Topics to be covered and reviewed in this Talk What is a dwarf galaxy, as opposed to a star cluster? What types of dwarf galaxies do exist (dwarf elliptical, dwarf spheroidal, ultra faint dwarf, ultra compact dwarf, dwarf irregular, etc.), what are the differences, and how are they related to each other (if at all)? What is the origin of (compact) dwarf galaxies (formation scenarios, primordial vs. secondary galaxies)? A bit discussion of black holes in the compact dwarfs.

Galaxy or Star Cluster? The term “galaxy” was introduced about 100 years ago to describe stellar systems similar to the Milky Way in luminosity and extension. Star clusters are collections of stars roughly born at same time, and gravitationally bound.

Most galaxies have substructures (other than stars) but some don’t - but very young star clusters have such substructure, too! Distinguishing them by mere morphological properties, size and magnitude is not feasible anymore.

galaxies from star clusters Searching a criterion to distinguish galaxies from star clusters

1st criteria: “Gilmore gap” (or valley)

While ...

2nd criteria: relaxation time The relation time is a measure of how long the star can remember its initial energy. We will see more stars are there in the galaxy, the longer the relation time is. Galaxies can be considered as stellar system where this can not expand to star clusters within the age of the Universe.

3rd criteria: no dark matter

Types of dwarf galaxies

A spread in iron abundance M54 – GCs before 2017

The stuff below the Gilmore gap:

Formation scenarios for UCDs 1. UCDs may simply be the most GCs

Formation scenarios for UCDs 2. UCDs may form from merging star cluster complexes.

Formation scenarios for UCDs 3. UCDs may be the surviving nuclei of dwarf galaxies, and most of them are found in galaxy clusters. Mass and gas are stripped when going toward the center of galaxy cluster.

The stuff below the Gilmore gap: Owing to their old ages and high masses, they probably have produced a large number of stellar-mass black holes.

Be contrast with the long-stand debate that interactions within its nucleus are expected to eject the majority of black holes after 1Gyrs