Reply to post: Re: 500 stars a galaxy?

Captain, dark energy sensor readings show dwarf galaxies orbiting the Milky Way

Trevor_Pott Gold badge

Re: 500 stars a galaxy?

It's about density and stellar population. Globular clusters are really densely packed, despite not having a black hole larger than a few stellar masses. They also tend to be metal poor stars and are much, much, much older.

These dwarf galaxylets are likely to be small stellar nurseries ejected from the Milky Way during a previous collision and are hence metal rich and pretty new. Previous discoveries of similar objects have shown them to be not all that densely packed and roughly in line with the age of the stars in the host galaxy.

Some speculate that these represent stellar nurseries that formed around black holes of a few stellar masses and remained gravitationally bound as their location within the galactic disk was perturbed. As such, when they were ejected they stayed together. By staying together they retained enough gravity (collectively) that they didn't achieve galactic escape velocity but were simply pulled into a wider orbit.

It is probably better not to view these as dwarf galaxies in the traditional sense as they don't have supermassive black holes and aren't remnants of larger galaxies that experienced a collision event. They are - for lack of a better term - larger objects in the Kuiper belt of the Milky Way. Part of it as much as Eris, Haumea, Makemake and Sedna are part of Sol's Kuiper belt, which is a part of our solar system.

There is no hard edge to the galaxy. it doesn't just stop at some arbitrary point. The number of systems and gases peter out with distance, but within the very low density fringed agglomerations appear. These are typically material kicked out from the main body, but there is still enough low density stuff out there - gases, rogue planets, individual systems, etc - that it can keep small clusters of stars young.

I hope that explains how these objects are (most of them, anyways,) likely to be different from the old, dense globular clusters that orbit much farther out.

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