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

[APOD Robot]

Wolf-Lundmark-Melotte

Explanation: Named for the three astronomers instrumental in its discovery and identification, Wolf - Lundmark - Melotte (WLM) is a lonely dwarf galaxy. Seen toward the mostly southern constellation Cetus, about 3 million light-years from the Milky Way, it is one of the most remote members of our local galaxy group. In fact, it may never have interacted with any other local group galaxy. Still, telltale pinkish star forming regions and hot, young, bluish stars speckle the isolated island universe. Older, cool yellowish stars fade into the small galaxy's halo, extending about 8,000 light-years across. This sharp portrait of WLM was captured by the 268-megapixel OmegaCAM widefield imager and survey telescope at ESO's Paranal Observatory.

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[ta152h0]

Wow, can be seen from space

[Ann]

Wow, can be seen from space

I'm not quite sure exactly what you mean, ta152h0, but I thought it was really funny!

It's a very fine picture. I'll probably come back and say something more about it when I have more time.

Ann

[RocketRon]

Can be seen/photoed clearly from on earth might be a more suitable comment.

https://apod.nasa.gov/apod/ap160407.html

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

[Nitpicker]

With no evidence whatsoever, I am tempted to think of it as the "missing link" between globular clusters and disc-shaped galaxies. But I'll refrain.

[De58te]

What exactly is the remotest galaxy in the Local Group? In the Local Group link, it would appear that WLM is closer to M33 than M33 is to the Milky Way. (An astronomer in M33 would think Leo 1 is far more remote than WLM.) Even M31 looks about the same distance to WLM as M31 is to the Milky Way. (An astronomer in M31 would measure in his telescope that WLM and the Milky Way are about the same distance away, except they are in different areas in the sky.)
By the way, although the picture is two dimensional, are only the Magellanic Clouds and Leo 1 on the right side of the Milky Way? What about the other galaxies? I thought there were some 26 or 29 galaxies in the Local Group?

[Chris Peterson]

With no evidence whatsoever, I am tempted to think of it as the "missing link" between globular clusters and disc-shaped galaxies. But I'll refrain. :ssmile:

I think there's a missing link between globular clusters and disk-shaped galaxies in the same way there's a missing link between rocks and cats.

[Chris Peterson]

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

It's really just a catchall designation for any galaxy that has lost much of its symmetry.

In some ways, morphological classification of galaxies is a scientific fossil (not unlike the morphological classification of species in biology). Generally of more interest these days is the physical evolution of galaxies- the processes that have led to any given appearance today.

[neufer]

What exactly is the remotest galaxy in the Local Group?

remote (adj.) mid-15c., from Middle French remot or directly from Latin remotus "afar off, remote, distant in place," past participle of removere "move back or away".

• Sextans B may or may not be the remotest galaxy in the Local Group:

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Sextans B is an irregular galaxy that may be part of the Local Group, or lie just beyond it. Sextans B is 4.44 million light-years away from Earth and thus is one of the most distant members of the Local Group, if it is indeed a member. It forms a pair with its neighbouring galaxy Sextans A. It is a type Ir IV-V galaxy according to the galaxy morphological classification scheme. Sextans B may also be gravitationally associated with the galaxies NGC 3109 and the Antlia Dwarf.

Sextans B has a uniform stellar population, but the interstellar medium in it may be inhomogeneous. Its mass is estimated to be about 2 × 10⁸ times the mass of the Sun, of which 5.5 × 10⁷ is in the form of atomic hydrogen. Star formation in the galaxy seems to have proceeded in distinct periods of low intensity, separated by shorter periods of no activity. The existence of Cepheid variables in the galaxy implies that Sextans B contains at least some young stars. The metallicity of Sextans B is rather low, with a value of approximately Z = 0.001. Sextans B is receding from the Milky Way with a speed of approximately 300 kilometres per second, and probably lies just outside the edge of the Local Group, so as its neighbour Sextans A.

Five planetary nebulae have been identified in Sextans B, which is one of the smallest galaxies where planetary nebulae have been observed. These appear point-like and can be identified by their spectral emission lines. It also contains a massive globular cluster.>>

[sillyworm]

"...10 million light years from the Milky Way..." ONE Light year = (approx?) 5,880,000,000,000 miles. I know this is just elementary astronomy....still...these distances blow my mind...and this is just a small stride in an immense Universe.

[NGC3314]

It's really just a catchall designation for any galaxy that has lost much of its symmetry.

In some ways, morphological classification of galaxies is a scientific fossil (not unlike the morphological classification of species in biology). Generally of more interest these days is the physical evolution of galaxies- the processes that have led to any given appearance today.

I'm gonna have to call you on these.

"Lost much of its symmetry"? That might make readers think it is normal for galaxies to have the kinds of symmetry that strike us. The system may never have been very symmetric, or (especially in the early Universe) not yet have established much symmetry. The majority of galaxies are small and have low luminosity, so if they have any ongoing star formation at all they will look pretty asymmetric and irregular simply because of small numbers.

As for morphological classification being a fossil - not nearly to the extent that your wording might suggest. Of course we want to get at the physical processes, but the reason for the success of, for example, Galaxy Zoo is that algorithmic decomposition of galaxy images has yet to keep up (although automated techniques are getting better). Edwin Hubble''s type bins (and their refinements) have survived for nearly a century because they happen to correlate so well with things we really want to know - dynamical components (thin disk/bulge, bar instabilities, history of stellar populations, resonant orbits, content of dust and gas).

[Ann]

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

[Wadsworth]

I recently had a retinal issue in my right eye, and this morning while looking at APOD I found myself thinking of how I would miss things like this if I lost my sight. I tried to describe the photo to myself to see if it would do it any justice. Hardly.

I found myself zooming in and staring at the image for several minutes lost in imagination and wonder. A great image in its entirety.
Don't take your sight for granite.

[Chris Peterson]

It's really just a catchall designation for any galaxy that has lost much of its symmetry.

In some ways, morphological classification of galaxies is a scientific fossil (not unlike the morphological classification of species in biology). Generally of more interest these days is the physical evolution of galaxies- the processes that have led to any given appearance today.

I'm gonna have to call you on these.

"Lost much of its symmetry"? That might make readers think it is normal for galaxies to have the kinds of symmetry that strike us.

Well, yes, that's exactly what people should think. The overwhelming majority of galaxies are symmetric. Asymmetric blobs are the exception.

[ta152h0]

I am hooked on the science channel and it is noted some things our ancestors have created on Earth surface can only be seen from space. " can only be seen from space " appears to be a recurring phrase.

[neufer]

With no evidence whatsoever, I am tempted to think of it as the "missing link"
between globular clusters and disc-shaped galaxies. But I'll refrain.

I think there's a missing link between globular clusters and disk-shaped galaxies
in the same way there's a missing link between rocks and cats.

[b][color=#0000FF]The globular cluster Omega Centauri. Credit ESO[/color][/b]

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<<Omega Centauri (ω Cen or NGC 5139) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 15,800 light-years, it is the largest globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars and a total mass equivalent to 4 million solar masses. Omega Centauri is so distinctive from the other galactic globular clusters that... it has been speculated that Omega Centauri may be the core of a dwarf galaxy that was disrupted and absorbed by the Milky Way. Indeed, Kapteyn's Star, which is currently only 13 light years away, is thought to originate from Omega Centauri. Omega Centauri's chemistry and motion in the Milky Way are also consistent with this picture. Like Mayall II, Omega Centauri has a range of metallicities and stellar ages that suggests that it did not all form at once (as globular clusters are thought to form) and may in fact be the remainder of the core of a smaller galaxy long since incorporated into the Milky Way. The novel Singularity (2012), by Ian Douglas, presents as fact that Omega Centauri and Kapteyn's Star originate from a disrupted dwarf galaxy, and this origin is central to the novel's plot.>>

[neufer]

"Lost much of its symmetry"?

That might make readers think it is normal for galaxies to have the kinds of symmetry that strike us.

Well, yes, that's exactly what people should think.

The overwhelming majority of galaxies are symmetric. Asymmetric blobs are the exception.

---

<<An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Collectively they are thought to make up about a quarter of all galaxies. Irregular galaxies are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force.

The irregular galaxy NGC 1427A will not survive long as an identifiable galaxy, passing through the Fornax cluster at nearly 600 kilometers per second. Galaxy clusters, like the Fornax cluster, contain hundreds or even thousands of individual galaxies. Within the Fornax cluster, there is a considerable amount of gas lying between the galaxies. When the gas within NGC 1427A collides with the Fornax gas, it is compressed to the point that it starts to collapse under its own gravity. This leads to formation of the myriad of new stars seen across NGC 1427A, which give the galaxy an overall arrowhead shape that appears to point in the direction of the galaxy's high-velocity motion. The tidal forces of nearby galaxies in the cluster may also play a role in triggering star formation on such a massive scale.>>

[Ann]

With no evidence whatsoever, I am tempted to think of it as the "missing link" between globular clusters and disc-shaped galaxies. But I'll refrain.

Click to view full size image

Globular cluster 47 Tucanae and an irregular galaxy,
the Small Magellanic Cloud. Photo: Eder Ivan.

Click to view full size image

Compact dwarf elliptical galaxy M32 and large disk galaxy M31.

Click to view full size image

Compact blue dwarf galaxy, starburst galaxy I Zwicky 18.
HST/NASA/ESA.

Click to view full size image

Dwarf spheroidal galaxy Leo 1.
Photo: Australian Astronomical Observatory.

Globular clusters contain only old stars, and they are usually very compact. Being compact, they have a high surface brightness. They are metal-poor, often extremely metal-poor. All their stars are usually of the same age, and in those cases when a globular contains stars of different ages, the younger ones are not much younger than the older ones. The star formation of globular clusters stopped very long ago. Globulars always seem to be satellites of galaxies, and I've never heard of a "free-floating globular". Globulars come in certain sizes. For example, the largest globular cluster of the Milky Way, Omega Centauri, has a diameter of roughly 150 light-years. There seems to be a limit to their mass, and Omega Centauri has a mass of 5 million suns.

Galaxies can be isolated, or they can exist in groups. Dwarf galaxies are often satellites of larger galaxies, but they, too, can be isolated. Galaxies often have a low surface brightness. Galaxies often contain stars of several generations, and all galaxies that are currently forming stars contain both young and old stars. Most galaxies, with a very, very few exceptions (e.g., I Zwicky 18) are dominated by stars that are lot more metal-rich than the stars of globular clusters. Galaxies, even dwarf galaxies, are usually larger and more massive than globular clusters. For example, compact dwarf elliptical galaxy M32 has a mass of 3 billion suns, compared with 5 million suns for globular cluster Omega Centauri. And puny dwarf spheroidal galaxy Leo 1 may have a mass of 20 million suns, compared with 5 million suns for Omega Centauri. As for the typical sizes of galaxies and globulars,galaxies are typically a lot larger. M32 is 8,000 light-years in diameter, compared with 150 light-years in diameter for Omega Centauri.

So I don't think that globulars and dwarf galaxies are very similar at all.

Ann

[geckzilla]

You'd be surprised to realize at how off your sense of scale gets when your brain isn't supplied adequate details. You put these dwarf galaxies in Hubble's line of sight and realize they are big enough to host their own globular clusters. There are a lot more stars in them than one might think, and they are nowhere near the same scale as globulars, despite how they may appear.

[bubba]

Anyone know what the small red streak on the lower left is? Glitch?

[bubba]

Phenomenal pic, BTW. I started counting the background galaxies, but decided that would take way too long.

[Nitpicker]

I am well aware of the size difference between globulars, dwarf galaxies and larger disc shaped galaxies.

Perhaps "missing link" was the wrong term to use to describe my thinking. Never mind.

[geckzilla]

I am well aware of the size difference between globulars, dwarf galaxies and larger disc shaped galaxies.

Perhaps "missing link" was the wrong term to use to describe my thinking. Never mind.

Sorry if my reply bothered you. It seems like it might have. I was speaking from personal experience, not trying to give a sort of lecture.

[Nitpicker]

Not bothered so much as misunderstood. I will try to flesh-out my thinking pattern. In a nutshell, early galaxies (spheroidal blobs of dark and conventional matter, that somehow formed out of the primordial universal soup created by the big bang) tended to create collections of (still largely pristine) globular clusters. And in those galaxies where discs formed, the discs formed later, apparently coinciding with the cessation of globular formation. Within the discs, open clusters tend to form in preference, and these have lower densities and seemingly larger variations in star masses, and perhaps therefore a greater tendency to evolve the discs into the complex forms we see today.

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.)

[neufer]

[b][color=#0000FF]Captain 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.)