Donatello II and the difference between dwarf galaxies and sparse clusters

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Donatello II and the difference between dwarf galaxies and sparse clusters

Post by Ann » Mon Feb 06, 2023 10:40 am

ESA/Hubble wrote:

If you cannot quite distinguish the clump of faint stars that is all we can see of Donatiello II in this image, then you are in good company. Donatiello II is one of three newly discovered galaxies that were so difficult to spot that they were all missed by an algorithm designed to search astronomical data for potential galaxy candidates. Even the best algorithms have their limitations when it comes to distinguishing very faint galaxies from individual stars and background noise. In these most challenging identification cases, discovery has to be done the old-fashioned way — by a dedicated human trawling through the data themselves...

I took a screenshot of the full size version of Donatiello II to make you all see a little better:

Donatiello II dwarf galaxy Hubble.png
Donatiello II.
NGC 2547 chaosrand.png
Open cluster NGC 2547. Credit: chaosrand at flickr.

So what do you think? The first thing that strikes me is that practically all the brightest objects in Donatiello II appear to be galaxies. There are also perhaps 20 red giants that clearly belong to the dwarf galaxy itself, and there are a few blue objects that most likely are hugely distant tiny galaxies from the Cosmic Noon, during the epoch when the Universe was bursting with the formation of tiny galaxies full of furiously massive Population III (?) stars.

Anyway. Dwarf galaxy Donatiello II appears to be more sparsely populated than many star clusters in the Milky Way. See for yourself. Open cluster NGC 3547 looks more well-populated than dwarf galaxy Donatiello II, doesn't it?

I guess that the question of what is a galaxy and what is not boils down to location, location, location. If a group of stars is inside a galaxy, and particularly if this group of stars formed together inside that galaxy, then it is a cluster. But if the group of stars is on its own in space, and if it formed outside any other galaxy, then it is a dwarf galaxy.

Note the cute heart shape of Donatiello II. Admittedly that lovely shape is created by the nearby dwarf galaxy overlapping several very distant background galaxies. Anyway, it's cute, and it is suitable for Valentine's Day, which is coming up soon! ❤️

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