I'd argue that both ovoid and ellipsoid are incorrect. I'm not aware of any ovoid galaxies, as the word implies an egg-like shape- larger at one end than the other. Neither is it clear that any elliptical galaxies are ellipsoidal. They are called "elliptical" because they appear, in two dimensions, similar to ellipses. In fact, most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape.

I don't think that oval, nor ovoid, nor even egg-like, implies a shape larger at one end than the other. Most generally to me, it just means that it is elongated from a circular or spherical form. In some technical contexts it means something more precise. So, I'm going back to "oval".

I didn't know that most elliptical galaxies are nearly flat disks. But I don't understand what you mean by: "most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape." By that description, I don't understand how it couldn't produce an ellipsoid. What am I not understanding?

[Chris Peterson]

I'd argue that both ovoid and ellipsoid are incorrect. I'm not aware of any ovoid galaxies, as the word implies an egg-like shape- larger at one end than the other. Neither is it clear that any elliptical galaxies are ellipsoidal. They are called "elliptical" because they appear, in two dimensions, similar to ellipses. In fact, most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape.

I don't think that oval, nor ovoid, nor even egg-like, implies a shape larger at one end than the other. Most generally to me, it just means that it is elongated from a circular or spherical form. In some technical contexts it means something more precise. So, I'm going back to "oval".

Perhaps. Every definition I can find for "oval" allows that the term is vague and doesn't strictly require an egg shape. However, every definition I find for "ovoid" does explicitly say that there's a taper.

I didn't know that most elliptical galaxies are nearly flat disks. But I don't understand what you mean by: "most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape." By that description, I don't understand how it couldn't produce an ellipsoid. What am I not understanding?

It means that if you cut an elliptical galaxy along its plane of symmetry, you get a circular face. Such a galaxy may be nearly flat, or have most of its stars in random inclinations (and all forms between). Such a shape is circular in one projection and... well, something else in other projections, something that isn't necessarily elliptical. Unless all cross-sections are elliptical, it isn't an ellipsoid.

[Nitpicker]

Perhaps. Every definition I can find for "oval" allows that the term is vague and doesn't strictly require an egg shape. However, every definition I find for "ovoid" does explicitly say that there's a taper.

<snip>

It means that if you cut an elliptical galaxy along its plane of symmetry, you get a circular face. Such a galaxy may be nearly flat, or have most of its stars in random inclinations (and all forms between). Such a shape is circular in one projection and... well, something else in other projections, something that isn't necessarily elliptical. Unless all cross-sections are elliptical, it isn't an ellipsoid.

I use the term "ovoid" as the 3-D version of "oval". No implication of a taper, but no implication that there isn't, either. I maintain this is the more common (and certainly the more general) definition.

Ellipsoids may be scalene, oblate, prolate, or spherical. It is only a scalene ellipsoid that always yields non-circular axial sections. The others can all yield circular sections. Based on other definitions I've just read of elliptical galaxies, it seems to me that they typically form something roughly approximating an oblate ellipsoid (a.k.a. an oblate spheroid). Sadly, this is not far removed from the shape of a lentil (lenticular) in 3-D. I'm glad I don't have to classify galaxies for a living, as I reckon I'd be no good at it.

[Ann]

Chris wrote:
Neither is it clear that any elliptical galaxies are ellipsoidal. They are called "elliptical" because they appear, in two dimensions, similar to ellipses. In fact, most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape.

Interesting, Chris. It is just possible that I'm more of a galaxy nerd than you are, and I haven't heard the claim that most elliptical galaxies are disks. Of course, my interest in elliptical galaxies is not great (for obvious color reasons), but that doesn't mean I'm not interested in what to me is radically new information about them.

Click to view full size image

The Perseus Cluster. Photo: Jim Misti.

My impression is that when I look at galaxy clusters full of elliptical galaxies, the largest galaxies always appear to be (fairly) round. If some of them are disk galaxies, we ought to see some of the largest ones as elongated disks. Yet we don't, in my opinion. So may I inquire about your sources that say that most elliptical galaxies are disks?

Ann

[Nitpicker]

And for completeness, I may as well state that I think an ovoid is more like a prolate ellipsoid, but not so much like an oblate, or scalene ellipsoid. So, that may rule out "ovoid" as a good description of the shape of an elliptical galaxy.

(But I've learnt something about the actual 3-D shape of elliptical galaxies.)

[Chris Peterson]

I use the term "ovoid" as the 3-D version of "oval". No implication of a taper, but no implication that there isn't, either. I maintain this is the more common (and certainly the more general) definition.

I don't think you'd be misunderstood. I'm just not sure it's technically correct.

Ellipsoids may be scalene, oblate, prolate, or spherical. It is only a scalene ellipsoid that always yields non-circular axial sections. The others can all yield circular sections. Based on other definitions I've just read of elliptical galaxies, it seems to me that they typically form something roughly approximating an oblate ellipsoid (a.k.a. an oblate spheroid).

Ellipsoids are characterized by having every cross-section through their center creating an ellipse. I don't think that's the case for the shape of elliptical galaxies, which is why I suggested that they aren't actually ellipsoids.

[Nitpicker]

Ellipsoids are characterized by having every cross-section through their center creating an ellipse.

That is only the case for scalene (or triaxial) ellipsoids.

[Chris Peterson]

Chris wrote:
Neither is it clear that any elliptical galaxies are ellipsoidal. They are called "elliptical" because they appear, in two dimensions, similar to ellipses. In fact, most are disks, which may be nearly flat or could be expanded out nearly to spheres. That does not produce an ellipsoidal shape.

Interesting, Chris. It is just possible that I'm more of a galaxy nerd than you are, and I haven't heard the claim that most elliptical galaxies are disks.

The entire class is defined based on the fact that they are observed to have elliptical shapes in projection, ranging from circular to cigar shaped. Outside of rare cases that actually are significantly asymmetrical (which isn't dynamically stable for long periods), that implies most are disk-like and how we see them depends on their angle with respect to us. Call that "disk-like", or "oblate-sphere-like" or whatever, the point is that most have a circular cross section along one axis.

My impression is that when I look at galaxy clusters full of elliptical galaxies, the largest galaxies always appear to be (fairly) round. If some of them are disk galaxies, we ought to see some of the largest ones as elongated disks. Yet we don't, in my opinion. So may I inquire about your sources that say that most elliptical galaxies are disks?

As I hope I made clear, I only use "disk" loosely to describe a shape that is circular on one axis and extended along the perpendicular- either a small amount, meaning that the object is quite disk-like (producing a cigar shaped galaxy when viewed at an angle) to spherical (producing a circular galaxy when viewed from any angle). An elliptical galaxy is very similar to the bulge of a spiral galaxy. Over time, any flat galaxy will evolve to a spherical shape, since that's what large, gravitationally bound particle systems do.

[Chris Peterson]

Ellipsoids are characterized by having every cross-section through their center creating an ellipse.

That is only the case for scalene (or triaxial) ellipsoids.

My understanding is that it is the case for all ellipsoids.

[Nitpicker]

It appears we have simply failed to agree on terms.

https://en.wikipedia.org/wiki/Ellipsoid

There are four distinct cases of which one is degenerate:

a>b>c — tri-axial or (rarely) scalene ellipsoid;
a=b>c — oblate ellipsoid of revolution (oblate spheroid);
a=b<c — prolate ellipsoid of revolution (prolate spheroid);
a=b=c — the degenerate case of a sphere;

Mathematical literature often uses 'ellipsoid' in place of 'tri-axial ellipsoid'. Scientific literature (particularly geodesy) often uses 'ellipsoid' in place of 'ellipsoid of revolution' and only applies the adjective 'tri-axial' when treating the general case. Older literature uses 'spheroid' in place of 'ellipsoid of revolution'.

Any planar cross section passing through the center of an ellipsoid forms an ellipse on its surface: this degenerates to a circle for sections normal to the symmetry axis of an ellipsoid of revolution (or all sections when the ellipsoid degenerates to a sphere.)

I am pleased with this discussion. For some reason, I had previously imagined non-spherical elliptical galaxies to be prolate. I now know that they are actually oblate. So, "oval" does indeed seem like a good descriptor, but "ovoid" does not (and not because of any taper, or lack of taper). Phew.

[rstevenson]

Although the semantic discussion is fun, let's not lose sight of the fact that the category of Elliptical Galaxy covers quite a variety of shapes, from almost perfectly spherical to more disk-like (or at least containing a disk-like structure. Here's one of the latter, M105, from Hubble...

Messier_105.jpg (14.91 KiB) Viewed 4817 times

Centaurus-A is also an elliptical, of a rather fantastic shape (This one from ESO.)...

It's currently thought that many elliptical galaxies contain embedded disks, though since the disks are only visible when they are fairly close to edge-on, it's difficult to give a close estimate of how many such there are.

So, oval all, which roundoid name we use hardly matters.

Rob

[Nitpicker]

Rob, I just looked up M104, only to be informed that it is classified as a spiral galaxy.

It makes me think that galaxies have perhaps been over-classified, or at least that their classifications are over-emphasised.

From now on, I'm thinking of all of them as oblate spheroids of some kind. It works for me.