How could ultracompact galaxies form?

Question

Normally, most galaxies such as our Milky Way tend to be diffused. Sure, the Milky Way, for example has 400 billion stars, but it is spread out over 150,000 light years. Which means that the Milky Way has a moderate star density, and is not clumped together.

However, I read this article which talked about something that left me shocked.

According to this article, this galaxy (UCD3) is actually 40,000x smaller than our Milky Way, but that is not the problem.

The problem is, that this galaxy's 100 million stars are packed tightly into a region only about 300 light-years across. For reference, the Small Magellanic Cloud is about 3,500 ly across.

Even more baffling is the fact that this UCD3, despite having much less mass than our Milky Way, has a supermassive black hole that clocks in at about 3.5 million solar masses. For comparison, the Milky Way's supermassive black hole, only weighs about 4.1 million solar masses.

This means that UCD3's SMBH contains about 4% of the mass of its host galaxy. Typically, most SMBHs contain less than 1% of their galaxy's mass. This makes UCD3 an extremely baffling galaxy.

How could these ultracompact galaxies have formed in the first place?

Answer 1

This makes UCD3 an extremely baffling galaxy.

The Universe is indeed full of baffling peculiarities!

How could these ultracompact galaxies have formed in the first place?

The origin and evolution of ultra-compact dwarf galaxies (UCDs) are open and active questions in astronomy/astrophysics. Do to their rather extreme compactness (not relativistic compactness!), a naive expectation is that UCDs are associated with starburst galaxies, however as the introduction of the nice and recent review by Jeřábková $\textit{et al.}$ (2018) states, "several possible formation scenarios have been proposed: (A) UCDs are the massive end of the distribution of GCs ...; (B) UCDs are merged star cluster complexes ...; (C) UCDs are the tidally stripped nuclei of dwarf galaxies ...; and (D) UCDs are remnants of primordial compact galaxies .... "

Even more baffling is the fact that this UCD3, despite having much less mass than our Milky Way, has a supermassive black hole that clocks in at about 3.5 million solar masses. For comparison, the Milky Way's supermassive black hole, only weighs about 4.1 million solar masses... This means that UCD3's SMBH contains about 4% of the mass of its host galaxy. Typically, most SMBHs contain less than 1% of their galaxy's mass

This is one of the main things that makes UCDs so interesting: they systematically are observed to have an elevated dynamical mass-to-light (M/L) ratios. Many UCDs have a large M/L ratio, so although this is different from the Milky Way, it should not be surprising since it is one of the defining properties of UCDs. Relative to AGN, a black hole mass to host galaxy stellar mass fraction of 1% is still really quite large! This fraction can be as large as 15% for some of these UCDs. These studies challenge the claim that the initial mass function of galaxies is universal. It is suggested that those with large fraction formed monolithically (scenario A above), and those with small fraction formed via mergers (scenario B above), but this is uncertain. The masses of UCDs can also be smaller than the one found in the article you cite, and yet still have an elevated M/L ratio. The

The Fornax galaxy cluster is where UCDs were originally discovered, but these were within the cluster core which is a very dense environment itself. This biased our understanding of how UCDs form. A recent study found 44 new UCDs in the Fornax cluster outside of the cluster core, bringing the total count to 104 UCDs, implying that they can just as "easily" form in lower density environments as well, e.g., via tidal stripping on elongated orbits. But this is uncertain. There are UCDs observed in the Virgo cluster as well, which contains the most dense UCD observed to date.