APOD: Wolf-Lundmark-Melotte (2017 May 19)

[neufer]

[b][color=#0000FF]Kapteyn Peter “Wrongway” Peachfuzz[/color][/b]

---

At first glance, it appeared to me that WLM might be the sort of galaxy that lies somewhere in between the early, primordial blobs that formed globulars, and a highly evolved disc galaxy. I can see how I might have been misunderstood in my original comment, but I'm also surprised that everyone seems to have misunderstood me. (Neufer was almost on the right track, but not quite [it can be hard to tell]. I confess that much of my personal ponderings on globulars has resulted from personal observation of Omega Cen, in which I am quite fascinated. But I don't pretend to fully understand how any globular came to be, exactly.)

[MarkBour]

A missing (hypertext) link between rocks and cats ...

West Midland Safari Park, Bewdley, Worcestershire, England

So, back to the point, though. There's no denying that there is a striking similarity between today's image and some images of globular clusters that have been featured. Taking a local patch of each is an example:

Capture2.PNG (219.41 KiB) Viewed 3803 times

Which is which?

Capture1.PNG (166.7 KiB) Viewed 3803 times

but I think more importantly would be the result of a "star-density" plot, which for both items looks something like a normal distribution along any line intersecting the center of the feature.

Nitpicker's thoughts about the evolution of each ... well, that's way beyond me.

[Nitpicker]

For the record (if there is one), I was not trying to point out the similarity between WLM and a globular cluster. I was suggesting (clumsily) that pristine WLM, appears to have a shape somewhere in between a spheroidal primordial blob and an evolved disc galaxy. And therefore, that WLM might show how a disc galaxy once looked, at the time when it stopped creating globulars and its disc began to form.

[Ann]

Mark, thanks for your closeups of a part of a globular cluster and a part of WLM! Which closeup is which?

To me it's obvious that the picture of a globular is the one at left, and the WLM image is the one at right. Why? It's because you can see a certain clumpiness in the distribution of blue stars in WLM, and that kind of thing simply doesn't happen in globulars with a purely old population. Also there is one - just one - extra bright red giant in the WLM image, which is unsurprising if we are looking at an at least partly young population, but in a globular cluster we expect a large number of bright but not super-bright red giants scattered all over the face of it. As for the globular cluster image, you can see how the stellar density gets higher in the upper left corner of the globular - we are getting closer to the center of it. Note how the space between the stars remains relatively black in the WLM image, but in the globular cluster image, the background turns brighter, almost slightly yellowish at the upper left of the image. That is because the overall number of stars becomes much higher there. And as we get closer to the center of the globular cluster, the number of discernible red giants also rises.

Ann

[Ann]

For the record (if there is one), I was not trying to point out the similarity between WLM and a globular cluster. I was suggesting (clumsily) that pristine WLM, appears to have a shape somewhere in between a spheroidal primordial blob and an evolved disc galaxy. And therefore, that WLM might show how a disc galaxy once looked, at the time when it stopped creating globulars and its disc began to form.

Click to view full size image

Mayall II, largest globular of Andromeda.
Michael Rich, Kenneth Mighell, and James D. Neill
(Columbia University), and Wendy Freedman
(Carnegie Observatories) and NASA

Nit, I am one of those who misunderstood you. Nevertheless, I think that one of the points I tried to make, namely that globulars only come in certain sizes and galaxies bright enough to have names are larger than globulars, is relevant when we talk about the shapes of galaxies and globulars. At left you can see a picture of Mayall II, the largest globular of Andromeda.

Wikipedia wrote about Mayall II:

Because of the widespread distribution of metallicity, indicating multiple star generations and a large stellar creation period, many contend that it is not a true globular cluster, but is actually the galactic core that remains of a dwarf galaxy consumed by Andromeda.

So the largest globular cluster of Andromeda may be the galactic core of a dwarf galaxy shattered by Andromeda, and the largest globular of the Milky Way may be the galactic core of a dwarf galaxy shattered by the Milky Way. However, Mayall II is larger than Omega Centauri. According to Wikipedia, the mass of Mayall II is 10 million solar masses, but according to the Wikipedia entry about Omega Centauri, the mass of Omega Centauri is 4 million solar masses.

Note that the picture of Mayall II shows it to be slightly elongated. I have read somewhere, though I'm unable to find it, that the largest and most massive globulars may tend to be slightly elongated.

I believe we should think of large (or at least non-dwarf) elliptical galaxies as galaxies that would have had flattened disks if their symmetrical rotation had not been upset by some mechanism (which at the same time rendered their remaining gas unsuitable for star formation).

But I also think it takes a certain combined mass to flatten a congregation of stars and a portion of gas and dust and make it become disk-shaped. The WLM dwarf galaxy does not appear to be very highly flattened, although it is very clearly elongated. As I noted in a post I made some days ago, dwarf galaxies almost never have central dust lanes. I believe that the formation of a central dust lane also requires a certain combined mass of the galaxy. So I would guess that WLM is massive enough to be clearly elongated, but far from massive enough to become a flattened disk galaxy with a dust lane.

And when we think of it that way, it is indeed possible to consider dwarf galaxies like WLM to be intermediate between globulars (which are too lightweight to be even noticeably elongated in most cases) and large disk galaxies (which are massive enough to become highly flattened and sport central dust lanes).

Ann

[Nitpicker]

That may all be true, Ann, but I don't think it changes the point I was attempting to make. You seem to be using the term "intermediate" in terms of size/mass, whereas I was suggesting it in terms of time. All galaxies probably started out spheroidal. The larger ones tend to form flattened discs (or some form of significant oblateness). I merely think the current shape of (pristine and somewhat oblate) WLM might be similar to the shape of a much larger galaxy, but before its disc formed. It hardly seems like a bold suggestion. But as I said at the beginning, it isn't based on evidence.

[Nitpicker]

I may be off the mark, Ann, but it seems to me that the existence of galactic dust lanes being a requirement for disc-shaped galaxies to form, seems somewhat more bold than my suggestion.

I would think it more likely that dust lanes form after the disc shapes have formed.

Edit: Re-reading your post, maybe you didn't state that requirement. I may just be confused.

[Ann]

I may be off the mark, Ann, but it seems to me that the existence of galactic dust lanes being a requirement for disc-shaped galaxies to form, seems somewhat more bold than my suggestion.

I would think it more likely that dust lanes form after the disc shapes have formed.

I don't pretend to say anything about why central dust lanes only seem to exist in large disk galaxies. Are the dust lanes a requirement for disk-shaped galaxies to form? Or do they form as a consequence of the formation of large flattened galaxies? How would I know?

I'm just saying that I take a huge interest in galaxies, and that's why I love to look at pictures of galaxies. I base what I said about the shape of galaxies in the nearby universe and the presence or absence of central dust lanes in nearby galaxies on having looked at, and often scrutinized, thousands of galaxy images. Nothing more.

Ann

[Nitpicker]

Okay. Sorry Ann. My edit to my previous post came too late.

(l've only recently acquired a smart phone and I should not be using it to read and post to this forum.)

[Chris Peterson]

All galaxies probably started out spheroidal.

I think all of the dominant models have galaxies starting out as disks. That is, you have a disk of baryonic material (hydrogen and helium) embedded in a sphere of dark matter, and at that point you get star formation beginning.

[neufer]

All galaxies probably started out spheroidal.

I think all of the dominant models have galaxies starting out as disks. That is, you have a disk of baryonic material (hydrogen and helium) embedded in a sphere of dark matter, and at that point you get star formation beginning.

<<The Hubble Ultra-Deep Field (HUDF) has revealed high rates of star formation during the very early stages of galaxy formation, within a billion years after the Big Bang. Galaxies at high redshifts have been confirmed to be smaller and less symmetrical than ones at lower redshifts, illuminating the rapid evolution of galaxies in the first couple of billion years after the Big Bang. The Hubble eXtreme Deep Field (HXDF), is an image of a portion of space in the center of the Hubble Ultra Deep Field image. The HXDF contains approximately 5,500 galaxies, the oldest of which are seen as they were 13.2 billion years ago. The red galaxies in the image are the remnants of galaxies after major collisions during their elderly years. Many of the smaller galaxies in the image are very young galaxies that eventually developed into major galaxies, similar to the Milky Way and other galaxies in our galactic neighborhood.>>

[Chris Peterson]

All galaxies probably started out spheroidal.

I think all of the dominant models have galaxies starting out as disks. That is, you have a disk of baryonic material (hydrogen and helium) embedded in a sphere of dark matter, and at that point you get star formation beginning.

<<The Hubble Ultra-Deep Field (HUDF) has revealed high rates of star formation during the very early stages of galaxy formation, within a billion years after the Big Bang. Galaxies at high redshifts have been confirmed to be smaller and less symmetrical than ones at lower redshifts, illuminating the rapid evolution of galaxies in the first couple of billion years after the Big Bang. The Hubble eXtreme Deep Field (HXDF), is an image of a portion of space in the center of the Hubble Ultra Deep Field image. The HXDF contains approximately 5,500 galaxies, the oldest of which are seen as they were 13.2 billion years ago. The red galaxies in the image are the remnants of galaxies after major collisions during their elderly years. Many of the smaller galaxies in the image are very young galaxies that eventually developed into major galaxies, similar to the Milky Way and other galaxies in our galactic neighborhood.>>

Yes. Not sure of your point, however, as it doesn't alter my observation.

[neufer]

All galaxies probably started out spheroidal.

I think all of the dominant models have galaxies starting out as disks. That is, you have a disk of baryonic material (hydrogen and helium) embedded in a sphere of dark matter, and at that point you get star formation beginning.

<<The Hubble Ultra-Deep Field (HUDF) has revealed high rates of star formation during the very early stages of galaxy formation, within a billion years after the Big Bang. Galaxies at high redshifts have been confirmed to be smaller and less symmetrical than ones at lower redshifts, illuminating the rapid evolution of galaxies in the first couple of billion years after the Big Bang. The Hubble eXtreme Deep Field (HXDF), is an image of a portion of space in the center of the Hubble Ultra Deep Field image. The HXDF contains approximately 5,500 galaxies, the oldest of which are seen as they were 13.2 billion years ago. The red galaxies in the image are the remnants of galaxies after major collisions during their elderly years. Many of the smaller galaxies in the image are very young galaxies that eventually developed into major galaxies, similar to the Milky Way and other galaxies in our galactic neighborhood.>>

Yes. Not sure of your point, however, as it doesn't alter my observation.

Most galaxies probably started out small & irregular.

[Chris Peterson]

Yes. Not sure of your point, however, as it doesn't alter my observation.

Most galaxies probably started out small & irregular.

That is not my reading of it.

Regions of star formation started out in irregular clumps. Irregular clumps within disk-like gas regions.

[Ann]

Click to view full size image

The Hubble eXtreme Deep Field.
NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California,
Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team)

If you take a look at the small blue very distant galaxies in the Hubble eXtreme Deep Field, you can see that most of them seem to have features. Most of them don't look like featureless blobs.

But I'll offer no other opinion on what the first galaxies or proto-galaxies looked like when they first formed.

Ann

[Chris Peterson]

Click to view full size image

The Hubble eXtreme Deep Field.
NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California,
Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team)

If you take a look at the small blue very distant galaxies in the Hubble eXtreme Deep Field, you can see that most of them seem to have features. Most of them don't look like featureless blobs.

Exactly. Most galaxies have features. They typically form as disks, but in higher density regions many collided to form ellipticals while still very young.

Note that what we see in images like this is actual galaxies. Not protogalaxies in the early stages of formation (which is where you have clumping inside what will become a galaxy after millions of years).

[neufer]

Not sure of your point, however, as it doesn't alter my observation.

Most galaxies probably started out small & irregular.

That is not my reading of it.

Regions of star formation started out in irregular clumps. Irregular clumps within disk-like gas regions.

So those "irregular clumps" don't qualify as dwarf galaxies

[Chris Peterson]

Most galaxies probably started out small & irregular.

That is not my reading of it.

Regions of star formation started out in irregular clumps. Irregular clumps within disk-like gas regions.

So those "irregular clumps" don't qualify as dwarf galaxies :?:

Not at all. They represent part of the galaxy formation process very early on, before the galaxies even existed.

It's certainly possible that some dwarf galaxies originated in such clumps that were ejected from their host regions. But the dwarfs are considerably evolved from those early clumps.

[neufer]

Regions of star formation started out in irregular clumps. Irregular clumps within disk-like gas regions.

So those "irregular clumps" don't qualify as dwarf galaxies

Not at all. They represent part of the galaxy formation process very early on, before the galaxies even existed. It's certainly possible that some dwarf galaxies originated in such clumps that were ejected from their host regions. But the dwarfs are considerably evolved from those early clumps.

• . http://asterisk.apod.com/viewtopic.php? ... 89#p270923

[b][color=#0000FF][size=85]You better hope I'm not DUCK/DUCT taping this![/size][/color][/b]

---

<<The DUCK test is a term for a form of abDUCTive reasoning. The test implies that a person can identify an unknown subject by observing that subject's habitual characteristics. It is sometimes used to counter abstruse, or even valid, arguments that something is not what it appears to be.

Indiana poet James Whitcomb Riley (1849–1916) may have coined the phrase when he wrote:

• When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.

Douglas Adams parodied this test in his book Dirk Gently's Holistic Detective Agency:

• If it looks like a duck, and quacks like a duck, we have at least to consider the possibility that we have a small aquatic bird of the family Anatidae on our hands.

Similarly, the term elephant test refers to situations in which an idea or thing, "is hard to describe, but instantly recognizable when spotted".>>

[Nitpicker]

I'm clearly reading the wrong articles on galaxy formation.

And I also want to know why globulars stopped forming.

[Nitpicker]

Is there an explanation within the dominant models, for why a galaxy first forms as a disc of baryons embedded in a spheroidal halo of dark matter, prior to any star formation?

Does this imply that the baryons had an initial angular momentum that the dark matter lacked?

[Chris Peterson]

Is there an explanation within the dominant models, for why a galaxy first forms as a disc of baryons embedded in a spheroidal halo of dark matter, prior to any star formation?

Does this imply that the baryons had an initial angular momentum that the dark matter lacked?

Dark matter has angular momentum, as well. But dark matter doesn't have electromagnetic interaction, so you have no mechanism to cause it to flatten into a disk. So the dark matter remains in a spherical halo. The gravitationally bound baryons, however, if sufficiently dense, do interact, and flatten out because of their fluid behavior.

This has to happen before many stars form, because once you have stars, you have no mechanism to pull them into a disk. Stars behave like independent particles, not like elements of a fluid.

[Chris Peterson]

And I also want to know why globulars stopped forming.

Maybe when we have a solid theory about how they form, we'll also understand why they stop forming.

[starsurfer]

It seems to have no structure to it ?
The wiki article calls it an 'irregular' galaxy, precisely what does that mean.

Click to view full size image

An irregular galaxy: The Small Magellanic Cloud.
ESA/Hubble and Digitized Sky Survey 2. Acknowledgements: Davide De Martin.

Click to view full size image

A spheroidal galaxy: NGC 205, satellite of M31.
Photo: Probably Adam Block.

An irregular galaxy is small, has no spiral arms, contains young stars (as well as old ones) and often contains emission nebulas. Irregular galaxies also often lack a bright core.

That's it, really!

So the Small Magellanic Cloud, which has a lot of star formation, several big nebulas, no bright core and an irregular shape, is a prototype irregular galaxy. NGC 205, the largest M31 satellite, has a very oval shape, a predominantly old population and a small relatively young central population along with a pair of central dust clouds, but no ongoing star formation and no emission nebulas. NGC 205 is a (dwarf) spheroidal galaxy.

Ann

The image of M110 isn't by Adam Block, it might be by Leonardo Orazi?

[Ann]

It might be by Jean-Charles Cuillandre (CFHT) und Giovanni Anselmi (Coelum Astronomia), Hawaiian Starlight.

Ann