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

[Nitpicker]

Sneaky Rob, changing from M104 to M105 like that.

[Chris Peterson]

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

That's certainly true.

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

Oblate, for sure. And there's nothing particularly wrong with thinking of them as oblate spheroids. But more formally, and considering the technical terms, no, I don't think they are. Oblate spheroids are a special case of ellipsoid, and again, I don't think elliptical galaxies are ellipsoids, because I don't think a sagittal section of a non-spherical elliptical galaxy bounds an ellipse (and regardless of how we want to look at different definitions of ellipsoid, that is an absolute requirement for an oblate spheroid). Dynamically, I think an elliptical galaxy will not have a random distribution of stellar inclination, but rather, orbits with small galactic radiuses will show more range than those with large ones, which I don't think leads to an elliptical cross-section.

Maybe the best way to think of all galaxies is as "oddly and unevenly squished spheroids".

[Ann]

I'd question the claim that Centaurus A has a disk. The "disk" of Cen A is the spiral galaxy that is in the process of colliding with the elliptical component of this galaxy. Until the spiral has been fully incorporated with the elliptical component, I don't think we can say that Cen A has a disk.

We are seeing relatively few elliptical galaxies that are in the process of colliding with spirals. Another such elliptical colliding with a spiral is Perseus A. But in Perseus A we don't see a disk.

Ann

[Nitpicker]

Maybe the best way to think of all galaxies is as "oddly and unevenly squished spheroids".

I'm certainly not thinking a perfect oblate spheroid, but I think a perfect oblate spheroid would probably enclose it. I'm failing to imagine the actual sectional shape when "orbits with small galactic radiuses [radii] will show more range than those with large ones". I don't quite get your meaning there.

[Ann]

Chris wrote:
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.

Is the argument that I've highlighted in color the same as saying that every circle is an ellipse, only a special case of it? In other words, are all stable galaxies disk galaxies, where the perfectly spherical ones are just special cases of disks?

M104 must be a special case, indeed. It is the only well-ordered galaxy I can think of that has both a very obvious "spherically elliptical" component as well as a very strong disk.

I certainly agree with you that elliptical galaxies are extremely similar to (or identical with) the central bulges in spiral galaxies. That's why I'm thinking of elliptical galaxies as spiral galaxies that have lost their arms.

Ann

[Nitpicker]

Chris wrote:
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).

Is the argument that I've highlighted in color the same as saying that every circle is an ellipse, only a special case of it?

Perhaps every sphere is kinda, sorta, oblate spheroidal?

[Chris Peterson]

Maybe the best way to think of all galaxies is as "oddly and unevenly squished spheroids".

I'm certainly not thinking a perfect oblate spheroid, but I think a perfect oblate spheroid would probably enclose it. I'm failing to imagine the actual sectional shape when "orbits with small galactic radiuses [radii] will show more range than those with large ones". I don't quite get your meaning there.

Well, take a spiral galaxy as an extreme case. It's basically an intersection of a sphere and a disk. There are components that are in orbits in the disk structure, and components in orbit in the sphere structure. In sagittal section, this is obviously not elliptical. I think that an elliptical galaxy may maintain some of that discontinuity, with large radius orbits being slightly flatter than you'd expect for an elliptical cross-section, and inner orbits being somewhat more inclined.

[Chris Peterson]

Chris wrote:
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.

Is the argument that I've highlighted in color the same as saying that every circle is an ellipse, only a special case of it? In other words, are all stable galaxies disk galaxies, where the perfectly spherical ones are just special cases of disks?

No. I just mean that elliptical galaxies are mostly circular radially, while axially they range in thickness from disk-like to sphere-like. They are all squished spheres of some sort (maybe ellipsoidal, maybe not), varying primarily in the degree of squish.

[Nitpicker]

Maybe the best way to think of all galaxies is as "oddly and unevenly squished spheroids".

I'm certainly not thinking a perfect oblate spheroid, but I think a perfect oblate spheroid would probably enclose it. I'm failing to imagine the actual sectional shape when "orbits with small galactic radiuses [radii] will show more range than those with large ones". I don't quite get your meaning there.

Well, take a spiral galaxy as an extreme case. It's basically an intersection of a sphere and a disk. There are components that are in orbits in the disk structure, and components in orbit in the sphere structure. In sagittal section, this is obviously not elliptical. I think that an elliptical galaxy may maintain some of that discontinuity, with large radius orbits being slightly flatter than you'd expect for an elliptical cross-section, and inner orbits being somewhat more inclined.

Certainly, all the images of non-spherical, roughly edge-on elliptical galaxies I can identify, appear to suggest an elliptical sagittal section (sometimes rather eccentric), at least in terms of the photographic appearance. Not sure how one could readily be more precise than that.

[geckzilla]

There are quite a few dust disks to be found in ellipticals. The ones I've seen have been quite small, though. Hubble can pick 'em up pretty easily. It's not like they go from spiral to lenticular to elliptical right away.


Footballs!

[rstevenson]

Right, Nit, M104 is a spiral. I posted M104 as the example by mistake. I meant M105, so I edited the post a bit later, and added the second example at that time.

As far as classification of galaxies based on shape go, it's worth remembering that the shape is what was seen through a telescope prior to Hubble and the VLA and other modern instruments. In other words, most objects were classified according to the shape of a 2D blob lacking much detail, without regard to the actual 3D shape of the galaxy -- since that was not known.

Rob

[DavidLeodis]

The "eastern side" in the "Giant elliptical galaxy M60 and spiral galaxy NGC 4647...are found in a region of space where galaxies tend to gather, on the eastern side of the nearby Virgo Galaxy Cluster" made me . The eastern side is presumably the more salubrious part for galaxies to be! The western side on the other hand must be a run down part, but perhaps its galaxies are worth buying while they will be very cheap!

[DesertNative]

Over time, will the random movement of stars in an elliptical galaxy ever resolve into a common movement like spiral galaxies? If not, why not?

[Chris Peterson]

Over time, will the random movement of stars in an elliptical galaxy ever resolve into a common movement like spiral galaxies? If not, why not?

Just the opposite. Spiral galaxies will lose their order and evolve into ellipticals. There is no mechanism in a galaxy to force any common movement. The trend is towards a spherical shape with stars at random inclinations.

[Beyond]

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.

Did someone mention footballs? (American)

Click to view full size image

[Ann]

Can't let go of this "disk component, elliptical component" of galaxies yet.

I said before that M104 is the only galaxy I know of that has both a strong disk component and a strong elliptical component. But M104 is not a "normal" galaxy - it is two galaxies in one.

Universe Today wrote:
“The Sombrero is more complex than previously thought,” said Dimitri Gadotti of the European Southern Observatory in Chile and lead author of the report. “The only way to understand all we know about this galaxy is to think of it as two galaxies, one inside the other.”

Although it might seem that the Sombrero is the result of a collision between two separate galaxies, that’s actually not thought to be the case. Such an event would have destroyed the disk structure that’s seen today; instead, it’s thought that the Sombrero accumulated a lot of extra gas billions of years ago when the Universe was populated with large clouds of gas and dust. The extra gas fell into orbit around the galaxy, eventually spinning into a flattened disk and forming new stars.

Click to view full size image

NGC 3521. Image Credit & Copyright: R Jay Gabany (Blackbird Obs.),
Collaboration: David Martinez-Delgado (MPIA, IAC), et al.

So M104 is not a "normal" galaxy, but NGC 3521 is, or so I think. This is a "mostly flocculent" galaxy, which means that it sports a lot of stubby arm fragments rather than long elegant arms. (It does have some longish arms too.)

NGC 3521 is surrounded by several shells of old stars, which are reminiscent of of the shells you sometimes find around large elliptical galaxies. Note that its yellow bulge is very puffed up, too.

Click to view full size image

NGC 1055. Source:
http://cas.sdss.org/dr5/en/tools/places/page2.asp

Another disk galaxy with a very puffed-up bulge is NGC 1055. The bulge of NGC 1055 is very peanut-shaped, which shows that this is a strongly barred galaxy. True elliptical galaxies don't have peanut shapes, because they don't have bars any more.