19 replies to this topic

[CygnuS]

I just found out that our MW Galaxy has 60 satellite galaxies and may have as many as 200. Are any of them smaller than the the largest globular cluster? Are any of them closer than the furthest globular cluster? 

[archer1960]

I'm not sure what you are intending to ask; many of those satellite galaxies ARE globular clusters.

[Tony Flanders]

I just found out that our MW Galaxy has 60 satellite galaxies and may have as many as 200. Are any of them smaller than the the largest globular cluster? Are any of them closer than the furthest globular cluster?

As archer1960 says or hints, the distinction between a globular cluster inside the Milky Way and the core of a dwarf galaxy in the process of being cannibalized by the Milky Way is unclear. There's little doubt that many if not most of our galaxy's globular clusters formed inside the galaxy itself, but Messier 54 in particular has long been suspected of being the core of a cannibalized galaxy due to the wide diversity of its stars. Subsequent studies revealed other globular clusters and individual stars that clearly belong to M54's parent galaxy due to their common motion through space. It's now called the Sagittarius Dwarf Spheroidal, and is widely considered to be the Milky Way's closest satellite galaxy. Overlapping, in fact.

But even by more conservative definitions, there's never been the slightest doubt that the Large Magellanic Cloud is a separate galaxy, and at 163,000 light-years it's roughly half the estimated 300,000 l-y distance to the well-known globular cluster NGC 2419, sometimes called the "Intergalactic Wanderer" despite the fact that it's known to be gravitationally bound to the Milky Way. And there are several other globulars even farther than NGC 2419.

For what it's worth, although the LMC is currently the closest big galaxy to our own, this appears to be its first close encounter with the Milky Way, making it unclear whether it can appropriately be called a "satellite galaxy."

As for whether some galaxies are "smaller" than some globular clusters -- almost certainly yes, though it's not clear exactly what "smaller" means in this context. Neither galaxies nor globular clusters have sharply defined edges, and it's very hard to estimate the mass of a dwarf galaxy. Especially since the whole subject of dark matter -- which likely constitutes much of the mass of most galaxies -- is somewhat controversial.

[CygnuS]

I'm not sure what you are intending to ask; many of those satellite galaxies ARE globular clusters.

This must be relatively new. Years ago I remember reading that the largest globular cluster is almost as big as a galaxy. 

It could be that my memory is slipping.

[CygnuS]

As archer1960 says or hints, the distinction between a globular cluster inside the Milky Way and the core of a dwarf galaxy in the process of being cannibalized by the Milky Way is unclear. 

Thank you for your informative response. Does that mean that they can't tell by the stars? Can an object be classified as a galaxy if all it's stars are still first generation? Can an object be classified as a globular if it's stars are NOT first generation? 

[CygnuS]

I'm not sure what you are intending to ask; many of those satellite galaxies ARE globular clusters.

I'm still a bit confused. I have been searching and can't find anything on the internet about this. As Tony points out, there are objects that are questionable. I can't find any object that is classified as both a dwarf galaxy and a globular. 

[Tony Flanders]

I can't find any object that is classified as both a dwarf galaxy and a globular.

That's correct. It's now universally accepted that giant galaxies like the Milky Way have grown dramatically over their lifetimes by consuming other galaxies. When that happens, the individual stars of the smaller galaxies eventually end up scattered throughout the Milky Way, though for the more recent cases they're still possible to identify by their abnormal motions. The globular clusters within the galaxies, however, generally remain intact due to their intense mutual attraction.

In addition, most observed galaxies -- though by no means all -- have cores that strongly resemble globular clusters. Like globular clusters, these cores remain intact after the parent galaxy is absorbed, and are now classified as Milky Way globulars.

So there are objects that are classified both as globulars and as galaxy cores, but not as globulars and entire galaxies.

The problem with ultra-small galaxies is that they're exceedingly hard to observe. There's little doubt that they outnumber "normal" galaxies by a fair margin, just as red dwarf stars outnumber "normal" stars by a fair margin. But in practice we can't detect them at all unless they're extremely nearby.

That's why the census of our Local Galaxy Group is considered to be incomplete. There are almost certainly many ultra-small galaxies yet to be discovered, even in our immediate neighborhood.
 

There are definitely galaxies with fewer observable stars than globular clusters. But stars are only part of the picture. A large fraction of most galaxies' mass is in the form of gas, which is exceedingly hard to detect from great distances. And of course there is (or maybe isn't) that elusive dark matter.

[KiwiObserver]

On the general question of star clusters vs galaxies, there is a recent discovery, User Major III / UNIONS 1, which is a system of I believe around 100 stars or less and possibly the smallest galaxy yet discovered. There is currently active debate about whether this system is truly a galaxy or whether it is instead a star cluster (which I think is the reason for the two names, one would be for a galaxy and the other for a star cluster). One of the things being looked into to identify it as a cluster or galaxy (as Tony mentions above) is its dark matter content. If the motions of its stars imply that there is a dominating dark matter component, that would imply the system formed within its own dark matter halo and would therefore be considered a galaxy. A lack of dark matter could suggest it is a star cluster, perhaps for example the result of a gas cloud which was pulled or stripped out of the Milky Way and which formed it's own stars i.e. the material was originally part of the MW halo rather than having formed within its own halo. A complicating factor for claiming the presence of dark matter is the potential presence of binaries, which would increase the apparent stellar velocities (due to the speeds the stars orbit each other) and potentially give a false impression of dark matter. 

 

Original discovery: https://arxiv.org/pdf/2311.10147

Edited by KiwiObserver, 15 June 2025 - 11:53 AM.

[Redbetter]

I was considering responding to this when it was first posted, before any replies had been made.  However, the first question is not entirely clear which can lead to at least three paths:

1. "smaller" in terms of size (diameter) is the most literal approach.  Most globulars are very compact even compared to the many dwarf galaxies.  So without even working through a list of candidates my guess is that very few MW satellite galaxies have an estimated real diameter less than that of even the largest globulars.   However, it does look there are exceptions on the low end for satellite galaxies. And of course this is complicated by the fact that several globulars are believed to be the stripped cores of what were dwarf/companion galaxies.  Tony has already pointed out the size problem, and this gets worse when one considers tidal streams from both galaxies and globulars.
2. When it comes to mass rather than physical size there are two ways to approach it.  Absolute magnitude (luminosity of regular matter) is the most direct/logical approach.  Probably the majority of Milky Way satellite galaxies have lower absolute magnitudes than the brightest Milky Way globulars--with the latter running from about -9 to -10.    Relatively few MW satellite galaxies are brighter.
3. Then there is mass...including the giant dark matter fudge factor.  Some of the ultra faint galaxies have proposed mass to light ratios in the hundreds or even thousands.  Globulars have little in the way of dark matter from what I understand.

The second question about distance is easier to answer.  Many satellite galaxies are closer than globular NGC 2419.  Pal 3 & 4 are slightly more distant than 2419; AM-1 is something like 400,000 light years and Laevens 1 is about 470,000 light years.

[CygnuS]

This place  always amazes with all its helpful and well informed members. A big thanks to Tony, Kiwi and Redbetter. Too often we see responses that are too technical and/or mathematical....but you 3 really know how to explain things. You 3 are great examples of why CN is so popular and successful. 

[CygnuS]

I was considering responding to this when it was first posted, before any replies had been made.  However, the first question is not entirely clear which can lead to at least three paths:

1. "smaller" in terms of size (diameter) is the most literal approach.  Most globulars are very compact even compared to the many dwarf galaxies.  So without even working through a list of candidates my guess is that very few MW satellite galaxies have an estimated real diameter less than that of even the largest globulars.   However, it does look there are exceptions on the low end for satellite galaxies. And of course this is complicated by the fact that several globulars are believed to be the stripped cores of what were dwarf/companion galaxies.  Tony has already pointed out the size problem, and this gets worse when one considers tidal streams from both galaxies and globulars.

Interesting. At first I was wondering if larger globular can evolve into small galaxies. After reading further I started wondering if small galaxies evolve into globulars. 

[Redbetter]

Interesting. At first I was wondering if larger globular can evolve into small galaxies. After reading further I started wondering if small galaxies evolve into globulars. 

I think it is more that once a small galaxy has been stripped of all but a concentrated portion of its central bulge, it masquerades as a globular.  Very diffuse tidal streams can help give some of this away, although globulars tend to leak away stars to some extent.  

 

I can't keep up with which globulars have been tentatively classified as stripped galaxy cores, or moved back into the globular camp.

 

And one of the things I have wondered about is, if there is a large unseen blob of dark matter hanging around the galaxy cores...what happens to it?  The dark matter blob/shell of the smaller galaxy shouldn't be doing anything other than interacting gravitationally with the visible and dark matter.  One would think that it will still be associated with the core and comoving with it...and therefore make it easy to tell a globular (largely lacking dark matter) from a stripped core.  If anything the mass/light ratio should be much higher for the stripped core than it would have been prior to interaction that stripped away all of its extended regular matter.

[archival]

We're getting more into conjecture land than science again.

 

Globular clusters are globular clusters and ever more shall be so, and dwarf satellite galaxies are dwarf satellite galaxies.

 

Dwarf satellite galaxies can, and do, have their own globular clusters and some of the Milky Way globular clusters are thought to be inherited from long gobbled up dwarf galaxies.  dE and dSph and dIrr cases with globular clusters are all known.

 

Now, thought is one thing, evidence is another.  the Sagittarius Dwarf discovered nearly 30 years ago now has been shown to be the source of up to nine (depending on your information source) globular clusters in the Milky Way, with the most prominent one being M54, yes, Messier 54 is, or rather was, extragalactic, a globular cluster formerly belonging to a satellite dwarf galaxy of the Milky Way.

 

Some globulars are now thought to have streams and tails - when you get to the distances of these things values of motion measured are about the scale of the errors and much statistical massaging is employed and sweeping assumptions derived from very small samples (as with the aforementioned UMa III paper, a dwarf galaxy assumed from data for 11 stars) often presented as concrete proof.  Globular clusters could have tails simply from their orbital motion through space.

 

If you look at images of dwarf Ellipticals they are usually core free if you check the images, ultra faint dwarfs usually more so.  Look at an image of the dwarf Spheroidal Fornax Dwarf, no dense globular cluster like core.  However this galaxy has a higher than normal proportion of its own globular clusters including the largest NGC 1049 which was known of before the galaxy itself was discovered!  Have the LMC or SMC got "cores", or at least globular cluster like cores?  The former has its own globular cluster and for the latter I've been able to find mention of at least one (some recent work suggests the SMC is too galaxies superposed in the line of sight with the newer bit sometimes called miniSMC, not sure how that will hold up, but apparently there has been at least one independent "confirmation").

 

Dark matter arguments are mostly theoretical.  None has ever been identified.  More properly when the well demonstrated and long used Virial Theorem is applied to the intragalaxy rotation curves as well as in groups and clusters the intergalaxy motions the observed measures do not comply with the basic laws of angular momentum, nor the relativistic gravity rules either.  The popular thought is dark matter, something capable of having gravitational influence yet not detectable any other way, so called "dark matter" because there isn't enough "bar*****c matter" (known stuff) to explain the deviations from the angular momentum predictions.  Nothing wrong with that, it might be true, insufficient data to say either way, so extrapolating from the not shown and then claiming a conclusion as fact is a bit naughty but very popular in modern professional astronomy, especially people who are mathematicians and physicists first.

 

All this would be fine, but the trouble with trying to pigeonhole in astronomy is that the more stuff you shove in the pigeonhole the more there's a chance bits will fall out (like a traditional physical pigeonhole full of uncollected mail, ironically enough usually mostly junk mail, if you'll forgive the simile).

 

Which enables me to claim that globular clusters are not the cores of absorbed dwarf satellite galaxies to the Milky Way only to have omega Centuari shoved in my face.  omega Centauri, unlike the vast majority of globular clusters, has marked metallicity variations within its stellar populations.  Some globular clusters can have evidence for a couple of discrete phases of starburst, both usually very old, but apparently omega Centauri has more markedly different ranges of metallicity in its stars.  This is more likely to happy in a galaxy core than in a globular cluster, so there is some thought that it is the remnant core of a Milky Way swallowed up dwarf satellite galaxy.

 

There are selection effects though, it is physically big and apparently bright (naked eye, named as a star, omega Centuari, until Halley discovered it was fuzzy long ago but likely not identified as to true nature until much later than that) and by those reasons it is bound to have been better studied than many other globular clusters.  Some of these points make it quite different from the majority of globlar clusters.

 

The more we learn the less possible it is to simplistically categorise things.

 

Milky Way globular clusters are not the cores of defunct dwarf satellite galaxies except for when they are.

[moefuzz]

I just found out that our MW Galaxy has 60 satellite galaxies and may have as many as 200. Are any of them smaller than the the largest globular cluster? Are any of them closer than the furthest globular cluster? 

CygnuS,

Before this thread goes to much further we need some clarity as some may imply that clusters are, can, or have been been galaxies.

 

 

Interesting. At first I was wondering if larger globular can evolve into small galaxies. After reading further I started wondering if small galaxies evolve into globulars.

 

 

 

It's important to consider that there are major differences between originating Globular Clusters and Galaxies.

 

Research shows that In ~most~ originating globular cluster's stars move in random and in diverse elliptical orbits around the cluster’s center of mass and as such are driven by the cluster’s gravity.

 

This creates a *pressure-supported system   rather than  a rotationally supported one like a galaxy’s (rotational) disk.

In Globulars, the stars motions are more akin to a swarm of bees,

with no coherent rotation or axis of spin.

 

 There are exceptions to the above in that some globular clusters show mild rotation

as in those that have been stripped.

 

Some clusters like Omega Centauri show a weak rotation but this isn't a dominant feature.

If and when rotation is present, it's a very slow and contributes little gravitation force to the cluster

 

Conversely and yet still comparatively,

small galaxies with disk-like structures very often show rotational patterns, where all matter, stars and gas orbit the galactic center in a coordinated way.

 

 

There's also a major difference in how each are formed.

Originating Globular Clusters form in a huge/initial burst of energy resulting in star formation thus creating a dense gravitationally bound cluster that more or less stabilizes without significant rotational or well defined angular momentum.

 

While in Galaxies, even in the small ones, they form in complex ways which includes accretion discs and mergers which is what leads Galaxies to spin/rotate.

 

Sure, there are Galaxies that are as small as the largest Globular clusters, but it's difficult to group the two differing styles of masses and lump either some or all into one group or another.

 

 

In essence, a cluster can never and will never form, in itself, into coherent discs that 'rotate' in a specific plain

just as a Galaxy, in itself, can not stop gravitational rotation with it's stars opting or choosing to randomly go off in or into their own course and plains.

 

A final determining factor should be that many originating globular clusters orbit and/or have been captured by a neighbouring Galaxy and never the other way around.

Sure, some may have started as small galaxies but this type would now have lost most all of it's rotational mass in essence just leaving a tight group of random stars, gas and dust with little to no rotation.

 

 

 

 

*Pressure supported system

 

Under certain conditions a merger between two galaxies can produce a elliptical patterns with ill or no defined  angular momentum, and thus ~could~ be defined as a pressure-supported system.

 

The difference will still be that the merger will always show significant (if not complex) rotation while a Globular Cluster shows little if any rotation Re: little to no mass centric spin. (the centripetal force pulls the mass inward to follow a curved path, while the mass appears to push outward)

 

 

 

moe

 

 

.

Edited by moefuzz, 17 June 2025 - 04:57 PM.

[CygnuS]

I love science. You just never know what's around the corner when you look into something. Sometimes the answer is surprising simple when you expect it to be complex. 

And sometimes you expect questions (like the OP) to have simple answers and they end up being complex. 

Thanks again to everybody who has contributed here. I have read every response twice and have learned a lot. 

[CygnuS]

This is odd. To read the responses here it seems like it would be impossible to tell the story of dwarf galaxies without mentioning globular clusters, and vice versa. 

I just got done reading a 7 page story in Astronomy magazine about dwarf galaxies around our MWG and not once were globulars mentioned. (How Weird is the Milky Way?....July '25 page 12) 

[Lard Greystoke]

All classifications are only approximations of reality.

[Tony Flanders]

All classifications are only approximations of reality.

Indeed. And with respect to astronomy in particular, it's really important to remember just how little we know. I think of astronomy as a vast range of unknown, with tiny pools of understanding here and there.

Nobody really knows how globular clusters form. There are plenty of plausible hypotheses, but that's not the same as knowing.

Likewise, nobody really knows how and why galaxies form. And the more we learn about the subject, the less good our hypotheses seem.

It's not like you can take all the components for a galaxy, mix them together, and watch a galaxy form on your lab bench, after all.

[Bubbagumps]

Most of our current theoretical understanding of galactic formation and evolution has come through the application of supercomputers and advanced computational dynamics combined with the application of various statistical methods. This isn't a subject that lends itself to neat and tidy models like planets orbiting a star. There are no analytical solutions to dynamical equations of motion. When you set out to understand and model galactic formation, you essentially have to model a non-linear chaotic physical system with trillions of degrees of freedom. Extremely small variations in initial conditions can have a huge impact on the evolution and structure of the system over time. You have to take into account relativistic effects as well. It is the advent of supercomputing that has allowed us to make a dent in the subject. 

 

Over the past couple of decades there has been a lot of progress in this area. There is a research group at CalTech under a researcher named Phil Hopkins. He is currently at the forefront of these efforts and he has produced some very impressive results. I would recommend checking out some of the video simulations. 

 

https://www.youtube....tYfc38bn0&t=23s

 

Caltech Working Group Web Page:

 

http://www.tapir.cal...ite/animations/

[CygnuS]

Indeed. And with respect to astronomy in particular, it's really important to remember just how little we know. I think of astronomy as a vast range of unknown, with tiny pools of understanding here and there.

 

That is a great quote Tony! 

It reminds me of another one. It can be found on page 1 of Edward Harrison's 567 page book "Cosmology" that is 25 years old. 

"Proud of their knowledge and confident of its final truth, the members of a society pity the ignorance of their ancestors and fail to forsee that their descendants will also pity them for their ignorance."